**Meet the editor**

Mohammed Muzibur Rahman received his B.Sc. and M.Sc. from Shahjalal University of Science and Technology, Sylhet, Bangladesh, in 1999 and 2001, respectively. He received his Ph.D. from the Chonbuk National University, South Korea, in 2007. After his Ph.D., he worked as a postdoctoral fellowship and assistant professor in pioneer research centers and universities located

in South Korea, Japan, and Saudi Arabia (2007–2011). Since 2011 he has worked as Associate Professor in the Center of Excellence for Advanced Materials Research and Chemistry Department at King Abdulaziz University, Saudi Arabia. He has published more than 225 research articles and several proceedings in well-known high-impact ISI journals; attended more than 60 international and domestic conferences; and published several book chapters and 10 books as an editor. His research work has been largely in the area of carbon nanotubes, nanotechnology, sensors, ionic liquids, surface chemistry, electrochemistry, instrumental science, nanomaterials, self-assembled monolayers, photochemistry, m-chips and devices, etc.

Contents

**Preface VII**

Víllora

Miyatake

Yibo Wu

**Section 1 State of the Art Preparation 1**

Chapter 2 **Phosphazene-Based Ionic Liquids 27**

**Nascent Steel Surface 47**

**Section 2 State of the Art Characterization 67**

Neerish Revaprasadu

**Section 3 State of the Art Polymerization 91**

Chapter 5 **Ionic Polymerization in Ionic Liquids 93**

Ahmet Karadağ and Hüseyin Akbaş

Chapter 4 **Progress in Green Solvents for the Stabilisation of**

Chapter 6 **Ionic Liquids for Desulphurization: A Review 107**

Chapter 1 **Biopolymeric Nanoparticle Synthesis in Ionic Liquids 3**

Chapter 3 **Tribochemical Reactions of Halogen-Free Ionic Liquids on**

Mercedes G. Montalbán, Guzmán Carissimi, A. Abel Lozano-Pérez, José Luis Cenis, Jeannine M. Coburn, David L. Kaplan and Gloria

Shouhei Kawada, Seiya Watanabe, Shinya Sasaki and Masaaki

**Nanomaterials: Imidazolium Based Ionic Liquids 69**

Zikhona Tshemese, Siphamandla C. Masikane, Sixberth Mlowe and

Syamsul Bahari Abdullah, Hanida Abdul Aziz and Zakaria Man

## Contents

#### **Preface XI**

#### **Section 1 State of the Art Preparation 1**


#### **X** Contents

#### **Section 4 State of the Art Applications 121**

Chapter 7 **Applications of Ionic Liquids in Elastomeric Composites: A Review 123** Anna Sowinska and Magdalena Maciejewska

#### Chapter 8 **Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System 145** Pius Dore Ola and Michiaki Matsumoto

Chapter 9 **Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane and Binary Mix Gas Hydrates 159** Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and Mohamad Azmi Bustam

Preface

It gives me immense pleasure to introduce this book, *Recent Advances in Ionic Liquids*, based on the state of the art of preparation, characterization, polymerization, as well as application of ionic liquids in various industrial settings. Discussion of these aspects develops through fun‐ damental and applied experimental routes by various methods and comprises the interfacing of the scientific and technological worlds. Usually, ionic liquids have incontestably attained achievement of their conventional essence and have taken new directions from preparation and polymerization to potential applications in the research and development areas of sci‐ ence. The new routes and developing edges branch out from time to time around this ad‐ vanced stage of stable ionic liquids. Advances in ionic liquids with instrumentation for evaluating the practical use of these materials now enables us to understand quite broadly almost all the events that take place for ionic liquids. Because of the exceptional and unique properties of ionic liquids, they can offer significant and great interest for developing green, efficient, and influential catalytic utilization and technologies. The latest advances in ionic liquid catalysis and applications are focused on different fields, *namely* preparation, proper‐ ties, utilization, polymerization, applications of ionic liquids, and catalytic conversion or pro‐ duction of useful chemicals in ionic liquids. Particularly via selected samples preparation of ionic liquids, the advantages and potential applications of ionic liquids in exploring cleaner and efficient polymerization and catalytic technologies and processes can be described.

In this book, Prof. Villora et al. review the synthesis of biopolymeric nanoparticles using ion‐ ic liquids, such as trimethylsilyl cellulose or silk fibroin. They describe how high-power ul‐ trasounds are capable of enhancing the dissolution process of silk proteins in ionic liquids and how silk fibroin nanoparticles can be directly obtained from the silk fibroin/ionic liquid

Prof. Karadag et al. focus on the synthesis and possible application of cyclo- and polyphos‐ phazene-based ionic liquids (PzILs). PzILs constitute an alternative class of phosphorous ni‐ trogen compounds, and their derivatives have been widely used in biologically active materials, electrolytes, lubricants, catalysts, or nanomaterials. Considerable information is available on substitution reactions taking place in the phosphorous atoms of poly- and cy‐ clophosphazenes, thus a wide variety of phosphazene derivatives have been obtained.

Prof. Kawada et al. describe the use of halogen-free anion-based ionic liquids as lubricants. In this approach, the study investigates the tribological performance and lubricating mecha‐ nisms of sulfur, phosphorus, and cyano-anion-based ionic liquids. Sulfur and phosphorous anion-based ionic liquids form reaction films on worn surfaces; the sulfur- and phosphoruscontaining films exhibit low friction coefficients and specific wear rates, respectively. The steric hindrance of the ionic liquids affects their tribochemical reaction behaviors. Cyano-

solution by rapid desolvation in polar organic solvents.

## Preface

**Section 4 State of the Art Applications 121**

**Two-Phase System 145**

Mohamad Azmi Bustam

**A Review 123**

**VI** Contents

**Hydrates 159**

Chapter 7 **Applications of Ionic Liquids in Elastomeric Composites:**

Anna Sowinska and Magdalena Maciejewska

Chapter 9 **Kinetic Assessment of Tetramethyl Ammonium Hydroxide**

**(Ionic Liquid) for Carbon Dioxide, Methane and Binary Mix Gas**

Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and

Chapter 8 **Metal Extraction with Ionic Liquids-Based Aqueous**

Pius Dore Ola and Michiaki Matsumoto

It gives me immense pleasure to introduce this book, *Recent Advances in Ionic Liquids*, based on the state of the art of preparation, characterization, polymerization, as well as application of ionic liquids in various industrial settings. Discussion of these aspects develops through fun‐ damental and applied experimental routes by various methods and comprises the interfacing of the scientific and technological worlds. Usually, ionic liquids have incontestably attained achievement of their conventional essence and have taken new directions from preparation and polymerization to potential applications in the research and development areas of sci‐ ence. The new routes and developing edges branch out from time to time around this ad‐ vanced stage of stable ionic liquids. Advances in ionic liquids with instrumentation for evaluating the practical use of these materials now enables us to understand quite broadly almost all the events that take place for ionic liquids. Because of the exceptional and unique properties of ionic liquids, they can offer significant and great interest for developing green, efficient, and influential catalytic utilization and technologies. The latest advances in ionic liquid catalysis and applications are focused on different fields, *namely* preparation, proper‐ ties, utilization, polymerization, applications of ionic liquids, and catalytic conversion or pro‐ duction of useful chemicals in ionic liquids. Particularly via selected samples preparation of ionic liquids, the advantages and potential applications of ionic liquids in exploring cleaner and efficient polymerization and catalytic technologies and processes can be described.

In this book, Prof. Villora et al. review the synthesis of biopolymeric nanoparticles using ion‐ ic liquids, such as trimethylsilyl cellulose or silk fibroin. They describe how high-power ul‐ trasounds are capable of enhancing the dissolution process of silk proteins in ionic liquids and how silk fibroin nanoparticles can be directly obtained from the silk fibroin/ionic liquid solution by rapid desolvation in polar organic solvents.

Prof. Karadag et al. focus on the synthesis and possible application of cyclo- and polyphos‐ phazene-based ionic liquids (PzILs). PzILs constitute an alternative class of phosphorous ni‐ trogen compounds, and their derivatives have been widely used in biologically active materials, electrolytes, lubricants, catalysts, or nanomaterials. Considerable information is available on substitution reactions taking place in the phosphorous atoms of poly- and cy‐ clophosphazenes, thus a wide variety of phosphazene derivatives have been obtained.

Prof. Kawada et al. describe the use of halogen-free anion-based ionic liquids as lubricants. In this approach, the study investigates the tribological performance and lubricating mecha‐ nisms of sulfur, phosphorus, and cyano-anion-based ionic liquids. Sulfur and phosphorous anion-based ionic liquids form reaction films on worn surfaces; the sulfur- and phosphoruscontaining films exhibit low friction coefficients and specific wear rates, respectively. The steric hindrance of the ionic liquids affects their tribochemical reaction behaviors. Cyanoanion-based ionic liquids also show low friction coefficients; however, their values are high‐ er than those of halogen anion-based ionic liquids.

Lastly, Khan et al. highlight the impact of ammonium-based ionic liquid TMAOH on the formation kinetics of pure carbon dioxide (CO2), methane (CH4), and their binary mixed gas (50–50 mole%) hydrates. Hydrate formation induction time, the initial apparent rate of for‐ mation, and the total gas consumed into hydrate are the kinetic parameters used to evaluate

This work aims to bridge the gap between undergraduate, graduate, and scientist in applied ionic liquids, to initiate researchers into ionic liquid study in as straightforward a way as possible, and to introduce researchers to the opportunities offered by applied science and technological fields. I have worked unswervingly to complete this work for the InTech open access publisher. I hope that this contribution would further enhance applied stable liquid materials in nano- and bioscience, biosynthesis, polymerization, desulfurization, stabiliza‐ tion, kinetic assessments, and metal extraction, especially in bringing new entrants into the applied and modified ionic liquids science and technology fields and helping scientists to

Center of Excellence for Advanced Materials Research (CEAMR) and

**Mohammed Muzibur Rahman**

Chemistry Department Faculty of Science

Preface IX

King Abdulaziz University Jeddah, Saudi Arabia

the performance of TMAOH on hydrate formation.

develop their own field of specialization.

In this approach, Prof. Mlowe et al. describe how the physicochemical properties of ionic liquids differ significantly depending on the anionic/cationic species and alkyl chain length. Ionic liquids have found application in many scientific fields, the most recent being good solvents and stabilizing agents in nanomaterial synthesis. Their studies have shown that ionic liquids not only stabilize as synthesized nanomaterials but also provide environmen‐ tally green routes towards nanomaterial engineering.

Prof. Wu discusses the advantages and limitations of the application of ionic liquids as sol‐ vents for ionic polymerization processes. The most important types of cationic monomers, such as styrene and its derivatives, vinyl ethers, and isobutylene, have been polymerized in ionic liquids, and even undergo living polymerization. From his study, it is concluded that ionic liquids seem unsuitable solvents for anionic polymerization.

Prof. Abdullah et al. describe the interaction mechanism of ionic liquids with sulfur in a model oil system. The interaction is predicted using COSMO-RS where the strength of the hydrogen bond of anions should be reduced to increase thiophene extraction capacity. The absorption capacity of sulfur compounds in ionic liquids is strongly dependent on the chem‐ ical structures, physical properties, and compactness between the cation and the anion of the ionic liquids. Finally, the extractive desulfurization process using selective ionic liquids as the extractant is still in need of further research, starting from the screening of suitable ionic liquids for desulfurization, synthesis of ionic liquids, physical property analysis of ionic liq‐ uids, single batch extraction study encompassing process optimization up to actual diesel application, and the regeneration of spent ionic liquids.

In this approach, Prof. Maciejewska et al. reviews the advantages of ionic liquids as func‐ tional additives for elastomeric composites, with special emphasis on their use as dispersing agents for fillers, components of conducting rubber composites, crosslinkers, or components of crosslinking systems. An analysis of the recent literature reports indicates that ionic liq‐ uids are widely used in elastomeric composites as dispersing agents of fillers, conductive additives, crosslinkers or components of the crosslinking system (vulcanization accelerators or activators), catalysts for the silanization reaction, solvents for the depolymerization of natural rubber, or for the production of highly stretchable ionogels. Because the structure of ionic liquids can be designed for specific applications, it can be expected that the use of ionic liquids in elastomeric composites will continue to increase.

Prof. Matsumoto summarizes the use of an ionic liquids-based aqueous two-phase system (ATPS) for the separation of metals used in various areas of human life. An ATPS composed of ionic liquids and a salting-out agent is excellent for metal ion separation because of its efficiency, selectivity, and environmental friendliness. Due to the temperature dependence of a mixture comprised of ionic liquids with water, it has been manipulated for metal ion extraction known as homogeneous liquid/liquid extraction (HLLE). HLLE also showed high efficiency and selectivity in metal ion extraction. Metal ions can be extracted by both an ATPS and HLLE, including transition metals, rare earth elements, and radioactive substan‐ ces. Results reveal that tetramethylammonium hydroxide (TMAOH) tends to delay hydrate formation for all the studied hydrate systems at all concentrations. The presence of TMAOH also reduces the total gas consumed into hydrates and the initial rate of hydrate formation in most of the studied systems.

Lastly, Khan et al. highlight the impact of ammonium-based ionic liquid TMAOH on the formation kinetics of pure carbon dioxide (CO2), methane (CH4), and their binary mixed gas (50–50 mole%) hydrates. Hydrate formation induction time, the initial apparent rate of for‐ mation, and the total gas consumed into hydrate are the kinetic parameters used to evaluate the performance of TMAOH on hydrate formation.

anion-based ionic liquids also show low friction coefficients; however, their values are high‐

In this approach, Prof. Mlowe et al. describe how the physicochemical properties of ionic liquids differ significantly depending on the anionic/cationic species and alkyl chain length. Ionic liquids have found application in many scientific fields, the most recent being good solvents and stabilizing agents in nanomaterial synthesis. Their studies have shown that ionic liquids not only stabilize as synthesized nanomaterials but also provide environmen‐

Prof. Wu discusses the advantages and limitations of the application of ionic liquids as sol‐ vents for ionic polymerization processes. The most important types of cationic monomers, such as styrene and its derivatives, vinyl ethers, and isobutylene, have been polymerized in ionic liquids, and even undergo living polymerization. From his study, it is concluded that

Prof. Abdullah et al. describe the interaction mechanism of ionic liquids with sulfur in a model oil system. The interaction is predicted using COSMO-RS where the strength of the hydrogen bond of anions should be reduced to increase thiophene extraction capacity. The absorption capacity of sulfur compounds in ionic liquids is strongly dependent on the chem‐ ical structures, physical properties, and compactness between the cation and the anion of the ionic liquids. Finally, the extractive desulfurization process using selective ionic liquids as the extractant is still in need of further research, starting from the screening of suitable ionic liquids for desulfurization, synthesis of ionic liquids, physical property analysis of ionic liq‐ uids, single batch extraction study encompassing process optimization up to actual diesel

In this approach, Prof. Maciejewska et al. reviews the advantages of ionic liquids as func‐ tional additives for elastomeric composites, with special emphasis on their use as dispersing agents for fillers, components of conducting rubber composites, crosslinkers, or components of crosslinking systems. An analysis of the recent literature reports indicates that ionic liq‐ uids are widely used in elastomeric composites as dispersing agents of fillers, conductive additives, crosslinkers or components of the crosslinking system (vulcanization accelerators or activators), catalysts for the silanization reaction, solvents for the depolymerization of natural rubber, or for the production of highly stretchable ionogels. Because the structure of ionic liquids can be designed for specific applications, it can be expected that the use of ionic

Prof. Matsumoto summarizes the use of an ionic liquids-based aqueous two-phase system (ATPS) for the separation of metals used in various areas of human life. An ATPS composed of ionic liquids and a salting-out agent is excellent for metal ion separation because of its efficiency, selectivity, and environmental friendliness. Due to the temperature dependence of a mixture comprised of ionic liquids with water, it has been manipulated for metal ion extraction known as homogeneous liquid/liquid extraction (HLLE). HLLE also showed high efficiency and selectivity in metal ion extraction. Metal ions can be extracted by both an ATPS and HLLE, including transition metals, rare earth elements, and radioactive substan‐ ces. Results reveal that tetramethylammonium hydroxide (TMAOH) tends to delay hydrate formation for all the studied hydrate systems at all concentrations. The presence of TMAOH also reduces the total gas consumed into hydrates and the initial rate of hydrate formation

er than those of halogen anion-based ionic liquids.

VIII Preface

tally green routes towards nanomaterial engineering.

application, and the regeneration of spent ionic liquids.

liquids in elastomeric composites will continue to increase.

in most of the studied systems.

ionic liquids seem unsuitable solvents for anionic polymerization.

This work aims to bridge the gap between undergraduate, graduate, and scientist in applied ionic liquids, to initiate researchers into ionic liquid study in as straightforward a way as possible, and to introduce researchers to the opportunities offered by applied science and technological fields. I have worked unswervingly to complete this work for the InTech open access publisher. I hope that this contribution would further enhance applied stable liquid materials in nano- and bioscience, biosynthesis, polymerization, desulfurization, stabiliza‐ tion, kinetic assessments, and metal extraction, especially in bringing new entrants into the applied and modified ionic liquids science and technology fields and helping scientists to develop their own field of specialization.

#### **Mohammed Muzibur Rahman**

Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

**Section 1**

**State of the Art Preparation**

## **State of the Art Preparation**

**Chapter 1**

Provisional chapter

**Biopolymeric Nanoparticle Synthesis in Ionic Liquids**

DOI: 10.5772/intechopen.78766

Recently, much research has focused on the use of biopolymers, which are regarded as biodegradable, natural, and environmentally friendly materials. In this context, biopolymeric nanoparticles have attracted great attention in the last few years due to their multiple applications especially in the field of biomedicine. Ionic liquids have emerged as promising solvents for use in a wide variety of chemical and biochemical processes for their extraordinary properties, which include negligible vapor pressure, high thermal and chemical stability, lower toxicity than conventional organic solvents, and the possibility of tuning their physical–chemical properties by choosing the appropriate cation and anion. We here review the published works concerning the synthesis of biopolymeric nanoparticles using ionic liquids, such as trimethylsilyl cellulose or silk fibroin. We also mention our recent studies describing how high-power ultrasounds are capable of enhancing the dissolution process of silk proteins in ionic liquids and how silk fibroin nanoparticles can be directly obtained from the silk fibroin/ionic liquid solution by rapid desolvation in polar organic solvents. As an example, their potential biomedical application of curcumin-

loaded silk fibroin nanoparticles for cancer therapy is also discussed.

1.1. Biopolymeric nanoparticles for biomedical applications

Keywords: ionic liquid, biopolymer, nanoparticle, synthesis, silk fibroin

During the last 30 years, nanotechnology has attracted much attention in many engineering fields including electronic, mechanical, biomedical, and space engineering. Among these fields,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Biopolymeric Nanoparticle Synthesis in Ionic Liquids

Mercedes G. Montalbán, Guzmán Carissimi,

Mercedes G. Montalbán, Guzmán Carissimi,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

A. Abel Lozano-Pérez, José Luis Cenis, Jeannine M. Coburn, David L. Kaplan and

A. Abel Lozano-Pérez, José Luis Cenis, Jeannine M. Coburn, David L. Kaplan and

http://dx.doi.org/10.5772/intechopen.78766

Gloria Víllora

Abstract

1. Introduction

Gloria Víllora

#### **Biopolymeric Nanoparticle Synthesis in Ionic Liquids** Biopolymeric Nanoparticle Synthesis in Ionic Liquids

DOI: 10.5772/intechopen.78766

Mercedes G. Montalbán, Guzmán Carissimi, A. Abel Lozano-Pérez, José Luis Cenis, Jeannine M. Coburn, David L. Kaplan and Gloria Víllora Mercedes G. Montalbán, Guzmán Carissimi, A. Abel Lozano-Pérez, José Luis Cenis, Jeannine M. Coburn, David L. Kaplan and Gloria Víllora

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78766

#### Abstract

Recently, much research has focused on the use of biopolymers, which are regarded as biodegradable, natural, and environmentally friendly materials. In this context, biopolymeric nanoparticles have attracted great attention in the last few years due to their multiple applications especially in the field of biomedicine. Ionic liquids have emerged as promising solvents for use in a wide variety of chemical and biochemical processes for their extraordinary properties, which include negligible vapor pressure, high thermal and chemical stability, lower toxicity than conventional organic solvents, and the possibility of tuning their physical–chemical properties by choosing the appropriate cation and anion. We here review the published works concerning the synthesis of biopolymeric nanoparticles using ionic liquids, such as trimethylsilyl cellulose or silk fibroin. We also mention our recent studies describing how high-power ultrasounds are capable of enhancing the dissolution process of silk proteins in ionic liquids and how silk fibroin nanoparticles can be directly obtained from the silk fibroin/ionic liquid solution by rapid desolvation in polar organic solvents. As an example, their potential biomedical application of curcuminloaded silk fibroin nanoparticles for cancer therapy is also discussed.

Keywords: ionic liquid, biopolymer, nanoparticle, synthesis, silk fibroin

#### 1. Introduction

#### 1.1. Biopolymeric nanoparticles for biomedical applications

During the last 30 years, nanotechnology has attracted much attention in many engineering fields including electronic, mechanical, biomedical, and space engineering. Among these fields,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nanotechnology has led to significant progress being made in biomedicine through the development of efficient structures for controlled and targeted drug or gene delivery, tissue engineering, the imaging of specific sites, regenerative medicine, biosensing, and probing of the DNA structure [1]. More specifically, the role of nanoparticles in the progress made in this field is of particular note, and treatments involving nanoparticles have been widely applied in cancer therapy, diabetes, allergy, infection, and inflammation [2]. The main advantages of nanoparticles used as drug carriers for the treatment of these diseases are the following: (a) a size range similar to that of proteins, (b) a large surface area, which allows the presence of different functional groups acting as ligands, (c) fast absorption and release properties, and (d) particle sizes and surface features that can be specifically designed.

stationary phase, supports for the enzyme immobilization, as liquid crystals, in technologies of separation, for nanomaterials synthesis as templates, in the synthesis of catalytic membranes,

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766 5

ILs may be excellent candidates for dissolving biopolymers and developing biomaterials mainly because of the flexibility that can be achieved by combining different cations and anions, and green solvent properties such as non-volatility, non-flammability, and recyclability. During recent years, biopolymers such as cellulose, xylan, starch, chitin/chitosan, keratin, silk fibroin (SF), and heparin have been transformed from ILs into films, scaffolds, membranes, fibers, and micro- or nanoparticles. In addition, composites formed by biopolymer/biopolymer or biopolymer/synthetic polymer mixtures can also be synthesized by the co-dissolution of polymers in ILs [1]. The interesting properties of ILs make them excellent media for the synthesis and stabilization of nanoparticles [15]. Their most noteworthy features for the syn-

a. ILs can contribute to the synthesis of small particles due to their generally low surface

b. ILs enhance the electronic and steric stabilization of nanoparticles and reduce particle growth, since their constituents (anion and cation) form a protective electrostatic shell that prevents agglomeration processes. In addition, nanoparticles are stable in solution due to

c. ILs have a significant effect on the shape of the synthesized nanoparticles because they are highly structured liquids which can form extended hydrogen-bonded networks.

d. ILs can be designed using different cation/anion combinations to achieve the desired

e. ILs can act as reactive agents, which may be of interest for the synthesis of nanoparticles

f. The choice of IL used for the synthesis of nanoparticles determines, at least partially, their

g. ILs possess negligible vapor pressure and are non-flammable, which allows safer operation at high temperature and under vacuum than with conventional organic solvents. h. ILs can be tailored following the 12 principles of green chemistry developed by Anastas and Warner [16] which, besides non-volatility, low toxicity, non-corrosiveness, and non-

In short, the synthesis of nanoparticles using ILs can be carried out by the following procedures that are more environmentally friendly than those using conventional organic solvents.

In this chapter, the processes used to synthesize biopolymer (polysaccharide or protein) nanoparticles using ILs are reviewed. To the best of our knowledge, the biopolymers used to obtain nanoparticles in processes involving ILs are cellulose, xylan, starch, chitosan, keratin, and silk fibroin. A previous work of the authors based on the synthesis of silk fibroin nanoparticles by dissolving the protein in several ILs is discussed. An example of the biomedical application of

curcumin-loaded silk fibroin nanoparticles obtained with this method is also shown.

the coordination of anion or cation through ionic or covalent bonds.

properties of density, viscosity, hydrophilicity, gas solubility, and so on.

flammability, do not include requiring auxiliary or separation solvents.

and in the generation of high conductivity materials [13, 14].

thesis of nanoparticles are as follows:

by chemical reactions.

properties, including water solubility.

tension, which leads to high nucleation rates.

Nanoparticles for therapeutic application can be synthesized from (natural or synthetic) polymers, ceramics, and metals [3]. Polymeric nanoparticles have been widely studied as nanocarriers of active molecules such as drugs and genes [4–9]. The main drawbacks in the use of polymeric nanoparticles are the difficulty of scaling up, and their low drug loading capacity and wide size distribution [2]. However, as nanoplatforms, they show great potential because they allow the targeted release of drugs to specific cells or tissues [10]. Moreover, in contrast to ceramic or metal nanoparticles, polymeric nanoparticles can be synthesized in a wide range of sizes and forms and can sustain localized drug therapeutic agents for weeks [3]. While the above features are common to both natural and synthetic polymers, natural polymers, also known as biopolymers, have some extra advantages such as their inherent biocompatibility and biodegradability, non-antigenicity, a high nutritional value, the abundance of their renewable sources, and an extraordinary binding capacity with several drugs [11]. Biopolymers are natural macromolecules and can be polysaccharides, proteins, polyphenols, polyesters, or polyamides. The synthesis of biopolymer-based materials, such as nanoparticles, is still difficult due to the low solubility of biopolymers in conventional organic solvents as a result of their highly crystalline structure [1]. Therefore, the search for new solvents capable of dissolving biopolymers successfully is a continuous challenge in order to achieve the industrial fabrication of biomaterials.

#### 1.2. Ionic liquids as media for the synthesis of nanoparticles

Ionic liquids (ILs) are organic salts that are liquid close to room temperature. They normally consist of an organic cation and a polyatomic inorganic anion. They have attracted great attention recently for use in a variety of chemical processes as "green" solvents. The most important advantage of ILs is their non-detectable vapor pressure, which makes them environmentally benign solvents compared with volatile organic solvents (VOSs). The growing awareness about the risk of using these solvents has led to a search for alternatives, and the discovery of ILs seemed to solve the problem [12]. They also show several advantages over classic organic solvents such as a good chemical and thermal stability, a high ionic conductivity, non-flammability, a large electrochemical window, solvation ability, and they can be used at high temperatures [13, 14]. Furthermore, the physical–chemical properties of ILs, such as their density, polarity, hydrophobicity, melting point, viscosity, and solvent properties, may be tuned by modifying the anion or the cation. Among other applications, ILs have been used as electrolytes, solvents, lubricants, matrices for mass spectrometry, for chromatography as stationary phase, supports for the enzyme immobilization, as liquid crystals, in technologies of separation, for nanomaterials synthesis as templates, in the synthesis of catalytic membranes, and in the generation of high conductivity materials [13, 14].

nanotechnology has led to significant progress being made in biomedicine through the development of efficient structures for controlled and targeted drug or gene delivery, tissue engineering, the imaging of specific sites, regenerative medicine, biosensing, and probing of the DNA structure [1]. More specifically, the role of nanoparticles in the progress made in this field is of particular note, and treatments involving nanoparticles have been widely applied in cancer therapy, diabetes, allergy, infection, and inflammation [2]. The main advantages of nanoparticles used as drug carriers for the treatment of these diseases are the following: (a) a size range similar to that of proteins, (b) a large surface area, which allows the presence of different functional groups acting as ligands, (c) fast absorption and release properties, and (d) particle sizes and

Nanoparticles for therapeutic application can be synthesized from (natural or synthetic) polymers, ceramics, and metals [3]. Polymeric nanoparticles have been widely studied as nanocarriers of active molecules such as drugs and genes [4–9]. The main drawbacks in the use of polymeric nanoparticles are the difficulty of scaling up, and their low drug loading capacity and wide size distribution [2]. However, as nanoplatforms, they show great potential because they allow the targeted release of drugs to specific cells or tissues [10]. Moreover, in contrast to ceramic or metal nanoparticles, polymeric nanoparticles can be synthesized in a wide range of sizes and forms and can sustain localized drug therapeutic agents for weeks [3]. While the above features are common to both natural and synthetic polymers, natural polymers, also known as biopolymers, have some extra advantages such as their inherent biocompatibility and biodegradability, non-antigenicity, a high nutritional value, the abundance of their renewable sources, and an extraordinary binding capacity with several drugs [11]. Biopolymers are natural macromolecules and can be polysaccharides, proteins, polyphenols, polyesters, or polyamides. The synthesis of biopolymer-based materials, such as nanoparticles, is still difficult due to the low solubility of biopolymers in conventional organic solvents as a result of their highly crystalline structure [1]. Therefore, the search for new solvents capable of dissolving biopolymers successfully is a continuous challenge in order to achieve the industrial

Ionic liquids (ILs) are organic salts that are liquid close to room temperature. They normally consist of an organic cation and a polyatomic inorganic anion. They have attracted great attention recently for use in a variety of chemical processes as "green" solvents. The most important advantage of ILs is their non-detectable vapor pressure, which makes them environmentally benign solvents compared with volatile organic solvents (VOSs). The growing awareness about the risk of using these solvents has led to a search for alternatives, and the discovery of ILs seemed to solve the problem [12]. They also show several advantages over classic organic solvents such as a good chemical and thermal stability, a high ionic conductivity, non-flammability, a large electrochemical window, solvation ability, and they can be used at high temperatures [13, 14]. Furthermore, the physical–chemical properties of ILs, such as their density, polarity, hydrophobicity, melting point, viscosity, and solvent properties, may be tuned by modifying the anion or the cation. Among other applications, ILs have been used as electrolytes, solvents, lubricants, matrices for mass spectrometry, for chromatography as

surface features that can be specifically designed.

4 Recent Advances in Ionic Liquids

fabrication of biomaterials.

1.2. Ionic liquids as media for the synthesis of nanoparticles

ILs may be excellent candidates for dissolving biopolymers and developing biomaterials mainly because of the flexibility that can be achieved by combining different cations and anions, and green solvent properties such as non-volatility, non-flammability, and recyclability. During recent years, biopolymers such as cellulose, xylan, starch, chitin/chitosan, keratin, silk fibroin (SF), and heparin have been transformed from ILs into films, scaffolds, membranes, fibers, and micro- or nanoparticles. In addition, composites formed by biopolymer/biopolymer or biopolymer/synthetic polymer mixtures can also be synthesized by the co-dissolution of polymers in ILs [1]. The interesting properties of ILs make them excellent media for the synthesis and stabilization of nanoparticles [15]. Their most noteworthy features for the synthesis of nanoparticles are as follows:


In short, the synthesis of nanoparticles using ILs can be carried out by the following procedures that are more environmentally friendly than those using conventional organic solvents.

In this chapter, the processes used to synthesize biopolymer (polysaccharide or protein) nanoparticles using ILs are reviewed. To the best of our knowledge, the biopolymers used to obtain nanoparticles in processes involving ILs are cellulose, xylan, starch, chitosan, keratin, and silk fibroin. A previous work of the authors based on the synthesis of silk fibroin nanoparticles by dissolving the protein in several ILs is discussed. An example of the biomedical application of curcumin-loaded silk fibroin nanoparticles obtained with this method is also shown.

#### 1.3. Methods of synthesis

One method used to obtain nanoparticles in ILs is synthesis via physical vapor deposition (PVP) since, due to the negligible vapor pressure of most ILs, they can be manipulated under high vacuum conditions even at high temperatures [17]. This method has been used to obtain metal nanoparticles through vaporization of a metal, an intermetallic phase, or a metal salt in the presence of an IL, for example, copper nanoparticles in the IL butyl-3-methylimidazolium hexafluorophosphate, [bmim<sup>+</sup> ][PF6 ]. By means of this technique, metal nanoparticles can be deposited not only on the IL but also onto materials dispersed in the IL, as occurs in the formation of Cu/ZnO nanocomposites in butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [bmim<sup>+</sup> ][NTf2 ]. Nevertheless, to our knowledge, the PVP method has never been applied to obtain nanoparticles from biopolymers in ILs.

glyosidic bond to form a linear structure. The degree of polymerization (DP) varies from the source but can be as high as 23,000 units. Cellulose has a ribbon-like structure, with two units per turn of 1.03 nm, dictated by the allowed angles of C1-O and O-C4 [24, 25]. In terms of

(1 GPa), when measured along the polymer length, than nylon, silk, chitin, collagen, tendon, or bone [26]. Cellulose is completely insoluble in water, mainly because its hydrogen-bonding capabilities are occupied by side chains, forming aggregates. Nevertheless, it has been demon-

cellulose even at high DP (6500 units), making it an appropriate reaction medium [24]. A

Swatloski et al. [20] carried out experiments using cellulose-dissolving pulps, fibrous cellulose, and Whatman cellulose filter papers. They prepared the cellulose solutions by the addition of the cellulose to the ionic liquids without pretreatment and heated on a heating plate or in a domestic microwave oven. These authors were able to dissolve 25 wt% cellulose in [bmim<sup>+</sup>

[Cl] using microwave heating. Nevertheless, ILs containing "non-coordinating" anions, including [BF4]- and [PF6]-, were nonsolvents, presumably because cellulose solubilizes through hydrogen bonding from hydroxyl functions to the anions of the solvent, [Cl]. The authors did not analyze the peak temperature reached during the microwave dissolution step

Han et al. [27] synthetized cellulose nanoparticles by dissolving microcrystalline cellulose

involves four steps: dissolution, regeneration, homogenization, and freeze-drying. Before pro-

dried to remove the remnants of water, whose presence can significantly impair cellulose solubility in IL by competing with the IL for hydrogen bonds to the cellulose microfibrils [20]. Then, the cellulose was dissolved in IL, using a magnetic hot plate stirrer with safety control, by stirring the 5% w/w disperse fibers for 5 h in a 125C oil bath. When a clear phase was obtained, the solution was slowly poured into distilled water and a white dispersion of nanoparticles immediately appeared. In order to remove IL, the dispersion was filtered, centrifuged, washed three times, and, eventually, dialyzed. To further enhance the suspension, the sample was run through a high-pressure homogenization, and, finally, the suspensions were quickly frozen by mixing acetone and dry ice in an ice pot and transferred to a freezedryer. Particles regenerated from MCC yield rods of 112 42 nm in length with an aspect ratio of 9.21 (Figure 1c) and spherical nanoparticles with a diameter of 118 32 nm (Figure 1e and f). Particles regenerated from CC formed only nanorods of 123 34 nm with an aspect ratio of

When infrared spectra were used to characterize the cellulose structure of samples, structural changes from a cellulose crystalline structure I to II were evident. For instance, after the regeneration of cotton cellulose, the -CH stretching vibration signals moved from 2901 to a

glycosidic linkage torsion angles. For both the MCC and MCC-NP, spectra showed strong

vibration from β-glycosidic linkage shifted from 897 to 894 cm<sup>1</sup>

, the CO stretching vibration at C-6 switched from 1033 to 1025 cm<sup>1</sup>

strated that the IL 1-butyl-3-methylimidazolium chloride ([bmim<sup>+</sup>

review of the solubility of cellulose in ILs is provided by Lee et al. [1].

), cellulose fibers are stiffer (Young's modulus 50–130 GPa) and stronger

][Cl]) is able to solubilize

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766

][Cl]. The preparation of cellulose nanoparticles

][Cl], MCC, or CC have to be vacuum-

]

7

, and COC

, probably due to changes in

density (1.5 Mgm<sup>3</sup>

nor the extent of cellulose degradation.

9.96 (Figure 1d).

lower 2893 cm<sup>1</sup>

(MCC) or cotton cellulose (CC) in [bmim<sup>+</sup>

ceeding to the dissolution process, solid state [bmim+

Microwave synthesis may be used to profit from the presence of large ions with high polarizability and conductivity of the ILs, which makes them very good media for absorbing microwaves, leading to high heating rates that result in the rapid formation of nuclei (nanoparticles) [18, 19]. Control over the particle size range can be achieved by changing the process temperature and time, as well as reactant concentrations and the choice of the anion and cation constituents of the IL. If the heat spreads uniformly through the sample, a narrow nanoparticle size distribution may be obtained. As an example, silver and gold nanoparticles have been obtained by microwave synthesis in 2-hydroxyethyl-N,N,N-trimethylammonium bis{(trifluoromethyl)sulfonyl}amide. Swatloski et al. [20] used microwave heating on a [bmim+ ][Cl]/cellulose solution to dissolve cellulose, but they did not analyze cellulose degradation. Phillips et al. [21] tried to dissolve silk fibroin in [bmim+ ][Cl] using a domestic microwave but were unsuccessful due to the thermal decomposition of the silk. However, our group has found that, by using a laboratory microwave with a strict control of the temperature and time, the integrity of the protein is preserved and nanoparticles can be obtained from silk fibroin-IL solutions by precipitation in polar organic solvents.

Ultrasound synthesis is another alternative that can be used as energy source for the preparation of many materials such as metal, oxide, sulfide, and carbide nanoparticles, and the use of ultrasound power has recently become popular in combination with ILs as the reaction medium [22]. We used high-power ultrasounds to enhance the dissolution process of silk proteins in ILs to obtain silk fibroin nanoparticles (SFNs) directly from the silk/ionic liquid solution (SIL) by rapid desolvation in polar organic solvents [23].

#### 2. Synthesis of biopolymer nanoparticles using ILs

#### 2.1. Cellulose

Cellulose is the main constituent of the vegetal kingdom and brown algae, making it the most abundant polysaccharide on earth. It plays a structural role in the cell wall and other surface structures of some amoebae, alveolates, chromists, and red and green algae. The molecular structure of this biopolymer is composed of D-glucopyranose units linked together by a β(1➔4) glyosidic bond to form a linear structure. The degree of polymerization (DP) varies from the source but can be as high as 23,000 units. Cellulose has a ribbon-like structure, with two units per turn of 1.03 nm, dictated by the allowed angles of C1-O and O-C4 [24, 25]. In terms of density (1.5 Mgm<sup>3</sup> ), cellulose fibers are stiffer (Young's modulus 50–130 GPa) and stronger (1 GPa), when measured along the polymer length, than nylon, silk, chitin, collagen, tendon, or bone [26]. Cellulose is completely insoluble in water, mainly because its hydrogen-bonding capabilities are occupied by side chains, forming aggregates. Nevertheless, it has been demonstrated that the IL 1-butyl-3-methylimidazolium chloride ([bmim<sup>+</sup> ][Cl]) is able to solubilize cellulose even at high DP (6500 units), making it an appropriate reaction medium [24]. A review of the solubility of cellulose in ILs is provided by Lee et al. [1].

1.3. Methods of synthesis

6 Recent Advances in Ionic Liquids

hexafluorophosphate, [bmim<sup>+</sup>

][NTf2

[21] tried to dissolve silk fibroin in [bmim+

precipitation in polar organic solvents.

2.1. Cellulose

applied to obtain nanoparticles from biopolymers in ILs.

fonyl}imide, [bmim<sup>+</sup>

][PF6

solution (SIL) by rapid desolvation in polar organic solvents [23].

2. Synthesis of biopolymer nanoparticles using ILs

One method used to obtain nanoparticles in ILs is synthesis via physical vapor deposition (PVP) since, due to the negligible vapor pressure of most ILs, they can be manipulated under high vacuum conditions even at high temperatures [17]. This method has been used to obtain metal nanoparticles through vaporization of a metal, an intermetallic phase, or a metal salt in the presence of an IL, for example, copper nanoparticles in the IL butyl-3-methylimidazolium

deposited not only on the IL but also onto materials dispersed in the IL, as occurs in the formation of Cu/ZnO nanocomposites in butyl-3-methylimidazolium bis{(trifluoromethyl)sul-

Microwave synthesis may be used to profit from the presence of large ions with high polarizability and conductivity of the ILs, which makes them very good media for absorbing microwaves, leading to high heating rates that result in the rapid formation of nuclei (nanoparticles) [18, 19]. Control over the particle size range can be achieved by changing the process temperature and time, as well as reactant concentrations and the choice of the anion and cation constituents of the IL. If the heat spreads uniformly through the sample, a narrow nanoparticle size distribution may be obtained. As an example, silver and gold nanoparticles have been obtained by microwave synthesis in 2-hydroxyethyl-N,N,N-trimethylammonium bis{(trifluoro-

lose solution to dissolve cellulose, but they did not analyze cellulose degradation. Phillips et al.

cessful due to the thermal decomposition of the silk. However, our group has found that, by using a laboratory microwave with a strict control of the temperature and time, the integrity of the protein is preserved and nanoparticles can be obtained from silk fibroin-IL solutions by

Ultrasound synthesis is another alternative that can be used as energy source for the preparation of many materials such as metal, oxide, sulfide, and carbide nanoparticles, and the use of ultrasound power has recently become popular in combination with ILs as the reaction medium [22]. We used high-power ultrasounds to enhance the dissolution process of silk proteins in ILs to obtain silk fibroin nanoparticles (SFNs) directly from the silk/ionic liquid

Cellulose is the main constituent of the vegetal kingdom and brown algae, making it the most abundant polysaccharide on earth. It plays a structural role in the cell wall and other surface structures of some amoebae, alveolates, chromists, and red and green algae. The molecular structure of this biopolymer is composed of D-glucopyranose units linked together by a β(1➔4)

methyl)sulfonyl}amide. Swatloski et al. [20] used microwave heating on a [bmim+

]. By means of this technique, metal nanoparticles can be

][Cl] using a domestic microwave but were unsuc-

][Cl]/cellu-

]. Nevertheless, to our knowledge, the PVP method has never been

Swatloski et al. [20] carried out experiments using cellulose-dissolving pulps, fibrous cellulose, and Whatman cellulose filter papers. They prepared the cellulose solutions by the addition of the cellulose to the ionic liquids without pretreatment and heated on a heating plate or in a domestic microwave oven. These authors were able to dissolve 25 wt% cellulose in [bmim<sup>+</sup> ] [Cl] using microwave heating. Nevertheless, ILs containing "non-coordinating" anions, including [BF4]- and [PF6]-, were nonsolvents, presumably because cellulose solubilizes through hydrogen bonding from hydroxyl functions to the anions of the solvent, [Cl]. The authors did not analyze the peak temperature reached during the microwave dissolution step nor the extent of cellulose degradation.

Han et al. [27] synthetized cellulose nanoparticles by dissolving microcrystalline cellulose (MCC) or cotton cellulose (CC) in [bmim<sup>+</sup> ][Cl]. The preparation of cellulose nanoparticles involves four steps: dissolution, regeneration, homogenization, and freeze-drying. Before proceeding to the dissolution process, solid state [bmim+ ][Cl], MCC, or CC have to be vacuumdried to remove the remnants of water, whose presence can significantly impair cellulose solubility in IL by competing with the IL for hydrogen bonds to the cellulose microfibrils [20]. Then, the cellulose was dissolved in IL, using a magnetic hot plate stirrer with safety control, by stirring the 5% w/w disperse fibers for 5 h in a 125C oil bath. When a clear phase was obtained, the solution was slowly poured into distilled water and a white dispersion of nanoparticles immediately appeared. In order to remove IL, the dispersion was filtered, centrifuged, washed three times, and, eventually, dialyzed. To further enhance the suspension, the sample was run through a high-pressure homogenization, and, finally, the suspensions were quickly frozen by mixing acetone and dry ice in an ice pot and transferred to a freezedryer. Particles regenerated from MCC yield rods of 112 42 nm in length with an aspect ratio of 9.21 (Figure 1c) and spherical nanoparticles with a diameter of 118 32 nm (Figure 1e and f). Particles regenerated from CC formed only nanorods of 123 34 nm with an aspect ratio of 9.96 (Figure 1d).

When infrared spectra were used to characterize the cellulose structure of samples, structural changes from a cellulose crystalline structure I to II were evident. For instance, after the regeneration of cotton cellulose, the -CH stretching vibration signals moved from 2901 to a lower 2893 cm<sup>1</sup> , the CO stretching vibration at C-6 switched from 1033 to 1025 cm<sup>1</sup> , and COC vibration from β-glycosidic linkage shifted from 897 to 894 cm<sup>1</sup> , probably due to changes in glycosidic linkage torsion angles. For both the MCC and MCC-NP, spectra showed strong

Figure 1. (Left) TEM images of MCC-NP (a), CC-NP (b), homogenized MCC-NP (c), homogenized CC-NP (d), and spherical NP from MCC (e, f). (Right) The dimension distribution of homogenized CC-NP, MCC-NP, and spherical MCC-NP. (a) and (b) width distribution of C-RCNs and M-RCNs; (c) and (d) length distribution of C-RCNs and M-RCNs; (e) diameter distribution of spherical RCNs. From Ref. [27] with permission of Elsevier Limited.

or dimethyl acetone and dialyzed against water. Nanoparticles of 170 nm are formed based on

Figure 2. FTIR spectra of raw cotton, C-RCNs, untreated MCC and M-RCNs. From Ref. [27] with permission of Elsevier

Cellulose-magnetite composites have also been prepared by suspension and dispersion of particles of magnetite in a homogeneous solution of cellulose in IL followed by regeneration in water and the subsequent preparation of films, flocs, fibers, or beads. The materials prepared were ferromagnetic, with a small superparamagnetic response. Characterization by Xray diffraction showed that the initial magnetite was chemically unaltered after encapsulation,

Xylan is the second most abundant polysaccharide in the vegetal kingdom after cellulose. It is the major component of hemicellulose, constituting 25–35% of the biomass of woody tissues of dicots and lignified tissues of monocots and up to 50% of some tissues of cereal grains [30]. It has a linear backbone constituted by β-D-xylosa units bound together by a β(1➔4) glyosidic bond. Xylan normally presents side chain sugars such as 4-O-methyl-glucuronic acid,

Gerick et al. [31] synthetized nanoparticles with a hydrodynamic radius of ca. 160 nm (Figure 3) of modified xylan with phenyl carbonate groups (DS up to 2.0). For the synthesis of xylan

of pyridine was added and stirred for 18 h at 80C. The solution was cooled to 25C under nitrogen atmosphere; then 15 mL of pyridine followed by 3.82 mL of phenyl chloroformate (60.8 mmol) were added. After 3 h of reaction time at 25C, the mixture was poured into 300 mL ice cold water. The precipitate was removed by filtration and washed twice with 150 mL of water and twice with 150 mL of ethanol. The crude product was dissolved in 40 mL of DMSO

][Cl] were stirred for 1 h at 80C. Afterwards, 5 mL

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766 9

the slow exchange of an organic solvent against an anti-solvent (non-solvent).

with an average particle size of approximately 25 nm [29].

2.2. Xylan

Limited.

galacturonic acid, and arabinose.

nanoparticles, 2 g of xylan and 18 g [bmim+

hydrogen bonded OH stretching vibrations in the range of 3000–3600 cm<sup>1</sup> . The signals at around 1430 and 2900 cm<sup>1</sup> were awarded to CH2 (C6) bending vibration and CH stretching vibration, respectively. Adsorbed water was noticed by a peak at 1644 cm<sup>1</sup> from O-H bending. Characteristic FTIR absorption bands related to the transition from cellulose I to cellulose II were also watched. A deeper analysis of the structural changes can be found in the author's publication [27] and references therein. The lack of absorption bands from [bmim+ ][Cl] suggests that the ILs were successfully removed. The properties of regenerated cellulose nanoparticles produced from microcrystalline cellulose and cotton using a combined IL and high-pressure homogenization treatment point to the potential for application in the biomedicine field as drug delivery systems, biomarkers, tablet excipients, and more (Figure 2).

Cellulose is susceptible to modification due to its hydroxyl groups; for example, silylation is one possible route for the synthesis of water-soluble derivatives, thus broadening the spectrum of possibilities. Silylation can take place in heterogeneous media like pyridine, or in homogeneous media, like methyl sulfoxide/LiCl. Nevertheless, in heterogeneous media, the reaction is not even at whole cellulose molecule and the use of homogeneous media is not applicable on a large scale as it is impossible to regenerate the media. Heinz's research group [28] prepared trimethylsilyl cellulose (TMSC) nanoparticles by taking advantage of the solubilizing power of IL and the self-assembly capabilities of TMSC. In brief, cellulose is first solubilized in [emim<sup>+</sup> ] [Ac] 10% w/w, and hexamethyldisilane is added in a 3–1 excess molar ration with respect to the glucose units of cellulose. After a reaction time of 1 h at 80C, a 2.65 of substitution (DS) was obtained. TMSC nanoparticles are dissolved in an organic solvent such as tetrahydrofuran

Figure 2. FTIR spectra of raw cotton, C-RCNs, untreated MCC and M-RCNs. From Ref. [27] with permission of Elsevier Limited.

or dimethyl acetone and dialyzed against water. Nanoparticles of 170 nm are formed based on the slow exchange of an organic solvent against an anti-solvent (non-solvent).

Cellulose-magnetite composites have also been prepared by suspension and dispersion of particles of magnetite in a homogeneous solution of cellulose in IL followed by regeneration in water and the subsequent preparation of films, flocs, fibers, or beads. The materials prepared were ferromagnetic, with a small superparamagnetic response. Characterization by Xray diffraction showed that the initial magnetite was chemically unaltered after encapsulation, with an average particle size of approximately 25 nm [29].

#### 2.2. Xylan

. The signals at

][Cl] suggests that

]

hydrogen bonded OH stretching vibrations in the range of 3000–3600 cm<sup>1</sup>

RCNs; (e) diameter distribution of spherical RCNs. From Ref. [27] with permission of Elsevier Limited.

8 Recent Advances in Ionic Liquids

tion [27] and references therein. The lack of absorption bands from [bmim+

delivery systems, biomarkers, tablet excipients, and more (Figure 2).

around 1430 and 2900 cm<sup>1</sup> were awarded to CH2 (C6) bending vibration and CH stretching vibration, respectively. Adsorbed water was noticed by a peak at 1644 cm<sup>1</sup> from O-H bending. Characteristic FTIR absorption bands related to the transition from cellulose I to cellulose II were also watched. A deeper analysis of the structural changes can be found in the author's publica-

Figure 1. (Left) TEM images of MCC-NP (a), CC-NP (b), homogenized MCC-NP (c), homogenized CC-NP (d), and spherical NP from MCC (e, f). (Right) The dimension distribution of homogenized CC-NP, MCC-NP, and spherical MCC-NP. (a) and (b) width distribution of C-RCNs and M-RCNs; (c) and (d) length distribution of C-RCNs and M-

the ILs were successfully removed. The properties of regenerated cellulose nanoparticles produced from microcrystalline cellulose and cotton using a combined IL and high-pressure homogenization treatment point to the potential for application in the biomedicine field as drug

Cellulose is susceptible to modification due to its hydroxyl groups; for example, silylation is one possible route for the synthesis of water-soluble derivatives, thus broadening the spectrum of possibilities. Silylation can take place in heterogeneous media like pyridine, or in homogeneous media, like methyl sulfoxide/LiCl. Nevertheless, in heterogeneous media, the reaction is not even at whole cellulose molecule and the use of homogeneous media is not applicable on a large scale as it is impossible to regenerate the media. Heinz's research group [28] prepared trimethylsilyl cellulose (TMSC) nanoparticles by taking advantage of the solubilizing power of IL and the self-assembly capabilities of TMSC. In brief, cellulose is first solubilized in [emim<sup>+</sup>

[Ac] 10% w/w, and hexamethyldisilane is added in a 3–1 excess molar ration with respect to the glucose units of cellulose. After a reaction time of 1 h at 80C, a 2.65 of substitution (DS) was obtained. TMSC nanoparticles are dissolved in an organic solvent such as tetrahydrofuran Xylan is the second most abundant polysaccharide in the vegetal kingdom after cellulose. It is the major component of hemicellulose, constituting 25–35% of the biomass of woody tissues of dicots and lignified tissues of monocots and up to 50% of some tissues of cereal grains [30]. It has a linear backbone constituted by β-D-xylosa units bound together by a β(1➔4) glyosidic bond. Xylan normally presents side chain sugars such as 4-O-methyl-glucuronic acid, galacturonic acid, and arabinose.

Gerick et al. [31] synthetized nanoparticles with a hydrodynamic radius of ca. 160 nm (Figure 3) of modified xylan with phenyl carbonate groups (DS up to 2.0). For the synthesis of xylan nanoparticles, 2 g of xylan and 18 g [bmim+ ][Cl] were stirred for 1 h at 80C. Afterwards, 5 mL of pyridine was added and stirred for 18 h at 80C. The solution was cooled to 25C under nitrogen atmosphere; then 15 mL of pyridine followed by 3.82 mL of phenyl chloroformate (60.8 mmol) were added. After 3 h of reaction time at 25C, the mixture was poured into 300 mL ice cold water. The precipitate was removed by filtration and washed twice with 150 mL of water and twice with 150 mL of ethanol. The crude product was dissolved in 40 mL of DMSO

study [38–40]. The drug-loading and -releasing capabilities of the starch nanoparticles were

Chitosan is a lineal polysaccharide constituted by randomly distributed β-(1 ! 4)-linked Dglucosamine and N-acetyl-D-glucosamine. Chitosan is obtained from the deacetylation of chitin via basic hydrolysis. Chitin is mostly present in the exoskeleton of crustaceans, insect wings, and cell walls of fungi and algae, among others. Chitosan nanoparticles are widely used in food and bioengineering industries for the encapsulation of active food ingredients, enzyme immobilization, as a carrier for controlled drug delivery, and in agriculture as a plant antimicrobial agent and growth promoter [41]. For instance, Torzsas et al. showed that chitosan may

Bharmoria et al. [43] reported the synthesis of chitosan nanoparticles by ionic cross-linking with IL for the first time. The simple, self-assembling methods consist of adding [bmim+

chitosan. The chitosan chains are attracted to IL micelles by charge interactions and aggregate in a gelated complex. Acetone is used as antisolvent to precipitate the nanoparticles, which have a mean hydrodynamic diameter in the range of 300–560 nm with Z potential above +58.5 mV, depending on the IL used. The authors assumed that the electrostatic and hydrophobic interaction produced between chitosan and IL directs the formation of chitosan nanoparticle, where IL aggregates act as templates. Scanning electron microscopy (SEM) revealed

Chitosan nanoparticles have previously been tested for cellular uptake and trafficking to lymph nodes with very promising results [44]. Furthermore, chitosan-based nanoparticles are of special interest for the oral administration of insulin, as subcutaneous administration suffers disadvantages such as patient noncompliance and occasional hypoglycemia. Moreover, these

last approaches do not mimic the normal physiological pattern of insulin release [45].

Figure 4. Hydrodynamic diameter (Dh) plots of chitosan nanoparticles formed with (A) [bmim<sup>+</sup>

][Cl�]. From Ref. [43] with permission of Elsevier Limited.

][Cl�] above the critical micelle concentration to an aqueous solution of

][C8OSO3

]

11

�] solutions have a greater

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766

][C8OSO3

�] and (B)

have an important role in protecting against colon cancer [42] (Figure 4).

that the nanoparticles obtained from chitosan-[bmim<sup>+</sup>

sphericity and a lower tendency to agglomerate (Figure 5).

tasted with mitoxantrone hydrochloride.

2.4. Chitosan

[C8OSO3

[omim<sup>+</sup>

�] or [omim<sup>+</sup>

Figure 3. (left) Scanning electron microscopy images of nanoparticles obtained by dialysis of xylan phenyl carbonate XPC 20. (right) Size distribution of XPC 20 nanoparticles obtained by dynamic light scattering. From Ref. [31] with permission of Elsevier Limited.

and then precipitated in 300 mL of ethanol. This functionalization allows the molecule to have a major electrophilic center for further functionalization with drugs, signaling molecules, recognition molecules, and so on. Many other xylan modifications/functionalizations are described in Petzold-Welcke's review [32]. For example, the modification of xylan with a methyl group can be used as polymeric tensides, and some esters showed good drug-carrying capabilities. Xylan sulfate may be applied as antiviral drugs and as blood coagulation inhibitor.

#### 2.3. Starch

Starch is produced by green plants to store energy. Granules of starch accumulate at high concentrations in reproductive structures like cereal grains (e.g., wheat, rice, maize, barley, rye, oats, millet, and sorghum) and in vegetative structures such as tubers (potatoes) and roots (cassava and taro) [33]. The two major forms of starch are amylose and amylopectin. The first is the liner polymer of D-glucopyranose units bound by α(1➔4) glyosidic bond. Its molecular weight varies from 104 to 105 with a DP of 250–1000 units. The second is a highly branched polymer of α(1➔4) glyosidic bonds and lateral chains of α(1➔6) with a molecular weight of 106 –108 , corresponding to a DP of 5000–50,000 units [34, 35].

Zhou et al. [36, 37] prepared starch nanoparticles with a controlled mean diameter of 64–255 nm with a water/ionic liquid emulsion (W/IL) cross-linked technique. To prepare the aqueous phase, 0.5 g of acid-treated granular starch was dissolved in 9.5 g of NaOH solution (20 M) and then added to 40 g of [bmim+ ][PF6 ] to form the water/IL microemulsion with 40 g of a mixture of TX-100 surfactant and 1-butanol cosurfactant (TX-100/1-butanol = 3:1, w/w) After several minutes of stirring, 1.84 of epichlorohydrin was added to the above microemulsion as a cross-linker. At this point, the mixture was stirred at 50C for 4 h. Next, this solution was allowed to reach room temperature and methanol was used as antisolvent to precipitate the starch nanoparticles. The precipitate was centrifuged and washed thoroughly with sufficient methanol and ethanol to eliminate unreacted epichlorohydrin, remaining [bmim<sup>+</sup> ][PF6 ], TX-100, and 1-butanol. Finally, the solid was dried in a vacuum for 24 h at 45C. More examples of starch nanoparticle synthesis in IL emulsions can be found in the study [38–40]. The drug-loading and -releasing capabilities of the starch nanoparticles were tasted with mitoxantrone hydrochloride.

#### 2.4. Chitosan

and then precipitated in 300 mL of ethanol. This functionalization allows the molecule to have a major electrophilic center for further functionalization with drugs, signaling molecules, recognition molecules, and so on. Many other xylan modifications/functionalizations are described in Petzold-Welcke's review [32]. For example, the modification of xylan with a methyl group can be used as polymeric tensides, and some esters showed good drug-carrying capabilities.

Figure 3. (left) Scanning electron microscopy images of nanoparticles obtained by dialysis of xylan phenyl carbonate XPC 20. (right) Size distribution of XPC 20 nanoparticles obtained by dynamic light scattering. From Ref. [31] with permission

Starch is produced by green plants to store energy. Granules of starch accumulate at high concentrations in reproductive structures like cereal grains (e.g., wheat, rice, maize, barley, rye, oats, millet, and sorghum) and in vegetative structures such as tubers (potatoes) and roots (cassava and taro) [33]. The two major forms of starch are amylose and amylopectin. The first is the liner polymer of D-glucopyranose units bound by α(1➔4) glyosidic bond. Its molecular weight varies from 104 to 105 with a DP of 250–1000 units. The second is a highly branched polymer of α(1➔4) glyosidic bonds and lateral chains of α(1➔6) with a molecular weight of

Zhou et al. [36, 37] prepared starch nanoparticles with a controlled mean diameter of 64–255 nm with a water/ionic liquid emulsion (W/IL) cross-linked technique. To prepare the aqueous phase, 0.5 g of acid-treated granular starch was dissolved in 9.5 g of NaOH solution

of a mixture of TX-100 surfactant and 1-butanol cosurfactant (TX-100/1-butanol = 3:1, w/w) After several minutes of stirring, 1.84 of epichlorohydrin was added to the above microemulsion as a cross-linker. At this point, the mixture was stirred at 50C for 4 h. Next, this solution was allowed to reach room temperature and methanol was used as antisolvent to precipitate the starch nanoparticles. The precipitate was centrifuged and washed thoroughly with sufficient methanol and ethanol to eliminate unreacted epichlorohydrin, remaining

45C. More examples of starch nanoparticle synthesis in IL emulsions can be found in the

], TX-100, and 1-butanol. Finally, the solid was dried in a vacuum for 24 h at

] to form the water/IL microemulsion with 40 g

][PF6

Xylan sulfate may be applied as antiviral drugs and as blood coagulation inhibitor.

, corresponding to a DP of 5000–50,000 units [34, 35].

(20 M) and then added to 40 g of [bmim+

2.3. Starch

of Elsevier Limited.

10 Recent Advances in Ionic Liquids

106 –108

[bmim<sup>+</sup>

][PF6

Chitosan is a lineal polysaccharide constituted by randomly distributed β-(1 ! 4)-linked Dglucosamine and N-acetyl-D-glucosamine. Chitosan is obtained from the deacetylation of chitin via basic hydrolysis. Chitin is mostly present in the exoskeleton of crustaceans, insect wings, and cell walls of fungi and algae, among others. Chitosan nanoparticles are widely used in food and bioengineering industries for the encapsulation of active food ingredients, enzyme immobilization, as a carrier for controlled drug delivery, and in agriculture as a plant antimicrobial agent and growth promoter [41]. For instance, Torzsas et al. showed that chitosan may have an important role in protecting against colon cancer [42] (Figure 4).

Bharmoria et al. [43] reported the synthesis of chitosan nanoparticles by ionic cross-linking with IL for the first time. The simple, self-assembling methods consist of adding [bmim+ ] [C8OSO3 �] or [omim<sup>+</sup> ][Cl�] above the critical micelle concentration to an aqueous solution of chitosan. The chitosan chains are attracted to IL micelles by charge interactions and aggregate in a gelated complex. Acetone is used as antisolvent to precipitate the nanoparticles, which have a mean hydrodynamic diameter in the range of 300–560 nm with Z potential above +58.5 mV, depending on the IL used. The authors assumed that the electrostatic and hydrophobic interaction produced between chitosan and IL directs the formation of chitosan nanoparticle, where IL aggregates act as templates. Scanning electron microscopy (SEM) revealed that the nanoparticles obtained from chitosan-[bmim<sup>+</sup> ][C8OSO3 �] solutions have a greater sphericity and a lower tendency to agglomerate (Figure 5).

Chitosan nanoparticles have previously been tested for cellular uptake and trafficking to lymph nodes with very promising results [44]. Furthermore, chitosan-based nanoparticles are of special interest for the oral administration of insulin, as subcutaneous administration suffers disadvantages such as patient noncompliance and occasional hypoglycemia. Moreover, these last approaches do not mimic the normal physiological pattern of insulin release [45].

Figure 4. Hydrodynamic diameter (Dh) plots of chitosan nanoparticles formed with (A) [bmim<sup>+</sup> ][C8OSO3 �] and (B) [omim<sup>+</sup> ][Cl�]. From Ref. [43] with permission of Elsevier Limited.

have two important synergistic effects on the mixture: rapid heating and efficient disruption of the fibers at the same time. By applying the precipitation method commonly used for the coagulation of an aqueous SF solution in a water-miscible organic solvent, it is possible to

The dissolution of B. mori SF using an oil bath as the heat source was investigated in the assembled state of the fibers. Table 1 summarizes the results. Complete dissolution took over 1 h at 100C, as described by Phillips et al. [21]. The saturated solubilities by weight for SF in 1 alkyl-3-methylimidazolium chlorides are dependent on the length of the alkyl substituent of

It was checked that 1-alkyl-3-methyl imidazolium chlorides are able to disrupt the hydrogen bonding in silkworm SF and they are good solvents for silk dissolution when the length of the alkyl chain is lower than eight carbons [21]. The hydrophobicity of the organic cation increases with the cation alkyl chain length, so that long-aliphatic-chain ILs cannot dissolve SF. Like-

Silk proteins were successfully dissolved in the 1-alkyl-3-methylimidazolium chlorides (where

saturated solubility by weight and the time required for silk dissolution in selected ILs are

In an oil bath at 100C, the thermal heating process of SF or SC dissolution in ILs takes hours, but instead, by using ultrasounds, a significant reduction is achieved in the time necessary to complete dissolution. The break of the β-sheet hydrogen bonds network of the proteins was

necessary to reach a concentration higher than 10% (w/w), since an increased viscosity repre-

IL SF solubility (wt%) IL SF solubility (wt%)

][Cl] Insoluble\* ETAN Insoluble\*

Table 1. Solubility (%wt) of SF in selected ILs at 90C (heated in an oil bath). From Ref. [23] with permission of Wiley

][Cl]; propyl [pmim<sup>+</sup>

][Cl]) using high-power ultrasounds and limiting the temperature at 100C. The

][Cl]; butyl [bmim+

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766

][Cl], for particle formation, it is not

][Cl] Insoluble\*

]TfO Insoluble\*

Insoluble\*

Insoluble\*

Insoluble\*

Insoluble\*

]EtSO4

]OctSO4

]PF6

]EtSO4

][Cl] or

13

wise, ILs with highly hydrophobic anions cannot act as solvents of SF.

][Cl]; ethyl [emim<sup>+</sup>

enhanced by the use of the high-power ultrasounds. Although the solubility of SF was highest in [emim+

][Cl] Soluble (>12%) [eim+

][Cl] Soluble (>23%) [emim+

] [Cl] Soluble (>15%) [emim+

][Cl] Soluble (>12%) [bmim+

][Cl] Soluble (>11%) [bBmim<sup>+</sup>

][Cl] Insoluble\* [3-MEP<sup>+</sup>

obtain particles of regenerated SF [52].

the imidazolium ring.

1-alkyl is: methyl [mim+

sents a handicap for handling.

hexyl [hmim<sup>+</sup>

[mim<sup>+</sup>

[emim<sup>+</sup>

[pmim<sup>+</sup>

[bmim+

[hmim<sup>+</sup>

[omim<sup>+</sup>

[dmim<sup>+</sup>

\*After 24 h at 90C.

Online Library.

listed in Table 2.

Figure 5. SEM images of nanoparticles formed with (A) [bmim<sup>+</sup> ][C8OSO3 ] and (B) [omim<sup>+</sup> ][Cl]. From Ref. [43] with permission of Elsevier Limited.

#### 2.5. Silk fibroin

Due to its excellent biocompatibility and mechanical properties, silk fibroin (SF) obtained from Bombyx mori cocoons is an attractive biomaterial for use in biomedical and tissue engineering applications [46]. This biomaterial, formulated as particles, has potential applications in medicine for its capacity to adsorb, transport, and deliver a wide range of bioactive molecules [47]. Silks are insoluble in most solvents, including water, dilute acid, and alkali. There are two classical solvent systems to dissolve SF: ionic hydro-alcoholic solutions, such as a CaCl2/ethanol/ water mixture (Ajisawa's reagent) [48], or ionic aqueous solutions, traditionally 9.3 M LiBr or 50 wt% CaCl2 solution [49]. These solutions require to be dialyzed for 48 h against ultra-pure water using a cellulose semi-permeable membrane (cut-off 3.5 KD) to remove salts, pigments, small peptides, and other impurities of the silk solution. Both processes are time-consuming, and the solutions are unstable and aggregate to a gel state. For long-term storage, aqueous solutions of SF can be lyophilized and redissolved in organic solvents such as 1,1,1,3,3,3 hexafluoroisopropanol (HFIP). However, these solvents are toxic and extremely corrosive, requiring considerable care in handling [50].

The suitability of imidazolium-based ionic liquid solvents, such as 1-butyl-3-methylimidazolium chloride ([bmim<sup>+</sup> ][Cl]), to form stable SF solutions was demonstrated [21]. Within an ionic liquid, the anion plays a larger role in dictating the ultimate solubility of the SF, attributed to the ability of the anion to disrupt the hydrogen bonding in the β-sheets of the silkworm SF to form solutions (mainly halogens or small carboxylates) [51]. The use of ILs as solvents has the advantage that the total number of steps required for the dissolution process is reduced and, furthermore, the cocoon can be dissolved directly because sericin was also dissolved in the selected ILs. However, complete dissolution of SF using the classic abovedescribed methods takes several hours, even with intense heating at 100C [21], resulting in the loss of protein integrity. Long treatments lead to breakage of the peptidic chains and poor mechanical properties of the resulting biomaterials. On the other hand, the process of silk dissolution may be improved by applying high-power ultrasounds to the SIL mixture to accelerate the process and by adding water to reduce the viscosity. High-power ultrasounds have two important synergistic effects on the mixture: rapid heating and efficient disruption of the fibers at the same time. By applying the precipitation method commonly used for the coagulation of an aqueous SF solution in a water-miscible organic solvent, it is possible to obtain particles of regenerated SF [52].

The dissolution of B. mori SF using an oil bath as the heat source was investigated in the assembled state of the fibers. Table 1 summarizes the results. Complete dissolution took over 1 h at 100C, as described by Phillips et al. [21]. The saturated solubilities by weight for SF in 1 alkyl-3-methylimidazolium chlorides are dependent on the length of the alkyl substituent of the imidazolium ring.

It was checked that 1-alkyl-3-methyl imidazolium chlorides are able to disrupt the hydrogen bonding in silkworm SF and they are good solvents for silk dissolution when the length of the alkyl chain is lower than eight carbons [21]. The hydrophobicity of the organic cation increases with the cation alkyl chain length, so that long-aliphatic-chain ILs cannot dissolve SF. Likewise, ILs with highly hydrophobic anions cannot act as solvents of SF.

Silk proteins were successfully dissolved in the 1-alkyl-3-methylimidazolium chlorides (where 1-alkyl is: methyl [mim+ ][Cl]; ethyl [emim<sup>+</sup> ][Cl]; propyl [pmim<sup>+</sup> ][Cl]; butyl [bmim+ ][Cl] or hexyl [hmim<sup>+</sup> ][Cl]) using high-power ultrasounds and limiting the temperature at 100C. The saturated solubility by weight and the time required for silk dissolution in selected ILs are listed in Table 2.

2.5. Silk fibroin

permission of Elsevier Limited.

12 Recent Advances in Ionic Liquids

requiring considerable care in handling [50].

Figure 5. SEM images of nanoparticles formed with (A) [bmim<sup>+</sup>

dazolium chloride ([bmim<sup>+</sup>

Due to its excellent biocompatibility and mechanical properties, silk fibroin (SF) obtained from Bombyx mori cocoons is an attractive biomaterial for use in biomedical and tissue engineering applications [46]. This biomaterial, formulated as particles, has potential applications in medicine for its capacity to adsorb, transport, and deliver a wide range of bioactive molecules [47]. Silks are insoluble in most solvents, including water, dilute acid, and alkali. There are two classical solvent systems to dissolve SF: ionic hydro-alcoholic solutions, such as a CaCl2/ethanol/ water mixture (Ajisawa's reagent) [48], or ionic aqueous solutions, traditionally 9.3 M LiBr or 50 wt% CaCl2 solution [49]. These solutions require to be dialyzed for 48 h against ultra-pure water using a cellulose semi-permeable membrane (cut-off 3.5 KD) to remove salts, pigments, small peptides, and other impurities of the silk solution. Both processes are time-consuming, and the solutions are unstable and aggregate to a gel state. For long-term storage, aqueous solutions of SF can be lyophilized and redissolved in organic solvents such as 1,1,1,3,3,3 hexafluoroisopropanol (HFIP). However, these solvents are toxic and extremely corrosive,

][C8OSO3

] and (B) [omim<sup>+</sup>

][Cl]. From Ref. [43] with

The suitability of imidazolium-based ionic liquid solvents, such as 1-butyl-3-methylimi-

an ionic liquid, the anion plays a larger role in dictating the ultimate solubility of the SF, attributed to the ability of the anion to disrupt the hydrogen bonding in the β-sheets of the silkworm SF to form solutions (mainly halogens or small carboxylates) [51]. The use of ILs as solvents has the advantage that the total number of steps required for the dissolution process is reduced and, furthermore, the cocoon can be dissolved directly because sericin was also dissolved in the selected ILs. However, complete dissolution of SF using the classic abovedescribed methods takes several hours, even with intense heating at 100C [21], resulting in the loss of protein integrity. Long treatments lead to breakage of the peptidic chains and poor mechanical properties of the resulting biomaterials. On the other hand, the process of silk dissolution may be improved by applying high-power ultrasounds to the SIL mixture to accelerate the process and by adding water to reduce the viscosity. High-power ultrasounds

][Cl]), to form stable SF solutions was demonstrated [21]. Within

In an oil bath at 100C, the thermal heating process of SF or SC dissolution in ILs takes hours, but instead, by using ultrasounds, a significant reduction is achieved in the time necessary to complete dissolution. The break of the β-sheet hydrogen bonds network of the proteins was enhanced by the use of the high-power ultrasounds.

Although the solubility of SF was highest in [emim+ ][Cl], for particle formation, it is not necessary to reach a concentration higher than 10% (w/w), since an increased viscosity represents a handicap for handling.


Table 1. Solubility (%wt) of SF in selected ILs at 90C (heated in an oil bath). From Ref. [23] with permission of Wiley Online Library.


Table 2. Solubility and time required for dissolution of silk proteins, in selected ILs. From Ref. [23] with permission of Wiley Online Library.

Silk protein integrity in the SIL solutions was confirmed by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS-PAGE). It was found that the peptidic chain fragmentation that occurs when the classical dissolution methods are used is drastically reduced in the process with ILs and high-power ultrasound. In Figure 6, the molecular masses of the fragments of the SIL solutions may be observed, which are practically the same than that of the SF present in the silkworm gland [53].

In brief, to the freshly prepared SIL solution, ultrapure water was added to reduce its viscosity, and the SIL solution was slowly dripped into 100 mL of vigorously stirred cold methanol. After a few drops, a milky-white suspension appeared and the suspension was stirred for 2 h. The particle suspension was recovered by centrifugation at 18,000 g for 15 min, at 4C. The supernatant (free of particles) was removed and reserved for subsequent recycling of the IL. The white precipitate was subjected to successive rinses with fresh methanol and ultrapure water. After lyophilizing the particles for 72 h at 0.5 mbar and 55C (Edwards Modulyo 4 K Freeze Dryer), SFNs were obtained in the form of a dry powder. A rotary evaporator at 80 mbar and

Figure 7. Scheme of the overall process of SF dissolution, using ILs and ultrasonication, and consequent SFNs prepara-

The liquid silk fibroin can be regenerated as nanoparticles by pouring SIL solution into an excess of a polar organic solvent. In this case, methanol was used for SF regeneration. When the IL is dissolved in methanol from the SIL solution, the SF changes from random coil and αhelix forms into anti-parallel β-sheet form by reconstitution of the hydrogen bonds network of

The particles were characterized by dynamic light scattering (DLS) and infrared spectroscopy (FTIR). The stability of the particles was tested in purified water at 25C and in Dulbecco's Modified Eagle Medium (DMEM) (without Fetal Bovine Serum (FBS) supplementation) at 37C. In MilliQ water at 25C, the SFNs had an average size of 170–184 nm measured by DLS. The results indicate that the particles were slightly larger (183–341 nm) when dispersed in DMEM (see Table 3); these values are almost identical to those described previously in the

[Cl] solution and SFNs regenerated from classical CaCl2/EtOH/H2O solution, obtained using the classical Zhang's method [52] is presented. As can be observed, β-sheet is predominant in particles with peaks at 1230 (Amide III, C-H Stretching), 1516 (Amide II, N-H Bending), and 1626 cm<sup>1</sup> (Amide I, C=O Stretching), which are typical of the β-sheet conformation [27]. The SFN profiles were similar to those of SFNs obtained by methanol immersion of SF dissolved

][Cl], SFNs regenerated from SF/[bmim+

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766 15

]

80C was used to recover the IL from the methanolic and water fractions.

tion from SIL solutions. From Ref. [23] with permission of Wiley Online Library.

the protein chains [52].

corresponding study [52, 54].

In Figure 8, a comparative FTIR spectrum of [bmim+

The overall process of SFNs preparation from SIL solutions is summarized in Figure 7. The procedure is based on the method described previously by Zhang et al., with modifications [52].

Figure 6. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein components of silk fibroin (SF) and white silk cocoons (SC) after the use of high-power ultrasounds in the solutions SC/IL or SF/IL with ILs: [bmim+ ][Cl], [pmim<sup>+</sup> ][Cl], [emim<sup>+</sup> ][Cl], [hmim<sup>+</sup> ][Cl] and [mim<sup>+</sup> ][Cl]. From Ref. [23] with permission of Wiley Online Library.

Figure 7. Scheme of the overall process of SF dissolution, using ILs and ultrasonication, and consequent SFNs preparation from SIL solutions. From Ref. [23] with permission of Wiley Online Library.

Silk protein integrity in the SIL solutions was confirmed by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS-PAGE). It was found that the peptidic chain fragmentation that occurs when the classical dissolution methods are used is drastically reduced in the process with ILs and high-power ultrasound. In Figure 6, the molecular masses of the fragments of the SIL solutions may be observed, which are practically the same than that of the SF

Table 2. Solubility and time required for dissolution of silk proteins, in selected ILs. From Ref. [23] with permission of

Silk fibroin (SF) Silk cocoon (SC) Solvent Solubility (%wt) Time (min.) Solubility (%wt) Time (min.)

][Cl] 12.5 0.1 4 12.5 0.1 17

][Cl] 23.0 0.3 17 18.7 0.6 67

][Cl] 15.2 0.3 14 17.6 0.1 27

][Cl] 12.7 0.6 5 12.9 0.4 24

][Cl] 10.9 0.2 8 11.1 0.3 20

Solubility is presented as the average value SD (standard deviation) (n = 3).

The overall process of SFNs preparation from SIL solutions is summarized in Figure 7. The procedure is based on the method described previously by Zhang et al., with modifications [52].

Figure 6. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein components of silk fibroin (SF) and white silk cocoons (SC) after the use of high-power ultrasounds in the solutions SC/IL or SF/IL with ILs: [bmim+

][Cl]. From Ref. [23] with permission of Wiley Online Library.

][Cl],

present in the silkworm gland [53].

[mim<sup>+</sup>

[emim+

[pmim<sup>+</sup>

[bmim+

[hmim<sup>+</sup>

[pmim<sup>+</sup>

][Cl], [emim<sup>+</sup>

][Cl], [hmim<sup>+</sup>

][Cl] and [mim<sup>+</sup>

Wiley Online Library.

14 Recent Advances in Ionic Liquids

In brief, to the freshly prepared SIL solution, ultrapure water was added to reduce its viscosity, and the SIL solution was slowly dripped into 100 mL of vigorously stirred cold methanol. After a few drops, a milky-white suspension appeared and the suspension was stirred for 2 h. The particle suspension was recovered by centrifugation at 18,000 g for 15 min, at 4C. The supernatant (free of particles) was removed and reserved for subsequent recycling of the IL. The white precipitate was subjected to successive rinses with fresh methanol and ultrapure water. After lyophilizing the particles for 72 h at 0.5 mbar and 55C (Edwards Modulyo 4 K Freeze Dryer), SFNs were obtained in the form of a dry powder. A rotary evaporator at 80 mbar and 80C was used to recover the IL from the methanolic and water fractions.

The liquid silk fibroin can be regenerated as nanoparticles by pouring SIL solution into an excess of a polar organic solvent. In this case, methanol was used for SF regeneration. When the IL is dissolved in methanol from the SIL solution, the SF changes from random coil and αhelix forms into anti-parallel β-sheet form by reconstitution of the hydrogen bonds network of the protein chains [52].

The particles were characterized by dynamic light scattering (DLS) and infrared spectroscopy (FTIR). The stability of the particles was tested in purified water at 25C and in Dulbecco's Modified Eagle Medium (DMEM) (without Fetal Bovine Serum (FBS) supplementation) at 37C. In MilliQ water at 25C, the SFNs had an average size of 170–184 nm measured by DLS. The results indicate that the particles were slightly larger (183–341 nm) when dispersed in DMEM (see Table 3); these values are almost identical to those described previously in the corresponding study [52, 54].

In Figure 8, a comparative FTIR spectrum of [bmim+ ][Cl], SFNs regenerated from SF/[bmim+ ] [Cl] solution and SFNs regenerated from classical CaCl2/EtOH/H2O solution, obtained using the classical Zhang's method [52] is presented. As can be observed, β-sheet is predominant in particles with peaks at 1230 (Amide III, C-H Stretching), 1516 (Amide II, N-H Bending), and 1626 cm<sup>1</sup> (Amide I, C=O Stretching), which are typical of the β-sheet conformation [27]. The SFN profiles were similar to those of SFNs obtained by methanol immersion of SF dissolved


also be present in the rest of the Asian continent and has been widely employed in Ayurvedic medicine for centuries [58]. Its most relevant pharmacological effects are its anti-inflammatory

Although curcumin is safe, nontoxic, and well tolerated in animal and human studies, it cannot be administered to patients directly due to its poor solubility in water [56] (estimated value: 3.12 mg/L at 25C [61]). In an attempt to enhance the therapeutic efficiency of curcumin, improvements in its bioavailability have been tried. Several nanocarriers such as solid lipid nanoparticles [62], natural [63] or synthetic [64] polymer nanoparticles, and inorganic nanoparticles [65] can be found in the study as examples of nanoplatforms for the intracellular delivery of curcumin. Recently, research interests focus on the use of biopolymers such as SF to encapsulate curcumin and other similar drugs [66]. By virtue of their small size, SFNs can penetrate thin capillaries, fostering the uptake of drugs by cells. In addition, these SFNs are potential targeted delivery systems because, for instance, they can deliver antitumor drugs to tumor cells. Several research groups have studied curcumin encapsulation in SFNs by different

The authors [5] studied the synthesis of curcumin-loaded SFNs (Curc-SFNs) to improve on

high-power ultrasounds to dissolve the SF. The synthesis of Curc-SFNs developed in this chapter is a more scalable and continuous processing option than those already published in the study. The drug was loaded into the SFNs by physical adsorption, Curc-SFNs 1, and by

For loading of curcumin by physical adsorption, 40 mL of a 1 mg/mL solution of curcumin in ethanol was used to resuspend 325 mg of SFNs obtained from an SF-IL solution. The suspension was ultrasonicated for 5 min and gently stirred at 30 rpm in a Tube Rotator for 24 h. Next, Curc-SFNs 1 were centrifuged for 15 min at 13,400 rpm. Finally, Curc-SFNs 1 were washed with water to eliminate the rest of ethanol. The drug loaded in the nanoparticles was indirectly determined by the measurement of the UV absorbance of curcumin at 421 nm in the centrifu-

To obtain Curc-SFNs 2 by coprecipitation, the drug was loaded in the nanoparticles throughout the synthesis step. In brief, an exact weighed amount of curcumin (25 mg) was dissolved in 3 mL of 0.1 M NaOH solution, and this solution was immediately dissolved in 5 g of a previously prepared SIL solution (10% wt.). The drug-SIL solution was heated to 60C to reduce the viscosity of the mixture and sprayed with nitrogen onto 100 mL of gently stirred ethanol. The orange suspension was stirred for 2 h before being centrifuged at 13,400 rpm for

][CH3COO]) and

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766 17

current methods, using IL (1-ethyl-3-methylimidazolium acetate, [emim<sup>+</sup>

gation supernatants (ethanol and water) and in the initial curcumin solution.

coprecipitation, Curc-SFNs 2, in order to obtain Curc-SFNs.

[59], anticancer [57], antioxidant [60], and antimicrobial [60] activities.

techniques [67, 68].

Figure 9. Chemical structure of curcumin [5].

a Z-average SD (n = 5) and accumulation times = 100. <sup>b</sup>

Average value.

Table 3. Comparative values for the particle size (diameter), polydispersity (PdI), and zeta potential of classical SFNs [52] obtained from CaCl2/EtOH/H2O solvent and SFNs produced from SIL solutions. From Ref. [23] with permission of Wiley Online Library.

Figure 8. Comparative FTIR spectra of (A) only [bmim+ ][Cl]; (B) SFNs regenerated from SF/[bmim<sup>+</sup> ][Cl] solution; (C) SFNs regenerated from classical CaCl2/EtOH/H2O solution [52]. From Ref. [23] with permission of Wiley Online Library.

in the Ajisawa solvent system [52, 55]. Characteristic signals of ILs (1572, 1465, and 1170 cm<sup>1</sup> ) were absent in the recorded spectrum of SFNs obtained from [bmim+ ][Cl], indicating that the IL was efficiently washed out from the SFNs.

#### 2.5.1. Synthesis of curcumin-loaded silk fibroin nanoparticles using ILs

Curcumin ((1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione, Figure 9) is a hydrophobic polyphenol derived from turmeric: the rhizome of the herb Curcuma longa [56]. From the chemical point of view, curcumin is a bis-α,β-unsaturated β-diketone (also called diferuloylmethane) that shows keto-enol tautomerism, with a stable enol form in alkaline media and a predominant keto form in acidic and neutral solutions. Commercial curcumin is a mixture of curcuminoids (approximately, 77% diferuloylmethane, 18% demethoxycurcumin, and 5% bisdemethoxycurcumin) [57]. C. longa mainly grows in China and India although it can

Figure 9. Chemical structure of curcumin [5].

in the Ajisawa solvent system [52, 55]. Characteristic signals of ILs (1572, 1465, and 1170 cm<sup>1</sup>

SFNs regenerated from classical CaCl2/EtOH/H2O solution [52]. From Ref. [23] with permission of Wiley Online Library.

][Cl]; (B) SFNs regenerated from SF/[bmim<sup>+</sup>

MilliQ water at 25C DMEM at 37C Solvent used Diametera (nm) PdI<sup>b</sup> Zpot<sup>a</sup> (mV) Diametera (nm) PdI<sup>b</sup> Zpot<sup>a</sup> (mV) CaCl2/EtOH/H2O 174 2 0.121 26.23 0.59 183 3 0.140 12.02 0.42

][Cl] 177 4 0.153 27.15 0.74 208 4 0.115 12.08 1.50

][Cl] 181 3 0.230 25.65 0.90 341 9 0.393 12.00 2.12

][Cl] 175 4 0.129 27.53 0.66 211 4 0.076 12.22 1.09

][Cl] 184 5 0.212 24.53 1.42 235 4 0.245 12.24 1.66

][Cl] 176 3 0.140 27.90 0.82 200 3 0.133 11.28 0.55

Table 3. Comparative values for the particle size (diameter), polydispersity (PdI), and zeta potential of classical SFNs [52] obtained from CaCl2/EtOH/H2O solvent and SFNs produced from SIL solutions. From Ref. [23] with permission of Wiley

Curcumin ((1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione, Figure 9) is a hydrophobic polyphenol derived from turmeric: the rhizome of the herb Curcuma longa [56]. From the chemical point of view, curcumin is a bis-α,β-unsaturated β-diketone (also called diferuloylmethane) that shows keto-enol tautomerism, with a stable enol form in alkaline media and a predominant keto form in acidic and neutral solutions. Commercial curcumin is a mixture of curcuminoids (approximately, 77% diferuloylmethane, 18% demethoxycurcumin, and 5% bisdemethoxycurcumin) [57]. C. longa mainly grows in China and India although it can

were absent in the recorded spectrum of SFNs obtained from [bmim+

2.5.1. Synthesis of curcumin-loaded silk fibroin nanoparticles using ILs

was efficiently washed out from the SFNs.

Figure 8. Comparative FTIR spectra of (A) only [bmim+

Z-average SD (n = 5) and accumulation times = 100. <sup>b</sup>

[mim<sup>+</sup>

16 Recent Advances in Ionic Liquids

[emim<sup>+</sup>

[pmim<sup>+</sup>

[bmim+

[hmim<sup>+</sup>

Average value.

Online Library.

a

)

][Cl], indicating that the IL

][Cl] solution; (C)

also be present in the rest of the Asian continent and has been widely employed in Ayurvedic medicine for centuries [58]. Its most relevant pharmacological effects are its anti-inflammatory [59], anticancer [57], antioxidant [60], and antimicrobial [60] activities.

Although curcumin is safe, nontoxic, and well tolerated in animal and human studies, it cannot be administered to patients directly due to its poor solubility in water [56] (estimated value: 3.12 mg/L at 25C [61]). In an attempt to enhance the therapeutic efficiency of curcumin, improvements in its bioavailability have been tried. Several nanocarriers such as solid lipid nanoparticles [62], natural [63] or synthetic [64] polymer nanoparticles, and inorganic nanoparticles [65] can be found in the study as examples of nanoplatforms for the intracellular delivery of curcumin. Recently, research interests focus on the use of biopolymers such as SF to encapsulate curcumin and other similar drugs [66]. By virtue of their small size, SFNs can penetrate thin capillaries, fostering the uptake of drugs by cells. In addition, these SFNs are potential targeted delivery systems because, for instance, they can deliver antitumor drugs to tumor cells. Several research groups have studied curcumin encapsulation in SFNs by different techniques [67, 68].

The authors [5] studied the synthesis of curcumin-loaded SFNs (Curc-SFNs) to improve on current methods, using IL (1-ethyl-3-methylimidazolium acetate, [emim<sup>+</sup> ][CH3COO]) and high-power ultrasounds to dissolve the SF. The synthesis of Curc-SFNs developed in this chapter is a more scalable and continuous processing option than those already published in the study. The drug was loaded into the SFNs by physical adsorption, Curc-SFNs 1, and by coprecipitation, Curc-SFNs 2, in order to obtain Curc-SFNs.

For loading of curcumin by physical adsorption, 40 mL of a 1 mg/mL solution of curcumin in ethanol was used to resuspend 325 mg of SFNs obtained from an SF-IL solution. The suspension was ultrasonicated for 5 min and gently stirred at 30 rpm in a Tube Rotator for 24 h. Next, Curc-SFNs 1 were centrifuged for 15 min at 13,400 rpm. Finally, Curc-SFNs 1 were washed with water to eliminate the rest of ethanol. The drug loaded in the nanoparticles was indirectly determined by the measurement of the UV absorbance of curcumin at 421 nm in the centrifugation supernatants (ethanol and water) and in the initial curcumin solution.

To obtain Curc-SFNs 2 by coprecipitation, the drug was loaded in the nanoparticles throughout the synthesis step. In brief, an exact weighed amount of curcumin (25 mg) was dissolved in 3 mL of 0.1 M NaOH solution, and this solution was immediately dissolved in 5 g of a previously prepared SIL solution (10% wt.). The drug-SIL solution was heated to 60C to reduce the viscosity of the mixture and sprayed with nitrogen onto 100 mL of gently stirred ethanol. The orange suspension was stirred for 2 h before being centrifuged at 13,400 rpm for 15 min, at 4�C. In this case, three washes with water were carried out to remove the IL. Lyophilization was carried out under the experimental conditions described earlier.

Drug loading content (DLC) and entrapment efficiency (EE) of Curc-SFNs 1 and Curc-SFNs 2 obtained were calculated according to the following expressions from the measurements of UV–Vis absorbance of curcumin:

$$\text{DLC} = \frac{\text{weight of the drug in nanopparticles}}{\text{weight of the nanopparticles}} \cdot 100\tag{1}$$

$$EE = \frac{\text{weight of the drug in nanoparticles}}{\text{weight of the fending drugs}} \cdot 100\tag{2}$$

previous studies which synthesize SFNs by classical methods [52, 70], which reflects the improvement in stability of the SFNs and hence of the Curc-SFNs obtained with this new

Biopolymeric Nanoparticle Synthesis in Ionic Liquids http://dx.doi.org/10.5772/intechopen.78766 19

The morphology of the nanoparticles was examined by field emission scanning electron microscopy (FESEM). As can be observed in Figure 10, SFNs and Curc-SFNs 1 present nanospherical morphology. However, Curc-SFNs 2 has elongated shape. FESEM micrographs showed smaller sizes than the DLS measurements. This could be due to the swelling of the nanoparticles in the water solution in DLS measurements. This difference has also been

The Curc-SFNs obtained in this work enhanced the antitumor activity of curcumin toward the two different tumor cell lines studied (hepatocellular carcinoma, Hep3B and human neuroblastoma, Kelly Cells), while the viability of the healthy cells (human bone marrow-derived mesenchymal stem cells, hBMSCs) did not decrease. This broadens the possibility of using these SFNs, which have been synthesized by an industrial process, as future systems for other

ILs are excellent candidates to participate in the synthesis of biopolymeric nanoparticles mainly because they can dissolve biopolymers due to their design flexibility by combining different cation and anion and their green solvent properties such as non-volatility, non-flammability, and recyclability. Different biopolymers, such as cellulose, xylan, starch, chitosan, keratin, and

procedure.

3. Conclusions

observed in previous works [52, 69].

drugs of hydrophilic or hydrophobic nature, such as curcumin.

Figure 10. FESEM pictures of (a) SFNs, (b) Curc-SFNs 1, and (c) Curc-SFNs 2 [5].

As can be seen in Table 4, DLC values in the physical adsorption assays were higher than in the coprecipitation experiments, probably due to the much higher initial curcumin/SF mass ratio in the physical adsorption experiments. Nevertheless, the EE values were about 50% for both types of nanoparticles. DLC and EE are in the same order or even higher than those found in the study [69, 70].

Furthemore, the mean hydrodynamic diameter (Z-average), the PdI, the zeta potential, and the electrophoretic mobility were measured by DLS. All measurements were performed in purified water at 25�C. The mean values of the measurements performed in triplicate are reported in Table 5. The results revealed that the Z-average of the SFNs was smaller than of the particles with curcumin, whereas the PdI values were similar and lower than 0.15 for all types of nanoparticles, resulting in size distributions practically monodisperse. The zeta potential of the nanoparticles with and without curcumin was highly negative and of the same order of magnitude, indicating their high colloidal stability and higher than the values found in


Mean values � SD (standard deviation) (n = 3).

Table 4. Drug loading and encapsulation efficiency of the Cur-SFNs [5].


Table 5. Physical characterization of the Curc-SFNs. From Ref. [5] with permission of MDPI.

Figure 10. FESEM pictures of (a) SFNs, (b) Curc-SFNs 1, and (c) Curc-SFNs 2 [5].

previous studies which synthesize SFNs by classical methods [52, 70], which reflects the improvement in stability of the SFNs and hence of the Curc-SFNs obtained with this new procedure.

The morphology of the nanoparticles was examined by field emission scanning electron microscopy (FESEM). As can be observed in Figure 10, SFNs and Curc-SFNs 1 present nanospherical morphology. However, Curc-SFNs 2 has elongated shape. FESEM micrographs showed smaller sizes than the DLS measurements. This could be due to the swelling of the nanoparticles in the water solution in DLS measurements. This difference has also been observed in previous works [52, 69].

The Curc-SFNs obtained in this work enhanced the antitumor activity of curcumin toward the two different tumor cell lines studied (hepatocellular carcinoma, Hep3B and human neuroblastoma, Kelly Cells), while the viability of the healthy cells (human bone marrow-derived mesenchymal stem cells, hBMSCs) did not decrease. This broadens the possibility of using these SFNs, which have been synthesized by an industrial process, as future systems for other drugs of hydrophilic or hydrophobic nature, such as curcumin.

#### 3. Conclusions

15 min, at 4�C. In this case, three washes with water were carried out to remove the IL.

Drug loading content (DLC) and entrapment efficiency (EE) of Curc-SFNs 1 and Curc-SFNs 2 obtained were calculated according to the following expressions from the measurements of

DLC <sup>¼</sup> weight of the drug in nanoparticles

EE <sup>¼</sup> weight of the drug in nanoparticles

As can be seen in Table 4, DLC values in the physical adsorption assays were higher than in the coprecipitation experiments, probably due to the much higher initial curcumin/SF mass ratio in the physical adsorption experiments. Nevertheless, the EE values were about 50% for both types of nanoparticles. DLC and EE are in the same order or even higher than those found

Furthemore, the mean hydrodynamic diameter (Z-average), the PdI, the zeta potential, and the electrophoretic mobility were measured by DLS. All measurements were performed in purified water at 25�C. The mean values of the measurements performed in triplicate are reported in Table 5. The results revealed that the Z-average of the SFNs was smaller than of the particles with curcumin, whereas the PdI values were similar and lower than 0.15 for all types of nanoparticles, resulting in size distributions practically monodisperse. The zeta potential of the nanoparticles with and without curcumin was highly negative and of the same order of magnitude, indicating their high colloidal stability and higher than the values found in

DLC (%) 6.63 � 0.09 2.47 � 0.11 EE (%) 53.75 � 0.81 48.84 � 2.67

Table 5. Physical characterization of the Curc-SFNs. From Ref. [5] with permission of MDPI.

Zeta potential (mV) Z-average (nm) Electrophoretic mobility (μmcm/Vs) PdI Curc-SFNs 1 �42.9 � 2.8 166.0 � 0.1 �3.362 � 0.264 0.114 � 0.003 Curc-SFNs 2 �45.9 � 5.0 171.2 � 2.6 �3.504 � 0.348 0.106 � 0.017 SFNs �41.3 � 0.6 157.9 � 1.5 �3.396 � 0.146 0.132 � 0.011

weight of the nanoparticles <sup>∙</sup><sup>100</sup> (1)

weight of the feeding drugs <sup>∙</sup><sup>100</sup> (2)

Curc-SFNs 1<sup>1</sup> Curc-SFNs 21

Lyophilization was carried out under the experimental conditions described earlier.

UV–Vis absorbance of curcumin:

18 Recent Advances in Ionic Liquids

in the study [69, 70].

Mean values � SD (standard deviation) (n = 3).

Mean values � SD (standard deviation).

Table 4. Drug loading and encapsulation efficiency of the Cur-SFNs [5].

1

ILs are excellent candidates to participate in the synthesis of biopolymeric nanoparticles mainly because they can dissolve biopolymers due to their design flexibility by combining different cation and anion and their green solvent properties such as non-volatility, non-flammability, and recyclability. Different biopolymers, such as cellulose, xylan, starch, chitosan, keratin, and silk fibroin, were used to obtain nanoparticles in processes involving ILs. We synthetized SFNs by a new methodology using high-power ultrasounds to enhance the dissolution of the protein in the IL. From SIL solutions, SFNs were obtained by regeneration of the SF in an organic polar solvent, and SFNs showed a high degree of β-sheet, similar to that of the SF native fibers. In this way, large amounts of silk can be turned into biomaterials directly from the dissolved SIL solution, for use in a wide range of applications. Focusing on the biomedical application, Curc-SFNs were successfully synthesized by two environmentally friendly procedures using ILs and high-power ultrasound to dissolve the SF. High DLC and EE values were obtained in both cases compared with those in the study. The SFNs and the Curc-SFNs obtained showed a narrow size distribution, with a hydrodynamic diameter of <175 nm, and high zeta potential (in absolute terms), which make them excellent nanocarriers for use in therapeutic treatments.

References

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ijms14011629

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## Acknowledgements

This work has been partially supported from the European Commission (FEDER/ERDF) and the Spanish MINECO (Ref. CTQ2014-57467-R and Ref. CTQ2017-87708-R) and the programme of support to the research of the Seneca Foundation of Science and Technology of Murcia, Spain (Ref. 19499/PI/14). The research contract of Dr. A. Abel Lozano-Pérez was partially supported (80%) by the ERDF/FEDER Operative Programme of the Region of Murcia (Project No. 14-20-01).

#### Conflict of interest

The authors declare no conflict of interest.

## Author details

Mercedes G. Montalbán<sup>1</sup> , Guzmán Carissimi<sup>2</sup> , A. Abel Lozano-Pérez<sup>3</sup> , José Luis Cenis<sup>3</sup> , Jeannine M. Coburn4,5, David L. Kaplan4 and Gloria Víllora<sup>2</sup> \*

\*Address all correspondence to: gvillora@um.es

1 Department of Chemical Engineering, University of Alicante, Alicante, Spain

2 Department of Chemical Engineering, Faculty of Chemistry, Regional Campus of International Excellence "Campus Mare Nostrum", University of Murcia, Murcia, Spain

3 Department of Biotechnology, Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario (IMIDA), La Alberca (Murcia), Spain

4 Department of Biomedical Engineering, Tufts University, Medford, MA, USA

5 Department of Biomedical Engineering, Worcester Polytechnic Institute, Worcester, MA, USA

#### References

silk fibroin, were used to obtain nanoparticles in processes involving ILs. We synthetized SFNs by a new methodology using high-power ultrasounds to enhance the dissolution of the protein in the IL. From SIL solutions, SFNs were obtained by regeneration of the SF in an organic polar solvent, and SFNs showed a high degree of β-sheet, similar to that of the SF native fibers. In this way, large amounts of silk can be turned into biomaterials directly from the dissolved SIL solution, for use in a wide range of applications. Focusing on the biomedical application, Curc-SFNs were successfully synthesized by two environmentally friendly procedures using ILs and high-power ultrasound to dissolve the SF. High DLC and EE values were obtained in both cases compared with those in the study. The SFNs and the Curc-SFNs obtained showed a narrow size distribution, with a hydrodynamic diameter of <175 nm, and high zeta potential (in absolute terms), which make them excellent nanocarriers for use in therapeutic treatments.

This work has been partially supported from the European Commission (FEDER/ERDF) and the Spanish MINECO (Ref. CTQ2014-57467-R and Ref. CTQ2017-87708-R) and the programme of support to the research of the Seneca Foundation of Science and Technology of Murcia, Spain (Ref. 19499/PI/14). The research contract of Dr. A. Abel Lozano-Pérez was partially supported (80%) by the ERDF/FEDER Operative Programme of the Region of Murcia (Project No. 14-20-01).

, A. Abel Lozano-Pérez<sup>3</sup>

\*

, José Luis Cenis<sup>3</sup>

,

Acknowledgements

20 Recent Advances in Ionic Liquids

Conflict of interest

Author details

USA

Mercedes G. Montalbán<sup>1</sup>

The authors declare no conflict of interest.

, Guzmán Carissimi<sup>2</sup>

1 Department of Chemical Engineering, University of Alicante, Alicante, Spain

4 Department of Biomedical Engineering, Tufts University, Medford, MA, USA

2 Department of Chemical Engineering, Faculty of Chemistry, Regional Campus of International Excellence "Campus Mare Nostrum", University of Murcia, Murcia, Spain

3 Department of Biotechnology, Instituto Murciano de Investigación y Desarrollo Agrario y

5 Department of Biomedical Engineering, Worcester Polytechnic Institute, Worcester, MA,

Jeannine M. Coburn4,5, David L. Kaplan4 and Gloria Víllora<sup>2</sup>

\*Address all correspondence to: gvillora@um.es

Alimentario (IMIDA), La Alberca (Murcia), Spain


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10.4172/2577-0543.1000116

24 Recent Advances in Ionic Liquids

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**Chapter 2**

**Provisional chapter**

**Phosphazene-Based Ionic Liquids**

**Phosphazene-Based Ionic Liquids**

DOI: 10.5772/intechopen.76613

This chapter presents the definition, synthesis, and possible application of cyclo and polyphosphazene-based ionic liquids (PzILs). PZILs constitute an alternative class of phosphorus nitrogen compounds and their derivatives have been widely used in biologically-active materials, electrolytes, lubricants, catalysts or nanomaterials. Considerable information is available on substitution reactions taking place at the phosphorus atoms of poly and cyclophosphazenes, thus, a wide variety of phosphazene derivatives have been obtained. However, quaternization of ring nitrogen atoms has received less attention. In addition, phosphazenes containing aliphatic and aromatic substituents with terminal tertiary amino groups are synthesized and subsequently quaternized with methyl iodide.

PzILs. In the compounds identified as protonic ionic liquids (PILs) or protic molten salts (PMOSs), the positively charged position is determined by X-ray diffraction study. PzILs

**Keywords:** phosphazene-based ionic liquid, biologically-active material, electrolyte,

Phosphazenes, which are cyclic or linear chain inorganic compounds formed by the bonding and repetition of phosphorus and nitrogen atoms with (P=N)n bonds, comprise an important class of inorganic compounds (**Figure 1**). There are many phosphazene compounds ranging from oligomers to polymers. Among the phosphazene compounds, the hexachlo-

ramer) derivatives have attracted considerable attention (**Figure 1a,b**) [1]. The reaction of

P3 Cl<sup>6</sup>

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Cl in a chlorinated solvent gives a mixture of colorless solids of the formula

, trimer) and octachlorocyclotetraphosphazene (N<sup>4</sup>

and N<sup>4</sup>

P4 Cl<sup>8</sup>

CF<sup>3</sup> )2

or NaBF<sup>4</sup>

gives the respective

P4 Cl<sup>8</sup> , tet-

are readily separated by

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

The successive metathesis with salts such as LiN(SO<sup>2</sup>

P3 Cl<sup>6</sup>

)n. As the most popular compounds N<sup>3</sup>

are also soluble in water and in many polar organic solvents.

Ahmet Karadağ and Hüseyin Akbaş

Ahmet Karadağ and Hüseyin Akbaş

http://dx.doi.org/10.5772/intechopen.76613

**Abstract**

lubricant, catalyst

rocyclotriphosphazene (N<sup>3</sup>

with NH<sup>4</sup>

**1. Introduction**

PCl<sup>5</sup>

(NPCl<sup>2</sup>


#### **Phosphazene-Based Ionic Liquids Phosphazene-Based Ionic Liquids**

#### Ahmet Karadağ and Hüseyin Akbaş Ahmet Karadağ and Hüseyin Akbaş

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.76613

#### **Abstract**

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[69] Xiao L, Lu G, Lu Q, Kaplan DL. Direct formation of silk nanoparticles for drug delivery. ACS Biomaterials Science & Engineering. 2016;2:2050-2057. DOI: 10.1021/acsbiomate-

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and Technology. 2016;32:10-20. DOI: 10.1016/j.jddst.2016.01.007

2015;103:1-9. DOI: 10.1016/j.supflu.2015.04.021

2014;26:7393-7398. DOI: 10.1002/adma.201403562

10.1021/acsbiomaterials.7b00153

rials.6b00457

26 Recent Advances in Ionic Liquids

This chapter presents the definition, synthesis, and possible application of cyclo and polyphosphazene-based ionic liquids (PzILs). PZILs constitute an alternative class of phosphorus nitrogen compounds and their derivatives have been widely used in biologically-active materials, electrolytes, lubricants, catalysts or nanomaterials. Considerable information is available on substitution reactions taking place at the phosphorus atoms of poly and cyclophosphazenes, thus, a wide variety of phosphazene derivatives have been obtained. However, quaternization of ring nitrogen atoms has received less attention. In addition, phosphazenes containing aliphatic and aromatic substituents with terminal tertiary amino groups are synthesized and subsequently quaternized with methyl iodide. The successive metathesis with salts such as LiN(SO<sup>2</sup> CF<sup>3</sup> )2 or NaBF<sup>4</sup> gives the respective PzILs. In the compounds identified as protonic ionic liquids (PILs) or protic molten salts (PMOSs), the positively charged position is determined by X-ray diffraction study. PzILs are also soluble in water and in many polar organic solvents.

DOI: 10.5772/intechopen.76613

**Keywords:** phosphazene-based ionic liquid, biologically-active material, electrolyte, lubricant, catalyst

#### **1. Introduction**

Phosphazenes, which are cyclic or linear chain inorganic compounds formed by the bonding and repetition of phosphorus and nitrogen atoms with (P=N)n bonds, comprise an important class of inorganic compounds (**Figure 1**). There are many phosphazene compounds ranging from oligomers to polymers. Among the phosphazene compounds, the hexachlorocyclotriphosphazene (N<sup>3</sup> P3 Cl<sup>6</sup> , trimer) and octachlorocyclotetraphosphazene (N<sup>4</sup> P4 Cl<sup>8</sup> , tetramer) derivatives have attracted considerable attention (**Figure 1a,b**) [1]. The reaction of PCl<sup>5</sup> with NH<sup>4</sup> Cl in a chlorinated solvent gives a mixture of colorless solids of the formula (NPCl<sup>2</sup> )n. As the most popular compounds N<sup>3</sup> P3 Cl<sup>6</sup> and N<sup>4</sup> P4 Cl<sup>8</sup> are readily separated by

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**2.1. Phosphazene-based ionic liquids in which quaternization occurs on the** 

The protonation of the ring was determined by the crystal structure of N<sup>3</sup>

)2 ] −

P3 Cl<sup>6</sup>

in the presence of water or HX yielded protonated phosphazenes P<sup>3</sup>

Ring and the exocyclic nitrogens are two possible basic sites of cyclophosphazenes. The formation of the protonation and the pKa′ values of cyclophosphazene derivatives were investigated in the literature [14–16]. The ring-nitrogen protonation of cyclophosphazene bases with

(**Figure 3**) [19, 20].

, AlBr<sup>3</sup>

·HAlCl<sup>4</sup>

, GaCl<sup>3</sup>

, N<sup>3</sup> P3 Cl<sup>6</sup>

ring and weakened the two P − N bonds that flank the proton-

with AlCl<sup>3</sup>

P3 Cl<sup>6</sup>

Despite the low basicity and nucleophilicity, various phosphazenium compounds have been obtained using potent electrophilic reagents based on carborane anions. (**Figure 5**) [18].

N-alkyl phosphazenium cations are obtained by alkylation of the ring nitrogen atom of cyclotriphosphazenes containing organoamino substituents with alkyl halides (**Figure 6**). As

COOH were supported by infrared and NMR data [17, 18].

P3 Cl<sup>2</sup>

N3 Cl<sup>6</sup>

·HGaCl<sup>4</sup>

under anaerobic conditions or

.HMX<sup>4</sup>

, N<sup>3</sup> P3 Cl<sup>6</sup>

(NHPr<sup>i</sup> )4 .HCI

Phosphazene-Based Ionic Liquids

http://dx.doi.org/10.5772/intechopen.76613

. The proton-

are

29

·HAlBr<sup>4</sup>

**nitrogen atom or phosphorus atom of phosphazene**

, and CH<sup>3</sup>

[N(POCl<sup>2</sup>

N3

P3 HCl<sup>4</sup> (NH<sup>2</sup> )2 ] + [N(POCl<sup>2</sup> )2 ] − .

P3 Cl<sup>6</sup> ·HMX<sup>4</sup>

, (a) N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlCl<sup>4</sup>

; (b) N<sup>3</sup> P3 Cl<sup>6</sup> ·HGaCl<sup>4</sup>

; (c) N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlBr<sup>4</sup> .

HCl, HBr, HF, HClO<sup>4</sup>

(NH<sup>2</sup> )2 ]+

ated nitrogen. The crystal structures of N<sup>3</sup>

Reportedly, the reactions of N<sup>3</sup>

ated specie distorted the P<sup>3</sup>

shown in **Figure 4** [21, 22].

**Figure 3.** The crystal structure of [N<sup>3</sup>

**Figure 4.** The crystal structures of N<sup>3</sup>

P3 HCl<sup>4</sup>

and [N<sup>3</sup>

**Figure 1.** Phosphazene structures.

distillation under reduced pressure. Cyclic phosphazene compounds containing five, six, seven and higher -P=N units are also known, but these compounds are not very common [2]. Polyphosphazenes are inorganic–organic polymers containing alternate phosphorus and nitrogen atoms, each skeletal phosphorus atom having bonds with one or more organic or inorganic substituent (**Figure 1c**) [3].

Phosphazenes exhibit highly customizable physical and chemical characteristics which depend on the substituents bonded to the phosphorus atom. Thus, they have found wide application in a variety of fields involving their use in rechargeable batteries [4], membranes [5] and lubricants [6], liquid crystals [7], anticancer agents [8], antibacterial reagents [9], flameretardants [10], biological materials [11], and synthetic bones [12].

This chapter deals with the structures and applications of PzILs which are formed by quaternization of the ring nitrogen or phosphorus in the phosphazene, or the nitrogen atom in the substituent bonded to the phosphorus atom.

#### **2. Phosphazene-based ionic liquids**

The PzIL consists of repeating phosphorus-nitrogen units having a pendant group bonded to the phosphorus atoms of the phosphazene. PzILs may have cyclic or linear structure (**Figure 2**). The positive charge is positioned in a substituent attached to the phosphorus atom, or in the skeleton nitrogen or phosphorus atom. The positive charge's position is reported to be effective on the stability, viscosity and other properties of PzIL [13].

**Figure 2.** General chemical structure of PzILs.

#### **2.1. Phosphazene-based ionic liquids in which quaternization occurs on the nitrogen atom or phosphorus atom of phosphazene**

Ring and the exocyclic nitrogens are two possible basic sites of cyclophosphazenes. The formation of the protonation and the pKa′ values of cyclophosphazene derivatives were investigated in the literature [14–16]. The ring-nitrogen protonation of cyclophosphazene bases with HCl, HBr, HF, HClO<sup>4</sup> , and CH<sup>3</sup> COOH were supported by infrared and NMR data [17, 18]. The protonation of the ring was determined by the crystal structure of N<sup>3</sup> P3 Cl<sup>2</sup> (NHPr<sup>i</sup> )4 .HCI and [N<sup>3</sup> P3 HCl<sup>4</sup> (NH<sup>2</sup> )2 ]+ [N(POCl<sup>2</sup> )2 ] − (**Figure 3**) [19, 20].

Reportedly, the reactions of N<sup>3</sup> P3 Cl<sup>6</sup> with AlCl<sup>3</sup> , AlBr<sup>3</sup> , GaCl<sup>3</sup> under anaerobic conditions or in the presence of water or HX yielded protonated phosphazenes P<sup>3</sup> N3 Cl<sup>6</sup> .HMX<sup>4</sup> . The protonated specie distorted the P<sup>3</sup> N3 ring and weakened the two P − N bonds that flank the protonated nitrogen. The crystal structures of N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlCl<sup>4</sup> , N<sup>3</sup> P3 Cl<sup>6</sup> ·HGaCl<sup>4</sup> , N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlBr<sup>4</sup> are shown in **Figure 4** [21, 22].

Despite the low basicity and nucleophilicity, various phosphazenium compounds have been obtained using potent electrophilic reagents based on carborane anions. (**Figure 5**) [18].

N-alkyl phosphazenium cations are obtained by alkylation of the ring nitrogen atom of cyclotriphosphazenes containing organoamino substituents with alkyl halides (**Figure 6**). As

**Figure 3.** The crystal structure of [N<sup>3</sup> P3 HCl<sup>4</sup> (NH<sup>2</sup> )2 ] + [N(POCl<sup>2</sup> )2 ] − .

distillation under reduced pressure. Cyclic phosphazene compounds containing five, six, seven and higher -P=N units are also known, but these compounds are not very common [2]. Polyphosphazenes are inorganic–organic polymers containing alternate phosphorus and nitrogen atoms, each skeletal phosphorus atom having bonds with one or more organic or

Phosphazenes exhibit highly customizable physical and chemical characteristics which depend on the substituents bonded to the phosphorus atom. Thus, they have found wide application in a variety of fields involving their use in rechargeable batteries [4], membranes [5] and lubricants [6], liquid crystals [7], anticancer agents [8], antibacterial reagents [9], flame-

This chapter deals with the structures and applications of PzILs which are formed by quaternization of the ring nitrogen or phosphorus in the phosphazene, or the nitrogen atom in the

The PzIL consists of repeating phosphorus-nitrogen units having a pendant group bonded to the phosphorus atoms of the phosphazene. PzILs may have cyclic or linear structure (**Figure 2**). The positive charge is positioned in a substituent attached to the phosphorus atom, or in the skeleton nitrogen or phosphorus atom. The positive charge's position is reported to be effec-

retardants [10], biological materials [11], and synthetic bones [12].

tive on the stability, viscosity and other properties of PzIL [13].

inorganic substituent (**Figure 1c**) [3].

**Figure 1.** Phosphazene structures.

28 Recent Advances in Ionic Liquids

substituent bonded to the phosphorus atom.

**2. Phosphazene-based ionic liquids**

**Figure 2.** General chemical structure of PzILs.

**Figure 4.** The crystal structures of N<sup>3</sup> P3 Cl<sup>6</sup> ·HMX<sup>4</sup> , (a) N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlCl<sup>4</sup> ; (b) N<sup>3</sup> P3 Cl<sup>6</sup> ·HGaCl<sup>4</sup> ; (c) N<sup>3</sup> P3 Cl<sup>6</sup> ·HAlBr<sup>4</sup> .

**Figure 5.** N-protonated, N-methylated, and N-silylated adducts of N<sup>3</sup> P3 Cl<sup>6</sup> .

**Figure 7.** Supramolecular structure of (1Me)I. The dashed lines show hydrogen bonds.

Phosphazene-Based Ionic Liquids

31

http://dx.doi.org/10.5772/intechopen.76613

[2Me](NO<sup>3</sup>

)3 .H<sup>2</sup> O.

**Figure 8.** The crystal structure of Ag<sup>2</sup>

**Figure 9.** Chemical structure of phosphazenium salt (PZN).

**Figure 6.** N-alkyl phosphazenium salts.

observed in the X-ray crystal structures, the associated P-N bonds of the alkylation of the ring N sites are significantly longer. Highly stable phosphazenium salts generate complex supramolecular networks with NH…X interactions in the solid state (**Figure 7**). N-Alkyl phosphazenium salts react with silver nitrate to form complexes with silver ions. Depending on the steric requirement of the RNH substituents, one or both of the free ring nitrogen sites are coordinated with silver ions (**Figure 8**) [23].

Industrial application of High performance nonmetallic molecular phosphazene catalysts involves the synthesis of polypropylene glycols (PPGs). Phosphazenium salts (PZN) have giant cations that are 10–12 Å in diameter and they exhibit unique catalytic behavior in various anionic organic reactions that are highly demanded in chemical industry (**Figure 9**). Reportedly, a phosphazenium ion (a macrocationic species), should considerably activate anionic active species [24, 25].

**Figure 7.** Supramolecular structure of (1Me)I. The dashed lines show hydrogen bonds.

**Figure 8.** The crystal structure of Ag<sup>2</sup> [2Me](NO<sup>3</sup> )3 .H<sup>2</sup> O.

**Figure 5.** N-protonated, N-methylated, and N-silylated adducts of N<sup>3</sup>

30 Recent Advances in Ionic Liquids

**Figure 6.** N-alkyl phosphazenium salts.

coordinated with silver ions (**Figure 8**) [23].

P3 Cl<sup>6</sup> .

observed in the X-ray crystal structures, the associated P-N bonds of the alkylation of the ring N sites are significantly longer. Highly stable phosphazenium salts generate complex supramolecular networks with NH…X interactions in the solid state (**Figure 7**). N-Alkyl phosphazenium salts react with silver nitrate to form complexes with silver ions. Depending on the steric requirement of the RNH substituents, one or both of the free ring nitrogen sites are

Industrial application of High performance nonmetallic molecular phosphazene catalysts involves the synthesis of polypropylene glycols (PPGs). Phosphazenium salts (PZN) have giant cations that are 10–12 Å in diameter and they exhibit unique catalytic behavior in various anionic organic reactions that are highly demanded in chemical industry (**Figure 9**). Reportedly, a phosphazenium ion (a macrocationic species), should considerably activate anionic active species [24, 25].

**Figure 9.** Chemical structure of phosphazenium salt (PZN).

**2.2. Phosphazene-based ionic liquids in which quaternization occurs on a pendant** 

Due to the more sterically suitable positions of the nitrogen atoms of the phosphazene ring, the alkylation occurred at the exocyclic nitrogen atoms Rapko and Feistel presented the parameters in their study on the dialkyl cation of hexakisdimethylamino cyclotriphosphazene

Allcock et al. synthesized phosphazenium iodide salts by quaternization of several cyclic phosphazenes either at side-group sites or at the skeletal nitrogen atom (**Figure 13**). With the exception of piperidino derivatives, in which case the reactive sites were the skeletal nitrogen

was not quaternized, since the side-group nitrogen atoms are directly attached to the phosp-

PzILs are prepared by quaternization of a wide variety of phosphazenes either at the sidegroup or at the skeletal nitrogen atoms. These ILs have been used as anticancer, antibacterial reagents [31–34], adsorbents and surface modifiers of fluorescent nanoparticles [35], lubricants [36, 37], chemosensors for metal ions [38], electrolyte solutions for energy storage devices [39–41], as gate dielectric layer for OFETs [42], or as polyelectrolytes [26, 43, 44].

Several studies have been performed on cyclotriphosphazene-based protic molten salts (PMOSs) synthesized with cyclotriphosphazenes and bulky organic acids. Recently, aminocyclotriphosphazenes have received greater attention due to their anti-cancer agent properties [45, 46]. In contrast to cyclotriphosphazene derivatives, there are not many studies related to cyclotriphosphazene salts as antimicrobial and anticancer agents [31–34]. Phosphazenium salts are very soluble in common apolar and polar organic solvents and some are quite soluble in water. Solubility in biological liquid is very important in pharmacological studies.

**Figure 12.** The synthesis of dimethyl hexakisdimethylamino cyclotriphosphonitrilium difluoroborate.

]), (**Figure 12**). The position of the alkylation was investigated by

P3 (OC<sup>6</sup> H5 )5 N(CH<sup>3</sup> ) 2

Phosphazene-Based Ionic Liquids

33

http://dx.doi.org/10.5772/intechopen.76613

**group of phosphazene bonded to a phosphorus atom**

hydrolytic degradation of the obtained cyclotriphosphazene salts [29].

atoms, quaternization occurred at the side-group sites. The compound N<sup>3</sup>

hazene ring due to their protected or inactivated nature [30].

**2.3. Applications of phosphazene-based ionic liquids**

*2.3.1. Anticancer, antibacterial reagents*

]2+[(BF<sup>4</sup> ˉ)2

([N<sup>3</sup> P3 (NMe<sup>2</sup> )6 (Me)<sup>2</sup>

**Figure 10.** Quaternization of the cyclotriphosphazenes.

**Figure 11.** Mono protonated cyclotriphosphazene, [HN<sup>3</sup> P3 (NMe<sup>2</sup> ) 4 (NHCH<sup>2</sup> CH<sup>2</sup> CH<sup>2</sup> NH)]<sup>2</sup> -[PtCl<sup>4</sup> ].

Organophosphazenes bearing -OR substituted groups are readily quaternized at the ring nitrogen atoms to form phosphazene cations with alkyl halides, methyl trifluoromethanesulfonate (CF<sup>3</sup> SO<sup>3</sup> CH<sup>3</sup> ) or trimethyloxonium tetrafluoroborate [(CH<sup>3</sup> )3 O(BF<sup>4</sup> )] (**Figure 10**). Lower electron densities at the ring nitrogen atoms render alkoxy or aryloxy-substituted cyclotriphosphazenes inert to iodomethane at room temperature or lead to the rearrangement of the alkoxyphosphazene to the N-alkyloxophosphazane at higher temperatures [26].

The transition metal chemistry of cyclophosphazenes has also attracted great interest. The nature of cyclophosphazene-adducted compounds usually depends both on the phosphazene base and the corresponding Lewis acid. For example, for the N<sup>6</sup> P6 (NMe<sup>2</sup> )12.CuCl<sup>2</sup> , Lewis acid is not bonded to a particular ring atom. However, it is located on the ring and is attached to more than one nitrogen atom [27]. A mono protonated (amino) *spiro* cyclic cyclotriphosphazene salt was synthesized and its crystal structure was clarified. The protonation occurs at one of the nitrogen atoms adjacent to the spiro phosphorus atom of the P<sup>3</sup> N3 ring (**Figure 11**). The protonation caused elongation of the P-N bonds in the ring and puckering of the phosphazene ring. In the crystal lattice, 2n cyclophosphazenium cations are connected by *n*[PtCl<sup>4</sup> ]2− anions with N-H…Cl hydrogen bonds to generate a linear polymeric structure [28]. A great number of cyclophosphazenium cations with metal anions, such as [HN<sup>3</sup> P3 (NMe<sup>2</sup> )6 ]2 [Mo<sup>6</sup> O19], [HN<sup>3</sup> P3 (NMe<sup>2</sup> )6 ]2 [CoCl<sup>4</sup> ], [MeN<sup>4</sup> P4 Me<sup>8</sup> ][Cr(CO)<sup>5</sup> I], [HN<sup>4</sup> P4 Me<sup>8</sup> ]2 [CoCl<sup>4</sup> ], [H<sup>2</sup> N4 P4 Me<sup>8</sup> ][PtCl<sup>4</sup> ] and [H<sup>2</sup> N5 P5 Me10][CuCl<sup>4</sup> ], have been obtained.

#### **2.2. Phosphazene-based ionic liquids in which quaternization occurs on a pendant group of phosphazene bonded to a phosphorus atom**

Due to the more sterically suitable positions of the nitrogen atoms of the phosphazene ring, the alkylation occurred at the exocyclic nitrogen atoms Rapko and Feistel presented the parameters in their study on the dialkyl cation of hexakisdimethylamino cyclotriphosphazene ([N<sup>3</sup> P3 (NMe<sup>2</sup> )6 (Me)<sup>2</sup> ]2+[(BF<sup>4</sup> ˉ)2 ]), (**Figure 12**). The position of the alkylation was investigated by hydrolytic degradation of the obtained cyclotriphosphazene salts [29].

Allcock et al. synthesized phosphazenium iodide salts by quaternization of several cyclic phosphazenes either at side-group sites or at the skeletal nitrogen atom (**Figure 13**). With the exception of piperidino derivatives, in which case the reactive sites were the skeletal nitrogen atoms, quaternization occurred at the side-group sites. The compound N<sup>3</sup> P3 (OC<sup>6</sup> H5 )5 N(CH<sup>3</sup> ) 2 was not quaternized, since the side-group nitrogen atoms are directly attached to the phosphazene ring due to their protected or inactivated nature [30].

#### **2.3. Applications of phosphazene-based ionic liquids**

PzILs are prepared by quaternization of a wide variety of phosphazenes either at the sidegroup or at the skeletal nitrogen atoms. These ILs have been used as anticancer, antibacterial reagents [31–34], adsorbents and surface modifiers of fluorescent nanoparticles [35], lubricants [36, 37], chemosensors for metal ions [38], electrolyte solutions for energy storage devices [39–41], as gate dielectric layer for OFETs [42], or as polyelectrolytes [26, 43, 44].

#### *2.3.1. Anticancer, antibacterial reagents*

**Figure 10.** Quaternization of the cyclotriphosphazenes.

32 Recent Advances in Ionic Liquids

**Figure 11.** Mono protonated cyclotriphosphazene, [HN<sup>3</sup>

romethanesulfonate (CF<sup>3</sup>

higher temperatures [26].

O19], [HN<sup>3</sup>

][PtCl<sup>4</sup>

P3 (NMe<sup>2</sup> )6 ]2 [CoCl<sup>4</sup>

> N5 P5

] and [H<sup>2</sup>

*n*[PtCl<sup>4</sup>

(NMe<sup>2</sup> )6 ]2 [Mo<sup>6</sup>

[H<sup>2</sup> N4 P4 Me<sup>8</sup> SO<sup>3</sup> CH<sup>3</sup>

zene base and the corresponding Lewis acid. For example, for the N<sup>6</sup>

one of the nitrogen atoms adjacent to the spiro phosphorus atom of the P<sup>3</sup>

Me10][CuCl<sup>4</sup>

Organophosphazenes bearing -OR substituted groups are readily quaternized at the ring nitrogen atoms to form phosphazene cations with alkyl halides, methyl trifluo-

P3 (NMe<sup>2</sup> ) 4 (NHCH<sup>2</sup>

(**Figure 10**). Lower electron densities at the ring nitrogen atoms render alkoxy or aryloxy-substituted cyclotriphosphazenes inert to iodomethane at room temperature or lead to the rearrangement of the alkoxyphosphazene to the N-alkyloxophosphazane at

The transition metal chemistry of cyclophosphazenes has also attracted great interest. The nature of cyclophosphazene-adducted compounds usually depends both on the phospha-

acid is not bonded to a particular ring atom. However, it is located on the ring and is attached to more than one nitrogen atom [27]. A mono protonated (amino) *spiro* cyclic cyclotriphosphazene salt was synthesized and its crystal structure was clarified. The protonation occurs at

The protonation caused elongation of the P-N bonds in the ring and puckering of the phosphazene ring. In the crystal lattice, 2n cyclophosphazenium cations are connected by

[28]. A great number of cyclophosphazenium cations with metal anions, such as [HN<sup>3</sup>

]2− anions with N-H…Cl hydrogen bonds to generate a linear polymeric structure

P4 Me<sup>8</sup>

], have been obtained.

][Cr(CO)<sup>5</sup>

], [MeN<sup>4</sup>

) or trimethyloxonium tetrafluoroborate [(CH<sup>3</sup>

CH<sup>2</sup> CH<sup>2</sup> NH)]<sup>2</sup>

> P6 (NMe<sup>2</sup>


N3

I], [HN<sup>4</sup>

P4 Me<sup>8</sup> ]2 [CoCl<sup>4</sup> ],

)3 O(BF<sup>4</sup> )]

)12.CuCl<sup>2</sup>

ring (**Figure 11**).

, Lewis

P3

Several studies have been performed on cyclotriphosphazene-based protic molten salts (PMOSs) synthesized with cyclotriphosphazenes and bulky organic acids. Recently, aminocyclotriphosphazenes have received greater attention due to their anti-cancer agent properties [45, 46]. In contrast to cyclotriphosphazene derivatives, there are not many studies related to cyclotriphosphazene salts as antimicrobial and anticancer agents [31–34]. Phosphazenium salts are very soluble in common apolar and polar organic solvents and some are quite soluble in water. Solubility in biological liquid is very important in pharmacological studies.

**Figure 12.** The synthesis of dimethyl hexakisdimethylamino cyclotriphosphonitrilium difluoroborate.

**Figure 13.** Various cyclotriphosphazenium iodide salts.

Therefore, studies on the biological and anti-cancer activities of the salts of cyclotriphosphazenes are likely to attract great interest because of their organic solvent/water solubility and various PMOS diversity with different properties.

Akbaş et al. prepared the salicylic acid salts (**1–6**) of pyrrolidine and piperidine substituted cyclotriphosphazenes (**Figure 14**). The crystallographic data of **5** clearly indicate that the nitrogen of the phosphazene ring was protonated (**Figure 15**). The antimicrobial and cytotoxic activities of the phosphazenium salts (**1–6**) were also investigated. Compounds **5** and **6** appear to be good candidates for anti-cancer agents because they have significant cytotoxic activity against DLD-1 cancer cells. All of the compounds have an antimicrobial effect on bacterial and yeast strains between 312 and 625 μM (bacterial strains) and 19.5–312 μM (yeast strains) and compounds **4–6** are found to be most effective against yeast strains [31].

Elmas et al. synthesized phosphazenium salts (**1a-4a**) from the reactions of the phosphazene bases (**1–4**) with gentisic acid (**Figure 16**). The crystallographic data of **4a** unambiguously indicate that the nitrogen of the phosphazene ring is monoprotonated (**Figure 17**). Also, In vitro antimicrobial activities of compounds were investigated and the PILs (**1a**, **3a** and **4a**)

gentisic, decanoic and boric acids (**Figure 18**). Their biological activity in cultured cell lines was investigated. The binding of **1**,**2** or **3** to calf thymus (CT-DNA) and bovine serum albumin (BSA) led to remarkable changes in spectral characteristics. The potent low cytotoxic, strong apoptotic, and effective DNA topoisomerase inhibitory characteristics of these PILs revealed

Okumuş et al. obtained the PMOSs from the reactions of tetrapyrrolidino- and tetrapiperidino-substituted cyclotriphosphazenes with the gentisic and γ-resorcylic acids (**Figure 19**). The crystallographic result of **1b** and **2b** exhibited that the N atom of the phosphazene ring adjacent to the spiro precursor was mono protonated (**Figure 20**). In addition, their

P3 (NC<sup>4</sup> H8 ) 6

Phosphazene-Based Ionic Liquids

35

http://dx.doi.org/10.5772/intechopen.76613

] with the

were found to be significantly active against *C. albicans* [32].

**Figure 16.** The synthesis of 4-fluorobenzylspiro(N/O)cyclotriphosphazenium salts.

**Figure 15.** The crystal structure of compound **5.**

that they can be a good candidate for anticancer drugs [33].

Akbaş et al. obtained the PILs or PMOSs (**1–3**) from the reactions of [N<sup>3</sup>

**Figure 14.** Salicylic acid salts of the mono(4-fluorobenzyl)spirocyclotriphosphazenes.

Phosphazene-Based Ionic Liquids http://dx.doi.org/10.5772/intechopen.76613 35

**Figure 15.** The crystal structure of compound **5.**

**Figure 14.** Salicylic acid salts of the mono(4-fluorobenzyl)spirocyclotriphosphazenes.

various PMOS diversity with different properties.

**Figure 13.** Various cyclotriphosphazenium iodide salts.

34 Recent Advances in Ionic Liquids

Therefore, studies on the biological and anti-cancer activities of the salts of cyclotriphosphazenes are likely to attract great interest because of their organic solvent/water solubility and

Akbaş et al. prepared the salicylic acid salts (**1–6**) of pyrrolidine and piperidine substituted cyclotriphosphazenes (**Figure 14**). The crystallographic data of **5** clearly indicate that the nitrogen of the phosphazene ring was protonated (**Figure 15**). The antimicrobial and cytotoxic activities of the phosphazenium salts (**1–6**) were also investigated. Compounds **5** and **6** appear to be good candidates for anti-cancer agents because they have significant cytotoxic activity against DLD-1 cancer cells. All of the compounds have an antimicrobial effect on bacterial and yeast strains between 312 and 625 μM (bacterial strains) and 19.5–312 μM (yeast

strains) and compounds **4–6** are found to be most effective against yeast strains [31].

**Figure 16.** The synthesis of 4-fluorobenzylspiro(N/O)cyclotriphosphazenium salts.

Elmas et al. synthesized phosphazenium salts (**1a-4a**) from the reactions of the phosphazene bases (**1–4**) with gentisic acid (**Figure 16**). The crystallographic data of **4a** unambiguously indicate that the nitrogen of the phosphazene ring is monoprotonated (**Figure 17**). Also, In vitro antimicrobial activities of compounds were investigated and the PILs (**1a**, **3a** and **4a**) were found to be significantly active against *C. albicans* [32].

Akbaş et al. obtained the PILs or PMOSs (**1–3**) from the reactions of [N<sup>3</sup> P3 (NC<sup>4</sup> H8 ) 6 ] with the gentisic, decanoic and boric acids (**Figure 18**). Their biological activity in cultured cell lines was investigated. The binding of **1**,**2** or **3** to calf thymus (CT-DNA) and bovine serum albumin (BSA) led to remarkable changes in spectral characteristics. The potent low cytotoxic, strong apoptotic, and effective DNA topoisomerase inhibitory characteristics of these PILs revealed that they can be a good candidate for anticancer drugs [33].

Okumuş et al. obtained the PMOSs from the reactions of tetrapyrrolidino- and tetrapiperidino-substituted cyclotriphosphazenes with the gentisic and γ-resorcylic acids (**Figure 19**). The crystallographic result of **1b** and **2b** exhibited that the N atom of the phosphazene ring adjacent to the spiro precursor was mono protonated (**Figure 20**). In addition, their

**Figure 17.** The crystal structure of compound **4a.**

**Figure 18.** The syntheses of the PILs (**1–3**) with [N<sup>3</sup> P3 (NC<sup>4</sup> H8 ) 6 ] and gentisic, decanoic and boric acids, respectively.

**Figure 19.** The gentisic and γ-resorcylic acid salts of the tetrapyrrolidino and tetrapiperidino mono(4-fluorobenzyl) spirocyclotriphosphazenes.

cytotoxic and antiproliferative activities against A549, Hep3B and normal FL cell lines were investigated. The findings also displayed that the PMOS (**1b-6b**) were strong antiproliferatives and they had excusable cytotoxic activities against the cells [34].

phosphazenes were well suited for coupling to the nanocrystalline surface and a strategy for surface modification of cyclotriphosphazenes containing cationic substituents was developed. A good solubility of nanoparticles in an aqueous medium is a significant precondition for the prediction of bioanalytical applications such as fluorescent immunoassays. This is the main purpose of specially designed molecule surface modifiers. The stable nanocrystallinker complexes in methanol were formed with compounds **3** and **4**, while the fully quaternized cyclic phosphazenes **1** and **2** interact strongly with nanoparticles, resulting in an agglomeration. It was found that aromatic groups may interfere with the UV absorption in

Phosphazene-Based Ionic Liquids

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nanocrystals, but no significant effect of aliphatic side chains was observed [35].

**Figure 20.** The crystal structure of **1b** and **2b.**

**Figure 21.** The synthesis of cyclotriphosphazenium iodide salts.

#### *2.3.2. Adsorbents and surface modifiers of fluorescent nanoparticles*

Veldboer et al., quaternized the cyclotriphosphazenes having terminal tertiary amino functions with methyl iodide (**Figure 21**) and the resulting salts were studied as surface modifiers for lanthanide phosphate nanoparticles. It was observed that the quaternized cyclic

**Figure 20.** The crystal structure of **1b** and **2b.**

**Figure 18.** The syntheses of the PILs (**1–3**) with [N<sup>3</sup>

spirocyclotriphosphazenes.

**Figure 17.** The crystal structure of compound **4a.**

36 Recent Advances in Ionic Liquids

P3 (NC<sup>4</sup> H8 ) 6

cytotoxic and antiproliferative activities against A549, Hep3B and normal FL cell lines were investigated. The findings also displayed that the PMOS (**1b-6b**) were strong antiprolifera-

**Figure 19.** The gentisic and γ-resorcylic acid salts of the tetrapyrrolidino and tetrapiperidino mono(4-fluorobenzyl)

Veldboer et al., quaternized the cyclotriphosphazenes having terminal tertiary amino functions with methyl iodide (**Figure 21**) and the resulting salts were studied as surface modifiers for lanthanide phosphate nanoparticles. It was observed that the quaternized cyclic

tives and they had excusable cytotoxic activities against the cells [34].

*2.3.2. Adsorbents and surface modifiers of fluorescent nanoparticles*

] and gentisic, decanoic and boric acids, respectively.

**Figure 21.** The synthesis of cyclotriphosphazenium iodide salts.

phosphazenes were well suited for coupling to the nanocrystalline surface and a strategy for surface modification of cyclotriphosphazenes containing cationic substituents was developed. A good solubility of nanoparticles in an aqueous medium is a significant precondition for the prediction of bioanalytical applications such as fluorescent immunoassays. This is the main purpose of specially designed molecule surface modifiers. The stable nanocrystallinker complexes in methanol were formed with compounds **3** and **4**, while the fully quaternized cyclic phosphazenes **1** and **2** interact strongly with nanoparticles, resulting in an agglomeration. It was found that aromatic groups may interfere with the UV absorption in nanocrystals, but no significant effect of aliphatic side chains was observed [35].

#### *2.3.3. Lubricants*

Omotowa et al. investigated the tribological properties of PzILs containing trimethylammonium and N-methylpyridinium chains (**Figure 22**). (Dimethylamino)ethoxy, pyridylmethoxy, or (dimethylamino)propoxy side groups linked to the phosphorus in the phosphazene ring were quaternized at the side group nitrogen with iodomethane to obtain polyiodo salts. Subsequently, polyquaternary PZILs were formed with salts such as LiN(SO<sup>2</sup> CF<sup>3</sup> ) 2 or NaBF<sup>4</sup> by the anions exchange reaction. These PzILs were investigated for use as lubricants for aircraft gas turbine engines and as additives in water lubrication of silicon nitride ceramics Friction and wear properties of water with **5–8 (**0.25 weight %) as boundary lubricant additives were tested on silicon nitride ceramic interfaces. It was observed that these PzILs lead to a decrease in the running-in period. The PzILs, **5–8**, are more viscous than the free cyclophosphazene bases and are highly viscous for use as oils. For a faster transition to low friction, ionic liquids with higher solubility must be used [36].

Recently, additives obtained from phosphazene having polar functions which can interact with tribological surfaces, have been developed. Singh et al. obtained The PzP(-NHP)<sup>6</sup> salt with N<sup>3</sup> P3 Cl<sup>6</sup> and 2,6-di-tert-butyl-4-(dimethylaminomethyl) phenol (**Figure 23**). This compound contains a phosphazene ring containing polar nitrogen and phosphorus atoms surrounded by hindered phenolic substituents with *tert*-butyl groups. Due to these properties, it can show an affinity for a metal surface to form a surface film which leads to anticorrosion, antiwear, and antifriction properties together with antioxidant characteristics. Report edly, the PzP(-NHP)<sup>6</sup> additive exhibits excellent antioxidant properties, and moderate anticorrosion, antiwear and antifriction properties. A doping concentration of 3000 ppm PzP(-NHP)<sup>6</sup> decreased the average wear scar diameter (AWSD) and average friction coefficient by 15.81% and ~27.27%, respectively, in comparison to those for the blank polyol base oil [37].

*2.3.4. Chemosensors for metal ions*

**Figure 23.** The structure of the PzP(-NHP)<sup>6</sup>

Çiftçi et al. obtained the quaternized cationic and zwitterionic derivatives of 3-[2-(diethylamino)ethyl]-7-oxy-4-methylcoumarin substituted trimeric and tetrameric derivatives with dimethyl sulfate and 1,3-propanesultone, respectively (**Figure 24**). Quaternized ionic and zwitterionic compounds display excellent solubility in water and the effects of metal ions on the fluorescent behavior of the cytophosphazene salts were investigated using these compounds

Phosphazene-Based Ionic Liquids

39

http://dx.doi.org/10.5772/intechopen.76613

salt.

**Figure 24.** The quaternization of coumarin-substituted cyclophosphazene derivatives.

**Figure 22.** PzILs containing trimethylammonium and N-methylpyridinium chains.

**Figure 23.** The structure of the PzP(-NHP)<sup>6</sup> salt.

#### *2.3.4. Chemosensors for metal ions*

**Figure 22.** PzILs containing trimethylammonium and N-methylpyridinium chains.

*2.3.3. Lubricants*

38 Recent Advances in Ionic Liquids

with N<sup>3</sup>

P3 Cl<sup>6</sup>

edly, the PzP(-NHP)<sup>6</sup>

Omotowa et al. investigated the tribological properties of PzILs containing trimethylammonium and N-methylpyridinium chains (**Figure 22**). (Dimethylamino)ethoxy, pyridylmethoxy, or (dimethylamino)propoxy side groups linked to the phosphorus in the phosphazene ring were quaternized at the side group nitrogen with iodomethane to obtain polyiodo salts. Subsequently,

exchange reaction. These PzILs were investigated for use as lubricants for aircraft gas turbine engines and as additives in water lubrication of silicon nitride ceramics Friction and wear properties of water with **5–8 (**0.25 weight %) as boundary lubricant additives were tested on silicon nitride ceramic interfaces. It was observed that these PzILs lead to a decrease in the running-in period. The PzILs, **5–8**, are more viscous than the free cyclophosphazene bases and are highly viscous for use as oils. For a faster transition to low friction, ionic liquids with higher solubility must be used [36]. Recently, additives obtained from phosphazene having polar functions which can interact with tribological surfaces, have been developed. Singh et al. obtained The PzP(-NHP)<sup>6</sup>

pound contains a phosphazene ring containing polar nitrogen and phosphorus atoms surrounded by hindered phenolic substituents with *tert*-butyl groups. Due to these properties, it can show an affinity for a metal surface to form a surface film which leads to anticorrosion, antiwear, and antifriction properties together with antioxidant characteristics. Report

rosion, antiwear and antifriction properties. A doping concentration of 3000 ppm PzP(-NHP)<sup>6</sup> decreased the average wear scar diameter (AWSD) and average friction coefficient by 15.81%

and ~27.27%, respectively, in comparison to those for the blank polyol base oil [37].

and 2,6-di-tert-butyl-4-(dimethylaminomethyl) phenol (**Figure 23**). This com-

additive exhibits excellent antioxidant properties, and moderate anticor-

CF<sup>3</sup> ) 2

or NaBF<sup>4</sup>

by the anions

salt

polyquaternary PZILs were formed with salts such as LiN(SO<sup>2</sup>

Çiftçi et al. obtained the quaternized cationic and zwitterionic derivatives of 3-[2-(diethylamino)ethyl]-7-oxy-4-methylcoumarin substituted trimeric and tetrameric derivatives with dimethyl sulfate and 1,3-propanesultone, respectively (**Figure 24**). Quaternized ionic and zwitterionic compounds display excellent solubility in water and the effects of metal ions on the fluorescent behavior of the cytophosphazene salts were investigated using these compounds

**Figure 24.** The quaternization of coumarin-substituted cyclophosphazene derivatives.

as chemosensors for metal ions. The results showed that cyclophosphazenium salts exhibit highly selective fluorescence chemosensor behavior for Fe3+ ions in aqueous solution [38].

#### *2.3.5. Electrolyte solutions for energy storage devices*

The ionic liquid also serves as an ion source for the formation of an electric double layer when electrolytes are used for the electrical double layer capacitor. Thus, an additional supporting electrolyte is not required. The PzIL is decomposed during combustion to produce a nitrogen gas, a phosphate ester, and the like. Because of this nitrogen gas, phosphate ester and the like, the ionic compound overcomes the risk of low combustion. Further, when the ionic compound contains a halogen, the halogen acts as an active radical during the accidental combustion to reduce the risk of burning. Moreover, when the ionic compound contains an organic substituent, the oxygen has a protective effect, as it forms a carbide during combustion. When the ionic compound is in a liquid state at room temperature, it can be used as an electrolyte for an electric double layer capacitor, a lithium-ion battery or a dye-sensitized solar cell, a reaction solvent for an organic synthesis, an extracting solvent for an organic compound and a magnetic fluid. If the ionic compound is in a solid state at room temperature, it can be used as a salt. It exhibits high non-combustibility in both of the liquid and solid states and can significantly suppress the risk of combustion in the application. For this purpose, various PzILs have been synthesized (**Figure 25**) [39–41].

off ratios of these OFETs are about 10<sup>2</sup>

by-layer assembly of a cationic (PAZ<sup>+</sup>

dc conductivity values of the PAZ<sup>+</sup>

**Figure 27.** Structures of the (PAZ<sup>+</sup>

*2.3.7. Polyelectrolytes*

increasing thickness and doping level of active organic layer [42].

**Figure 26.** The chemical structure of mono(4-fluorobenzyl)cyclotriphosphazene ionic liquids.

Linear polyphosphazenes containing quaternary ammonium side groups have the potential for application as a polycation component in the formation of ordering polyelectrolyte multilayers. Polyelectrolyte multilayers comprise of ionically modified polyphosphazenes by layer-

) and an anionic (PAZ−

of the PAH/PSS multilayers when these multilayers were compared to those of poly(sodium-

Polyelectrolytes were obtained by quaternization of the poly- alkoxy- and aryloxy- phosphazenes with strong alkylating reagents (**Figure 28**). Because of the lower electron donating abilities of the alkoxy and aryloxy side groups compared to the alkylamino side groups, quaternization is only carried out with methyl trifluoromethanesulfonate (MeOTf). Quaternary poly(alkoxyphosphazenes)

Because of the atomic polarization of the iodide anion, it has been found that the iodide salts (ionomer) of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) (**Figure 29**) have a high

) (**3**) polyphosphazenes.

have a high ionic conductivity without the need for plasticizers or additional salts [26].

/PAZ−

4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) [43].

) (**2**) and (PAZ−

. The low value of on–off ratio could be caused by the

films were found to be ten times greater than those

) polyphosphazene (**Figure 27**). The

Phosphazene-Based Ionic Liquids

41

http://dx.doi.org/10.5772/intechopen.76613

#### *2.3.6. A gate dielectric layer for OFETs*

Organic field effect transistors (OFETs) are very attractive with their potential applications in a wide area, as flexible and low cost electronic devices. Conventional electrolytes are not stable with their solvents and it is difficult to stabilize the electrolyte concentration. However, ionic liquids are attractive as a gate dielectric layer for OFETs with superior properties such as high thermal and chemical stability, non-volatility, non-toxicity and high polarizability. For this purpose, PzILs have been synthesized. The chain nitrogen atoms of free cyclotriphosphazene bases were quaternized by treatment with methyl iodide to give phosphazenium salts, PzIL1-PzIL4. Subsequently, polyquaternary PZILs have been formed with LiN(SO<sup>2</sup> CF<sup>3</sup> )2 (**Figure 26**). These PzILs have been used as the dielectric layer in OFETs. Due to the high dielectric effect of PzILs, the fabricated OFETs have operated in the low voltage ranges. On/

**Figure 25.** The conversion of chloropentafluorocyclotriphosphazenes to PzILs.

Phosphazene-Based Ionic Liquids http://dx.doi.org/10.5772/intechopen.76613 41

**Figure 26.** The chemical structure of mono(4-fluorobenzyl)cyclotriphosphazene ionic liquids.

off ratios of these OFETs are about 10<sup>2</sup> . The low value of on–off ratio could be caused by the increasing thickness and doping level of active organic layer [42].

#### *2.3.7. Polyelectrolytes*

**Figure 25.** The conversion of chloropentafluorocyclotriphosphazenes to PzILs.

as chemosensors for metal ions. The results showed that cyclophosphazenium salts exhibit highly selective fluorescence chemosensor behavior for Fe3+ ions in aqueous solution [38].

The ionic liquid also serves as an ion source for the formation of an electric double layer when electrolytes are used for the electrical double layer capacitor. Thus, an additional supporting electrolyte is not required. The PzIL is decomposed during combustion to produce a nitrogen gas, a phosphate ester, and the like. Because of this nitrogen gas, phosphate ester and the like, the ionic compound overcomes the risk of low combustion. Further, when the ionic compound contains a halogen, the halogen acts as an active radical during the accidental combustion to reduce the risk of burning. Moreover, when the ionic compound contains an organic substituent, the oxygen has a protective effect, as it forms a carbide during combustion. When the ionic compound is in a liquid state at room temperature, it can be used as an electrolyte for an electric double layer capacitor, a lithium-ion battery or a dye-sensitized solar cell, a reaction solvent for an organic synthesis, an extracting solvent for an organic compound and a magnetic fluid. If the ionic compound is in a solid state at room temperature, it can be used as a salt. It exhibits high non-combustibility in both of the liquid and solid states and can significantly suppress the risk of combustion in the application. For this purpose, various PzILs

Organic field effect transistors (OFETs) are very attractive with their potential applications in a wide area, as flexible and low cost electronic devices. Conventional electrolytes are not stable with their solvents and it is difficult to stabilize the electrolyte concentration. However, ionic liquids are attractive as a gate dielectric layer for OFETs with superior properties such as high thermal and chemical stability, non-volatility, non-toxicity and high polarizability. For this purpose, PzILs have been synthesized. The chain nitrogen atoms of free cyclotriphosphazene bases were quaternized by treatment with methyl iodide to give phosphazenium salts, PzIL1-PzIL4. Subsequently, polyquaternary PZILs have been formed with LiN(SO<sup>2</sup>

(**Figure 26**). These PzILs have been used as the dielectric layer in OFETs. Due to the high dielectric effect of PzILs, the fabricated OFETs have operated in the low voltage ranges. On/

CF<sup>3</sup> )2

*2.3.5. Electrolyte solutions for energy storage devices*

40 Recent Advances in Ionic Liquids

have been synthesized (**Figure 25**) [39–41].

*2.3.6. A gate dielectric layer for OFETs*

Linear polyphosphazenes containing quaternary ammonium side groups have the potential for application as a polycation component in the formation of ordering polyelectrolyte multilayers. Polyelectrolyte multilayers comprise of ionically modified polyphosphazenes by layerby-layer assembly of a cationic (PAZ<sup>+</sup> ) and an anionic (PAZ− ) polyphosphazene (**Figure 27**). The dc conductivity values of the PAZ<sup>+</sup> /PAZ− films were found to be ten times greater than those of the PAH/PSS multilayers when these multilayers were compared to those of poly(sodium-4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) [43].

Polyelectrolytes were obtained by quaternization of the poly- alkoxy- and aryloxy- phosphazenes with strong alkylating reagents (**Figure 28**). Because of the lower electron donating abilities of the alkoxy and aryloxy side groups compared to the alkylamino side groups, quaternization is only carried out with methyl trifluoromethanesulfonate (MeOTf). Quaternary poly(alkoxyphosphazenes) have a high ionic conductivity without the need for plasticizers or additional salts [26].

Because of the atomic polarization of the iodide anion, it has been found that the iodide salts (ionomer) of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) (**Figure 29**) have a high

**Figure 27.** Structures of the (PAZ<sup>+</sup> ) (**2**) and (PAZ− ) (**3**) polyphosphazenes.

[2] Jaeger R, Gleria M. Poly(Organophosphazene)s and related compounds: Synthesis, properties and applications. Progress in Polymer Science. 1998;**23**:179-276. DOI: 10.1016/

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[3] Allcock HR. Recent developments in polyphosphazene materials science. Current Opinion in Solid State & Materials Science. 2066;**10**(5-6):231-240. DOI: 10.1016/j.cossms.

[4] Xu G, Lu Q, Yu B, Wen L. Inorganic polymer phosphazene disulfide as cathode material for rechargeable lithium batteries. Solid State Ionics. 2006;**177**:305-309. DOI: 10.1016/j.

[5] Singh A, Krogman NR, Sethurman S, Nair LS, Sturgeon JL, Brown PW, Laurencin CT, Allcock HR. Effect of side group chemistry on the properties of biodegradable l-alanine cosubstituted polyphosphazenes. Biomacromolecules. 2006;**7**:914-918. DOI: 10.1021/

[6] Keller MA, Saba CS. Oxidative stability and degradation mechanism of a cyclotriphosphazene lubricant. Analytical Chemistry. 1996;**68**(19):3489-3492. DOI: 10.1021/ac960632x [7] Davarcı D, Beşli S, Demirbaş E. Synthesis of a series of triple-bridged cyclotriphosphazene hexa-alkoxy derivatives and investigation of their structural and mesomorphic properties. Liquid Crystals. 2013;**40**(5):624-631. DOI: 10.1080/02678292.2013.773093 [8] Brandt K, Bartczak TJ, Kruszynski R, Porwolik-Czomperlik I. AIDS-related lymphoma screen results and molecular structure determination of a new crown ether bearing aziridinylcyclophosphazene, potentially capable of ion-regulated DNA cleavage action.

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[9] Çil E, Tanyıldızı MA, Ozen F, Boybay M, Arslan M, Görgülü AO. Synthesis, characterization, and biological–pharmacological evaluation of new phosphazenes bearing dioxybiphenyl and schiff base groups. Archiv der Pharmazie - Chemistry in Life Sciences.

[10] Sun J, Yu Z, Wang X, Wu D. Synthesis and performance of cyclomatrix polyphosphazene derived from trispiro-cyclotriphosphazene as a halogen-free nonflammable material. ACS Sustainable Chemistry & Engineering. 2014;**2**:231-238. DOI: 10.1021/sc400283d

[11] Allcock HR, Kwon S. Covalent linkage of proteins to surface-modified poly (organophosphazenes): Immobilization of glucose-6-phosphate dehydrogenase and trypsin.

[12] Greish YE, Bender JD, Lakshmi S, Brown PW, Allcock HR, Laurencin CT.Low temperature formation of hydroxyapatite-poly(alkyl oxybenzoate)phosphazene composites for biomedical applications. Biomaterials. 2005;**26**:1-9. DOI: 10.1016/j.biomaterials.2004.02.016

[13] Gering K L, Harrup M K, Rollins H W. Ionic liquids, electrolyte solutions including the ionic liquids, and energy storage devices including the ionic liquids. Pat. Appl. Pub.

[14] Feakins D, Last WA, Shaw RA. 855. Structure and basicity. Part II. The basicity of fully aminolysed cyclotriphosphazatrienes and cyclotetraphosphazatetraenes in nitrobenzene and water. Journal of the Chemical Society. 1964:4464-4471. DOI: 10.1039/JR9640004464

Macromolecules. 1986;**19**:1502-1508. DOI: 10.1021/ma00160a002

S0079-6700(97)00027-0

2007.06.001

ssi.2005.10.029

bm050752r

**Figure 28.** Quaternization of polymers.

**Figure 29.** Synthetic route for the polyphosphazenes and their salts.

frequency dielectric constant, ε∞ (highest value ε∞=11). These MEEP-based polyphosphazene salts have a room temperature dc conductivity of 10−6 S·cm−1. If the segmental mobility can be increased, they may have a potential for application in iodide conducting solar cells [44].

#### **3. Conclusions**

In this chapter, we reported a literature review about phosphazene-based ionic liquids (PzILs), which have received considerable attention in recent years. The design and synthesis of PzILs were introduced, and the recent applications (since 2004) were analyzed and discussed. We believe that further studies on the synthesis and application of new PzILs will be performed in the near future.

#### **Author details**


#### **References**

[1] Chandrasekhar V, Thilagar P, Pandian BM. Cyclophosphazene-based multi-site coordination ligands. Coordination Chemistry Reviews. 2007;**251**:1045-1074. DOI: 10.1016/j. ccr.2006.07.005


**Figure 28.** Quaternization of polymers.

42 Recent Advances in Ionic Liquids

**3. Conclusions**

in the near future.

**Author details**

**References**

ccr.2006.07.005

Ahmet Karadağ1,2\* and Hüseyin Akbaş<sup>2</sup>

\*Address all correspondence to: ahmet.karadag@gop.edu.tr

1 Department of Biotechnology, Bartın University, Bartın, Turkey

2 Department of Chemistry, Gaziosmanpaşa University, Tokat, Turkey

**Figure 29.** Synthetic route for the polyphosphazenes and their salts.

frequency dielectric constant, ε∞ (highest value ε∞=11). These MEEP-based polyphosphazene salts have a room temperature dc conductivity of 10−6 S·cm−1. If the segmental mobility can be increased, they may have a potential for application in iodide conducting solar cells [44].

In this chapter, we reported a literature review about phosphazene-based ionic liquids (PzILs), which have received considerable attention in recent years. The design and synthesis of PzILs were introduced, and the recent applications (since 2004) were analyzed and discussed. We believe that further studies on the synthesis and application of new PzILs will be performed

[1] Chandrasekhar V, Thilagar P, Pandian BM. Cyclophosphazene-based multi-site coordination ligands. Coordination Chemistry Reviews. 2007;**251**:1045-1074. DOI: 10.1016/j.


[15] Feakins D, Last WA, Neemuchwala N, Shaw RA. 503. Structure and basicity. Part III. The basicity of homogeneously substituted cyclotriphosphazatrienes and cyclotetraphos-phazatetraenes. Journal of the Chemical Society. 1965:2804-2811. DOI: 10.1039/ JR9650002804

[27] Marsh WC, Trotter J. Crystal and molecular structure of chloro[dodeca(dimethylamino)-

[28] Chandrasekaran A. A salt of a protonated (amino)spirocyclic cyclotriphosphazene. Acta

[29] Rapko JN, Feistel GR. The synthesis and structure of a dialkyl cation of hexakisdimethylaminocyclotriphosphazatriene. Chemical Communications (London). 1968:474-475.

[30] Allcock HR, Levin ML, Austin PE. Quaternized cyclic and high polymeric phosphazenes and their interactions with tetracyanoquinodimethane. Inorganic Chemistry.

[31] Akbaş H, Okumuş A, Karadağ A, Kılıç Z, Hökelek T, Koç LY, Açık L, Aydın B, Türk M. Phosphorus–nitrogen compounds part 32. Structural and thermal characterizations, antimicrobial and cytotoxic activities, and in vitro DNA binding of the phosphazenium salts. Journal of Thermal Analysis and Calorimetry. 2016;**123**:1627-1641. DOI: 10.1007/

[32] Elmas G, Okumuş A, Kılıç Z, Gönder LY, Açık L, Hökelek T. The syntheses and structural characterizations, antimicrobial activity and in vitro DNA binding of 4-fluorobenzylspiro(N/O)cyclotriphosphazenes and their phosphazenium salts. JOTCSA.

[33] Akbaş H, Karadağ A, Aydın A, Destegül A, Kılıç Z. Synthesis, structural and thermal properties of the hexapyrrolidinocyclotriphosphazenes-based protic molten salts: Antiproliferative effects against HT29, HeLa, and C6 cancer cell lines. Journal of

[34] Okumuş A, Akbaş H, Karadağ A, Aydın A, Kılıç Z, Hökelek T. Antiproliferative effects against A549, Hep3B and FL cell lines of cyclotriphosphazene-based novel protic molten salts: Spectroscopic, crystallographic and thermal results. ChemistrySelect.

[35] Veldboer K, Karataş Y, Vielhaber T, Karst U, Wiemhöfer H. Cyclic phosphazenes for the surface modification of lanthanide phosphate-based nanoparticles. Zeitschrift für Anorganische und Allgemeine Chemie. 2008;**634**:2175-2180. DOI: 10.1002/zaac.200800297

[36] Omotowa BA, Phillips BS, Zabinski JS, Shreeve JM. Phosphazene-based ionic liquids: Synthesis, temperature-dependent viscosity, and effect as additives in water lubrication of silicon nitride ceramics. Inorganic Chemistry. 2004;**43**(17):5466-5471. DOI: 10.1021/

[37] Singh RK, Kukrety A, Saxena RC, Chouhan A, Jain SL, Ray SS. Phosphazene-based novel organo-inorganic hybrid salt: Synthesis, characterization and performance evaluation as multifunctional additive in polyol. RSC Advances. 2017;**7**:13390-13397. DOI: 10.1039/

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P6 (NMe<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.76613

Phosphazene-Based Ionic Liquids

)12CuIICl]<sup>+</sup>

45

cyclohexaphosphazene-NNNN]-copper(II)dichlorocuprate(I) ([N<sup>6</sup>

Cryst. 1994;**C50**:1692-1694. DOI: 10.1107/S0108270194003859

CuICl<sup>2</sup> –

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C6RA26186H

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2017;**2**(18):4988-4999. DOI: 10.1002/slct.201700497


[27] Marsh WC, Trotter J. Crystal and molecular structure of chloro[dodeca(dimethylamino) cyclohexaphosphazene-NNNN]-copper(II)dichlorocuprate(I) ([N<sup>6</sup> P6 (NMe<sup>2</sup> )12CuIICl]<sup>+</sup> CuICl<sup>2</sup> – ). Journal of the Chemical Society A. 1971:1482-1486. DOI: 10.1039/J19710001482

[15] Feakins D, Last WA, Neemuchwala N, Shaw RA. 503. Structure and basicity. Part III. The basicity of homogeneously substituted cyclotriphosphazatrienes and cyclotetraphos-phazatetraenes. Journal of the Chemical Society. 1965:2804-2811. DOI: 10.1039/

[16] Feakins D, Last WA, Nabi SN, Shaw RA. Structure and basicity. Part IV. Aminochlorocy clotriphosphazatrienes. Journal of the Chemical Society A. 1966:1831-1834. DOI: 10.1039/

[17] Moeller T, Kokalis SG. The Lewis-base behaviour of some hexa-n-alkylamino triphosphoniıtriles. Journal of Inorganic and Nuclear Chemistry. 1963;**25**(7):875-881. DOI:

[18] Zhang Y, Tham FS, Reed CA. Phosphazene cations. Inorganic Chemistry. 2006;**45**:10446-

[19] Mani NV, Wagner AJ. The crystal structure of compounds with (N-P)n rings. VIII. Dichlo

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[21] Tun ZM, Heston AJ, Panzner MJ, Medvetz DA, Wright BD, Savant D, Dudipala VR, Banerjee D, Rinaldi PL, Youngs WJ, Tessier CA. Group 13 Lewis acid adducts of [PCl<sup>2</sup>

[22] Tun ZM, Heston AJ, Panzner MJ, Scionti V, Medvetz DA, Wright BD, Johnson NA, Li L, Wesdemiotis C, Savant D, Rinaldi PL, Youngs WJ, Tessier CA. Group 13 super-

[23] Benson MA, Zacchini S, Boomishankar R, Chan Y, Steiner A. Alkylation and acylation of cyclotriphosphazenes. Inorganic Chemistry. 2017;**46**(17):7097-7108. DOI: 10.1021/

[24] Furuyama R, Fujita T, Funaki SF, Nobori T, Nagata T, Fujiwara K. New high-performance catalysts developed at mitsui chemicals for polyolefins and organic synthesis. Catalysis Surveys from Asia. 2004;**8**:61-71. DOI: 10.1023/B:CATS.0000015115.09940.4e [25] Nobori T, Hayashi T, Shibahara A, Saeki T, Yamasaki S, Ohkubo K. Development of novel molecular catalysts "phosphazene catalysts" for commercial production of highly advanced polypropylene glycols. Catalysis Surveys from Asia. 2010;**14**:164-167. DOI:

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P3 Cl<sup>2</sup>

N3 Cl<sup>4</sup> (NH<sup>2</sup> ) 2

. Inorganic Chemistry. 2016;**55**(7):3283-3293. DOI: 10.1021/acs.

(NH<sup>2</sup> )2 ]+

N3 HCl<sup>4</sup> (NHPri)<sup>4</sup>

)2 ]− ). Zeits-

] with [HN(POCl<sup>2</sup>

[N(POCl<sup>2</sup>

.HCl.

)2 ];

N]<sup>3</sup> .

rotetrakisisopropylaminocyclotriphosphazatriene hydrochloride, N<sup>3</sup>

Inorganic Chemistry. 2011;**50**(18):8937-8945. DOI: 10.1021/ic201075z

Acta Cryst. 1971;**B27**:51-58. DOI: 10.1107/S0567740871001699

[20] Alberti M, Marecek A, Zak Z, Pastera P. Reaction of [P<sup>3</sup>

N]<sup>3</sup>

the crystal structure of the phosphazenium salt ([P<sup>3</sup>

JR9650002804

44 Recent Advances in Ionic Liquids

J19660001831

zaac.19956211027

acid adducts of [PCl<sup>2</sup>

10.1007/s10563-010-9098-0

inorgchem.5b02341

ic7009463

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10448. DOI: 10.1021/ic062077f


[38] Çiftçi GY, Şenkuytu E, Bulut M, Durmuş M. Novel coumarin substituted water soluble cyclophosphazenes as "turn-off" type fluorescence chemosensors for detection of Fe3+ ions in aqueous media. Journal of Fluorescence. 2015;**25**:1819-1830. DOI: 10.1007/ s10895-015-1672-4

**Chapter 3**

**Provisional chapter**

**Tribochemical Reactions of Halogen-Free Ionic Liquids**

**Tribochemical Reactions of Halogen-Free Ionic Liquids** 

Ionic liquids are expected to show applicability as novel lubricants. However, halogen anion-based ionic liquids cause severe corrosive wear. To preclude this, this chapter describes the use of halogen-free anion-based ionic liquids as lubricants. The study investigated the tribological performances and lubricating mechanisms of sulfur, phosphorus, and cyanoanion-based ionic liquids. Sulfur and phosphorus anion-based ionic liquids formed reaction films on worn surfaces; the sulfur- and phosphorus-containing films exhibited low-friction coefficients and specific wear rates, respectively. The steric hindrance of the ionic liquids affected their tribochemical reaction behaviors. Cyanoanionbased ionic liquids also showed low-friction coefficients; however, their values were higher than those of halogen anion-based ionic liquids. To achieve low friction, tribochemical reaction of the ionic liquids and adsorption of anions on the worn surface were required. The stability of the cyanoanion-based ionic liquids against the nascent steel surface was related to the thermal stability. These halogen-free anion-based ionic liquids and formed tribolayer films differ in physical and chemical properties. When these ionic liquids are applied as lubricants in the industry, it is important to choose ionic liquids

**Keywords:** tribology, lubricants, halogen-free ionic liquids, tribochemical reaction,

Global warming is an important issue that cannot be ignored. The Intergovernmental Panel on Climate Change (IPCC) has reported increases in the global average temperature [1]. This

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.77352

**on Nascent Steel Surface**

**on Nascent Steel Surface**

Shouhei Kawada, Seiya Watanabe, Shinya Sasaki and Masaaki Miyatake

and Masaaki Miyatake

**Abstract**

http://dx.doi.org/10.5772/intechopen.77352

depending on the sliding conditions.

quadrupole mass spectrometer

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Shouhei Kawada, Seiya Watanabe, Shinya Sasaki


#### **Tribochemical Reactions of Halogen-Free Ionic Liquids on Nascent Steel Surface Tribochemical Reactions of Halogen-Free Ionic Liquids on Nascent Steel Surface**

DOI: 10.5772/intechopen.77352

Shouhei Kawada, Seiya Watanabe, Shinya Sasaki and Masaaki Miyatake Shouhei Kawada, Seiya Watanabe, Shinya Sasaki and Masaaki Miyatake

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77352

#### **Abstract**

[38] Çiftçi GY, Şenkuytu E, Bulut M, Durmuş M. Novel coumarin substituted water soluble cyclophosphazenes as "turn-off" type fluorescence chemosensors for detection of Fe3+ ions in aqueous media. Journal of Fluorescence. 2015;**25**:1819-1830. DOI: 10.1007/

[40] Otsuki M, Kanno H. Non-aqueous electrolyte for battery and non-aqueous electrolyte battery comprising the same as well as electrolyte for electric double layer capacitor and electric double layer capacitor comprising the same. US Patent 2011;7,951;**495**:1-10 [41] Gering KL, Harrup MK, Rollins HW. Ionic liquids, electrolyte solutions including the ionic liquids, and energy storage devices including the ionic liquids. Pat. Appl. Pub.

[42] Akbaş H, Karadag A, Destegül A, Çakırlar Ç, Yerli Y, Tekin K C, Malayoğlu U, Kılıç Z. Syntheses, spectroscopic, thermal and dielectric properties of phosphazene based ionic liquids: Tribological behavior and OFET application. Journal of Molecular Liquids.

[43] Akgöl Y, Hofmann C, Karataş Y, Cramer C, Wiemhöfer H, Schönhoff M. Conductivity spectra of polyphosphazene-based polyelectrolyte multilayers. The Journal of Physical

[44] Bartels J, Hess A, Shiau H, Allcock HR, Colby RH, Runt J. Synthesis, morphology, and ion conduction of polyphosphazene ammonium iodide ionomers. Macromolecules.

[45] Görgülü AO, Koran K, Özen F, Tekin S, Sandal S. Synthesis, structural characterization and anti-carcinogenic activity of new cyclotriphosphazenes containing dioxybiphenyl and chalcone groups. Journal of Molecular Structure. 2015;**1087**:1-10. DOI: 10.1016/j.

[46] Akbaş H, Okumuş A, Kılıç Z, Hökelek T, Süzen Y, Koç LY, Açık L, Çelik ZB. Phosphorusnitrogen compounds part 27. Syntheses, structural characterizations, antimicrobial and cytotoxic activities, and DNA interactions of new phosphazenes bearing secondary amino and pendant (4-fluorobenzyl)spiro groups. European Journal of Medicinal

[39] Otsuki M, Kanno H. Ionic compound. US Patent. 2010;**7**(718,826):1-10

Chemistry. B. 2007;**111**:8532-8539. DOI: 10.1021/jp068872w

Chemistry. 2013;**70**:294-307. DOI: 10.1016/j.ejmech.2013.09.046

2015;**48**:111-118. DOI: 10.1021/ma501634b

s10895-015-1672-4

46 Recent Advances in Ionic Liquids

2018:In Review

molstruc.2015.01.033

2013; US 2013/0089793A1:1-11

Ionic liquids are expected to show applicability as novel lubricants. However, halogen anion-based ionic liquids cause severe corrosive wear. To preclude this, this chapter describes the use of halogen-free anion-based ionic liquids as lubricants. The study investigated the tribological performances and lubricating mechanisms of sulfur, phosphorus, and cyanoanion-based ionic liquids. Sulfur and phosphorus anion-based ionic liquids formed reaction films on worn surfaces; the sulfur- and phosphorus-containing films exhibited low-friction coefficients and specific wear rates, respectively. The steric hindrance of the ionic liquids affected their tribochemical reaction behaviors. Cyanoanionbased ionic liquids also showed low-friction coefficients; however, their values were higher than those of halogen anion-based ionic liquids. To achieve low friction, tribochemical reaction of the ionic liquids and adsorption of anions on the worn surface were required. The stability of the cyanoanion-based ionic liquids against the nascent steel surface was related to the thermal stability. These halogen-free anion-based ionic liquids and formed tribolayer films differ in physical and chemical properties. When these ionic liquids are applied as lubricants in the industry, it is important to choose ionic liquids depending on the sliding conditions.

**Keywords:** tribology, lubricants, halogen-free ionic liquids, tribochemical reaction, quadrupole mass spectrometer

#### **1. Introduction**

Global warming is an important issue that cannot be ignored. The Intergovernmental Panel on Climate Change (IPCC) has reported increases in the global average temperature [1]. This

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

meeting reported that the global average temperature increased by 0.85°C from 1880 to 2012 [1]. Some measures must be taken quickly to resolve this problem. The *Conférence de Paris de 2015 sur le climat* (COP21) established the target values of CO2 release reduction, which is the most significant matter in global warming. The United States of America and Japan each set the goal to reduce CO2 release by more than 25% by 2025–2030. Meanwhile, the European Union, Russian Federation, China, and India set significant reduction targets exceeding 35%. To achieve these targets, high efficiency of mechanical systems is required. One method to increase efficiency is the reduction of friction loss in sliding parts. In order to achieve this, the development of new technologies, such as novel lubricants, materials, and lubrication state controls, are needed. Regarding novel materials, the research on the tribological performance of diamond-like carbon (DLC) has been the most energetic [2–5]. The development of novel synthetic oils as lubricants has also been remarkable. Among these, ionic liquids such as lubricants have received much attention [6–14]. Ionic liquids are organic salts consisting of cations and anions that form liquid phases at temperatures below 100°C. They have the attractive physical and chemical properties of high thermal stability, low vapor pressure, and flame resistance [15–17]. In addition, their properties can be controlled by changing the combinations of cations and anions [8]. In the tribology field, ionic liquids were first considered for use in extreme environmental conditions, such as high temperatures, vacuum, and high contact pressures, where existing lubricants cannot be used [7, 8]. Ionic liquids exhibit high heat resistance compared to existing lubricants such as perfluoropolyether (PFPE) and poly-αolefin [7]. Recently, reports on ionic liquids under ordinary temperatures and pressures have increased [6, 11–13]. The increase in the variety of ionic liquids triggered this trend. However, the detailed lubricating mechanisms and the relationship between the chemical structures of ionic liquids and tribological performances are still unclear. To apply ionic liquids as lubricants, it is necessary to understand lubricating mechanisms.

or both. From this information, it is considered that sulfur or phosphorus anion-based ionic liquids can be applied in the industry as alternatives to halogen anion-based ionic liquids. This section investigated tribological performances and the effect of alkyl chain lengths of the

Tribochemical Reactions of Halogen-Free Ionic Liquids on Nascent Steel Surface

Four kinds of sulfur and three kinds of phosphorus anion-based ionic liquids were used as lubricants. **Table 1** lists the chemical names and molecular structures of these ionic liquids:

lium methyl sulfate ([EMIM][MSU]), 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM] [ESU]), 1-ethyl-3-methylimidazolium n-octyl sulfate ([EMIM][OSU]), 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM][DMP]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and 1-ethyl-3-methylimidazolium dibutyl phosphate ([EMIM][DBP]).

[OSU] were purchased from Merck Chemicals, Germany, as "Synthesis (S)" grade (halide content <1000 ppm, water content <10,000 ppm). [EMIM][MSU], [EMIM][DMP], [EMIM] [DEP], and [EMIM][DBP] were purchased from IoLiTec, Germany, as "HP" grade (water content <5000 ppm). In addition, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]

All ionic liquids are liquid in phase at room temperature. **Table 2** lists the viscosities and thermal decomposition temperatures of the used ionic liquids. The viscosities of all the ionic liquids were measured using a tuning-fork vibration-type viscometer (SV-1A, A&D Company, Japan). The thermal decomposition temperatures of the ionic liquids were defined as the points at which 10% weight loss occurred by thermogravimetric analysis. The programming

The tribological performances of all ionic liquids were evaluated using a ball-on-disk sliding tester [9]. For the specimens, a *ϕ* 24 mm × *t* 7.9 mm disk and a *ϕ* 4-mm ball of bearing steel (AISI 52100, hardness of HRC 60) were used. The surface roughness of the disk specimens and corre-

solution of 1:1 petroleum benzine and acetone for 20 min. The sliding tests were performed with a normal load of 3.5 N and sliding speed of 52.3 mm/s for 2 h under vacuum conditions (2.0 × 10−5 Pa). After the sliding tests, the worn surfaces of the ball specimens were observed and the specific wear rates were measured by optical microscopy (OM, VHX-100, Keyence, Japan).

The tribochemical reactions of the ionic liquids were estimated using Q-MS (MKS Instruments, Inc.). The measurable mass-to-charge ratio (m/*e*) range of the Q-MS was 1–200, using an ion

]), 1-ethyl-3-methylimidazo-

http://dx.doi.org/10.5772/intechopen.77352

49

], [EMIM][ESU], and [EMIM]

, and the measurement range was 50–500°C.

0.05 ± 0.01 μm. They were ultrasonically cleaned twice with a mixed

1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO<sup>4</sup>

All ionic liquids were commercial materials. [EMIM][HSO<sup>4</sup>

]), containing halogens, was used for comparison.

*2.1.2. Physical and chemical properties of ionic liquids*

rate was 10°C/min, the environment was N2

anions on their tribochemical reactions.

**2.1. Experimental details**

*2.1.1. Ionic liquids*

[PF<sup>6</sup>

*2.1.3. Sliding tests*

*2.1.4. Analysis*

sponding balls were each *R*<sup>a</sup>

Regarding the tribological performances of ionic liquids, most investigations have used fluoride or chloride anion-based ionic liquids, such as chloride [Cl], tetrafluoroborate [BF<sup>4</sup> ], hexafluorophosphate [PF<sup>6</sup> ], and bis(trifluoromethane)sulfonamide [TFSI] [7, 8, 10–13]. These kinds of anions form metallic halides on worn surfaces, and these reactants achieve good tribological performances [7, 8]. These anions exhibit lower friction coefficients than PFPE [7]. However, the hydrolysis of metallic halides causes severe corrosive wear [8]. In addition, [BF<sup>4</sup> ] and [PF<sup>6</sup> ] anions generate hydrogen fluoride by hydrolysis with water or atmospheric moisture [18–20]. To preclude the appearance of these bad properties, this chapter describes experiments considering the application of two kinds of halogen-free ionic liquids. One is sulfur or phosphorus anion-based ionic liquids, which form reaction films on worn surfaces. The other is cyanoanion-based ionic liquids, which form adsorption films on worn surfaces. The tribological performances of these ionic liquids were investigated by sliding tests. In addition, the relationship between the chemical structures of the ionic liquids and tribochemical reaction is discussed by using quadrupole mass spectrometry (Q-MS).

#### **2. Sulfur or phosphorus anion-based ionic liquids**

Sulfur and phosphorus are widely used as extreme-pressure additives, such as zinc dialkyl dithiophosphate (ZDDP) [21–23]. These elements exhibit low-friction coefficients, low wear, or both. From this information, it is considered that sulfur or phosphorus anion-based ionic liquids can be applied in the industry as alternatives to halogen anion-based ionic liquids. This section investigated tribological performances and the effect of alkyl chain lengths of the anions on their tribochemical reactions.

#### **2.1. Experimental details**

#### *2.1.1. Ionic liquids*

meeting reported that the global average temperature increased by 0.85°C from 1880 to 2012 [1]. Some measures must be taken quickly to resolve this problem. The *Conférence de Paris de* 

most significant matter in global warming. The United States of America and Japan each set

Union, Russian Federation, China, and India set significant reduction targets exceeding 35%. To achieve these targets, high efficiency of mechanical systems is required. One method to increase efficiency is the reduction of friction loss in sliding parts. In order to achieve this, the development of new technologies, such as novel lubricants, materials, and lubrication state controls, are needed. Regarding novel materials, the research on the tribological performance of diamond-like carbon (DLC) has been the most energetic [2–5]. The development of novel synthetic oils as lubricants has also been remarkable. Among these, ionic liquids such as lubricants have received much attention [6–14]. Ionic liquids are organic salts consisting of cations and anions that form liquid phases at temperatures below 100°C. They have the attractive physical and chemical properties of high thermal stability, low vapor pressure, and flame resistance [15–17]. In addition, their properties can be controlled by changing the combinations of cations and anions [8]. In the tribology field, ionic liquids were first considered for use in extreme environmental conditions, such as high temperatures, vacuum, and high contact pressures, where existing lubricants cannot be used [7, 8]. Ionic liquids exhibit high heat resistance compared to existing lubricants such as perfluoropolyether (PFPE) and poly-αolefin [7]. Recently, reports on ionic liquids under ordinary temperatures and pressures have increased [6, 11–13]. The increase in the variety of ionic liquids triggered this trend. However, the detailed lubricating mechanisms and the relationship between the chemical structures of ionic liquids and tribological performances are still unclear. To apply ionic liquids as lubri-

Regarding the tribological performances of ionic liquids, most investigations have used fluoride or chloride anion-based ionic liquids, such as chloride [Cl], tetrafluoroborate [BF<sup>4</sup>

kinds of anions form metallic halides on worn surfaces, and these reactants achieve good tribological performances [7, 8]. These anions exhibit lower friction coefficients than PFPE [7]. However, the hydrolysis of metallic halides causes severe corrosive wear [8]. In addition,

moisture [18–20]. To preclude the appearance of these bad properties, this chapter describes experiments considering the application of two kinds of halogen-free ionic liquids. One is sulfur or phosphorus anion-based ionic liquids, which form reaction films on worn surfaces. The other is cyanoanion-based ionic liquids, which form adsorption films on worn surfaces. The tribological performances of these ionic liquids were investigated by sliding tests. In addition, the relationship between the chemical structures of the ionic liquids and tribochemical reac-

Sulfur and phosphorus are widely used as extreme-pressure additives, such as zinc dialkyl dithiophosphate (ZDDP) [21–23]. These elements exhibit low-friction coefficients, low wear,

], and bis(trifluoromethane)sulfonamide [TFSI] [7, 8, 10–13]. These

] anions generate hydrogen fluoride by hydrolysis with water or atmospheric

release by more than 25% by 2025–2030. Meanwhile, the European

release reduction, which is the

],

*2015 sur le climat* (COP21) established the target values of CO2

cants, it is necessary to understand lubricating mechanisms.

tion is discussed by using quadrupole mass spectrometry (Q-MS).

**2. Sulfur or phosphorus anion-based ionic liquids**

the goal to reduce CO2

48 Recent Advances in Ionic Liquids

hexafluorophosphate [PF<sup>6</sup>

] and [PF<sup>6</sup>

[BF<sup>4</sup>

Four kinds of sulfur and three kinds of phosphorus anion-based ionic liquids were used as lubricants. **Table 1** lists the chemical names and molecular structures of these ionic liquids: 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO<sup>4</sup> ]), 1-ethyl-3-methylimidazolium methyl sulfate ([EMIM][MSU]), 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM] [ESU]), 1-ethyl-3-methylimidazolium n-octyl sulfate ([EMIM][OSU]), 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM][DMP]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and 1-ethyl-3-methylimidazolium dibutyl phosphate ([EMIM][DBP]). All ionic liquids were commercial materials. [EMIM][HSO<sup>4</sup> ], [EMIM][ESU], and [EMIM] [OSU] were purchased from Merck Chemicals, Germany, as "Synthesis (S)" grade (halide content <1000 ppm, water content <10,000 ppm). [EMIM][MSU], [EMIM][DMP], [EMIM] [DEP], and [EMIM][DBP] were purchased from IoLiTec, Germany, as "HP" grade (water content <5000 ppm). In addition, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF<sup>6</sup> ]), containing halogens, was used for comparison.

#### *2.1.2. Physical and chemical properties of ionic liquids*

All ionic liquids are liquid in phase at room temperature. **Table 2** lists the viscosities and thermal decomposition temperatures of the used ionic liquids. The viscosities of all the ionic liquids were measured using a tuning-fork vibration-type viscometer (SV-1A, A&D Company, Japan). The thermal decomposition temperatures of the ionic liquids were defined as the points at which 10% weight loss occurred by thermogravimetric analysis. The programming rate was 10°C/min, the environment was N2 , and the measurement range was 50–500°C.

#### *2.1.3. Sliding tests*

The tribological performances of all ionic liquids were evaluated using a ball-on-disk sliding tester [9]. For the specimens, a *ϕ* 24 mm × *t* 7.9 mm disk and a *ϕ* 4-mm ball of bearing steel (AISI 52100, hardness of HRC 60) were used. The surface roughness of the disk specimens and corresponding balls were each *R*<sup>a</sup> 0.05 ± 0.01 μm. They were ultrasonically cleaned twice with a mixed solution of 1:1 petroleum benzine and acetone for 20 min. The sliding tests were performed with a normal load of 3.5 N and sliding speed of 52.3 mm/s for 2 h under vacuum conditions (2.0 × 10−5 Pa). After the sliding tests, the worn surfaces of the ball specimens were observed and the specific wear rates were measured by optical microscopy (OM, VHX-100, Keyence, Japan).

#### *2.1.4. Analysis*

The tribochemical reactions of the ionic liquids were estimated using Q-MS (MKS Instruments, Inc.). The measurable mass-to-charge ratio (m/*e*) range of the Q-MS was 1–200, using an ion

**2.2. Results**

[EMIM][HSO<sup>4</sup>

[PF<sup>6</sup>

while H<sup>2</sup>

The friction coefficient of [EMIM][HSO<sup>4</sup>

*2.2.2. Tribochemical reaction behaviors via Q-MS*

the cation is slight. In addition, outgassing of H<sup>2</sup>

outgassing were as follows: CH3

S (m/*e* = 34), CH3

*2.2.1. Tribological performances of sulfur and phosphorus anion-based ionic liquids*

**Ionic liquid Viscosity [mPa s] Decomposition temperature [°C]**

**40°C 70°C**

] 327.8 93.6 312.8

[EMIM][MSU] 41.0 17.6 309.6 [EMIM][ESU] 41.8 18.2 314.9 [EMIM][OSU] 169.2 47.9 296.6 [EMIM][DMP] 75.5 28.4 294.2 [EMIM][DEP] 110.3 37.6 229.8 [EMIM][DBP] 251.6 58.6 355.5

exhibit higher friction coefficients and specific wear rates than [BMIM][PF<sup>6</sup>

[DMP] and [EMIM][DEP] exhibit almost the same value as [BMIM][PF<sup>6</sup>

ball specimens. Chemical wear is confirmed with [EMIM][HSO<sup>4</sup>

O (m/*e* = 31), C2

phorus anion-based ionic liquids, respectively. For [EMIM][HSO<sup>4</sup>

the initial sliding period. On the other hand, the outgassing of SO2

**Figure 1** shows the average friction coefficients in the last 5 min of testing and the specific wear rates of the ball specimens. The results for the sulfur anion-based ionic liquids confirm the relationship between the alkyl chain lengths of the anions and the liquids' tribological performances. The friction coefficients are increased as the alkyl chains become longer. Meanwhile, the specific wear rates of the ball specimens are decreased as the alkyl chains become longer.

**Table 2.** The viscosities and thermal decomposition temperatures of the sulfur and phosphorus anion-based ionic liquids.

]. However, the specific wear rate is very high. Other sulfur anion-based ionic liquids

phorus anion-based ionic liquids, the friction coefficients and specific wear rates both show increasing tendencies as the alkyl chains become longer. The friction coefficients of [EMIM]

[EMIM][DMP] exhibits a very low specific wear rate. [EMIM][DMP] showed the greatest tribological performance of all the ionic liquids. **Figure 2** shows the worn surface images of the

The outgassing of the ionic liquids due to sliding was measured by Q-MS. The main kinds of

were derived from the anions [24]. Q-MS was used to track the detailed outgassing behaviors of these materials. **Figures 3** and **4** show the outgassing behaviors of the sulfur and phos-

H6

O (m/*e* = 45), SO2

(m/*e* = 15) and C2

detected throughout the sliding period. For [EMIM][MSU], the outgassing of CH3

H5

] is approximately 0.04, smaller than that of [BMIM]

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51

]. For the phos-

]. Furthermore, the

(m/*e* = 79)

] and [EMIM][MSU].

(m/*e* = 30) were derived from the cations,

(m/*e* = 64), and PO3

S derived from the anion is significant in

], outgassing derived from

derived from the anion is

, C2 H6 , and

**Table 1.** Names and molecular structures of the sulfur and phosphorus anion-based ionic liquids.

source sensitivity of 3.8 × 10−7 A/Pa with a secondary electron multiplier. The partial pressure of the ions was converted from the ion currents using the conversion rate of N2 by the Q-MS software. The m/*e* ratios, derived from the decomposition of ionic liquids before and after the friction tests, were measured. The temporal resolution was approximately 1 s. After the sliding tests, the disk specimens were ultrasonically cleaned with a mixed solution of 1:1 petroleum benzine and acetone for 10 min. The disk specimens were analyzed by X-ray photoelectron spectroscopy (XPS, QUANTERAII, ULVAC-PHI, Inc., Japan) with a monochromatic Al Kα X-ray source (1486.6 eV). All the spectra were referenced relative to the C 1 s peak (285.0 eV).


**Table 2.** The viscosities and thermal decomposition temperatures of the sulfur and phosphorus anion-based ionic liquids.

#### **2.2. Results**

source sensitivity of 3.8 × 10−7 A/Pa with a secondary electron multiplier. The partial pressure

software. The m/*e* ratios, derived from the decomposition of ionic liquids before and after the friction tests, were measured. The temporal resolution was approximately 1 s. After the sliding tests, the disk specimens were ultrasonically cleaned with a mixed solution of 1:1 petroleum benzine and acetone for 10 min. The disk specimens were analyzed by X-ray photoelectron spectroscopy (XPS, QUANTERAII, ULVAC-PHI, Inc., Japan) with a monochromatic Al Kα X-ray source (1486.6 eV). All the spectra were referenced relative to the C 1 s peak (285.0 eV).

by the Q-MS

of the ions was converted from the ion currents using the conversion rate of N2

**Table 1.** Names and molecular structures of the sulfur and phosphorus anion-based ionic liquids.

**Chemical name Molecular structure**

1-Ethyl-3-methylimidazolium hydrogen sulfate

50 Recent Advances in Ionic Liquids

1-Ethyl-3-methylimidazolium methyl sulfate

1-Ethyl-3-methylimidazolium ethyl sulfate

1-Ethyl-3-methylimidazolium n-octyl sulfate

1-Ethyl-3-methylimidazolium dimethyl phosphate

1-Ethyl-3-methylimidazolium diethyl phosphate

1-Ethyl-3-methylimidazolium dibutyl phosphate

#### *2.2.1. Tribological performances of sulfur and phosphorus anion-based ionic liquids*

**Figure 1** shows the average friction coefficients in the last 5 min of testing and the specific wear rates of the ball specimens. The results for the sulfur anion-based ionic liquids confirm the relationship between the alkyl chain lengths of the anions and the liquids' tribological performances. The friction coefficients are increased as the alkyl chains become longer. Meanwhile, the specific wear rates of the ball specimens are decreased as the alkyl chains become longer. The friction coefficient of [EMIM][HSO<sup>4</sup> ] is approximately 0.04, smaller than that of [BMIM] [PF<sup>6</sup> ]. However, the specific wear rate is very high. Other sulfur anion-based ionic liquids exhibit higher friction coefficients and specific wear rates than [BMIM][PF<sup>6</sup> ]. For the phosphorus anion-based ionic liquids, the friction coefficients and specific wear rates both show increasing tendencies as the alkyl chains become longer. The friction coefficients of [EMIM] [DMP] and [EMIM][DEP] exhibit almost the same value as [BMIM][PF<sup>6</sup> ]. Furthermore, the [EMIM][DMP] exhibits a very low specific wear rate. [EMIM][DMP] showed the greatest tribological performance of all the ionic liquids. **Figure 2** shows the worn surface images of the ball specimens. Chemical wear is confirmed with [EMIM][HSO<sup>4</sup> ] and [EMIM][MSU].

#### *2.2.2. Tribochemical reaction behaviors via Q-MS*

The outgassing of the ionic liquids due to sliding was measured by Q-MS. The main kinds of outgassing were as follows: CH3 (m/*e* = 15) and C2 H6 (m/*e* = 30) were derived from the cations, while H<sup>2</sup> S (m/*e* = 34), CH3 O (m/*e* = 31), C2 H5 O (m/*e* = 45), SO2 (m/*e* = 64), and PO3 (m/*e* = 79) were derived from the anions [24]. Q-MS was used to track the detailed outgassing behaviors of these materials. **Figures 3** and **4** show the outgassing behaviors of the sulfur and phosphorus anion-based ionic liquids, respectively. For [EMIM][HSO<sup>4</sup> ], outgassing derived from the cation is slight. In addition, outgassing of H<sup>2</sup> S derived from the anion is significant in the initial sliding period. On the other hand, the outgassing of SO2 derived from the anion is detected throughout the sliding period. For [EMIM][MSU], the outgassing of CH3 , C2 H6 , and

**Figure 1.** The average friction coefficients in the last 5 min of testing and specific wear rates of the tested ionic liquids. (a) Sulfur anion-based ionic liquids and (b) phosphorus anion-based ionic liquids.

**Figure 2.** The worn surface images of ball specimens tested with the ionic liquids. (a) [EMIM][HSO<sup>4</sup> ], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][EDP], and (g) [EMIM][DBP].

**Figure 3.** The outgassing behaviors of sulfur anion-based ionic liquids. (a) [EMIM][HSO<sup>4</sup>

[ESU], and (d) [EMIM][OSU].

], (b) [EMIM][MSU], (c) [EMIM]

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**Figure 3.** The outgassing behaviors of sulfur anion-based ionic liquids. (a) [EMIM][HSO<sup>4</sup> ], (b) [EMIM][MSU], (c) [EMIM] [ESU], and (d) [EMIM][OSU].

**Figure 2.** The worn surface images of ball specimens tested with the ionic liquids. (a) [EMIM][HSO<sup>4</sup>

**Figure 1.** The average friction coefficients in the last 5 min of testing and specific wear rates of the tested ionic liquids.

(c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][EDP], and (g) [EMIM][DBP].

(a) Sulfur anion-based ionic liquids and (b) phosphorus anion-based ionic liquids.

52 Recent Advances in Ionic Liquids

], (b) [EMIM][MSU],

CH3

[HSO<sup>4</sup>

is not detected.

*2.2.3. XPS analysis*

the peak of FePO<sup>4</sup>

**2.3. Lubricating mechanisms**

CH3 , C2 H6 , CH3

O is detected with unstable behavior. The outgassing of H2

liquids, respectively. For the sulfur anion-based ionic liquids, the FeSO<sup>4</sup>

sliding period, like that of [EMIM][HSO<sup>4</sup>

ing period. However, the outgassing of H<sup>2</sup>

O, and C2

also shows unstable behavior. The outgassing of CH3

H5

observed on the worn surface; however, FeS and FeS2

(133–134 eV) [28–30].

S and SO2

is hardly detected, unlike with [EMIM]

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(162–163 eV) peaks are not observed

]. For [EMIM][ESU], the outgassing of CH3

Tribochemical Reactions of Halogen-Free Ionic Liquids on Nascent Steel Surface

H5

O and C2

O is slight and shows stable behavior, but the outgassing of SO2

S and SO2

**Figure 5** shows the S 2*p* and P 2*p* spectra of the sulfur and phosphorus anion-based ionic

[25–27]. On the other hand, the worn surfaces of phosphorus anion-based ionic liquids show

ited a very low-friction coefficient, but a high specific wear rate and chemical wear. It is well known that chemical wear induces a low-friction coefficient and high wear volume. Among the phosphorus anion-based ionic liquids, [EMIM][DMP] exhibited the greatest tribological performance. Q-MS analysis indicated that the outgassing derived from the anion was small compared to that from the cation. This result indicated that the anion components remained on and reacted with the worn surface. It is well known that reaction films derived from sulfur

**Figure 5.** The spectra of S 2*p* and P 2*p* from worn surfaces treated with different ionic liquids. (a) [EMIM][HSO<sup>4</sup>

[EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][EDP], and (g) [EMIM][DBP].

From the results of the sliding tests with sulfur anion-based ionic liquids, [EMIM][HSO<sup>4</sup>

] and [EMIM][MSU]. For [EMIM][OSU], it is interesting to note that the outgassing of

is high in the initial

peak (168–169 eV) is

] exhib-

], (b)

O is high in the initial slid-

and C2

H6

55

**Figure 4.** The outgassing behaviors of phosphorus anion-based ionic liquids. (a) [EMIM][DMP], (b) [EMIM][DEP], and (c) [EMIM][DBP].

CH3 O is detected with unstable behavior. The outgassing of H2 S and SO2 is high in the initial sliding period, like that of [EMIM][HSO<sup>4</sup> ]. For [EMIM][ESU], the outgassing of CH3 and C2 H6 also shows unstable behavior. The outgassing of CH3 O and C2 H5 O is high in the initial sliding period. However, the outgassing of H<sup>2</sup> S and SO2 is hardly detected, unlike with [EMIM] [HSO<sup>4</sup> ] and [EMIM][MSU]. For [EMIM][OSU], it is interesting to note that the outgassing of CH3 , C2 H6 , CH3 O, and C2 H5 O is slight and shows stable behavior, but the outgassing of SO2 is not detected.

#### *2.2.3. XPS analysis*

**Figure 4.** The outgassing behaviors of phosphorus anion-based ionic liquids. (a) [EMIM][DMP], (b) [EMIM][DEP], and

(c) [EMIM][DBP].

54 Recent Advances in Ionic Liquids

**Figure 5** shows the S 2*p* and P 2*p* spectra of the sulfur and phosphorus anion-based ionic liquids, respectively. For the sulfur anion-based ionic liquids, the FeSO<sup>4</sup> peak (168–169 eV) is observed on the worn surface; however, FeS and FeS2 (162–163 eV) peaks are not observed [25–27]. On the other hand, the worn surfaces of phosphorus anion-based ionic liquids show the peak of FePO<sup>4</sup> (133–134 eV) [28–30].

#### **2.3. Lubricating mechanisms**

From the results of the sliding tests with sulfur anion-based ionic liquids, [EMIM][HSO<sup>4</sup> ] exhibited a very low-friction coefficient, but a high specific wear rate and chemical wear. It is well known that chemical wear induces a low-friction coefficient and high wear volume. Among the phosphorus anion-based ionic liquids, [EMIM][DMP] exhibited the greatest tribological performance. Q-MS analysis indicated that the outgassing derived from the anion was small compared to that from the cation. This result indicated that the anion components remained on and reacted with the worn surface. It is well known that reaction films derived from sulfur

**Figure 5.** The spectra of S 2*p* and P 2*p* from worn surfaces treated with different ionic liquids. (a) [EMIM][HSO<sup>4</sup> ], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][EDP], and (g) [EMIM][DBP].

achieve low friction, while those derived from phosphorous achieve low wear volumes [21– 24]. These reaction films were confirmed by XPS analysis. Regarding the tribochemical reaction behaviors, ionic liquids with short alkyl chain lengths (e.g., [EMIM][HSO<sup>4</sup> ] and [EMIM][MSU]) showed high reactivities with the worn surfaces. It is possible that the heat of friction dominates the tribochemical reactions of the lubricants. However, no relationship between thermal stability and reactivity existed. It is well known that the sliding surface has a high chemical activity derived from the uncovered nascent surface, which functions as a catalytic substance [24, 31]. Thus, the ionic liquids with short alkyl chain lengths easily underwent catalytic degradation on the nascent steel surfaces because the ionic liquids made easy contact with the chemically active sites of the nascent steel surfaces and then achieved low-friction coefficients. For long alkyl chain lengths, the steric hindrance of the anions induced by the alkyl chains decreased the degree of contact with the nascent surface and slowed the tribochemical reactions.

#### **2.4. Summary**

The sulfur anion-based ionic liquids exhibited low-friction coefficients, while the phosphorus anion-based ionic liquids exhibited low specific wear rates. The sulfur and phosphorus anionbased ionic liquids reacted with the steel surface to achieve low friction and low wear volume, respectively. Ionic liquids with short alkyl chain lengths easily underwent catalytic degradation on the nascent steel surfaces and reacted easily.

#### **3. Cyanoanion-based ionic liquids**

The cyanoanion-based ionic liquids consist of light elements (e.g., hydrogen, boron, carbon, and nitrogen). These ionic liquids are expected to reduce environmental burdens when compared to halogen, sulfur, and phosphorus anion-based ionic liquids. However, information on the tribological performances of these ionic liquids is scarce. This section reports the lubricating mechanism of cyanoanion-based ionic liquids.

#### **3.1. Experimental details**

#### *3.1.1. Lubricants*

Six kinds of cyanoanion-based ionic liquids were used as lubricants. **Table 3** lists the chemical names and molecular structures of the used ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCN]), 1-butyl-3-methyl pyrrolidinium dicyanamide ([BMPL][DCN]), 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][TCC]), 1-butyl-3-methyl pyrrolidinium tricyanomethanide ([BMPL][TCC]), 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]), and 1-butyl-3-methyl pyrrolidinium tetracyanoborate ([BMPL][TCB]). All ionic liquids were commercial materials. [EMIM][DCN], [BMPL][DCN], [EMIM][TCB], and [BMPL][TCB] were purchased from Merck Chemicals, Germany, as "High Purity (HP)" grade (halide content <100 ppm, water content <1000 ppm). [EMIM][TCC] and [BMPL][TCC] were purchased from IoLiTec, Germany, as "HP" grade (water content <1000 ppm). In addition, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), containing halogen elements, was used for comparison.

*3.1.2. Physical and chemical properties of ionic liquids*

**Chemical name Molecular structure**

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1-Ethyl-3-methylimidazolium dicyanamide

1-Butyl-3-methyl pyrrolidinium dicyanamide

1-Ethyl-3-methylimidazolium tricyanomethanide

1-Butyl-3-methyl pyrrolidinium tricyanomethanide

1-Ethyl-3-methylimidazolium tetracyanoborate

1-Butyl-3-methyl pyrrolidinium tetracyanoborate

Testing was performed in the same methods outlined in Section 2.1.3.

**Table 3.** Chemical names and molecular structures of cyanoanion-based ionic liquids.

the same as those given in Section 2.1.2.

*3.1.3. Sliding tests*

All ionic liquids are liquid in phase at room temperature. **Table 4** lists the viscosities and thermal decomposition temperatures of the used ionic liquids. The measurement methods were

**Table 3.** Chemical names and molecular structures of cyanoanion-based ionic liquids.

#### *3.1.2. Physical and chemical properties of ionic liquids*

All ionic liquids are liquid in phase at room temperature. **Table 4** lists the viscosities and thermal decomposition temperatures of the used ionic liquids. The measurement methods were the same as those given in Section 2.1.2.

#### *3.1.3. Sliding tests*

achieve low friction, while those derived from phosphorous achieve low wear volumes [21– 24]. These reaction films were confirmed by XPS analysis. Regarding the tribochemical reaction

showed high reactivities with the worn surfaces. It is possible that the heat of friction dominates the tribochemical reactions of the lubricants. However, no relationship between thermal stability and reactivity existed. It is well known that the sliding surface has a high chemical activity derived from the uncovered nascent surface, which functions as a catalytic substance [24, 31]. Thus, the ionic liquids with short alkyl chain lengths easily underwent catalytic degradation on the nascent steel surfaces because the ionic liquids made easy contact with the chemically active sites of the nascent steel surfaces and then achieved low-friction coefficients. For long alkyl chain lengths, the steric hindrance of the anions induced by the alkyl chains decreased the

The sulfur anion-based ionic liquids exhibited low-friction coefficients, while the phosphorus anion-based ionic liquids exhibited low specific wear rates. The sulfur and phosphorus anionbased ionic liquids reacted with the steel surface to achieve low friction and low wear volume, respectively. Ionic liquids with short alkyl chain lengths easily underwent catalytic degrada-

The cyanoanion-based ionic liquids consist of light elements (e.g., hydrogen, boron, carbon, and nitrogen). These ionic liquids are expected to reduce environmental burdens when compared to halogen, sulfur, and phosphorus anion-based ionic liquids. However, information on the tribological performances of these ionic liquids is scarce. This section reports the lubri-

Six kinds of cyanoanion-based ionic liquids were used as lubricants. **Table 3** lists the chemical names and molecular structures of the used ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCN]), 1-butyl-3-methyl pyrrolidinium dicyanamide ([BMPL][DCN]), 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][TCC]), 1-butyl-3-methyl pyrrolidinium tricyanomethanide ([BMPL][TCC]), 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]), and 1-butyl-3-methyl pyrrolidinium tetracyanoborate ([BMPL][TCB]). All ionic liquids were commercial materials. [EMIM][DCN], [BMPL][DCN], [EMIM][TCB], and [BMPL][TCB] were purchased from Merck Chemicals, Germany, as "High Purity (HP)" grade (halide content <100 ppm, water content <1000 ppm). [EMIM][TCC] and [BMPL][TCC] were purchased from IoLiTec, Germany, as "HP" grade (water content <1000 ppm). In addition, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), containing halogen ele-

] and [EMIM][MSU])

behaviors, ionic liquids with short alkyl chain lengths (e.g., [EMIM][HSO<sup>4</sup>

degree of contact with the nascent surface and slowed the tribochemical reactions.

tion on the nascent steel surfaces and reacted easily.

cating mechanism of cyanoanion-based ionic liquids.

**3. Cyanoanion-based ionic liquids**

**3.1. Experimental details**

ments, was used for comparison.

*3.1.1. Lubricants*

**2.4. Summary**

56 Recent Advances in Ionic Liquids

Testing was performed in the same methods outlined in Section 2.1.3.


**Table 4.** The viscosities and thermal decomposition temperatures of the used ionic liquids.

#### *3.1.4. Analysis*

The tribochemical reactions of the ionic liquids were estimated using Q-MS (MKS Instruments, Inc.). After the sliding tests, the disk specimens were ultrasonically cleaned with a mixed solution of 1:1 petroleum benzine and acetone for 10 min. The worn disk surfaces were analyzed by time-of-flight secondary-ion mass spectrometry (ToF-SIMS). The primary ion source was Au3 + , impact energy was 30 kV, measured area was 300 × 300 μm, mass resolution was 1955 *m*/δm, spatial resolution was 3 μm, and dosage was 4.09 × 10<sup>10</sup> ion/cm2 .

tendencies similar to those of [EMIM][TCC]. For [EMIM][TCB], the amount of outgassing is initially large before stabilizing. With [BMPL][TCB], much outgassing is detected in the first

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**Figure 6.** The average of friction coefficients in the last 5 min and specific wear rates of the tested ionic liquids.

Q-MS indicated that the anions remained on the worn surfaces. To obtain information from the worn surfaces, mapping imaging was conducted. **Figure 8** shows mapping images of the surfaces tested with each ionic liquid. For [EMIM][TCB], which showed the highest friction coefficient, both ions show low intensities on the worn area. However, for the other ionic liquids, the adsorption of anions on the worn areas is confirmed. The cations are not observed

From the results of the sliding tests, [EMIM][TCB] exhibited the highest friction coefficient. Other ionic liquids exhibited low-friction coefficients. The [BMPL] cation group exhibited a low specific wear rate when compared to the [EMIM] cation group. Q-MS analysis indicated that the anions remained on the worn surfaces. Adsorption film formation was confirmed by ToF-SIMS analysis. Regarding the tribochemical reaction behavior, [EMIM][TCC], [BMPL] [TCC], and [BMPL][TCB] decomposed easily on the nascent steel surfaces and exhibited lowfriction coefficients. However, [EMIM][TCB] was stable when compared to the other ionic liquids and exhibited a high-friction coefficient. These ionic liquids did not have large differences in chemical structure. Thus, the effect of steric hindrance on the tribochemical reaction was very small and another influence was extant. It is considered that the stability of the cyanoanion-based ionic liquids was important in the tribochemical reactions. [EMIM][TCB] had the highest thermal stability of the cyanoanion-based ionic liquids, and it is believed that

30 min of sliding, before stabilizing.

on the worn surfaces, unlike the anions.

**3.3. Lubricating mechanisms**

*3.2.3. ToF-SIMS analysis*

#### **3.2. Results**

#### *3.2.1. Tribological performance*

**Figure 6** shows the average friction coefficients in the last 5 min of testing and specific wear rates of the ball specimens. [EMIM][TCB] exhibits the highest friction coefficient of 0.18. The other cyanoanion-based ionic liquids exhibit equivalent low-friction coefficients of approximately 0.07. However, this value is higher than that of [BMIM][PF<sup>6</sup> ]. For the specific wear rate, the [BMPL] cation exhibits a smaller value than the [EMIM] cation for the same anion. In addition, chemical wear was observed with none of the ionic liquids, thus, imaging of the ball specimens was omitted.

#### *3.2.2. Tribochemical reaction via Q-MS*

The main kinds of outgassing are as follows: CH3 (m/*e* = 15), C2 H6 (m/*e* = 30), C3 H8 (m/*e* = 44), and C<sup>4</sup> H10 (m/*e* = 58) are derived from the cations [31]. Q-MS traced the detailed outgassing behaviors of these materials. Outgassing derived from anions was not confirmed. This indicated that anions remained on the worn surfaces. **Figure 7** shows the outgassing behavior of each ionic liquid. When [EMIM][DCN] and [BMPL][DCN] are used, the outgassing shows stable behavior throughout the sliding period. With [EMIM][TCC], the outgassing behavior is particularly interesting. Much outgassing with sliding is detected until 30 min have elapsed, then the outgassing behavior is stabilized. For [BMPL][TCC], the outgassing behavior shows

**Figure 6.** The average of friction coefficients in the last 5 min and specific wear rates of the tested ionic liquids.

tendencies similar to those of [EMIM][TCC]. For [EMIM][TCB], the amount of outgassing is initially large before stabilizing. With [BMPL][TCB], much outgassing is detected in the first 30 min of sliding, before stabilizing.

#### *3.2.3. ToF-SIMS analysis*

*3.1.4. Analysis*

58 Recent Advances in Ionic Liquids

was Au3 +

**3.2. Results**

and C<sup>4</sup>

*3.2.1. Tribological performance*

ball specimens was omitted.

*3.2.2. Tribochemical reaction via Q-MS*

The main kinds of outgassing are as follows: CH3

The tribochemical reactions of the ionic liquids were estimated using Q-MS (MKS Instruments, Inc.). After the sliding tests, the disk specimens were ultrasonically cleaned with a mixed solution of 1:1 petroleum benzine and acetone for 10 min. The worn disk surfaces were analyzed by time-of-flight secondary-ion mass spectrometry (ToF-SIMS). The primary ion source

**Figure 6** shows the average friction coefficients in the last 5 min of testing and specific wear rates of the ball specimens. [EMIM][TCB] exhibits the highest friction coefficient of 0.18. The other cyanoanion-based ionic liquids exhibit equivalent low-friction coefficients of approxi-

rate, the [BMPL] cation exhibits a smaller value than the [EMIM] cation for the same anion. In addition, chemical wear was observed with none of the ionic liquids, thus, imaging of the

(m/*e* = 15), C2

H10 (m/*e* = 58) are derived from the cations [31]. Q-MS traced the detailed outgassing behaviors of these materials. Outgassing derived from anions was not confirmed. This indicated that anions remained on the worn surfaces. **Figure 7** shows the outgassing behavior of each ionic liquid. When [EMIM][DCN] and [BMPL][DCN] are used, the outgassing shows stable behavior throughout the sliding period. With [EMIM][TCC], the outgassing behavior is particularly interesting. Much outgassing with sliding is detected until 30 min have elapsed, then the outgassing behavior is stabilized. For [BMPL][TCC], the outgassing behavior shows

H6

(m/*e* = 30), C3

1955 *m*/δm, spatial resolution was 3 μm, and dosage was 4.09 × 10<sup>10</sup> ion/cm2

**Table 4.** The viscosities and thermal decomposition temperatures of the used ionic liquids.

**Ionic liquid Viscosity [mPa s] Decomposition temperature [°C]**

**40°C 70°C**

[EMIM][DCN] 9.7 5.1 298.23 [BMPL][DCN] 15.4 7.1 283.73 [EMIM][TCC] 10.0 5.4 349.0 [BMPL][TCC] 19.4 9.1 320.8 [EMIM][TCB] 10.6 5.2 412.3 [BMPL][TCB] 25.1 10.3 376.7

mately 0.07. However, this value is higher than that of [BMIM][PF<sup>6</sup>

, impact energy was 30 kV, measured area was 300 × 300 μm, mass resolution was

.

]. For the specific wear

H8

(m/*e* = 44),

Q-MS indicated that the anions remained on the worn surfaces. To obtain information from the worn surfaces, mapping imaging was conducted. **Figure 8** shows mapping images of the surfaces tested with each ionic liquid. For [EMIM][TCB], which showed the highest friction coefficient, both ions show low intensities on the worn area. However, for the other ionic liquids, the adsorption of anions on the worn areas is confirmed. The cations are not observed on the worn surfaces, unlike the anions.

#### **3.3. Lubricating mechanisms**

From the results of the sliding tests, [EMIM][TCB] exhibited the highest friction coefficient. Other ionic liquids exhibited low-friction coefficients. The [BMPL] cation group exhibited a low specific wear rate when compared to the [EMIM] cation group. Q-MS analysis indicated that the anions remained on the worn surfaces. Adsorption film formation was confirmed by ToF-SIMS analysis. Regarding the tribochemical reaction behavior, [EMIM][TCC], [BMPL] [TCC], and [BMPL][TCB] decomposed easily on the nascent steel surfaces and exhibited lowfriction coefficients. However, [EMIM][TCB] was stable when compared to the other ionic liquids and exhibited a high-friction coefficient. These ionic liquids did not have large differences in chemical structure. Thus, the effect of steric hindrance on the tribochemical reaction was very small and another influence was extant. It is considered that the stability of the cyanoanion-based ionic liquids was important in the tribochemical reactions. [EMIM][TCB] had the highest thermal stability of the cyanoanion-based ionic liquids, and it is believed that

[EMIM][TCB] was stable against nascent surface. From these results, it was considered that realizing low friction required the tribological decomposition of the ionic liquids and adsorp-

Tribochemical Reactions of Halogen-Free Ionic Liquids on Nascent Steel Surface

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61

**Figure 8.** The mapping images (300 × 300 μm) of each ion on the worn surfaces by ToF-SIMS.

The tribological performances were different according to the structures of the cyanoanionbased ionic liquids. To achieve low friction, the tribochemical reaction of the ionic liquids and adsorption of anions were required. The stability of the cyanoanion-based ionic liquids against the nascent steel surfaces was related to the results of thermal stability for each

This chapter reported investigations of the tribological performances of halogen-free ionic liquids and discussed the lubricating mechanisms of such liquids. As compared with halogen anion-based ionic liquids, the sulfur anion-based ionic liquids exhibited low-friction coefficients, and the phosphorus anion-based ionic liquids exhibited low specific wear rates. [EMIM][DMP] exhibited a particularly low-friction coefficient and specific wear rate. The main kind of outgassing under sliding was from the cation component. The anion remained on and reacted with the worn surface. The anion with short alkyl chain length reacted easily with the worn surface and achieved high tribological performance. Sulfur and phosphorus

The cyanoanion-based ionic liquids also showed low-friction coefficients of less than 0.1; however, these remained higher than those of halogen anion-based ionic liquids. To achieve low

anion-based ionic liquids show potential as novel lubricants.

tion of anions.

**3.4. Summary**

**4. Conclusions**

liquid.

**Figure 7.** The outgassing behaviors of cyanoanion-based ionic liquids. (a) [EMIM][DCN], (b) [BMPL][DCN], (c) [EMIM] [TCC], (d) [BMPL][TCC], (e) [EMIM][TCB], and (f) [BMPL][TCB].


**Figure 8.** The mapping images (300 × 300 μm) of each ion on the worn surfaces by ToF-SIMS.

[EMIM][TCB] was stable against nascent surface. From these results, it was considered that realizing low friction required the tribological decomposition of the ionic liquids and adsorption of anions.

#### **3.4. Summary**

The tribological performances were different according to the structures of the cyanoanionbased ionic liquids. To achieve low friction, the tribochemical reaction of the ionic liquids and adsorption of anions were required. The stability of the cyanoanion-based ionic liquids against the nascent steel surfaces was related to the results of thermal stability for each liquid.

#### **4. Conclusions**

**Figure 7.** The outgassing behaviors of cyanoanion-based ionic liquids. (a) [EMIM][DCN], (b) [BMPL][DCN], (c) [EMIM]

[TCC], (d) [BMPL][TCC], (e) [EMIM][TCB], and (f) [BMPL][TCB].

60 Recent Advances in Ionic Liquids

This chapter reported investigations of the tribological performances of halogen-free ionic liquids and discussed the lubricating mechanisms of such liquids. As compared with halogen anion-based ionic liquids, the sulfur anion-based ionic liquids exhibited low-friction coefficients, and the phosphorus anion-based ionic liquids exhibited low specific wear rates. [EMIM][DMP] exhibited a particularly low-friction coefficient and specific wear rate. The main kind of outgassing under sliding was from the cation component. The anion remained on and reacted with the worn surface. The anion with short alkyl chain length reacted easily with the worn surface and achieved high tribological performance. Sulfur and phosphorus anion-based ionic liquids show potential as novel lubricants.

The cyanoanion-based ionic liquids also showed low-friction coefficients of less than 0.1; however, these remained higher than those of halogen anion-based ionic liquids. To achieve low friction, tribochemical reactions with the worn surface and the adsorption of anions on the worn surface were required. The thermal stability and tribochemical reactivity were found to be related.

[3] Okubo H, Oshima K, Tsuboi R, Tadokoro C, Sasaki S. Effects of hydrogen on frictional properties of DLC films. Tribology Online. 2015;**10**:397-403. DOI: 10.2474/trol.10.397 [4] Okubo H, Sasaki S. *In situ* Raman observation of structural transformation of diamondlike carbon films lubricated with MoDTC solution: Mechanism of wear acceleration of DLC films lubricated with MoDTC solution. Tribology International. 2017;**113**:399-410.

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http://dx.doi.org/10.5772/intechopen.77352

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[5] Okubo H, Watanabe S, Tadokoro C, Sasaki S. Ultralow friction of a tetrahedral amorphous carbon film lubricated with an environmentally friendly ester-based oil. Tribology

[6] Okubo H, Kawada S, Watanabe S, Sasaki S. Tribological performance of halogen-free ionic liquids in steel–steel and DLC–DLC contacts. Tribology Transactions. 2018;(1):71-

[7] Suzuki A, Shinka Y, Masuko M. Tribological characteristics of imidazolium-based room temperature ionic liquids under high vacuum. Tribology Letters. 2007;**24**:307-313. DOI:

[8] Kondo Y, Yagi S, Koyama T, Tsuboi R, Sasaki S. Lubricity and corrosiveness of ionic liquids for steel-on-steel sliding contacts. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 2012;**226**:991-1006. DOI: 10.1177/

[9] Kawada S, Watanabe S, Kondo Y, Tsuboi R, Sasaki S. Tribochemical reactions of ionic liquids under vacuum conditions. Tribology Letters. 2014;**54**:309-315. DOI: 10.1007/s11249-

[10] Ye C, Liu W, Chen Y, Yu L. Room-temperature ionic liquids: a novel versatile lubricant.

[11] Kamimura H, Kubo T, Minami I, Mori S. Effect and mechanism of additives for ionic liquids as new lubricants. Tribology International. 2007;**40**:620-625. DOI: 10.1016/j.triboint.

[12] Mahrove M, Pagano F, Pejaković V, Valea A, Kalin M, Igartua A, Tojo E. Pyridinium based dicationic ionic liquids as base lubricants or lubricant additives. Tribology International.

[13] Pejaković V, Kronberger M, Kalin M. Influence of temperature of tribological behaviour of ionic liquids as lubricants and lubricant additives. Lubrication Science. 2014;**26**:107-

[14] Kawada S, Sato K, Watanabe S, Sasaki S. Lubricating property of cyano-based ionic liquids against hard materials. Journal of Mechanical Science and Technology. 2017;**31**:5745-

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79. DOI: 10.1080/10402004.2016.1272731

10.1007/s11249-007-9235-8

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The sulfur and phosphorus anion-based ionic liquids and cyanoanion-based ionic liquids formed different tribological films. These films had different physical and chemical properties. When these ionic liquids are applied as lubricants in the industry, it is important to select the ionic liquid type depending on the sliding condition. For example, because the sulfur and phosphorus anion-based ionic liquids have high viscosities, they are suitable for sliding in the low-velocity regime. In addition, they are suitable for high contact pressures because they form reaction films. On the other hand, the cyanoanion-based ionic liquids are expected to show applicability for sliding parts exposed to large temperature changes, because their viscosity indices are high.

### **Acknowledgements**

This work was supported by a Grant-in-Aid for JSPS Fellows No. 15 J05958 and JSPS KAKE-NHI Grant Numbers, JP16H02310, JP26630041.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Shouhei Kawada<sup>1</sup> \*, Seiya Watanabe2 , Shinya Sasaki1 and Masaaki Miyatake<sup>1</sup>

\*Address all correspondence to: s-kawada@rs.tus.ac.jp

1 Tokyo University of Science, Tokyo, Japan

2 Kungliga Tekniska Högskolan, Stockholm, Sweden

#### **References**


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friction, tribochemical reactions with the worn surface and the adsorption of anions on the worn surface were required. The thermal stability and tribochemical reactivity were found to

The sulfur and phosphorus anion-based ionic liquids and cyanoanion-based ionic liquids formed different tribological films. These films had different physical and chemical properties. When these ionic liquids are applied as lubricants in the industry, it is important to select the ionic liquid type depending on the sliding condition. For example, because the sulfur and phosphorus anion-based ionic liquids have high viscosities, they are suitable for sliding in the low-velocity regime. In addition, they are suitable for high contact pressures because they form reaction films. On the other hand, the cyanoanion-based ionic liquids are expected to show applicability for sliding parts exposed to large temperature changes, because their

This work was supported by a Grant-in-Aid for JSPS Fellows No. 15 J05958 and JSPS KAKE-

, Shinya Sasaki1

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62 Recent Advances in Ionic Liquids

viscosity indices are high.

**Acknowledgements**

**Conflict of interest**

**Author details**

Shouhei Kawada<sup>1</sup>

**References**

NHI Grant Numbers, JP16H02310, JP26630041.

The authors declare no conflict of interest.

\*, Seiya Watanabe2

\*Address all correspondence to: s-kawada@rs.tus.ac.jp

2 Kungliga Tekniska Högskolan, Stockholm, Sweden

1 Tokyo University of Science, Tokyo, Japan


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**Section 2**

**State of the Art Characterization**

**State of the Art Characterization**

**Chapter 4**

**Provisional chapter**

**Progress in Green Solvents for the Stabilisation of**

**Progress in Green Solvents for the Stabilisation of** 

DOI: 10.5772/intechopen.80062

For over a decade, ionic liquids (ILs) have attracted enormous attention from scientists across the globe. The history of these compounds traces back to 1914 where the inception of the first IL with a melting point of 12°C was made. Years later, a progression of the remarkable related compounds have been discovered. Out of many analogous compounds realized from time to time, the imidazolium class of ionic liquid is the most studied because of their air and moisture stability. The physicochemical properties of ILs differ significantly depending on the anionic/cationic species and alkyl chain length. ILs have found application in many scientific fields the most recent being good solvents and stabilizing agents in the nanomaterial synthesis. Studies have showed that ILs not only stabilize as synthesized nanomaterials but also provide environmentally green routes

**Keywords:** ionic liquids, green solvent, nanomaterials, imidazolium-based ionic liquid,

Nanomaterials have penetrated numerous multidisciplinary research fields in both science and engineering domains, leading to the development of next generation products which have already made a debut in the commercial marketplace [1]. To date, synthetic methods and protocols remain of fundamental importance in accessing and harnessing unique properties of materials as their particle sizes approach the nanometer size regime. Suspended nanomaterials are usually desired due to their suitability with various applications and they are

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Nanomaterials: Imidazolium Based Ionic Liquids**

**Nanomaterials: Imidazolium Based Ionic Liquids**

Zikhona Tshemese, Siphamandla C. Masikane, Sixberth Mlowe and Neerish Revaprasadu

Zikhona Tshemese, Siphamandla C. Masikane, Sixberth Mlowe and Neerish Revaprasadu

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80062

towards nanomaterials engineering.

stabilization, capping agents

**Abstract**

**1. Introduction**

#### **Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids**

DOI: 10.5772/intechopen.80062

Zikhona Tshemese, Siphamandla C. Masikane, Sixberth Mlowe and Neerish Revaprasadu Zikhona Tshemese, Siphamandla C. Masikane, Sixberth Mlowe and Neerish Revaprasadu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80062

#### **Abstract**

For over a decade, ionic liquids (ILs) have attracted enormous attention from scientists across the globe. The history of these compounds traces back to 1914 where the inception of the first IL with a melting point of 12°C was made. Years later, a progression of the remarkable related compounds have been discovered. Out of many analogous compounds realized from time to time, the imidazolium class of ionic liquid is the most studied because of their air and moisture stability. The physicochemical properties of ILs differ significantly depending on the anionic/cationic species and alkyl chain length. ILs have found application in many scientific fields the most recent being good solvents and stabilizing agents in the nanomaterial synthesis. Studies have showed that ILs not only stabilize as synthesized nanomaterials but also provide environmentally green routes towards nanomaterials engineering.

**Keywords:** ionic liquids, green solvent, nanomaterials, imidazolium-based ionic liquid, stabilization, capping agents

#### **1. Introduction**

Nanomaterials have penetrated numerous multidisciplinary research fields in both science and engineering domains, leading to the development of next generation products which have already made a debut in the commercial marketplace [1]. To date, synthetic methods and protocols remain of fundamental importance in accessing and harnessing unique properties of materials as their particle sizes approach the nanometer size regime. Suspended nanomaterials are usually desired due to their suitability with various applications and they are

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

easily prepared from chemical approaches. These approaches provide opportunities in tailoring and tuning the properties of the nanomaterials to suit desired applications by tweaking reaction parameters. Two main reaction parameters have received attention, namely, (1) the type and nature of the precursor/s [2–4], as well as (2) the nature of the solvent used during and after completion of the fabrication process [5–7]. Nanomaterials have, however, received their share of potential negative implications on the environment and its inhabitants [8]. Thus, efforts in devising eco-friendly reaction protocols have been seen as a proper response. In this chapter, imidazolium-based ILs are reviewed as green solvents over conventional organic solvents for use in the synthesis of nanomaterials.

Ionic liquids (ILs) are generally molten salts composed of organic cations and organic or inorganic anions, with characteristic melting points below 100°C. They possess unique physicochemical properties such as low viscosity [12], negligible vapor pressure [13], nonflammability [14], good thermal and chemical stability [15], high ionic conductivity [16], and tunable solubility for both organic and inorganic molecules. They are thus regarded as environmentally-friendly and may lead to the formation of novel materials that have never been achieved by conventional solvents or even water. ILs are normally characterized by their cationic components of which imidazolium [17], pyridinium [18], ammonium [19], pyrrolidinium [20], piperidinium [21] and phosphonium [22] cations with different alkyl chain lengths being the most common (**Figure 1**). Examples of some commonly used anions are shown in **Figure 2**. ILs can be classified further into groups, *viz.* polyionic liquids (PILs) [23, 24], room temperature ionic liquids [25–27], task-specific ionic liquids [28, 29] and supported ionic liquid membranes [30, 31]. IL composites are also known, such as the metal organic

Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids

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71

The complexity and suitability of ILs in various applications have been debated over the years. Many questions have been asked about the complexity (and sometimes even the simplicity) associated with the usefulness of ionic liquids in various fields of science specifically their use as solvents. However, the hybrid ionic nature of ILs and the resultant intermolecular interactions

frameworks-supported ILs [32, 33].

**Figure 2.** Most familiar anionic specie used in ILs synthesis.

The general use of conventional organic solvents in various laboratory and industrial processes has presented issues over the years. For example, in pharmaceutical production, the high energy consumption (*ca.* 60% of the overall energy) and 50% of the post-treatment greenhouse gas emissions are attributed to the incorporation of organic solvents in the production process [9, 10]. Thus, the search for alternative green solvents that do not compromise the process and quality of the end-product has become popular. As a result, different tools, methods and guidelines have been established to ascertain the proper selection of alternative solvents for synthesis; Ghandi [11] has categorized these solvents into three classes, namely: (i) preferred, (ii) usable and (iii) desirable. Ionic liquids are among solvents have become a central focus as alternatives to toxic solvents and subsequently exploited in various reactions.

Ionic liquids (ILs) are generally molten salts composed of organic cations and organic or inorganic anions, with characteristic melting points below 100°C. They possess unique physicochemical properties such as low viscosity [12], negligible vapor pressure [13], nonflammability [14], good thermal and chemical stability [15], high ionic conductivity [16], and tunable solubility for both organic and inorganic molecules. They are thus regarded as environmentally-friendly and may lead to the formation of novel materials that have never been achieved by conventional solvents or even water. ILs are normally characterized by their cationic components of which imidazolium [17], pyridinium [18], ammonium [19], pyrrolidinium [20], piperidinium [21] and phosphonium [22] cations with different alkyl chain lengths being the most common (**Figure 1**). Examples of some commonly used anions are shown in **Figure 2**. ILs can be classified further into groups, *viz.* polyionic liquids (PILs) [23, 24], room temperature ionic liquids [25–27], task-specific ionic liquids [28, 29] and supported ionic liquid membranes [30, 31]. IL composites are also known, such as the metal organic frameworks-supported ILs [32, 33].

The complexity and suitability of ILs in various applications have been debated over the years. Many questions have been asked about the complexity (and sometimes even the simplicity) associated with the usefulness of ionic liquids in various fields of science specifically their use as solvents. However, the hybrid ionic nature of ILs and the resultant intermolecular interactions

**Figure 2.** Most familiar anionic specie used in ILs synthesis.

easily prepared from chemical approaches. These approaches provide opportunities in tailoring and tuning the properties of the nanomaterials to suit desired applications by tweaking reaction parameters. Two main reaction parameters have received attention, namely, (1) the type and nature of the precursor/s [2–4], as well as (2) the nature of the solvent used during and after completion of the fabrication process [5–7]. Nanomaterials have, however, received their share of potential negative implications on the environment and its inhabitants [8]. Thus, efforts in devising eco-friendly reaction protocols have been seen as a proper response. In this chapter, imidazolium-based ILs are reviewed as green solvents over conventional organic

The general use of conventional organic solvents in various laboratory and industrial processes has presented issues over the years. For example, in pharmaceutical production, the high energy consumption (*ca.* 60% of the overall energy) and 50% of the post-treatment greenhouse gas emissions are attributed to the incorporation of organic solvents in the production process [9, 10]. Thus, the search for alternative green solvents that do not compromise the process and quality of the end-product has become popular. As a result, different tools, methods and guidelines have been established to ascertain the proper selection of alternative solvents for synthesis; Ghandi [11] has categorized these solvents into three classes, namely: (i) preferred, (ii) usable and (iii) desirable. Ionic liquids are among solvents have become a central focus as alternatives to toxic solvents and subsequently exploited in various reactions.

solvents for use in the synthesis of nanomaterials.

70 Recent Advances in Ionic Liquids

**Figure 1.** Commonly used cations for ILs synthesis.

give rise to a complex set of phenomena, creating an area of study that is both interesting and challenging. Furthermore, they have been extensively studied and researched as potential solvents for inorganic nanomaterial synthesis, organic chemical reactions, polymer synthesis and electrochemical applications [34–38]. Thus, the aim of this chapter is to rather highlight the recent developments and breakthroughs in exploiting the interesting physicochemical properties of ILs in the world of nanomaterials. The scope will be restricted to the use of room temperature ILs as green solvents for the synthesis and stabilization of inorganic nanomaterials. Prior to this, the physicochemical properties of ILs will be briefly outlined.

practical point of view, ILs are desirable in their molten state e.g. transportation of ILs across multiple unit operations in industrial applications is most effective in their liquid phase. ILs exhibiting of significantly lower melting points or "room temperature ILs" (Tm < 25°C) are critically important to researchers searching for new industrial applications [49]. One main reason for their demand is their usage as solvents or absorbents for separation tasks involving

Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids

states. They are also widely considered as liquid solvents to promote chemical reactions [50].

ILs are generally denser than water, with values ranging from 1 to 1.6 g cm−3 and their densities decrease with increase in alkyl chain length in the cation [51]. For example, ILs composed of

ILs are also affected by the nature of anions, e.g. the densities of 1-butyl-3-methylimidazolium-

1.12, 1.21, 1.36 and 1.43 g cm−3, respectively [44, 53]. The order of increasing density with respect

The electrochemical window allows ILs to be used in the electrodeposition of semiconductors and metals. The electrochemical window, by definition, is the electrochemical potential range over which the electrolyte is neither reduced nor oxidized at an electrode. Thus, the magnitude of the electrochemical value determines the solvent's electrochemical stability. The electrodeposition of elements and compounds in water is limited by its low electrochemical window of only about 1.2 V. On the contrary, ILs have significantly larger electrochemical windows,

of ILs have opened the new horizon for the electrodeposition-assisted synthesis of metals and semiconductors at room temperature, which were previously achieved only from molten salts

Most solvents are thermally stable at high temperatures only for short durations; long time exposure habitually leads undesirable decomposition [43]. On the other hand, ILs are known to be thermally stable up to 450°C. Their thermal stability is limited by the strength of their hydrocarbon bonds [43]. It is common knowledge that ILs with larger proton transfer energies prematurely decompose before reaching their boiling points with varying decomposition temperatures ranging between 100 and 360°C [56]. Protic ILs with a bis(trifluoromethane) sulfonamide anion and alkylammonium, imidazolium and a range of other heterocyclic cations are identified as the most stable ILs due to their bond strength resulting from resonance stabilization [57]. Specifically, ILs such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate, 1-butyl-3-methyl-imdazolium tetrafluoroborate and 1,2-dimethyl-3-propyl imidazolium

SO3 ] <sup>−</sup> < [C3 F7 CO2 ] <sup>−</sup> < [(CF3

, PF<sup>6</sup>

, [BMIm]<sup>+</sup>

CO2 ] <sup>−</sup> < [CF3

SO3 −

and [BEIm]<sup>+</sup>

, TFA and Tf2

at a platinum electrode, 4.10 V for [BMIm]BF4

N at a glassy carbon electrode [54, 55]. Generally, the wide electrochemical windows

), liquid (e.g. toluene), or solid (e.g. cellulose)

http://dx.doi.org/10.5772/intechopen.80062

anion display densities of 1.39, 1.33, 1.29

, respectively [52]. The densities of

N in literature are recorded to be

, in Ref. [52].

73

and 5.5 V for

SO2 ) 2 N]<sup>−</sup>

selective dissolution of solutes in gas (e.g. CO2

substituted imidazolium-type cations and CF3

type ILs with different anions, such as BF4

SO3 ] <sup>−</sup> ≈ [BF4 ] <sup>−</sup> < [CF3

, [EEIm]<sup>+</sup>

and 1.27 g cm−3 for [EMIm]<sup>+</sup>

**2.5. Electrochemical window**

for example, 4.15 V for [BMIm]PF<sup>6</sup>

at high reaction temperatures.

**2.6. Thermal stability**

to anions is: [CH3

[BMP]Tf2

**2.4. Density**

#### **2. Physicochemical properties of ILs**

The physicochemical properties of ILs (e.g. viscosity, melting point, hydrophobicity, solvation, and catalytic activity) can be tuned by changing the alkyl chain length in the cationic and/or the anion groups [39, 40]. Task specific or targeting ILs can be synthesized by careful choice and combination of the cationic and anionic groups, to suit a specific application. A catalysis-themed review article by Sheldon [35], for example, outlines the potential use of ILs as both reaction media and catalysts. The former was, however, found to have a profound effect on the catalytic activities and selectivity.

#### **2.1. Viscosity**

Greater van der Waals forces and hydrogen bonding are typical attributes of materials exhibiting high viscosities [41, 42]. Generally, ILs are highly viscous than most common molecular solvents, with viscosity ranging between 10 and 500 mPa s at room temperature [43]. The cation structure influences the IL viscosity with minimal values reported for ethylmethylimidazolium, EtMeIm<sup>+</sup> ILs. Furthermore, EtMeIm<sup>+</sup> ILs have adequate side chain mobility and low molecular weight. Generally, the viscosity of ILs increases with alkyl chain length and degree of fluorination [44]. Anions also have an influence over the viscosity of ILs, e.g. [BMIm]PF<sup>6</sup> = 312 mPa [45], [BMIm]BF4 = 154 mPa [46] and [BMIm]TF2 N = 52 mPa [44].

#### **2.2. Conductivity**

ILs have relatively good ionic conductivities compared to organic solvents and electrolytic systems (up to about 10 mS cm−1) [43]. Susan et al. [47] first demonstrated that fused ammonium salts can be used as proton conductors in polymer membrane fuel cells [47]. According to Greaves et al. [48], "ionic conductivity is a transport property and is governed by the degree of dissociation of the ions, viscosity, ion mobility, and ionic charge". These aforementioned factors depend on the effective ion sizes and shapes. Furthermore, ionic conductivity of ILs is inversely proportional to viscosity and molar volume [48].

#### **2.3. Melting point**

The general description of ILs is that of ionic salts that are in a liquid phase below an arbitrary temperature of 100°C; the majority of ILs are solids at standard room temperature. From a practical point of view, ILs are desirable in their molten state e.g. transportation of ILs across multiple unit operations in industrial applications is most effective in their liquid phase. ILs exhibiting of significantly lower melting points or "room temperature ILs" (Tm < 25°C) are critically important to researchers searching for new industrial applications [49]. One main reason for their demand is their usage as solvents or absorbents for separation tasks involving selective dissolution of solutes in gas (e.g. CO2 ), liquid (e.g. toluene), or solid (e.g. cellulose) states. They are also widely considered as liquid solvents to promote chemical reactions [50].

#### **2.4. Density**

give rise to a complex set of phenomena, creating an area of study that is both interesting and challenging. Furthermore, they have been extensively studied and researched as potential solvents for inorganic nanomaterial synthesis, organic chemical reactions, polymer synthesis and electrochemical applications [34–38]. Thus, the aim of this chapter is to rather highlight the recent developments and breakthroughs in exploiting the interesting physicochemical properties of ILs in the world of nanomaterials. The scope will be restricted to the use of room temperature ILs as green solvents for the synthesis and stabilization of inorganic nanomateri-

The physicochemical properties of ILs (e.g. viscosity, melting point, hydrophobicity, solvation, and catalytic activity) can be tuned by changing the alkyl chain length in the cationic and/or the anion groups [39, 40]. Task specific or targeting ILs can be synthesized by careful choice and combination of the cationic and anionic groups, to suit a specific application. A catalysis-themed review article by Sheldon [35], for example, outlines the potential use of ILs as both reaction media and catalysts. The former was, however, found to have a profound

Greater van der Waals forces and hydrogen bonding are typical attributes of materials exhibiting high viscosities [41, 42]. Generally, ILs are highly viscous than most common molecular solvents, with viscosity ranging between 10 and 500 mPa s at room temperature [43]. The cation structure influences the IL viscosity with minimal values reported for ethylmethyl-

and low molecular weight. Generally, the viscosity of ILs increases with alkyl chain length and degree of fluorination [44]. Anions also have an influence over the viscosity of ILs, e.g.

ILs have relatively good ionic conductivities compared to organic solvents and electrolytic systems (up to about 10 mS cm−1) [43]. Susan et al. [47] first demonstrated that fused ammonium salts can be used as proton conductors in polymer membrane fuel cells [47]. According to Greaves et al. [48], "ionic conductivity is a transport property and is governed by the degree of dissociation of the ions, viscosity, ion mobility, and ionic charge". These aforementioned factors depend on the effective ion sizes and shapes. Furthermore, ionic conductivity of ILs is

The general description of ILs is that of ionic salts that are in a liquid phase below an arbitrary temperature of 100°C; the majority of ILs are solids at standard room temperature. From a

ILs have adequate side chain mobility

N = 52 mPa [44].

ILs. Furthermore, EtMeIm<sup>+</sup>

[BMIm]PF<sup>6</sup> = 312 mPa [45], [BMIm]BF4 = 154 mPa [46] and [BMIm]TF2

inversely proportional to viscosity and molar volume [48].

als. Prior to this, the physicochemical properties of ILs will be briefly outlined.

**2. Physicochemical properties of ILs**

effect on the catalytic activities and selectivity.

**2.1. Viscosity**

72 Recent Advances in Ionic Liquids

imidazolium, EtMeIm<sup>+</sup>

**2.2. Conductivity**

**2.3. Melting point**

ILs are generally denser than water, with values ranging from 1 to 1.6 g cm−3 and their densities decrease with increase in alkyl chain length in the cation [51]. For example, ILs composed of substituted imidazolium-type cations and CF3 SO3 − anion display densities of 1.39, 1.33, 1.29 and 1.27 g cm−3 for [EMIm]<sup>+</sup> , [EEIm]<sup>+</sup> , [BMIm]<sup>+</sup> and [BEIm]<sup>+</sup> , respectively [52]. The densities of ILs are also affected by the nature of anions, e.g. the densities of 1-butyl-3-methylimidazoliumtype ILs with different anions, such as BF4 , PF<sup>6</sup> , TFA and Tf2 N in literature are recorded to be 1.12, 1.21, 1.36 and 1.43 g cm−3, respectively [44, 53]. The order of increasing density with respect to anions is: [CH3 SO3 ] <sup>−</sup> ≈ [BF4 ] <sup>−</sup> < [CF3 CO2 ] <sup>−</sup> < [CF3 SO3 ] <sup>−</sup> < [C3 F7 CO2 ] <sup>−</sup> < [(CF3 SO2 ) 2 N]<sup>−</sup> , in Ref. [52].

#### **2.5. Electrochemical window**

The electrochemical window allows ILs to be used in the electrodeposition of semiconductors and metals. The electrochemical window, by definition, is the electrochemical potential range over which the electrolyte is neither reduced nor oxidized at an electrode. Thus, the magnitude of the electrochemical value determines the solvent's electrochemical stability. The electrodeposition of elements and compounds in water is limited by its low electrochemical window of only about 1.2 V. On the contrary, ILs have significantly larger electrochemical windows, for example, 4.15 V for [BMIm]PF<sup>6</sup> at a platinum electrode, 4.10 V for [BMIm]BF4 and 5.5 V for [BMP]Tf2 N at a glassy carbon electrode [54, 55]. Generally, the wide electrochemical windows of ILs have opened the new horizon for the electrodeposition-assisted synthesis of metals and semiconductors at room temperature, which were previously achieved only from molten salts at high reaction temperatures.

#### **2.6. Thermal stability**

Most solvents are thermally stable at high temperatures only for short durations; long time exposure habitually leads undesirable decomposition [43]. On the other hand, ILs are known to be thermally stable up to 450°C. Their thermal stability is limited by the strength of their hydrocarbon bonds [43]. It is common knowledge that ILs with larger proton transfer energies prematurely decompose before reaching their boiling points with varying decomposition temperatures ranging between 100 and 360°C [56]. Protic ILs with a bis(trifluoromethane) sulfonamide anion and alkylammonium, imidazolium and a range of other heterocyclic cations are identified as the most stable ILs due to their bond strength resulting from resonance stabilization [57]. Specifically, ILs such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate, 1-butyl-3-methyl-imdazolium tetrafluoroborate and 1,2-dimethyl-3-propyl imidazolium bis(trifluorosulfonyl)imide are thermally stable up to 445, 423 and 457°C, respectively [43]. The anionic components is known to predominantly contribute to the thermal stability of the ILs over the cationic counterparts. Furthermore, the hydrophilicity of the anions decreases thermal stability of the ILs [58].

ammonium, phosphonium and imidazolium moieties [65]. A graphical illustration of this

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75

Steric stabilization of nanoparticles is another significant parameter. This usually happens through the use of surfactants, alcohols and a variety of polymers/oligomers. These compounds are adsorbed on the nanoparticle's surface, thereby forming a protective layer [66]. When the nanoparticles are sterically stabilized, their free motion in solution is limited. Furthermore, the thickness of the protective layer has an important role in the stabilization process and greatly depends on the alkyl chain length and nature of the stabilizing agent, where applicable [65].

**4.3. A combination of electrostatic and steric stabilization of nanomaterials by ILs**

**4.4. Practical examples of imidazolium-based ILs used in the synthesis of** 

Both electrostatic and steric stabilization processes can co-exist in the stabilization of nanoparticles in solution phase. Ionic surfactants with long alkyl chains ordinarily provide this type of stabilization. This is mostly achieved through ILs which have polar heads groups which generate electric double layer around the nanoparticle and a lipophilic tail group providing the steric repulsions between nanoparticles [65]. Examples of IL-stabilized nanoparticles are

The diversity of ILs presents a challenge is identifying the relationship/trend between their physiochemical properties and morphological (also crystal phase) control of the nanomaterials. It is for these reasons ILs have been used in conjunction with conventional organic solvents to synthesize nanomaterials [68, 74, 75]. Since the influence of ILs on the

**Figure 3.** Electrostatic stabilization of nanoparticles by the imidazolium based ILs. Reproduced from Ref. [65] with

kind of stabilization is shown in **Figure 3**.

listed in **Table 1**.

**nanomaterials**

permission granted by Springer.

**4.2. Steric stabilization of nanomaterials by ILs**

### **3. Imidazolium-based ionic liquids**

ILs with the 1,3-dialkylimidazolium cation are by far the most studied class of ILs due to their interesting properties. The class can interact with various chemical species, as they offer hydrophobic or hydrophilic regions and a high directional polarizability. The structural organization of these solvents can be used as 'entropic drivers' for spontaneous, well-defined, and prolonged ordering of nanoscale structures. Certainly, the unique combination of flexibility towards other molecules (and phases) with strong hydrogen-bond-driven structures make ILs potential key tools in the preparation of a new generation of chemical nanostructures [13].

## **4. Imidazolium-based ionic liquids in nanomaterial synthesis**

Generally, ILs have chemical attributes such as non-volatility, negligible vapor pressure and most importantly, high thermal stability [59]. The majority of the fabrication protocols require heat to be applied, thus controlling both nucleation and growth rates to ensure monodispersity in suspended nanomaterials [60]. Functional groups of organic solvents determine stability and coordination mode towards nanomaterials [5]. Similarly, the composition of the ILs (combination of organic cation with organic or inorganic anion) play a major role in determining the final chemical attributes. The early reports have demonstrated ILs as dual-action reagents, i.e. as both solvent media and template or structure/shape-directing agent [61, 62]. Furthermore, the recovery of ILs after completion of nanomaterials synthesis strengthens their classification as eco-friendly solvents. The negligible vapor pressure feature enables reactions in ILs to proceed in ambient pressure when subjected to synthetic protocols common for hydro- and solvothermal methods, hence the reactions are rather termed ionothermal synthesis [59].

In recent years the advantages of incorporating ILs in the synthesis of inorganic nanomaterials have been thoroughly demonstrated, due to their unique physicochemical properties [63, 64]. Several ILS-incorporated methods and protocols have been reported for the fabrication of inorganic nanomaterials. Synthetic reactions, in the presence of ILs, offers nanomaterials to display exceptional and/or unique properties [34].

#### **4.1. Electrostatic stabilization of nanomaterials by ILs**

Ionic compounds such as carboxylates, polyoxoanions and fluorides usually generate double layers between the nanoparticles, thereby introducing repulsive forces between individual nanoparticles. Thus, electrostatic repulsions prevent aggregations and agglomeration of nanoparticles suspended in the solution phase. This type of electrostatic stabilization is sensitive to reaction parameters such as pH, concentration and temperature. This has been demonstrated in metal nanoparticles stabilized by ILs composed of sulfonium, tertiary butyl ammonium, phosphonium and imidazolium moieties [65]. A graphical illustration of this kind of stabilization is shown in **Figure 3**.

#### **4.2. Steric stabilization of nanomaterials by ILs**

bis(trifluorosulfonyl)imide are thermally stable up to 445, 423 and 457°C, respectively [43]. The anionic components is known to predominantly contribute to the thermal stability of the ILs over the cationic counterparts. Furthermore, the hydrophilicity of the anions decreases

ILs with the 1,3-dialkylimidazolium cation are by far the most studied class of ILs due to their interesting properties. The class can interact with various chemical species, as they offer hydrophobic or hydrophilic regions and a high directional polarizability. The structural organization of these solvents can be used as 'entropic drivers' for spontaneous, well-defined, and prolonged ordering of nanoscale structures. Certainly, the unique combination of flexibility towards other molecules (and phases) with strong hydrogen-bond-driven structures make ILs potential key tools in the preparation of a new generation of chemical nanostructures [13].

Generally, ILs have chemical attributes such as non-volatility, negligible vapor pressure and most importantly, high thermal stability [59]. The majority of the fabrication protocols require heat to be applied, thus controlling both nucleation and growth rates to ensure monodispersity in suspended nanomaterials [60]. Functional groups of organic solvents determine stability and coordination mode towards nanomaterials [5]. Similarly, the composition of the ILs (combination of organic cation with organic or inorganic anion) play a major role in determining the final chemical attributes. The early reports have demonstrated ILs as dual-action reagents, i.e. as both solvent media and template or structure/shape-directing agent [61, 62]. Furthermore, the recovery of ILs after completion of nanomaterials synthesis strengthens their classification as eco-friendly solvents. The negligible vapor pressure feature enables reactions in ILs to proceed in ambient pressure when subjected to synthetic protocols common for hydro- and solvothermal methods, hence the reactions are rather termed ionothermal synthesis [59].

In recent years the advantages of incorporating ILs in the synthesis of inorganic nanomaterials have been thoroughly demonstrated, due to their unique physicochemical properties [63, 64]. Several ILS-incorporated methods and protocols have been reported for the fabrication of inorganic nanomaterials. Synthetic reactions, in the presence of ILs, offers nanomaterials to

Ionic compounds such as carboxylates, polyoxoanions and fluorides usually generate double layers between the nanoparticles, thereby introducing repulsive forces between individual nanoparticles. Thus, electrostatic repulsions prevent aggregations and agglomeration of nanoparticles suspended in the solution phase. This type of electrostatic stabilization is sensitive to reaction parameters such as pH, concentration and temperature. This has been demonstrated in metal nanoparticles stabilized by ILs composed of sulfonium, tertiary butyl

**4. Imidazolium-based ionic liquids in nanomaterial synthesis**

thermal stability of the ILs [58].

74 Recent Advances in Ionic Liquids

**3. Imidazolium-based ionic liquids**

display exceptional and/or unique properties [34].

**4.1. Electrostatic stabilization of nanomaterials by ILs**

Steric stabilization of nanoparticles is another significant parameter. This usually happens through the use of surfactants, alcohols and a variety of polymers/oligomers. These compounds are adsorbed on the nanoparticle's surface, thereby forming a protective layer [66]. When the nanoparticles are sterically stabilized, their free motion in solution is limited. Furthermore, the thickness of the protective layer has an important role in the stabilization process and greatly depends on the alkyl chain length and nature of the stabilizing agent, where applicable [65].

#### **4.3. A combination of electrostatic and steric stabilization of nanomaterials by ILs**

Both electrostatic and steric stabilization processes can co-exist in the stabilization of nanoparticles in solution phase. Ionic surfactants with long alkyl chains ordinarily provide this type of stabilization. This is mostly achieved through ILs which have polar heads groups which generate electric double layer around the nanoparticle and a lipophilic tail group providing the steric repulsions between nanoparticles [65]. Examples of IL-stabilized nanoparticles are listed in **Table 1**.

#### **4.4. Practical examples of imidazolium-based ILs used in the synthesis of nanomaterials**

The diversity of ILs presents a challenge is identifying the relationship/trend between their physiochemical properties and morphological (also crystal phase) control of the nanomaterials. It is for these reasons ILs have been used in conjunction with conventional organic solvents to synthesize nanomaterials [68, 74, 75]. Since the influence of ILs on the

**Figure 3.** Electrostatic stabilization of nanoparticles by the imidazolium based ILs. Reproduced from Ref. [65] with permission granted by Springer.


tetrafluoroborate IL to form IL-ZnO composites for potential activity enhancement of doxorubicin and dasatinib for breast anticancer remedies. In their study the outcome showed that ZnO-NPs/BMTFB/CPE reveals two different oxidation signals for the simultaneous examination of doxorubicin and dasatinib, which eventually showed a good linear relationship between the oxidation peak current of doxorubicin and its concentration. They concluded that ZnO-NPs/BMTFB/CPE showed good capacity for analysis of doxorubicin and dasatinib in injection and serum samples. Husanu et al. [82] reported an easy synthetic method focusing on preparing citrate-based ILs which showed a dual-action feature, an ability to act as both capping agents and reducing agents towards the preparation of inorganic nanomaterials.

**Figure 4.** Synthesis of ferric giniite crystals exhibiting different shapes as a result of varying reaction parameters.

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Although the topic "ILs in the synthesis of nanomaterials" is not of novel origin, the 'novelty' is observed where a new class of ILs is prepared through the incorporation of new anionic groups. This subsequently results in the synthesis of nanomaterials bearing desired properties. For example; 3-(hydroxypropyl)-3-methylimidazolium bis(salicylato)borate (OHMimBScB) IL has been used for the covalent grafting of carbon quantum dots via a bottom up approach [83]. Since ILs have strong adsorption capabilities, the surface on the carbon quantum dots was protected against friction, which then improved their stability efficiency in tribological evaluations. Saien and Hashemi [84] used 1-hexadecyl-3-methyl imidazolium chloride IL as a

O4

study by Okoli et al. [85] it was found that the sole use of ILs without additional co-surfactants is capable of producing good quality (monodispersity with narrow size distribution) metal

nanoparticles in a biphasic oil/water system. In a

surfactant to study its interaction with Fe3

alloy nanoparticles which display enhanced catalytic activities.

Reproduced from Ref. [70] with permission from the Royal Society of Chemistry.

**Table 1.** Examples of nanoparticles stabilized by imidazolium based ILs.

properties of nanomaterials is not predictable at this stage, there are efforts in conducting investigations for the sake of rather establishing databases. A good example is the work by Duan et al. [76] where ILs are demonstrated as both solvent and shape-directing agent in the synthesis of γ-AlOOH and well-dispersed NH4 Al(OH)2 CO3 nanostructures. The same group extended the study on ferric giniite crystals (**Figure 4**) which revealed interesting photocatalytic behavior which could only be linked to exposed facets of the crystals, as opposed to the surface area [77]. There are reports that have documented evidence of imidazolium-based ILs having acted as capping agents [78, 79]. Zheng and co-workers [79, 80] have given plausible reasons that support capping through interaction modes of imidazolium-based ILs with TiO2 nanoparticles, for example, hydrogen bonding between the positively charged (due to delocalization) position-2 H atom in the imidazole ring with O on the TiO2 surface.

IL-nanomaterial composites have also been reported for use in biological applications, e.g. the study by Alavi-Tabari et al. [81] demonstrates the use of 1-butyl-3-methylimidazolium Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids http://dx.doi.org/10.5772/intechopen.80062 77

**Figure 4.** Synthesis of ferric giniite crystals exhibiting different shapes as a result of varying reaction parameters. Reproduced from Ref. [70] with permission from the Royal Society of Chemistry.

tetrafluoroborate IL to form IL-ZnO composites for potential activity enhancement of doxorubicin and dasatinib for breast anticancer remedies. In their study the outcome showed that ZnO-NPs/BMTFB/CPE reveals two different oxidation signals for the simultaneous examination of doxorubicin and dasatinib, which eventually showed a good linear relationship between the oxidation peak current of doxorubicin and its concentration. They concluded that ZnO-NPs/BMTFB/CPE showed good capacity for analysis of doxorubicin and dasatinib in injection and serum samples. Husanu et al. [82] reported an easy synthetic method focusing on preparing citrate-based ILs which showed a dual-action feature, an ability to act as both capping agents and reducing agents towards the preparation of inorganic nanomaterials.

properties of nanomaterials is not predictable at this stage, there are efforts in conducting investigations for the sake of rather establishing databases. A good example is the work by Duan et al. [76] where ILs are demonstrated as both solvent and shape-directing agent in

group extended the study on ferric giniite crystals (**Figure 4**) which revealed interesting photocatalytic behavior which could only be linked to exposed facets of the crystals, as opposed to the surface area [77]. There are reports that have documented evidence of imidazolium-based ILs having acted as capping agents [78, 79]. Zheng and co-workers [79, 80] have given plausible reasons that support capping through interaction modes of

the positively charged (due to delocalization) position-2 H atom in the imidazole ring with

IL-nanomaterial composites have also been reported for use in biological applications, e.g. the study by Alavi-Tabari et al. [81] demonstrates the use of 1-butyl-3-methylimidazolium

Al(OH)2

CO3

nanoparticles, for example, hydrogen bonding between

nanostructures. The same

the synthesis of γ-AlOOH and well-dispersed NH4

**Table 1.** Examples of nanoparticles stabilized by imidazolium based ILs.

imidazolium-based ILs with TiO2

**IL used NP** 

76 Recent Advances in Ionic Liquids

**synthesized**

CdS, PbS 2–15,

102–160

Al2 O3 , TiO2 ,

Fe<sup>3</sup> O4 **Size (nm)**

**IL name and reference**

ZnSe 70–100 1-butyl-3-methylimidazolium methyl sulfate [68]

PbS 100 1-n-butyl-3-methylimidazolium hexafluorophosphate

Ni 4.9–5.9 1-alkyl-3-methylimidazolium N-bis(trifluoromethane

Ir 2.4–2.6 1-n-butyl-3-methyl trifluoromethane sulfonate [73]

N-bis(trifluoromethylsulfonyl)imide [72]

sulfonyl) [71]

Au 140 n-butyltrimethylammonium

bromide [67]

[69]

80–100 1-methacryloyloxypropyl-3-methylimidazolium

1-ethyl-3-methylimidazolium methanesulfonate [70]

surface.

O on the TiO2

Although the topic "ILs in the synthesis of nanomaterials" is not of novel origin, the 'novelty' is observed where a new class of ILs is prepared through the incorporation of new anionic groups. This subsequently results in the synthesis of nanomaterials bearing desired properties. For example; 3-(hydroxypropyl)-3-methylimidazolium bis(salicylato)borate (OHMimBScB) IL has been used for the covalent grafting of carbon quantum dots via a bottom up approach [83]. Since ILs have strong adsorption capabilities, the surface on the carbon quantum dots was protected against friction, which then improved their stability efficiency in tribological evaluations. Saien and Hashemi [84] used 1-hexadecyl-3-methyl imidazolium chloride IL as a surfactant to study its interaction with Fe3 O4 nanoparticles in a biphasic oil/water system. In a study by Okoli et al. [85] it was found that the sole use of ILs without additional co-surfactants is capable of producing good quality (monodispersity with narrow size distribution) metal alloy nanoparticles which display enhanced catalytic activities.

**Figure 5.** The electrochemical exfoliation of graphite into (a and b) carbon nanoparticles, (c and d) carbon nanoribbons and (e and f) graphene sheets. Reprinted with permission from Ref. [87]. Copyright 2009 American Chemical Society.

properties of the nanomaterials [91, 92]. Various synthetic routes such as colloidal methods use solvent-mediated chemical procedures to form colloids via self-assembly or other processes related to the tuning of physicochemical properties [93]. The former process usually proceeds via the homogeneous nucleation of particles and subsequent particle growth by

**Figure 6.** Mechanism of nanoparticle production using colloidal methods. The methods differ in the way the starting molecules are generated by chemical reaction/precipitation. The nuclei may be amorphous or crystalline and lead to amorphous or crystalline nanoparticles. Because of their intrinsic instability, the nanoparticles may form agglomerates

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Imidazolium-based ILs have been reported to improve the electric conductivity of multi-walled carbon nanotubes for electrochemical or electro catalytic applications [65]. Covalent interaction linkage ensures improved oxidation and hydrogenation reactions. Imidazolium-based

**Figure 7.** Tuning of nanoparticles size by imidazolium based ionic liquid (reprinted with permission from Refs. [65, 95, 97]).

condensation, coagulation and arrest of particles (**Figure 6**).

that can be easily redispersed or form non-dispersible aggregate clusters.

A pre-concentration technique based on the use of graphene oxide (GO) functionalized with an IL was established for the determination of Hg traces in water [86]. In this study, the hybrid IL-GO nanomaterial was fabricated by a simple procedure, where the IL used was 1-butyl-3-dodecylimidazolium bromide ([C4 C12im]Br). The incorporation of [C4 C12im]Br led to high sorption performances on Hg. Furthermore, functionalization of the imidazolium ring with long alkyl chains increased the retention competence of GO for the analyte, consequently providing additional physicochemical properties that were beneficial for the extraction of Hg and its control at trace levels.

Other interesting reports include the use of ILs as a solvent medium in the electrochemical exfoliation of graphite to obtain nanomaterials of significant technological importance i.e. fluorescent carbon nanoribbons, nanoparticles and graphene (**Figure 5**) [87]. ILs are most suited for this task due to their broad electrochemical window, as well as high dielectric constant necessary to counteract stacking in nanomaterials resulting from van der Waals interactions. Furthermore, imidazolium-based IL-capped carbon quantum dots have been prepared by pyrolysis [88]. Recently, imidazolium-based IL has been used to exfoliate 2D transition metal chalcogenide, WS2 nanosheets [89]. Prechtl et al. [90] demonstrated the use of ILs as both stabilizing and reducing agents in the preparation of metal nanoparticles from organometallic complexes. Similarly, citrate-modified ILs were recently used to prepare IL-capped Ag nanoparticles [90].

#### **5. Tuning the properties of nanomaterials (NMs) through the use of IBILs**

Stabilization of nanoparticles using different media has been necessary to suit various applications. The medium plays a key role in modifying and tuning the chemical and physical Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids http://dx.doi.org/10.5772/intechopen.80062 79

**Figure 6.** Mechanism of nanoparticle production using colloidal methods. The methods differ in the way the starting molecules are generated by chemical reaction/precipitation. The nuclei may be amorphous or crystalline and lead to amorphous or crystalline nanoparticles. Because of their intrinsic instability, the nanoparticles may form agglomerates that can be easily redispersed or form non-dispersible aggregate clusters.

properties of the nanomaterials [91, 92]. Various synthetic routes such as colloidal methods use solvent-mediated chemical procedures to form colloids via self-assembly or other processes related to the tuning of physicochemical properties [93]. The former process usually proceeds via the homogeneous nucleation of particles and subsequent particle growth by condensation, coagulation and arrest of particles (**Figure 6**).

**Figure 5.** The electrochemical exfoliation of graphite into (a and b) carbon nanoparticles, (c and d) carbon nanoribbons and (e and f) graphene sheets. Reprinted with permission from Ref. [87]. Copyright 2009 American Chemical Society.

A pre-concentration technique based on the use of graphene oxide (GO) functionalized with an IL was established for the determination of Hg traces in water [86]. In this study, the hybrid IL-GO nanomaterial was fabricated by a simple procedure, where the IL used was 1-butyl-

sorption performances on Hg. Furthermore, functionalization of the imidazolium ring with long alkyl chains increased the retention competence of GO for the analyte, consequently providing additional physicochemical properties that were beneficial for the extraction of Hg

Other interesting reports include the use of ILs as a solvent medium in the electrochemical exfoliation of graphite to obtain nanomaterials of significant technological importance i.e. fluorescent carbon nanoribbons, nanoparticles and graphene (**Figure 5**) [87]. ILs are most suited for this task due to their broad electrochemical window, as well as high dielectric constant necessary to counteract stacking in nanomaterials resulting from van der Waals interactions. Furthermore, imidazolium-based IL-capped carbon quantum dots have been prepared by pyrolysis [88]. Recently, imidazolium-based IL has been used to exfoliate 2D transition metal chalcogenide,

 nanosheets [89]. Prechtl et al. [90] demonstrated the use of ILs as both stabilizing and reducing agents in the preparation of metal nanoparticles from organometallic complexes. Similarly, citrate-modified ILs were recently used to prepare IL-capped Ag nanoparticles [90].

**5. Tuning the properties of nanomaterials (NMs) through the use of** 

Stabilization of nanoparticles using different media has been necessary to suit various applications. The medium plays a key role in modifying and tuning the chemical and physical

C12im]Br). The incorporation of [C4

C12im]Br led to high

3-dodecylimidazolium bromide ([C4

and its control at trace levels.

78 Recent Advances in Ionic Liquids

WS2

**IBILs**

Imidazolium-based ILs have been reported to improve the electric conductivity of multi-walled carbon nanotubes for electrochemical or electro catalytic applications [65]. Covalent interaction linkage ensures improved oxidation and hydrogenation reactions. Imidazolium-based

**Figure 7.** Tuning of nanoparticles size by imidazolium based ionic liquid (reprinted with permission from Refs. [65, 95, 97]).

ILs provide surface stabilization of nanoparticles over a long period of time with insignificant or no change in particle size and size distribution, providing re-usable or recyclability capabilities in applications such as catalysis [65]. Photocatalyst stabilized by imidazolium-based ILs are known to bind strongly and could be easily separated from the reaction mixture without contamination or significant loss in catalytic performance [65].

the decrease of Ag nanoparticle sizes with an increase of the molecular weight of P[ViEtIm]

The type of functional group present in the IBIL determines the mode of interaction with the nanoparticle. Furthermore, tweaking of the reaction parameters has become a common practice to manipulate these interaction modes, thus, enabling particle size and shape-directing capabilities. The chemical structure of 1-ethyl-3-methylimidazolium IL, in the recent study, afforded covalent, coordinate, electrostatic/steric or weak chemisorption interactions via their cations and counter anions (**Figure 8**) to produce CdS and PbS nanoparticles [70, 98]. The 1-ethyl-3 methylimidazolium IL efficiently produced very small and well dispersed CdS quantum dots in the size range of 2.3–4.3 ± 0.265 nm, attributed to the presence of both cationic and anionic components [70]. Cubic-shaped and highly crystalline PbS nanoparticles in the particle size range of 64 ± 18 nm were synthesized using the same 1-ethyl-3-methylimidazolium IL (**Figure 8**) [98].

Ionic liquids (ILs), imidazolium-based ILs (IBILs) in particular, have been exploited in various applications including its use as both reactants and stabilizing agents in the synthesis of functional nanomaterials. The remarkable merit of IBILs and ILs in general is their diversified design which involves a vast selection of cations and anions species. The physicochemical properties of ILs play a major role in improving, modifying and tuning important properties of nanomaterials such as particle size and morphology to suit various applications. Interestingly, the physicochemical properties of ILs further provide unique stability for nanomaterials, which guarantees longer shelf life. In addition, ILs are known to be less toxic, thus enabling them to be used as alternative eco-friendly candidates in various studies as means of applying green chemistry principles. The properties of ILs change with cation/anion composition and there is currently no model to predict its interactions and subsequent influence on the properties of nanomaterials. Hence, an increasing interest observed in evaluating ILs as both reactants and stabilizing agent in the synthesis of differ-

Z. Tshemese, S. Mlowe, N. Revaprasadu and S.C. Masikane wishes to thank the National

Research Foundation—South Africa (NRF) for financial support.

CN] IL. In another report, Vijayakrishna et al. [97] studied the effect of alkyl chain lengths in the imidazolium-based poly-ILs of the general formula (P[ViRIm][OH]) where R = ethyl, butyl and pentyl, on particle sizes of magnetic Ni nanoparticles; average particle sizes

Progress in Green Solvents for the Stabilisation of Nanomaterials: Imidazolium Based Ionic Liquids

http://dx.doi.org/10.5772/intechopen.80062

81

[BH3

**6. Conclusion**

ent classes of nanomaterials.

**Acknowledgements**

**Conflict of interest**

We declare no conflict of interest.

increased with alkyl chain length (**Figure 7**).

Interestingly, small-sized Cu nanoparticles (6.6 nm) have reportedly been synthesized *in situ* using [BMIM][PF<sup>6</sup> ] and [BMIM][BF4 ] ILs as stabilizing agent and later used in cycloaddition catalytic reactions between azides and alkynes [94]. The particle size of imidazolium-based IL-capped nanoparticles can be easily tuned by varying the molecular weight, alkyl chain lengths and substituents on the imidazolium ring [71, 95]. Gracia et al. [96] demonstrated

**Figure 8.** Top = optical properties and transmission electron microscopy image of 1-ethyl-3-methylimidazolium methanesulfonate stabilized CdS nanoparticles. Bottom = scanning electron microscope image, structure of the IL and X-ray diffraction patterns of 1-ethyl-3-methylimidazolium methanesulfonate stabilized PbS nanoparticles (reprinted with permission from Refs. [70, 98]).

the decrease of Ag nanoparticle sizes with an increase of the molecular weight of P[ViEtIm] [BH3 CN] IL. In another report, Vijayakrishna et al. [97] studied the effect of alkyl chain lengths in the imidazolium-based poly-ILs of the general formula (P[ViRIm][OH]) where R = ethyl, butyl and pentyl, on particle sizes of magnetic Ni nanoparticles; average particle sizes increased with alkyl chain length (**Figure 7**).

The type of functional group present in the IBIL determines the mode of interaction with the nanoparticle. Furthermore, tweaking of the reaction parameters has become a common practice to manipulate these interaction modes, thus, enabling particle size and shape-directing capabilities. The chemical structure of 1-ethyl-3-methylimidazolium IL, in the recent study, afforded covalent, coordinate, electrostatic/steric or weak chemisorption interactions via their cations and counter anions (**Figure 8**) to produce CdS and PbS nanoparticles [70, 98]. The 1-ethyl-3 methylimidazolium IL efficiently produced very small and well dispersed CdS quantum dots in the size range of 2.3–4.3 ± 0.265 nm, attributed to the presence of both cationic and anionic components [70]. Cubic-shaped and highly crystalline PbS nanoparticles in the particle size range of 64 ± 18 nm were synthesized using the same 1-ethyl-3-methylimidazolium IL (**Figure 8**) [98].

#### **6. Conclusion**

ILs provide surface stabilization of nanoparticles over a long period of time with insignificant or no change in particle size and size distribution, providing re-usable or recyclability capabilities in applications such as catalysis [65]. Photocatalyst stabilized by imidazolium-based ILs are known to bind strongly and could be easily separated from the reaction mixture with-

Interestingly, small-sized Cu nanoparticles (6.6 nm) have reportedly been synthesized *in situ*

catalytic reactions between azides and alkynes [94]. The particle size of imidazolium-based IL-capped nanoparticles can be easily tuned by varying the molecular weight, alkyl chain lengths and substituents on the imidazolium ring [71, 95]. Gracia et al. [96] demonstrated

**Figure 8.** Top = optical properties and transmission electron microscopy image of 1-ethyl-3-methylimidazolium methanesulfonate stabilized CdS nanoparticles. Bottom = scanning electron microscope image, structure of the IL and X-ray diffraction patterns of 1-ethyl-3-methylimidazolium methanesulfonate stabilized PbS nanoparticles (reprinted

] ILs as stabilizing agent and later used in cycloaddition

out contamination or significant loss in catalytic performance [65].

] and [BMIM][BF4

using [BMIM][PF<sup>6</sup>

80 Recent Advances in Ionic Liquids

with permission from Refs. [70, 98]).

Ionic liquids (ILs), imidazolium-based ILs (IBILs) in particular, have been exploited in various applications including its use as both reactants and stabilizing agents in the synthesis of functional nanomaterials. The remarkable merit of IBILs and ILs in general is their diversified design which involves a vast selection of cations and anions species. The physicochemical properties of ILs play a major role in improving, modifying and tuning important properties of nanomaterials such as particle size and morphology to suit various applications. Interestingly, the physicochemical properties of ILs further provide unique stability for nanomaterials, which guarantees longer shelf life. In addition, ILs are known to be less toxic, thus enabling them to be used as alternative eco-friendly candidates in various studies as means of applying green chemistry principles. The properties of ILs change with cation/anion composition and there is currently no model to predict its interactions and subsequent influence on the properties of nanomaterials. Hence, an increasing interest observed in evaluating ILs as both reactants and stabilizing agent in the synthesis of different classes of nanomaterials.

#### **Acknowledgements**

Z. Tshemese, S. Mlowe, N. Revaprasadu and S.C. Masikane wishes to thank the National Research Foundation—South Africa (NRF) for financial support.

#### **Conflict of interest**

We declare no conflict of interest.

#### **Author details**

Zikhona Tshemese1 , Siphamandla C. Masikane1 , Sixberth Mlowe1,2\* and Neerish Revaprasadu1

\*Address all correspondence to: sixb2809@gmail.com


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**Section 3**

**State of the Art Polymerization**

**State of the Art Polymerization**

**Chapter 5**

**Provisional chapter**

**Ionic Polymerization in Ionic Liquids**

**Ionic Polymerization in Ionic Liquids**

DOI: 10.5772/intechopen.77183

Ionic liquids have emerged as a new class of solvents for ionic polymerization due to their low volatility, chemical stability, high conductivity, wide electrochemical window. The advantages and limitations of application of ionic liquids as solvents for ionic polymerization processes are critically discussed in this chapter. The field of cationic polymerization in ionic liquid has undergone rapid growth in recent years. The most important types of cationic monomers, such as styrene and its derivatives, vinyl ethers and isobutylene have been polymerized in ionic liquids; even undergo living polymerization. Corresponding elementary reactions of cationic polymerization in ionic liquids were proposed. Methyl methacrylate and styrene can undergo anionic polymerization in ionic liquids. However, ionic liquids seem unsuitable solvents for anionic polymerization. **Keywords:** ionic liquid, cationic polymerization, anionic polymerization, elementary

Ionic liquids are organic salts, and their physical and chemical properties can be fine-tuned by selection of the cation and anion. The most significant properties of ionic liquids are their negligible vapor pressure. So, ionic liquids have been recognized as green solvents alternative to volatile organic solvents. Application of ionic liquids in chemical processes has blossomed within the last decade. Although radical polymerization, electrochemical polymerization, and polycondensation in ionic liquids have been investigated by many researchers, there has been little study on the application of ionic liquids in ionic polymerization. Ionic liquids are regarded as highly polar but non-coordinating solvents, enabling them as ideal solvents for

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77183

reactions, ionic environment

Yibo Wu

Yibo Wu

**Abstract**

**1. Introduction**

ionic polymerization.

#### **Ionic Polymerization in Ionic Liquids Ionic Polymerization in Ionic Liquids**

#### Yibo Wu Yibo Wu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77183

#### **Abstract**

Ionic liquids have emerged as a new class of solvents for ionic polymerization due to their low volatility, chemical stability, high conductivity, wide electrochemical window. The advantages and limitations of application of ionic liquids as solvents for ionic polymerization processes are critically discussed in this chapter. The field of cationic polymerization in ionic liquid has undergone rapid growth in recent years. The most important types of cationic monomers, such as styrene and its derivatives, vinyl ethers and isobutylene have been polymerized in ionic liquids; even undergo living polymerization. Corresponding elementary reactions of cationic polymerization in ionic liquids were proposed. Methyl methacrylate and styrene can undergo anionic polymerization in ionic liquids. However, ionic liquids seem unsuitable solvents for anionic polymerization.

DOI: 10.5772/intechopen.77183

**Keywords:** ionic liquid, cationic polymerization, anionic polymerization, elementary reactions, ionic environment

#### **1. Introduction**

Ionic liquids are organic salts, and their physical and chemical properties can be fine-tuned by selection of the cation and anion. The most significant properties of ionic liquids are their negligible vapor pressure. So, ionic liquids have been recognized as green solvents alternative to volatile organic solvents. Application of ionic liquids in chemical processes has blossomed within the last decade. Although radical polymerization, electrochemical polymerization, and polycondensation in ionic liquids have been investigated by many researchers, there has been little study on the application of ionic liquids in ionic polymerization. Ionic liquids are regarded as highly polar but non-coordinating solvents, enabling them as ideal solvents for ionic polymerization.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Ionic polymerization is referred to as a classsic cationic when the active terminal group is positively charged, or as a pseudocationic if this group forms the positive end of active dipole. By the same token we refer to ionic polymerization as a classic anionic when the charge of the active group is negative, or as a pseudoanionic when the active group forms the negative end of an active dipole. Whenever electrically charged end-groups are formed, suitable counterions have to be present in the polymerizing system to ensure its electric neutrality. So, the counterions chaperon the growing active to form ion-pairs during the initiation and propagation reaction. These ion-pairs usually exist in three thermodynamically distinct forms, which are referred to as tight ion-pairs, loose ion-pairs and free ion-pairs, and plays an important role for determination of polymerization characteristic. For example, in cationic polymerization, if this counterion is too nucleophilic, it will attack the carbenium ion to form a covalently bond, which in principle does not initiate to cationic monomers. So, the ion environment of ionic liquid must affect balance of ion pairs owing to the high polarity and high charge density, and then influence polymerization process.

polymerizations have great theoretical and practical significance for the development of environment-friendly and low-energy consumption. The most important monomers polymerizing through cationic mechanism are styrene and its derivatives, vinyl ethers and isobutylene (IB). The commonness of these cationic monomers is that the active species can be stabilized by the substituents on the olefinic group. And the way of stabilization of the corresponding carbocation are shown in **Table 1** [1]. We discuss cationic polymerization of

Styrene is a well-known, commercially available vinyl monomer that undergoes polymerization via cationic as well as radical, anionic, and coordination pathways. More recently, developed air- and water-stable neutral ionic liquids (for instance, 1-butyl-3-methylimidazoliumhexafluoro-

polymerization of styrene in N-butyl-N-methylpyrrolidiniumbis-(trifluoromethanesulfonyl)

]) [2–4] and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)-

Ionic Polymerization in Ionic Liquids http://dx.doi.org/10.5772/intechopen.77183 95

])) [5] have been applied in cationic polymerization. Moreover, cationic

]) [6] ionic liquid with organoborate acids as initiators indicates some living/

these monomers in ionic liquids respectively in following sections.

**Table 1.** Cationic monomers and stabilization of the corresponding active species.

**2.1. Styrene and its derivatives**

phospate ([BMIM][PF6

controlled characteristics.

amide ([P6,6,6,14][NTf2

amide ([P14][NTf2

Moreover, properties of ionic liquids such as melting point, polarity, viscosity, and solubility of monomers, which can be fine-tuned by the adequate selection of the cation and anion constituents, also significantly affect the feasibility and regularity of ionic polymerization. High polarity of ionic liquid contributes to stabilize the cationic active center, but it also exhibits the low solubility for cationic monomers. Thus, desirable ionic liquids hope to achieve polar and non-polar balance scale, which can provide a moderately polar environment to solubilize cationic monomers and a polar environment to stabilize carbocation. In this chapter, the mechanism of polymerization, advantages and limitations of application of ionic liquids as solvents for ionic polymerization processes are critically discussed.

#### **2. Cationic polymerization in ionic liquids**

Cationic polymerization is an important technique to produce (co)polymers with predictable molecular weight and monomer sequence. Especially, living/controlled cationic polymerization represents an attractive technique for the synthesis of well-defined polymers, such as telechelic, star polymer, graft copolymer, etc. Industrialization products, such as bromide functionalized poly(isobutylene-*co*-*p*-methylstyrene) random copolymer, butyl rubber and poly (styrene-*b*-isobutylene-*b*-styrene) thermoplastic elastomer, play an important role in the tire industry, coronary stent, chewing gum and other fields. However, during the preparation of these products by traditional cationic polymerization, halogenated alkane (e.g., methyl chloride) usually used as solvents and cause environmental pollution due to its toxicity, volatile and corrosively. So it should be replace by green solvents. In addition, various Lewis acids (e.g., BF3 , SnCl4 , TiCl4 , AlCl3 OBu2 , and AlCl3 ) were used as coinitiator in cationic polymerization. It is difficult to separate these Lewis acid catalysts from the reaction products, and reuse/ disposal of these catalysts is also a big challenge to industry.

Ionic liquid is regarded as the ideal medium of cationic polymerization, which can be recycled and no pollution to the environment. So, applications of ionic liquids in cationic polymerizations have great theoretical and practical significance for the development of environment-friendly and low-energy consumption. The most important monomers polymerizing through cationic mechanism are styrene and its derivatives, vinyl ethers and isobutylene (IB). The commonness of these cationic monomers is that the active species can be stabilized by the substituents on the olefinic group. And the way of stabilization of the corresponding carbocation are shown in **Table 1** [1]. We discuss cationic polymerization of these monomers in ionic liquids respectively in following sections.

#### **2.1. Styrene and its derivatives**

Ionic polymerization is referred to as a classsic cationic when the active terminal group is positively charged, or as a pseudocationic if this group forms the positive end of active dipole. By the same token we refer to ionic polymerization as a classic anionic when the charge of the active group is negative, or as a pseudoanionic when the active group forms the negative end of an active dipole. Whenever electrically charged end-groups are formed, suitable counterions have to be present in the polymerizing system to ensure its electric neutrality. So, the counterions chaperon the growing active to form ion-pairs during the initiation and propagation reaction. These ion-pairs usually exist in three thermodynamically distinct forms, which are referred to as tight ion-pairs, loose ion-pairs and free ion-pairs, and plays an important role for determination of polymerization characteristic. For example, in cationic polymerization, if this counterion is too nucleophilic, it will attack the carbenium ion to form a covalently bond, which in principle does not initiate to cationic monomers. So, the ion environment of ionic liquid must affect balance of ion pairs owing to the high polarity and high charge den-

Moreover, properties of ionic liquids such as melting point, polarity, viscosity, and solubility of monomers, which can be fine-tuned by the adequate selection of the cation and anion constituents, also significantly affect the feasibility and regularity of ionic polymerization. High polarity of ionic liquid contributes to stabilize the cationic active center, but it also exhibits the low solubility for cationic monomers. Thus, desirable ionic liquids hope to achieve polar and non-polar balance scale, which can provide a moderately polar environment to solubilize cationic monomers and a polar environment to stabilize carbocation. In this chapter, the mechanism of polymerization, advantages and limitations of application of ionic liquids as

Cationic polymerization is an important technique to produce (co)polymers with predictable molecular weight and monomer sequence. Especially, living/controlled cationic polymerization represents an attractive technique for the synthesis of well-defined polymers, such as telechelic, star polymer, graft copolymer, etc. Industrialization products, such as bromide functionalized poly(isobutylene-*co*-*p*-methylstyrene) random copolymer, butyl rubber and poly (styrene-*b*-isobutylene-*b*-styrene) thermoplastic elastomer, play an important role in the tire industry, coronary stent, chewing gum and other fields. However, during the preparation of these products by traditional cationic polymerization, halogenated alkane (e.g., methyl chloride) usually used as solvents and cause environmental pollution due to its toxicity, volatile and corrosively. So it should be replace by green solvents. In addition, various Lewis acids

tion. It is difficult to separate these Lewis acid catalysts from the reaction products, and reuse/

Ionic liquid is regarded as the ideal medium of cationic polymerization, which can be recycled and no pollution to the environment. So, applications of ionic liquids in cationic

) were used as coinitiator in cationic polymeriza-

sity, and then influence polymerization process.

94 Recent Advances in Ionic Liquids

solvents for ionic polymerization processes are critically discussed.

**2. Cationic polymerization in ionic liquids**

(e.g., BF3

, SnCl4

, TiCl4

, AlCl3

OBu2

disposal of these catalysts is also a big challenge to industry.

, and AlCl3

Styrene is a well-known, commercially available vinyl monomer that undergoes polymerization via cationic as well as radical, anionic, and coordination pathways. More recently, developed air- and water-stable neutral ionic liquids (for instance, 1-butyl-3-methylimidazoliumhexafluorophospate ([BMIM][PF6 ]) [2–4] and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl) amide ([P6,6,6,14][NTf2 ])) [5] have been applied in cationic polymerization. Moreover, cationic polymerization of styrene in N-butyl-N-methylpyrrolidiniumbis-(trifluoromethanesulfonyl) amide ([P14][NTf2 ]) [6] ionic liquid with organoborate acids as initiators indicates some living/ controlled characteristics.

**Table 1.** Cationic monomers and stabilization of the corresponding active species.

In the traditional cationic polymerization of styrene, the lack of strongly electron-donating groups renders the growing carbocation unstable and thus results in side reactions, such as chain transfer accompanied by *β* proton elimination and Friedel–Crafts alkylation on the phenyl ring of the monomer unit. However, the polymerization mechanism of styrene and its derivatives cationic polymerizations in ionic liquids is still vague.

initiating systems. Considering that the effect of polymerization by the initiator, styrene cationic polymerization initiated by water exhibited lower yield and lower *M*n compared with a,a-dimethylbenzyl chloride and 2-chloro-2,4,4-trimethylpentane. It was speculated that the low initiating efficiency might occur by the formation of hydrogen bonding for water in the imidazole-based ionic liquid. Water is more likely to initiate cationic polymerization in

The polymerization rate of styrene and *p*-methylstyrene in ionic liquids rely mainly on solvent polarity and viscosity. We found that initial polymerization rate of styrene in ionic liquid was similar to that in dichloromethane which resulting from interactions between

hand, the high the polarity of solvent give rise to the faster the reaction rate in cationic polymerization; on the other hand, the high viscosity of ionic liquid reduces the monomer diffusion and thus slow down the rate of polymerization. So, the polymerization rate in ionic liquids was similar to that in dichloromethane, which was a consequence of viscosity

Styrene and *p*-methylstyrene cationic polymerizations proceeded in a milder exothermic manner in ionic liquids than in traditional organic solvents. The milder reactions in ionic liquid may be due to the relatively higher heat capacity of the ionic liquid. The relatively higher heat capacity of ionic liquid could absorb more heat during the cationic polymerization.

On basis of terminal structure and kinetics of polymerization, we proposed the corresponding elementary reactions of styrene and *p*-methylstyrene cationic polymerization in ionic liquids,

**Figure 2.** Temperature changes vs. time plots for cationic polymerization of styrene with CumCl/Lewis acid initiating system in ionic liquid and dichloromethane at −15°C operating temperature, (black square) dichloromethane, (red circle)

] ionic liquid had a consider-

Ionic Polymerization in Ionic Liquids http://dx.doi.org/10.5772/intechopen.77183 97

<sup>T</sup> as compared to dichloromethane. On the one

viscosity and polarity factors of ionic liquids. The [bmim][PF6

ably higher normalized solvent polarity EN

as shown in **Figures 1** and **2**, respectively.

and polarity of ionic liquid.

[Bmim][PF6

].

organic solvents.

Usually, the low solubilities of the monomers in ionic liquids impede the efficiency of these polymerization reactions. In order to search for suitable ionic liquids for styrene and its derivatives, we firstly screened and selected target ionic liquids by quantum chemically based computations (the COSMO-RS method) [7]. The COSMO-RS method was successfully used to screen potential ionic liquids as solvents with respect to the solubility of *p*-methylstyrene used in cationic polymerization (**Figure 1**). We also have demonstrated that COSMO-RS is a valuable tool for the preliminary screening of solvents for cationic monomers without the need for extensive experimental data. The guiding principle of the selection of suitable ionic liquids for use in cationic polymerization was obtained from COSMO-RS calculations. The monomer solubilities in ionic liquids are highly dependent on physical and chemical properties which determine by anion and cation in ionic liquid. Larger nonpolar regions of the cation or anion in ionic liquids result in higher monomer solubility for cationic monomers, for example, longer alkyl chains of the cation or anion contributes to higher *p*-methylstyrene solubilities in imidazolium-based ionic liquids.

It is instructive to compare the organic solvents and ionic liquids in cationic polymerizations. So, we comprehensively compared the cationic polymerizations of styrene [8] and *p*-methylstyrene [9] in ionic liquids with those in organic solvents employing a series of

**Figure 1.** Predicted solubilities of *p*-methylstyrene in 1750 types (50 cations × 35 anions) of ionic liquids.

initiating systems. Considering that the effect of polymerization by the initiator, styrene cationic polymerization initiated by water exhibited lower yield and lower *M*n compared with a,a-dimethylbenzyl chloride and 2-chloro-2,4,4-trimethylpentane. It was speculated that the low initiating efficiency might occur by the formation of hydrogen bonding for water in the imidazole-based ionic liquid. Water is more likely to initiate cationic polymerization in organic solvents.

In the traditional cationic polymerization of styrene, the lack of strongly electron-donating groups renders the growing carbocation unstable and thus results in side reactions, such as chain transfer accompanied by *β* proton elimination and Friedel–Crafts alkylation on the phenyl ring of the monomer unit. However, the polymerization mechanism of styrene and its

Usually, the low solubilities of the monomers in ionic liquids impede the efficiency of these polymerization reactions. In order to search for suitable ionic liquids for styrene and its derivatives, we firstly screened and selected target ionic liquids by quantum chemically based computations (the COSMO-RS method) [7]. The COSMO-RS method was successfully used to screen potential ionic liquids as solvents with respect to the solubility of *p*-methylstyrene used in cationic polymerization (**Figure 1**). We also have demonstrated that COSMO-RS is a valuable tool for the preliminary screening of solvents for cationic monomers without the need for extensive experimental data. The guiding principle of the selection of suitable ionic liquids for use in cationic polymerization was obtained from COSMO-RS calculations. The monomer solubilities in ionic liquids are highly dependent on physical and chemical properties which determine by anion and cation in ionic liquid. Larger nonpolar regions of the cation or anion in ionic liquids result in higher monomer solubility for cationic monomers, for example, longer alkyl chains of the cation or anion contributes to higher *p*-methylstyrene solubilities in

It is instructive to compare the organic solvents and ionic liquids in cationic polymerizations. So, we comprehensively compared the cationic polymerizations of styrene [8] and *p*-methylstyrene [9] in ionic liquids with those in organic solvents employing a series of

**Figure 1.** Predicted solubilities of *p*-methylstyrene in 1750 types (50 cations × 35 anions) of ionic liquids.

derivatives cationic polymerizations in ionic liquids is still vague.

imidazolium-based ionic liquids.

96 Recent Advances in Ionic Liquids

The polymerization rate of styrene and *p*-methylstyrene in ionic liquids rely mainly on solvent polarity and viscosity. We found that initial polymerization rate of styrene in ionic liquid was similar to that in dichloromethane which resulting from interactions between viscosity and polarity factors of ionic liquids. The [bmim][PF6 ] ionic liquid had a considerably higher normalized solvent polarity EN <sup>T</sup> as compared to dichloromethane. On the one hand, the high the polarity of solvent give rise to the faster the reaction rate in cationic polymerization; on the other hand, the high viscosity of ionic liquid reduces the monomer diffusion and thus slow down the rate of polymerization. So, the polymerization rate in ionic liquids was similar to that in dichloromethane, which was a consequence of viscosity and polarity of ionic liquid.

Styrene and *p*-methylstyrene cationic polymerizations proceeded in a milder exothermic manner in ionic liquids than in traditional organic solvents. The milder reactions in ionic liquid may be due to the relatively higher heat capacity of the ionic liquid. The relatively higher heat capacity of ionic liquid could absorb more heat during the cationic polymerization.

On basis of terminal structure and kinetics of polymerization, we proposed the corresponding elementary reactions of styrene and *p*-methylstyrene cationic polymerization in ionic liquids, as shown in **Figures 1** and **2**, respectively.

**Figure 2.** Temperature changes vs. time plots for cationic polymerization of styrene with CumCl/Lewis acid initiating system in ionic liquid and dichloromethane at −15°C operating temperature, (black square) dichloromethane, (red circle) [Bmim][PF6 ].

In initiation reactions, ion-pairs of carbocation and counterion were first formed by complexation reactions between coinitiator Lewis acid and initiator. Comparing with cation of ionic liquid, the carbocation in ion-pairs was easier to attack the styrene or *p*-methylstyrene to initiate cationic polymerization. Also, because of anion of ionic liquid was very weakly nucleophilic species, the counterion of ion-pairs was more closely to carbocation and made them approachable to interact with growing active center. Therefore, we assumed that at least one portion of chain termination reactions directly took place toward counterion rather than anion of ionic liquid. So, the anions or cations of the ionic liquids did not participate in any elementary reactions in the whole cationic polymerization. Despite all this, the anion of ionic liquid could stabilize the propagating carbocation active by dispersing the charge of carbocation. But it was still insufficient to stabilize the propagating carbocation to achieve a controlled/living cationic polymerization.

metal halide-based counterion was directly responsible for stereoregulation. Due to the existence interaction between growing carbocation with anion of ionic liquid, the interaction between the growing carbocation and counteranion become weaker. So, sterical hindrance of counteranion was reduced in ionic liquid which led to lower stereoregulation (**Figure 4**).

MacFarlane [5] has successfully demonstrated that the a controlled cationic polymerization

relatively easy to achieve styrene cationic polymerization. The molecular weights of polysty-

with narrow polydispersity and were predominantly syndiotactic. In another study cationic

The cationic polymerization of vinyl ethers under "non-living " conditions, has been known for many years and been used commercially [10]. These polymerizations are characterized by extremely high polymerization rates and the occurrence of chain transfer and termination reactions with the formation of different kinds of unsaturated end-groups. In additional, vinyl ethers are among the most reactive monomers in conventional (dry conditions) cationic polymerization, even more reactive than pMOS. So, chain transfer reaction is more likely to occur in vinyl ethers cationic polymerization. Low temperatures are usually employed in an attempt to reduce side reactions that destroy the propagating centers. In order to meet

**Figure 4.** Proposed mechanism for the cationic polymerization of *p*-methylstyrene with the CumOH/ BF3

] ionic liquid using bis(oxalato)boric acid

] ionic liquid allowed

Ionic Polymerization in Ionic Liquids http://dx.doi.org/10.5772/intechopen.77183 99

]), supercritical CO2

OEt2

initiating

mpyr][NTf2

] ionic liquid increased with decreasing HBOB concentration

in ionic liquid ([bmim][PF6

) was investigated [4]. The only conclusion was that polymeriza-

mpyr][NTf2

of styrene has been achieved in [C4

renes obtained in [C4

**2.2. Vinyl ethers**

system in NTf2

−1 based ionic liquids.

and organic solvent (CH2

(HBOB) as initiators. The hydrophobic nature of the [C4

mpyr][NTf2

Cl2

tion rates and molecular weights were higher than in organic solvent.

polymerization of styrene initiated with AlCl3

The terminal structure of polystyrenes analyzed by 1 H-NMR spectroscopy and MALDI-TOF spectra which clearly indicated that main chain termination reactions in ionic liquid directly took place toward halide-based counterion or toward Friedel-Crafts reaction, rather than β-hydrogen elimination reaction (**Figure 3**).

The sterical hindrance of counteranion influenced the insertion of monomer molecules into the propagating carbocation. Thus, the interaction between propagating carbocation and

**Figure 3.** Polymerization pathway of cationic polymerization of styrene with CumCl/Lewis acid in [bmim][PF6 ].

metal halide-based counterion was directly responsible for stereoregulation. Due to the existence interaction between growing carbocation with anion of ionic liquid, the interaction between the growing carbocation and counteranion become weaker. So, sterical hindrance of counteranion was reduced in ionic liquid which led to lower stereoregulation (**Figure 4**).

MacFarlane [5] has successfully demonstrated that the a controlled cationic polymerization of styrene has been achieved in [C4 mpyr][NTf2 ] ionic liquid using bis(oxalato)boric acid (HBOB) as initiators. The hydrophobic nature of the [C4 mpyr][NTf2 ] ionic liquid allowed relatively easy to achieve styrene cationic polymerization. The molecular weights of polystyrenes obtained in [C4 mpyr][NTf2 ] ionic liquid increased with decreasing HBOB concentration with narrow polydispersity and were predominantly syndiotactic. In another study cationic polymerization of styrene initiated with AlCl3 in ionic liquid ([bmim][PF6 ]), supercritical CO2 and organic solvent (CH2 Cl2 ) was investigated [4]. The only conclusion was that polymerization rates and molecular weights were higher than in organic solvent.

#### **2.2. Vinyl ethers**

In initiation reactions, ion-pairs of carbocation and counterion were first formed by complexation reactions between coinitiator Lewis acid and initiator. Comparing with cation of ionic liquid, the carbocation in ion-pairs was easier to attack the styrene or *p*-methylstyrene to initiate cationic polymerization. Also, because of anion of ionic liquid was very weakly nucleophilic species, the counterion of ion-pairs was more closely to carbocation and made them approachable to interact with growing active center. Therefore, we assumed that at least one portion of chain termination reactions directly took place toward counterion rather than anion of ionic liquid. So, the anions or cations of the ionic liquids did not participate in any elementary reactions in the whole cationic polymerization. Despite all this, the anion of ionic liquid could stabilize the propagating carbocation active by dispersing the charge of carbocation. But it was still insufficient to stabilize the propagating carbocation to achieve a con-

spectra which clearly indicated that main chain termination reactions in ionic liquid directly took place toward halide-based counterion or toward Friedel-Crafts reaction, rather than

The sterical hindrance of counteranion influenced the insertion of monomer molecules into the propagating carbocation. Thus, the interaction between propagating carbocation and

**Figure 3.** Polymerization pathway of cationic polymerization of styrene with CumCl/Lewis acid in [bmim][PF6

H-NMR spectroscopy and MALDI-TOF

].

trolled/living cationic polymerization.

98 Recent Advances in Ionic Liquids

β-hydrogen elimination reaction (**Figure 3**).

The terminal structure of polystyrenes analyzed by 1

The cationic polymerization of vinyl ethers under "non-living " conditions, has been known for many years and been used commercially [10]. These polymerizations are characterized by extremely high polymerization rates and the occurrence of chain transfer and termination reactions with the formation of different kinds of unsaturated end-groups. In additional, vinyl ethers are among the most reactive monomers in conventional (dry conditions) cationic polymerization, even more reactive than pMOS. So, chain transfer reaction is more likely to occur in vinyl ethers cationic polymerization. Low temperatures are usually employed in an attempt to reduce side reactions that destroy the propagating centers. In order to meet

**Figure 4.** Proposed mechanism for the cationic polymerization of *p*-methylstyrene with the CumOH/ BF3 OEt2 initiating system in NTf2 −1 based ionic liquids.

the requirements of low temperature for vinyl ethers cationic polymerizations, we mainly focused on the low-melting-point ionic liquids, such as [omim][BF4 ] ionic liquid.

In order to further understand the ion environment and its effect on cationic polymerization, we compared the characteristics of IBVE cationic polymerization in organic molecule medium. **Table 2** showed the data of IBVE polymerization in dichloromethane in the same condition as [omim][BF4 ]. Comparing with those obtained in ionic liquid, the yields of poly(IBVE)s obtained in organic molecule medium were lower (~30%). These indicated that IBVE cationic polymerizations were more likely to participate chain transfer and chain termination reactions in organic molecule medium. In addition, The *M*n of poly(IBVE)s obtained in ionic liquid were much higher than that in organic molecule medium. The ionic liquid had a considerably higher normalized solvent polarity EN T as compared to organic molecule medium, such as dichloromethane. Usually, the higher the polarity of solvent gives in cationic polymerization, the higher the *M*n of poly(IBVE) obtained.

Like that of styrene, the cationic polymerization of IBVE in dichloromethane system proceeded in a highly exothermic manner. For example, in IBVE-HCl/TiCl4 initiating system, the exothermic peak reached to 24°C. However, the exothermic peak in [omim][BF<sup>4</sup> ] ionic liquid reached to 9°C with ~90% monomer conversion. The temperature rise and fall periods were slower than that in dichloromethane. IBVE cationic polymerization in ionic liquid proceed in mild exothermic reactions may be due to the relatively higher viscosity and higher heat capacity of the ionic liquids. The high viscosity could slow down the reaction rate; the relatively higher heat capacity could absorb more heat during the catonic polymerization.

The long-lived species were observed in monomer addition experiments. The [omim][BF4

**Figure 5.** The optimized geometries of metal halide-based counterion, propagating carbocation and anion of ionic liquid.

However, [omim][BF4

**2.3. Isobutylene**

widely used in the world.

ionic liquid did not participate elementary reactions during IBVE cationic polymerization.

tion between propagating carbocation and counterion. We want to understand what was the cause of production of the long-lived species. The density functional theory was used to study

According to the geometry, the propagating carbocations of the poly(IBVE)s in [omim][BF4]

It was also noted that the charge on the propagating carbocation in ionic liquid was separated by its interaction with anion of ionic liquid leading to form relative stabilized propagating carbocation. However, these interactions were still insufficient to stabilize the propagating

Among cationic monomers, isobutylene is no doubt the most extensively studied one as it polymerizes only by cationic mechanism. In comparison with silicones or polyphosphazenes, polyisobutylene-based products exhibit such unique properties as chemical resistance, low permeability, good thermal and oxidative stability, mechanical dampening. The high- (*M*<sup>n</sup> > 120,000 g/mol), medium-(*M*<sup>n</sup> = 40,000–100,000 g/mol) and, low-molecular weight (*M*<sup>n</sup> < 5000 g/mol) polyisobutylenes (PIB) have been achieved commercial success and were

Low molecular weight polyisobutylene possessing an exo-olefin terminal group (so-called highly reactive polyisobutylene, HR PIB) is a key intermediate in the manufacturing of

interacted with not only metal halide-based counterions, but also soft Lewis basic BF4

the interactions among propagating active center, counterion and ionic liquid.

carbocation to achieving a controlled polymerization.

] should affect not only the stability of active center but also the interac-

]

Ionic Polymerization in Ionic Liquids http://dx.doi.org/10.5772/intechopen.77183 101

anions.

From reaction kinetics analysis, we found that the first-order plots of ln([M0]/[M]) vs. time were not linear for isobutyl vinyl ether (IBVE) cationic polymerization in [omim][BF4 ] ionic liquid. Analyzing from end microstructure, the side reactions took place by chain-breaking via predominant β-proton elimination from –CH<sup>2</sup> – in the growing carbocation and then by protic reinitiation to create a new polymer chain, resulting in the formation of polymer chains with exo-olefin terminal group. Once 2,6-di-tert-butylpyridine was used; β-proton had been trapped. Thus, the polymerization in ionic liquid exhibited some characteristics of a living/ controlled process (**Figure 5**).


Conditions: Initiator: [IBVE-HCl] = 0.003 M, [IBVE] = 1.04 M, the molar ratio of coinitiator to IBVE-HCl = 16, T = 0°C; Mn(theor) = 34,300 g/mol.

**Table 2.** Cationic polymerizations of IBVE using various coinitiators in ionic liquid and dichloromethane.

**Figure 5.** The optimized geometries of metal halide-based counterion, propagating carbocation and anion of ionic liquid.

The long-lived species were observed in monomer addition experiments. The [omim][BF4 ] ionic liquid did not participate elementary reactions during IBVE cationic polymerization. However, [omim][BF4 ] should affect not only the stability of active center but also the interaction between propagating carbocation and counterion. We want to understand what was the cause of production of the long-lived species. The density functional theory was used to study the interactions among propagating active center, counterion and ionic liquid.

According to the geometry, the propagating carbocations of the poly(IBVE)s in [omim][BF4] interacted with not only metal halide-based counterions, but also soft Lewis basic BF4 anions. It was also noted that the charge on the propagating carbocation in ionic liquid was separated by its interaction with anion of ionic liquid leading to form relative stabilized propagating carbocation. However, these interactions were still insufficient to stabilize the propagating carbocation to achieving a controlled polymerization.

#### **2.3. Isobutylene**

the requirements of low temperature for vinyl ethers cationic polymerizations, we mainly

In order to further understand the ion environment and its effect on cationic polymerization, we compared the characteristics of IBVE cationic polymerization in organic molecule medium. **Table 2** showed the data of IBVE polymerization in dichloromethane in the same condition

obtained in organic molecule medium were lower (~30%). These indicated that IBVE cationic polymerizations were more likely to participate chain transfer and chain termination reactions in organic molecule medium. In addition, The *M*n of poly(IBVE)s obtained in ionic liquid were much higher than that in organic molecule medium. The ionic liquid had a considerably

dichloromethane. Usually, the higher the polarity of solvent gives in cationic polymerization,

Like that of styrene, the cationic polymerization of IBVE in dichloromethane system pro-

reached to 9°C with ~90% monomer conversion. The temperature rise and fall periods were slower than that in dichloromethane. IBVE cationic polymerization in ionic liquid proceed in mild exothermic reactions may be due to the relatively higher viscosity and higher heat capacity of the ionic liquids. The high viscosity could slow down the reaction rate; the relatively

From reaction kinetics analysis, we found that the first-order plots of ln([M0]/[M]) vs. time

liquid. Analyzing from end microstructure, the side reactions took place by chain-breaking

protic reinitiation to create a new polymer chain, resulting in the formation of polymer chains with exo-olefin terminal group. Once 2,6-di-tert-butylpyridine was used; β-proton had been trapped. Thus, the polymerization in ionic liquid exhibited some characteristics of a living/

**Entry Solvent Coinitiator Time (min) Conv. (%) Mn Mw Mw/Mn**

2 Dichloromethane SnCl4 2.5 27 16,490 31,500 1.91 3 Dichloromethane TiCl4 2.5 29 13,200 24,000 1.82

Cl3 1.5 20 21,880 40,700 1.86

Cl3 10.0 82 38,010 65,380 1.72

] SnCl4 15.0 76 21,070 38,140 1.81

] TiCl4 15.0 83 18,140 36,600 2.02

Conditions: Initiator: [IBVE-HCl] = 0.003 M, [IBVE] = 1.04 M, the molar ratio of coinitiator to IBVE-HCl = 16, T = 0°C;

**Table 2.** Cationic polymerizations of IBVE using various coinitiators in ionic liquid and dichloromethane.

were not linear for isobutyl vinyl ether (IBVE) cationic polymerization in [omim][BF4

]. Comparing with those obtained in ionic liquid, the yields of poly(IBVE)s

] ionic liquid.

initiating system, the

] ionic liquid

] ionic

T as compared to organic molecule medium, such as

– in the growing carbocation and then by

focused on the low-melting-point ionic liquids, such as [omim][BF4

ceeded in a highly exothermic manner. For example, in IBVE-HCl/TiCl4

exothermic peak reached to 24°C. However, the exothermic peak in [omim][BF<sup>4</sup>

higher heat capacity could absorb more heat during the catonic polymerization.

as [omim][BF4

100 Recent Advances in Ionic Liquids

higher normalized solvent polarity EN

the higher the *M*n of poly(IBVE) obtained.

via predominant β-proton elimination from –CH<sup>2</sup>

] Al2

Et3

Et3

controlled process (**Figure 5**).

1 Dichloromethane Al2

4 [omim][BF4

5 [omim][BF4

6 [omim][BF4

Mn(theor) = 34,300 g/mol.

Among cationic monomers, isobutylene is no doubt the most extensively studied one as it polymerizes only by cationic mechanism. In comparison with silicones or polyphosphazenes, polyisobutylene-based products exhibit such unique properties as chemical resistance, low permeability, good thermal and oxidative stability, mechanical dampening. The high- (*M*<sup>n</sup> > 120,000 g/mol), medium-(*M*<sup>n</sup> = 40,000–100,000 g/mol) and, low-molecular weight (*M*<sup>n</sup> < 5000 g/mol) polyisobutylenes (PIB) have been achieved commercial success and were widely used in the world.

Low molecular weight polyisobutylene possessing an exo-olefin terminal group (so-called highly reactive polyisobutylene, HR PIB) is a key intermediate in the manufacturing of motor oil and fuel additives, with worldwide production in excess of 750,000 tons per year. The cationic polymerizations of HR PIB in ionic liquids were described only in a patent literatures [11–14] and only limited information was available. The use of acidic chloroaluminate ionic liquids as catalysts for low molecular weight polyisobutylene (*M*<sup>n</sup> = 1000 g/mol) in n-heptane should be particularly noteworthy. But, unfortunately, little information about the molecular weight distribution and microstructure of polyisobutylene oligomers was available. Recently, Kostjuk [15] has present new catalysts for the synthesis of HR PIB based on the combination of a chloroaluminate ionic liquids and diisopropyl ether, which allow to synthesize PIBs with a high content of exo-olefin end groups (≥90%) and relatively narrow MWD (Mw/Mn ≤ 2.0) (**Figure 6**).

**3. Anionic polymerization in ionic liquids**

Generally, anionic polymerizations need strict dehydration and oxygen-free conditions. Ionic liquids can be easy to dry under vacuum at high temperature due to their negligible volatility [17, 18]. So, cumbersome handling procedures for conventional volatile solvent such as distillation in the presence of drying agents was no need for ionic liquid. In order to understand mechanism and characteristics of anionic polymerization in ionic liquids, the methyl methacrylate (MMA) anionic polymerizations were carried out in ionic liquids by using alkyl lithium initiators such as n-butyllithium (n-BuLi) and diphenylhexyl lithium (DPHLi). The results were compared with those obtained for polymerization reactions in conventional solvents such as tetrahydrofuran (THF) and toluene. The MMA anionic polymerization in [NTf2

based ionic liquids did not yield polymers because the initiator were be deactivated by attacking on the trifluoromethyl group. However, as compared with in tetrahydrofuran, the MMA

mim][PF6

sion. The reaction between the initiator and the imidazolium cation and high polymerization

ium initiator withdrawn hydrogen atom at the imidazolium ring, which was the main cause

Another side-reaction in anionic MMA polymerization initiated by alkyl lithium initiators was observed by Kubisa [19]. Analyzing the terminal structure by MALDI-TOF, it was found that chain transfers to ionic liquids were easy to occur at the early stages of propagation polymerization. The chain transfer reaction in ionic liquid is shown in **Figure 7**. Although MMA anionic polymerization put a limit on molecular weights (Mn < 2000) in ionic liquid, all the PMMAs contained ionic end-groups derived from ionic liquids, which

in mm triads. These results indicated that the propagation carbocation of PMMA in [C4

]

103

] was rich

mim]

] ionic liquid with lower monomer conver-

H-NMR spectrum, the alkyl lith-

mim][PF6

Ionic Polymerization in Ionic Liquids http://dx.doi.org/10.5772/intechopen.77183

**3.1. Methyl methacrylate**

anionic polymerization proceeded in [C4

should be pay enough attention.

[PF6

temperature resulted in lower yields. Analyzing from the 1

] has a similar terminal structure with that in toluene.

of deactivating initiator. The tacticity of PMMA initiating by DPHLi in [C4

**Figure 7.** Chain transfer to ionic liquid in anionic polymerization of methyl methacrylate.

Indeed, we did not find suitable ionic liquids for isobutylene cationic polymerization to synthesize the high-, medium-*M*n polyisobutylene. Generally, isobutylene is hard to dissolve in ionic liquids. The number of possible combinations of anions and cations in ionic liquids are very numerous. The properties of ionic liquids can be fine-tuned by selection of the cation and anion. It is difficult to discuss their properties in general because their properties depend on the structure of cation and anion. So, in order to screen the potential neutral ionic liquid solvent for isobutylene cationic polymerization, we used density functional theory calculations to investigate the inter-ionic interactions of ionic liquids and the interactions of ionic liquids with isobutylene [16]. The geometry was explained by the change of total energy, inter-molecular distance and ESP charge. The most stable gas-phase structures of ion pairs (IPs) and IPs-IB indicated that hydrogen bonding with the C2-hydrogen on the imidazole ring played a dominating role in the formation of IPs. The addition of IB did not change the dominant interactions of IPs. Compared with previous literature, the dissolution mechanism of IB in ionic liquids is that IB molecules occupy the free space of the cavities which are primarily created by small angular rearrangements of the anions. The potential solvent for IB polymerization is the ionic liquid with weaker interactions of anion and ion pair with IB. This work was motivated by the selection of ionic liquids as polymerization solvents. This study will also provide a broad range for future studies on cationic polymerizations in ionic liquids.

**Figure 6.** The electrostatic potentials (ESP) surface of isobutylene (IB, a) and [Bmim] + (b).

#### **3. Anionic polymerization in ionic liquids**

#### **3.1. Methyl methacrylate**

motor oil and fuel additives, with worldwide production in excess of 750,000 tons per year. The cationic polymerizations of HR PIB in ionic liquids were described only in a patent literatures [11–14] and only limited information was available. The use of acidic chloroaluminate ionic liquids as catalysts for low molecular weight polyisobutylene (*M*<sup>n</sup> = 1000 g/mol) in n-heptane should be particularly noteworthy. But, unfortunately, little information about the molecular weight distribution and microstructure of polyisobutylene oligomers was available. Recently, Kostjuk [15] has present new catalysts for the synthesis of HR PIB based on the combination of a chloroaluminate ionic liquids and diisopropyl ether, which allow to synthesize PIBs with a high content of exo-olefin end groups (≥90%) and relatively narrow

Indeed, we did not find suitable ionic liquids for isobutylene cationic polymerization to synthesize the high-, medium-*M*n polyisobutylene. Generally, isobutylene is hard to dissolve in ionic liquids. The number of possible combinations of anions and cations in ionic liquids are very numerous. The properties of ionic liquids can be fine-tuned by selection of the cation and anion. It is difficult to discuss their properties in general because their properties depend on the structure of cation and anion. So, in order to screen the potential neutral ionic liquid solvent for isobutylene cationic polymerization, we used density functional theory calculations to investigate the inter-ionic interactions of ionic liquids and the interactions of ionic liquids with isobutylene [16]. The geometry was explained by the change of total energy, inter-molecular distance and ESP charge. The most stable gas-phase structures of ion pairs (IPs) and IPs-IB indicated that hydrogen bonding with the C2-hydrogen on the imidazole ring played a dominating role in the formation of IPs. The addition of IB did not change the dominant interactions of IPs. Compared with previous literature, the dissolution mechanism of IB in ionic liquids is that IB molecules occupy the free space of the cavities which are primarily created by small angular rearrangements of the anions. The potential solvent for IB polymerization is the ionic liquid with weaker interactions of anion and ion pair with IB. This work was motivated by the selection of ionic liquids as polymerization solvents. This study will also provide a broad range for future studies on cationic polymer-

MWD (Mw/Mn ≤ 2.0) (**Figure 6**).

102 Recent Advances in Ionic Liquids

izations in ionic liquids.

**Figure 6.** The electrostatic potentials (ESP) surface of isobutylene (IB, a) and [Bmim] + (b).

Generally, anionic polymerizations need strict dehydration and oxygen-free conditions. Ionic liquids can be easy to dry under vacuum at high temperature due to their negligible volatility [17, 18]. So, cumbersome handling procedures for conventional volatile solvent such as distillation in the presence of drying agents was no need for ionic liquid. In order to understand mechanism and characteristics of anionic polymerization in ionic liquids, the methyl methacrylate (MMA) anionic polymerizations were carried out in ionic liquids by using alkyl lithium initiators such as n-butyllithium (n-BuLi) and diphenylhexyl lithium (DPHLi). The results were compared with those obtained for polymerization reactions in conventional solvents such as tetrahydrofuran (THF) and toluene. The MMA anionic polymerization in [NTf2 ] based ionic liquids did not yield polymers because the initiator were be deactivated by attacking on the trifluoromethyl group. However, as compared with in tetrahydrofuran, the MMA anionic polymerization proceeded in [C4 mim][PF6 ] ionic liquid with lower monomer conversion. The reaction between the initiator and the imidazolium cation and high polymerization temperature resulted in lower yields. Analyzing from the 1 H-NMR spectrum, the alkyl lithium initiator withdrawn hydrogen atom at the imidazolium ring, which was the main cause of deactivating initiator. The tacticity of PMMA initiating by DPHLi in [C4 mim][PF6 ] was rich in mm triads. These results indicated that the propagation carbocation of PMMA in [C4 mim] [PF6 ] has a similar terminal structure with that in toluene.

Another side-reaction in anionic MMA polymerization initiated by alkyl lithium initiators was observed by Kubisa [19]. Analyzing the terminal structure by MALDI-TOF, it was found that chain transfers to ionic liquids were easy to occur at the early stages of propagation polymerization. The chain transfer reaction in ionic liquid is shown in **Figure 7**. Although MMA anionic polymerization put a limit on molecular weights (Mn < 2000) in ionic liquid, all the PMMAs contained ionic end-groups derived from ionic liquids, which should be pay enough attention.

**Figure 7.** Chain transfer to ionic liquid in anionic polymerization of methyl methacrylate.

#### **3.2. Styrene**

Styrene anionic polymerization initiated by butyllithium (BuLi) or sodium acetate (NaAc) in phosphonium-based ionic liquids has been reported by MacFarlane [20]. Although relatively high initiator concentration was used, polystyrene yield was still lower (10 ∼ 20%). Yields could be improved by addition of butyl imidazolium butane sulfonate zwitterion. Moreover, molecular weights kept to high (up to 400,000) with molecular weight distribution in the range of 1.4–2.1. This indicated that phosphonium-based ionic liquids were more suitable as solvents for styrene anionic polymerization than imidazolium-based ionic liquids.

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[2] Biedroñ T, Kubisa P. Cationic polymerization of styrene in a neutral ionic liquid. Journal of Polymer Science, Part A: Polymer Chemistry. 2004;**42**:3230-3235. DOI: 10.1002/pola.

[3] Baœko M, Biedroñ T, Kubisa P. Polymerization processes in ionic liquids. Cationic polymerization of styrene. Macromolecular Symposia. 2006;**240**:107-113. DOI: 10.1002/

[4] Bueno C, Cabral V, Cardozo-Filho L, Dias M, Antunes O. Cationic polymerization of styrene in scCO2 and [bmim][PF6]. Journal of Supercritical Fluids. 2009;**48**:183-187. DOI:

[5] Vijayaraghavan R, MacFarlane D. Organoborate acids as initiators for cationic polymerization of styrene in an ionic liquid medium. Macromolecules. 2007;**40**:6515-6520. DOI:

[6] Vijayaraghavan R, Macfarlane DR. Living cationic polymerisation of styrene in an ionic

[7] Zhang XQ, Guo WL, Wu YB, Li W, Li SX, Shang YW, Zhang JH. Solubility of monomers for chain polymerization in ionic liquids predicted by the conductor-like screening model for real solvents. Industrial and Engineering Chemistry Research. 2017;**56**:14694-

[8] Han L, Wu YB, Yang D, Wang H, Zhang XQ, Wei XL, Guo WL, Li SX. Characteristics and mechanism of styrene cationic polymerization in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. RSC Advances. 2016;**6**:105322-105330. DOI: 10.1039/

[9] Zhang XQ, Guo WL, Wu YB, Gong LF, Li W, Li XN, Li SX, Shang YW, Yang D, Wang H. Cationic polymerization of p-methylstyrenein selected ionic liquids and polymeriza-

tionmechanism. Polymer Chemistry. 2016;**7**:5099-5112. DOI: 10.1039/c6py00796a [10] Wu YB, Han L, Zhang XQ, Mao J, Gong LF, Guo WL, Gu K, Li SX. Cationic polymerization of isobutyl vinyl ether in an imidazole-based ionic liquid: Characteristics and

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[12] Abdul-Sada A, Ambler PW, Hodgson PKG, Seddon KR, Stewart NJ. (BP Chem. Ltd)

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liquid. Chemical Communications. 2004;**6**:700-701. DOI: 10.1039/b315100j

**32**(2):220-246. DOI: 10.1016/j.progpolymsci.2007.01.001

#### **4. Conclusion**

In recent years, an enormous progress has been made in the development of ionic polymerizations in ionic liquid. The polymerization reactions for cationic and anionic monomers were realized in ionic liquids. Some cationic polymerizations in ionic liquid indicated some living/ controlled characteristics. However, only the control of molecular weight and polydispersity have been achieved. Telechelic polymers, macromonomers, block copolymers, polymer networks, star-shaped polymers have not been described in ionic liquids. So, in development of mechanism of living cationic polymerization, scientists need to strengthen scientific research further. Ionic liquids seem unsuitable solvents for anionic polymerization. Is it possible to find suitable ionic liquids for anionic polymerization by "designer-solvents"? The further theoretical studies are needed and the guiding roles of the relevant theories should be brought into full play.

#### **Acknowledgements**

This work was supported by the National Science Foundation of China (51573020), Beijing Natural Science Foundation (2172022).

#### **Conflict of interest**

The authors report no conflicts of interest in this work.

#### **Author details**

Yibo Wu

Address all correspondence to: wuyibo@bipt.edu.cn

Department of Material Science and Engineering, Beijing Key Lab of Special Elastomeric Composite Materials, Beijing Institute of Petrochemical Technology, Beijing, China

#### **References**

**3.2. Styrene**

104 Recent Advances in Ionic Liquids

**4. Conclusion**

**Acknowledgements**

**Conflict of interest**

**Author details**

Yibo Wu

Natural Science Foundation (2172022).

The authors report no conflicts of interest in this work.

Address all correspondence to: wuyibo@bipt.edu.cn

Styrene anionic polymerization initiated by butyllithium (BuLi) or sodium acetate (NaAc) in phosphonium-based ionic liquids has been reported by MacFarlane [20]. Although relatively high initiator concentration was used, polystyrene yield was still lower (10 ∼ 20%). Yields could be improved by addition of butyl imidazolium butane sulfonate zwitterion. Moreover, molecular weights kept to high (up to 400,000) with molecular weight distribution in the range of 1.4–2.1. This indicated that phosphonium-based ionic liquids were more suitable as

In recent years, an enormous progress has been made in the development of ionic polymerizations in ionic liquid. The polymerization reactions for cationic and anionic monomers were realized in ionic liquids. Some cationic polymerizations in ionic liquid indicated some living/ controlled characteristics. However, only the control of molecular weight and polydispersity have been achieved. Telechelic polymers, macromonomers, block copolymers, polymer networks, star-shaped polymers have not been described in ionic liquids. So, in development of mechanism of living cationic polymerization, scientists need to strengthen scientific research further. Ionic liquids seem unsuitable solvents for anionic polymerization. Is it possible to find suitable ionic liquids for anionic polymerization by "designer-solvents"? The further theoretical studies are needed and the guiding roles of the relevant theories should be brought into full play.

This work was supported by the National Science Foundation of China (51573020), Beijing

Department of Material Science and Engineering, Beijing Key Lab of Special Elastomeric Composite Materials, Beijing Institute of Petrochemical Technology, Beijing, China

solvents for styrene anionic polymerization than imidazolium-based ionic liquids.


[15] Vasilenko IV, Berezianko IA, Shiman DI, Kostjuk SV. New catalysts for the synthesis of highly reactive polyisobutylene: Chloroaluminate imidazole-based ionic liquids in the presence of diisopropyl ether. Polymer Chemistry. 2016;**7**:5615-5619. DOI: 10.1039/ c6py01325b

**Chapter 6**

Provisional chapter

**Ionic Liquids for Desulphurization: A Review**

DOI: 10.5772/intechopen.79281

The literature survey has shown that not much work has been reported on the interaction mechanism of ionic liquids (ILs) with sulfur in model oil system. In recently published work, the interaction was predicted using COSMO-RS where the strength of hydrogen bond of anion should be reduced in order to increase thiophene extraction capacity. On the other hand, the same researchers also found that the smaller sized cations would lead to higher selectivity, which could lower the capacity and vice versa. While others have reported that the absorption capacity of sulfur compounds in ILs are strongly dependent on the chemical structures, physical properties and compactness between the cation and the anion of the ILs. However, these conclusions lead to a broad selection of ILs for

Keywords: ionic liquids, sulfur compounds, extractive desulphurization, absorption

Within recent years, ILs has gained increasing interest for application to different kinds of processes, amongst those is as separation media for LLE processes. Basically the optimization in LLE process or technique is mostly influenced by the interaction mechanism between the solute and solvent. Therefore, for desulphurization process it is vital to identify the interaction mechanism between sulfur compounds (solute) and ILs (solvent) since the interaction mechanism will determine the extraction efficiency and recycling capability of the ILs. Since the number of conceivable combinations between cations and anions are almost unlimited, and sole experimental screening is impossible, the use of simulation tools becomes important. Since ILs are a relatively new class of compounds, the use of common activity coefficient model for

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Ionic Liquids for Desulphurization: A Review

Syamsul Bahari Abdullah, Hanida Abdul Aziz and

Syamsul Bahari Abdullah, Hanida Abdul Aziz and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79281

extractive desulphurization process.

1. How do ILs interact with sulfur compounds?

Zakaria Man

Zakaria Man

Abstract

capacity


#### **Chapter 6** Provisional chapter

#### **Ionic Liquids for Desulphurization: A Review** Ionic Liquids for Desulphurization: A Review

DOI: 10.5772/intechopen.79281

Syamsul Bahari Abdullah, Hanida Abdul Aziz and Zakaria Man Syamsul Bahari Abdullah, Hanida Abdul Aziz and Zakaria Man

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79281

#### Abstract

[15] Vasilenko IV, Berezianko IA, Shiman DI, Kostjuk SV. New catalysts for the synthesis of highly reactive polyisobutylene: Chloroaluminate imidazole-based ionic liquids in the presence of diisopropyl ether. Polymer Chemistry. 2016;**7**:5615-5619. DOI: 10.1039/

[16] Li XN, Guo WL, Wu YB, Li W, Gong LF, Zhang XQ, Li SX, Shang YW, Yang D, Wang H. Investigation of the interactions between 1-butyl-3-methylimidazolium-based ionic liquids and isobutylene using density functional theory. Journal of Molecular Modeling.

[17] Kubisa P.Ionic liquids as solvents for polymerization processes–progress and challenges. Progress in Polymer Science. 2009;**34**:1333-1347. DOI: 10.1016/j.progpolymsci.2009.09.001

[18] Kokubo H, Watanabe M. Anionic polymerization of methyl methacrylate in an ionic liquid. Polymers for Advanced Technologies. 2008;**19**:1441-1444. DOI: 10.1002/pat.1210

[19] Biedron T, Kubisa P. Chain transfer to ionic liquid in an anionic polymerization of methyl methacrylate. Journal of Polymer Science Part A: Polymer Chemistry. 2007;**45**:4168-4172.

[20] Vijayaraghavan R, Pringle JM, MacFarlane DR. Anionic polymerization of styrene in ionic liquids. European Polymer Journal. 2008;**44**(6):1758-1762. DOI: 10.1016/ j.eurpolymj.2008.

c6py01325b

106 Recent Advances in Ionic Liquids

DOI: 10.1002/pola.22256

02.028

2018;**24**:83. DOI: 10.1007/s00894-018-3586-y

The literature survey has shown that not much work has been reported on the interaction mechanism of ionic liquids (ILs) with sulfur in model oil system. In recently published work, the interaction was predicted using COSMO-RS where the strength of hydrogen bond of anion should be reduced in order to increase thiophene extraction capacity. On the other hand, the same researchers also found that the smaller sized cations would lead to higher selectivity, which could lower the capacity and vice versa. While others have reported that the absorption capacity of sulfur compounds in ILs are strongly dependent on the chemical structures, physical properties and compactness between the cation and the anion of the ILs. However, these conclusions lead to a broad selection of ILs for extractive desulphurization process.

Keywords: ionic liquids, sulfur compounds, extractive desulphurization, absorption capacity

#### 1. How do ILs interact with sulfur compounds?

Within recent years, ILs has gained increasing interest for application to different kinds of processes, amongst those is as separation media for LLE processes. Basically the optimization in LLE process or technique is mostly influenced by the interaction mechanism between the solute and solvent. Therefore, for desulphurization process it is vital to identify the interaction mechanism between sulfur compounds (solute) and ILs (solvent) since the interaction mechanism will determine the extraction efficiency and recycling capability of the ILs. Since the number of conceivable combinations between cations and anions are almost unlimited, and sole experimental screening is impossible, the use of simulation tools becomes important. Since ILs are a relatively new class of compounds, the use of common activity coefficient model for

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

example group contribution method like UNIFAC is complicated, because it requires the input of interaction parameters, which for ILs have not been fully determined thus far. In order to describe the interaction mechanism and behavior of ILs, the dielectric continuum model COSMO-RS has been introduced, and it is gaining more interest for ILs prediction for various applications [1, 2].

calculations are done in a Turbomole program package. The geometries of all molecules involved in these calculations are first fully optimized and the calculations are only performed once for each compound. The result of the COSMO calculation which is the charge distribution on the molecular surface is stored in the so-called COSMO-files, which are collected in the database. COSMO-RS calculations are then performed using COSMOtherm program, which provides an efficient and flexible implementation of the COSMO-RS method. Thus in combination with a large database of solvents including ILs, COSMO-RS allows fast and efficient

Ionic Liquids for Desulphurization: A Review http://dx.doi.org/10.5772/intechopen.79281 109

In COSMO-RS, the bulk of a liquid phase is considered to be built of closely packed molecular cavities, and each molecule is divided into discrete segments, where each segment is assigned with a screening charge density, σi. Then, the interactions between the molecules are reduced to the interactions of the molecular segments, or rather the interactions of the screening charge densities. In order to describe the entire molecule and molecular properties the screening charge density distribution of a molecule, the so called σ-profile as shown in Figure 1 is used [6].

Initially, the assumption has been made that a liquid consists of close packed molecules, as a logical consequence, the properties of this liquid can also be described by means of the σprofiles. Next, based on the σ-profiles, the σ-potential, μ (σ) of a molecule is calculated. The σpotential is the central equation in COSMO-RS where all other equations for the calculation of thermodynamic data are based on. Additionally, electrostatic interactions (Emisfit) and hydrogen bond interactions (EHB) between the molecular surfaces pieces are described in dependence of σ. Therewith, the screening charge distribution profile holds all the information

large scale solvent screening [4–6].

which is necessary for COSMO-RS [5–7].

Figure 1. Screening charge distribution and σ-profile of BT.

#### 1.1. Interaction mechanism in COSMO-RS

COSMO-RS is independent of specific interaction parameters; therefore it is a promising approach for ILs. The name of COSMO-RS is derived from "Conductor-like-Screening-Model" (COSMO) and its extension RS stands for "real solvents". This approach belongs to the class of quantum chemistry of continuum solvation models (CSMs). CSMs are an extension of the basic quantum chemistry where a molecule in solution is described through a quantum chemical calculation of the solute molecule with an approximate representation of the surrounding solvent as a continuum. The solute is treated as if embedded in a dielectric medium via a molecular surface or cavity that is constructed around the molecule [3].

COSMO-RS uses only structural information of the molecules for the priori prediction of activity coefficients and other thermophysical data; thus the program is independent of specific interaction parameters. In COSMO-RS, a number of quantum chemical calculations are combined with statistical thermodynamics in order to enable the prediction of thermodynamic properties without any experimental data [4].

COSMO-RS is a combination of electrostatic theory of locally interacting molecular surface descriptors, which are computed by quantum chemical method (QM) with exact statistical thermodynamics methodology. In other words, it integrates concepts from quantum chemistry, dielectric continuum models, electrostatic forces interactions and statistical thermodynamics. It is based upon information evaluated by QM-COSMO calculations, which describe discrete surface around a molecule embedded in a virtual conductor. It treats a liquid as an ensemble of closely packed ideally screened molecules, where the molecular surface is in close contact with one another. Assuming that each molecule is still enclosed by virtual conductor, the interaction energies of the surface pairs are defined in terms of screening charge densities (SCDs), where σ and σ' of the respective surface segments. The SCDs measure electrostatic screening of the solute molecule by its surrounding and the back-polarization of the solute molecule [2–5].

Meanwhile, the statistical thermodynamic provides a link between the microscopic surface interaction energy and the macroscopic thermodynamic properties of a liquid. Since in COSMO-RS all molecular interactions are viewed as consisting of local pair-wise interactions of surface segments, the statistical averaging can be done in the ensemble of interacting surface pieces. In order to describe the composition of the surface segment ensemble which depends on σ, it is sufficient to consider histograms of the SCDs, the so-called σ-profiles. Such probability distribution gives the relative amount of surface with polarity σ for a molecule [5].

The COSMO-RS prediction that starts with QM-COSMO calculation is performed on the density functional theory (DFT) level, utilizing the BP functional with RI (resolution of identity) approximation and a triple-ζ valence polarized (TZVP) basis set. These QM-COSMO calculations are done in a Turbomole program package. The geometries of all molecules involved in these calculations are first fully optimized and the calculations are only performed once for each compound. The result of the COSMO calculation which is the charge distribution on the molecular surface is stored in the so-called COSMO-files, which are collected in the database. COSMO-RS calculations are then performed using COSMOtherm program, which provides an efficient and flexible implementation of the COSMO-RS method. Thus in combination with a large database of solvents including ILs, COSMO-RS allows fast and efficient large scale solvent screening [4–6].

example group contribution method like UNIFAC is complicated, because it requires the input of interaction parameters, which for ILs have not been fully determined thus far. In order to describe the interaction mechanism and behavior of ILs, the dielectric continuum model COSMO-RS has been introduced, and it is gaining more interest for ILs prediction for various

COSMO-RS is independent of specific interaction parameters; therefore it is a promising approach for ILs. The name of COSMO-RS is derived from "Conductor-like-Screening-Model" (COSMO) and its extension RS stands for "real solvents". This approach belongs to the class of quantum chemistry of continuum solvation models (CSMs). CSMs are an extension of the basic quantum chemistry where a molecule in solution is described through a quantum chemical calculation of the solute molecule with an approximate representation of the surrounding solvent as a continuum. The solute is treated as if embedded in a dielectric medium via a

COSMO-RS uses only structural information of the molecules for the priori prediction of activity coefficients and other thermophysical data; thus the program is independent of specific interaction parameters. In COSMO-RS, a number of quantum chemical calculations are combined with statistical thermodynamics in order to enable the prediction of thermodynamic

COSMO-RS is a combination of electrostatic theory of locally interacting molecular surface descriptors, which are computed by quantum chemical method (QM) with exact statistical thermodynamics methodology. In other words, it integrates concepts from quantum chemistry, dielectric continuum models, electrostatic forces interactions and statistical thermodynamics. It is based upon information evaluated by QM-COSMO calculations, which describe discrete surface around a molecule embedded in a virtual conductor. It treats a liquid as an ensemble of closely packed ideally screened molecules, where the molecular surface is in close contact with one another. Assuming that each molecule is still enclosed by virtual conductor, the interaction energies of the surface pairs are defined in terms of screening charge densities (SCDs), where σ and σ' of the respective surface segments. The SCDs measure electrostatic screening of the solute

molecule by its surrounding and the back-polarization of the solute molecule [2–5].

Meanwhile, the statistical thermodynamic provides a link between the microscopic surface interaction energy and the macroscopic thermodynamic properties of a liquid. Since in COSMO-RS all molecular interactions are viewed as consisting of local pair-wise interactions of surface segments, the statistical averaging can be done in the ensemble of interacting surface pieces. In order to describe the composition of the surface segment ensemble which depends on σ, it is sufficient to consider histograms of the SCDs, the so-called σ-profiles. Such probability distribution gives the relative amount of surface with polarity σ for a molecule [5].

The COSMO-RS prediction that starts with QM-COSMO calculation is performed on the density functional theory (DFT) level, utilizing the BP functional with RI (resolution of identity) approximation and a triple-ζ valence polarized (TZVP) basis set. These QM-COSMO

molecular surface or cavity that is constructed around the molecule [3].

applications [1, 2].

108 Recent Advances in Ionic Liquids

1.1. Interaction mechanism in COSMO-RS

properties without any experimental data [4].

In COSMO-RS, the bulk of a liquid phase is considered to be built of closely packed molecular cavities, and each molecule is divided into discrete segments, where each segment is assigned with a screening charge density, σi. Then, the interactions between the molecules are reduced to the interactions of the molecular segments, or rather the interactions of the screening charge densities. In order to describe the entire molecule and molecular properties the screening charge density distribution of a molecule, the so called σ-profile as shown in Figure 1 is used [6].

Initially, the assumption has been made that a liquid consists of close packed molecules, as a logical consequence, the properties of this liquid can also be described by means of the σprofiles. Next, based on the σ-profiles, the σ-potential, μ (σ) of a molecule is calculated. The σpotential is the central equation in COSMO-RS where all other equations for the calculation of thermodynamic data are based on. Additionally, electrostatic interactions (Emisfit) and hydrogen bond interactions (EHB) between the molecular surfaces pieces are described in dependence of σ. Therewith, the screening charge distribution profile holds all the information which is necessary for COSMO-RS [5–7].

Figure 1. Screening charge distribution and σ-profile of BT.

Activity coefficient at infinite dilution, ln (γ<sup>i</sup> inf) is an important parameter in order to study the deviation from ideal behavior in a mixture of ILs + sulfur compound in hydrocarbon. Basically, it provides information regarding non-ideality of the chosen species in a mixture. The value describes the extreme case in which only solute-solvent interaction contributes to non-ideality that has practical implications in chemical and industrial processes. In the case of desulphurization, it provides information about interaction between solvent, where in this case is ILs (solvent) and solute i.e. sulfur compounds. This is a useful tool for solvent selection for extractive desulphurization process. The separation factor of species to be separated at infinite dilution is sufficient for determining the suitability of an IL as solvent for selective extraction. Experimentally, the activity coefficient at infinite dilution of some ILs in hydrocarbons, polar and non-polar solvents is measured using either gas–liquid chromatography or the dilutor technique [5–8].

• Aromatic ring current effect (i.e., π-π interaction and CH-π interaction) occurs between

Ionic Liquids for Desulphurization: A Review http://dx.doi.org/10.5772/intechopen.79281 111

• Electrostatic field effect (i.e., Columbic interaction) occurs when bonding between the anion and cation of ILs becomes weaker because of their structures (most probably due to the length of substituted alkyl side-chain on the cation), which makes it more easier for

• Hydrogen bonding effect occurs due to the H-bond donation of cation part of ILs to the

The CH-π interaction between the imidazolium cation and aromatic ring of sulfur compound becomes one of the major mechanisms during sulfur extraction as indicated by chemical quantum simulation [10, 11] and NMR observations [12]. By using quantum chemical calculation approach (namely ab initio calculations correlated with experimental results), it was suggested that the positively charged atoms of the imidazolium cation can be the most approachable to the negatively charged atoms of the sulfur compounds, producing a maximal Columbic interaction [13, 14]. On the other hand, the formation of hydrogen bonding between acidic hydrogen of the imidazolium cation and the sulfur compound is weak due to poor Hbond acceptor by the sulfur compound, but becomes stronger with increasing alkyl side-chain length. The anion and dilution effects (the dilution of ILs by sulfur compound insertion) are not the dominant factors in determining the absorption capacity and selectivity of sulfur

Meanwhile, the specific π-π interaction due to aromatic current effects was first predicted between imidazolium cation and sulfur compound (thiophene) using NMR analysis approach [12]. The aromatic current effect is largely affected by the size of the cation itself and the length of alkyl side-chain substituted on the cation. Since then, it was predicted by many researchers that the stronger selective extraction of aromatic sulfur compounds resulted from the π-π

There was also a suggestion that π-π interaction between the unsaturated bonds of sulfur and the imidazole ring leads to the formation of liquid clathrate. Liquid clathrate is a semi-ordered liquid formed by associative interactions between ILs and aromatic sulfur compounds which

interaction between the imidazolium-based ILs and aromatic sulfur ring [17–21].

Figure 2. Possible contributing theories of interaction mechanism in extractive desulphurization by ILs.

aromatic-type-cation of ILs and aromatic sulfur compound.

insertion or interaction of aromatic sulfur compound in/with ILs.

compounds in model oil/imidazolium based-ILs systems [12, 15, 16].

sulfur atom of aromatic sulfur.

• Anion effect. • Dilution effect.

Several thermodynamic models are available such as NRTL and UNIFAC for predicting activity coefficient at infinite dilution, but the accuracy of the measurement needs to be improved in order to enhance the prediction. Besides that, new experimental data are required to generate quantitative interaction parameter, which hinders the use of these models [8]. On the other hand, COSMO-RS is a novel and efficient model for priori prediction of activity coefficient at infinite dilution for a mixture of ILs from thermodynamic aspects as it relies on optimized molecular structure as the only information; no experimental data is needed [9].

The predicted activity coefficient values obtained through COSMO-RS using different or modified parameterization have been done by Banerjee group to predict potential ILs for separation of sulfur compounds (thiophene, BT and DBT), by means of selectivity, capacity and performance index at infinite dilution. In the first study, they selectively screened out 264 suitable ILs (from 24 anions and 11 cations) and found that smaller sized cations have higher selectivity, but lower capacity and vice versa [10]. They identified that for fluorinated anions, the removal of sulfur compound (thiophene) increases with the increase of the van der Waals volume. While a smaller cation with a sterically shielded large anion gave high extraction efficiency. In a second study they screened out 168 suitable ILs based on the permutations of 28 anions and 6 cations, and found that the cation without aromatic ring combined with anions having sterical shielding effect such as thiocyanate, acetate and chloride proved to be the most favorable ILs [11]. However, their predictions were not consistent with the literatures. This shows that COSMO-RS has a limitation to some extent. For example, COSMO-RS may not be able to represent the π-π interaction effect which has resulted in inconsistent result between prediction and experiment. Therefore, there is a need to introduce new predictive approach for selecting appropriate ILs for desulphurization via interaction mechanism.

#### 1.2. Interaction mechanism in extractive desulphurization

ILs consist of complex ions with multiple types of interaction, where each solute molecule will possess somewhat different solute-solvent interactions due to the various acidic, basic, electron donating and electron withdrawing properties. There are several possible contributing mechanism theories in extractive desulphurization as listed in Figure 2.


Activity coefficient at infinite dilution, ln (γ<sup>i</sup>

dilutor technique [5–8].

110 Recent Advances in Ionic Liquids

inf) is an important parameter in order to study the

deviation from ideal behavior in a mixture of ILs + sulfur compound in hydrocarbon. Basically, it provides information regarding non-ideality of the chosen species in a mixture. The value describes the extreme case in which only solute-solvent interaction contributes to non-ideality that has practical implications in chemical and industrial processes. In the case of desulphurization, it provides information about interaction between solvent, where in this case is ILs (solvent) and solute i.e. sulfur compounds. This is a useful tool for solvent selection for extractive desulphurization process. The separation factor of species to be separated at infinite dilution is sufficient for determining the suitability of an IL as solvent for selective extraction. Experimentally, the activity coefficient at infinite dilution of some ILs in hydrocarbons, polar and non-polar solvents is measured using either gas–liquid chromatography or the

Several thermodynamic models are available such as NRTL and UNIFAC for predicting activity coefficient at infinite dilution, but the accuracy of the measurement needs to be improved in order to enhance the prediction. Besides that, new experimental data are required to generate quantitative interaction parameter, which hinders the use of these models [8]. On the other hand, COSMO-RS is a novel and efficient model for priori prediction of activity coefficient at infinite dilution for a mixture of ILs from thermodynamic aspects as it relies on optimized molecular structure as the only information; no experimental data is needed [9].

The predicted activity coefficient values obtained through COSMO-RS using different or modified parameterization have been done by Banerjee group to predict potential ILs for separation of sulfur compounds (thiophene, BT and DBT), by means of selectivity, capacity and performance index at infinite dilution. In the first study, they selectively screened out 264 suitable ILs (from 24 anions and 11 cations) and found that smaller sized cations have higher selectivity, but lower capacity and vice versa [10]. They identified that for fluorinated anions, the removal of sulfur compound (thiophene) increases with the increase of the van der Waals volume. While a smaller cation with a sterically shielded large anion gave high extraction efficiency. In a second study they screened out 168 suitable ILs based on the permutations of 28 anions and 6 cations, and found that the cation without aromatic ring combined with anions having sterical shielding effect such as thiocyanate, acetate and chloride proved to be the most favorable ILs [11]. However, their predictions were not consistent with the literatures. This shows that COSMO-RS has a limitation to some extent. For example, COSMO-RS may not be able to represent the π-π interaction effect which has resulted in inconsistent result between prediction and experiment. Therefore, there is a need to introduce new predictive approach for

ILs consist of complex ions with multiple types of interaction, where each solute molecule will possess somewhat different solute-solvent interactions due to the various acidic, basic, electron donating and electron withdrawing properties. There are several possible contributing mech-

selecting appropriate ILs for desulphurization via interaction mechanism.

1.2. Interaction mechanism in extractive desulphurization

anism theories in extractive desulphurization as listed in Figure 2.

• Dilution effect.

The CH-π interaction between the imidazolium cation and aromatic ring of sulfur compound becomes one of the major mechanisms during sulfur extraction as indicated by chemical quantum simulation [10, 11] and NMR observations [12]. By using quantum chemical calculation approach (namely ab initio calculations correlated with experimental results), it was suggested that the positively charged atoms of the imidazolium cation can be the most approachable to the negatively charged atoms of the sulfur compounds, producing a maximal Columbic interaction [13, 14]. On the other hand, the formation of hydrogen bonding between acidic hydrogen of the imidazolium cation and the sulfur compound is weak due to poor Hbond acceptor by the sulfur compound, but becomes stronger with increasing alkyl side-chain length. The anion and dilution effects (the dilution of ILs by sulfur compound insertion) are not the dominant factors in determining the absorption capacity and selectivity of sulfur compounds in model oil/imidazolium based-ILs systems [12, 15, 16].

Meanwhile, the specific π-π interaction due to aromatic current effects was first predicted between imidazolium cation and sulfur compound (thiophene) using NMR analysis approach [12]. The aromatic current effect is largely affected by the size of the cation itself and the length of alkyl side-chain substituted on the cation. Since then, it was predicted by many researchers that the stronger selective extraction of aromatic sulfur compounds resulted from the π-π interaction between the imidazolium-based ILs and aromatic sulfur ring [17–21].

There was also a suggestion that π-π interaction between the unsaturated bonds of sulfur and the imidazole ring leads to the formation of liquid clathrate. Liquid clathrate is a semi-ordered liquid formed by associative interactions between ILs and aromatic sulfur compounds which

Figure 2. Possible contributing theories of interaction mechanism in extractive desulphurization by ILs.

separate the cation-anion packing interactions to a sufficient degree resulting in the formation of localized cage-structures; in this case ILs are trapping the aromatic sulfur compounds. With too little interaction, the ILs are simply completely miscible or immiscible with the aromatic sulfur compound, whereas, if the cation-anion interaction of ILs are too great, then crystallization of the ILs occurs [22–25]. Since the aromatic sulfur compounds e.g. DBT, BT, 3-methylthiophene are conjugated structure, the lone pairs on the sulfur atom or the π-electrons on the aromatic sulfur compound ring preferentially insert into the molecular structure of the ILs. The steric effect between the interacting compounds also influences the interaction mechanism involved [26, 27].

ILs Experimental description

GC-analysis Extraction condition Speed: 1200 rpm Time: 15 min Vol. Ratio: 1/1

113

Ionic Liquids for Desulphurization: A Review http://dx.doi.org/10.5772/intechopen.79281

> GC–MS and HPLC Extraction condition; Time: 60 min Settling: 15 min Temp.: 40C (equal volume ratio)

[3-mebupy][N(CN)2] [bmim][C(CN)3] [4-mebupy][N(CN)2] [4-mebupy][SCN] [bmim][N(CN)2] [bmim][SCN] [emim][N(CN)2] [omim][BF4] [opy][BF4] [beim][DBP] [bmim][DBP] [eeim][DEP] [hpy][BF4] [omim][DMP] [emim][DEP] [obim][DBP] [beim][DEP] [oeim][DEP] [emim][DMP] [hmim][DMP] [hbim][DBP] [bbim][DBP] [heim][DEP] [bmim][DMP] [bpy][BF4] [mmim][DMP] [emim][DBP] [bmim][BF4]

Hansmeier et al., Green chemistry

[C4mim][BF4] [C4mim][OcSO4] [C4mim][CF3SO3] [C4mim][PF6] [C4mim][NTf2] [C4mim][SCN] [C4mim][CH3CO2] [C4py][NTf2] [C4py][BF4] [C4 4

mpy][NTf2]

mpy][SCN]

mpy][NTf2]

mpy][SCN]

mpy][CF3SO3]

2,4dmpy][NTf2]

2,5dmpy][NTf2] [C4mpyrr][NTf2]

Holbrey et al., Green Chemistry

Table 1. Results of DBT removal using some ILs in extractive desulphurization.

mpy][CF3SO3]

[C4 4 mpy][BF4]

[C4 4

[C4 4

[C4 3

[C4 3 mpy][BF4]

[C4 3

[C4 3

[C4

[C4

#### 2. Selection of ILs for extractive desulphurization

Preliminary selection and screening of suitable ILs by relying on physical, chemical and thermodynamic properties have been intensively investigated and reported in literatures. However, the reported predictive tools for selecting potential ILs are still not satisfactory, as these tools still lack the capability to identify the correct combination of cations and anions matchup for a particular application; this needs further investigations.

#### 2.1. Predictive approach

Due to the enormous number of possible combinations of cations and anions to produce ILs, an accurate prediction for a particular application is necessary. Predictive approach will reduce cost and time as opposed to trial and error using experimental work. One of the predictive approaches is COSMO-RS which is based on quantum chemistry approach. Recently this approach is being applied especially in predicting physical, chemical and thermodynamic properties plus interaction mechanism of potential ILs [25–28]. A recent study which employed COSMO-RS was carried out by Banerjee group, in which different or modified parameterization were used to predict potential ILs for diesel desulphurization, by means of selectivity and capacity at infinite dilution. They selectively screened out 168 suitable ILs (from 28 anions and 6 cations) mostly for extracting thiophene, BT and DBT from simulated diesel composition [11– 29]. The attempted investigation via COSMO-RS showed that 4-ethyl-4-methylmorpholinium gave the best performance for desulphurization in combination with anions such as thiocyanate (CNS), acetate (CH3COO), bis(trifluoromethylsulfonyl)imide (NTf2) and triflate (CF3SO3). However, their predictions did not match well with the experimental results from the literatures; for instance Holbrey et al. who reported that (CF3SO3) and (NTf2) anions showed poor results in removing DBT from n-dodecane phase, while Wang et al. indicated that (CH3COO) anion gave average performance in removing thiophene from n-heptane phase.

#### 2.2. Experimental approach

The screening of appropriate combination of cation/anion for ILs was first attempted by Bosmann and co-worker. They justified that from three types of cations ([emim], [bmim] and


Table 1. Results of DBT removal using some ILs in extractive desulphurization.

separate the cation-anion packing interactions to a sufficient degree resulting in the formation of localized cage-structures; in this case ILs are trapping the aromatic sulfur compounds. With too little interaction, the ILs are simply completely miscible or immiscible with the aromatic sulfur compound, whereas, if the cation-anion interaction of ILs are too great, then crystallization of the ILs occurs [22–25]. Since the aromatic sulfur compounds e.g. DBT, BT, 3-methylthiophene are conjugated structure, the lone pairs on the sulfur atom or the π-electrons on the aromatic sulfur compound ring preferentially insert into the molecular structure of the ILs. The steric effect between the interacting compounds also influences the interaction mechanism involved [26, 27].

Preliminary selection and screening of suitable ILs by relying on physical, chemical and thermodynamic properties have been intensively investigated and reported in literatures. However, the reported predictive tools for selecting potential ILs are still not satisfactory, as these tools still lack the capability to identify the correct combination of cations and anions

Due to the enormous number of possible combinations of cations and anions to produce ILs, an accurate prediction for a particular application is necessary. Predictive approach will reduce cost and time as opposed to trial and error using experimental work. One of the predictive approaches is COSMO-RS which is based on quantum chemistry approach. Recently this approach is being applied especially in predicting physical, chemical and thermodynamic properties plus interaction mechanism of potential ILs [25–28]. A recent study which employed COSMO-RS was carried out by Banerjee group, in which different or modified parameterization were used to predict potential ILs for diesel desulphurization, by means of selectivity and capacity at infinite dilution. They selectively screened out 168 suitable ILs (from 28 anions and 6 cations) mostly for extracting thiophene, BT and DBT from simulated diesel composition [11– 29]. The attempted investigation via COSMO-RS showed that 4-ethyl-4-methylmorpholinium gave the best performance for desulphurization in combination with anions such as thiocyanate (CNS), acetate (CH3COO), bis(trifluoromethylsulfonyl)imide (NTf2) and triflate (CF3SO3). However, their predictions did not match well with the experimental results from the literatures; for instance Holbrey et al. who reported that (CF3SO3) and (NTf2) anions showed poor results in removing DBT from n-dodecane phase, while Wang et al. indicated that (CH3COO)

2. Selection of ILs for extractive desulphurization

matchup for a particular application; this needs further investigations.

anion gave average performance in removing thiophene from n-heptane phase.

The screening of appropriate combination of cation/anion for ILs was first attempted by Bosmann and co-worker. They justified that from three types of cations ([emim], [bmim] and

2.1. Predictive approach

112 Recent Advances in Ionic Liquids

2.2. Experimental approach

[omim]) with [BF4] as anion and seven types of anions ([PF6], [CF3SO3], [BF4], [Cl], [MeSO4], [MeSO3] and [OSO4]) with [bmim] as cation, [omim] and [OSO4] depicted better extractability for DBT removal. It was later proved that the combination of these cation-anion, [omim][OSO4] has high viscosity at ambient conditions. Further work was carried out which indicated that [bmim][OSO4] has the best extractability of some sulfur compounds (Eβer et al.; Nie et al.). Later, Holbrey and co-worker screened out 20 ILs for extracting DBT and revealed that 1-butyldimethylpyridinium bis(trifluoromethylsulfonyl)imide ([bdmpy][NTf2]) yielded the highest DBT removal (83%) from n-dodecane. Recently, [bmim] tricyanomethane ([C(CN)3] has been found to yield higher DBT removal (86%) as compared to previous works [28, 29]. The result of both research studies are summarized in Table 1.

Their negligible vapor pressure allows the extracted product to be separated from the ILs through low pressure distillation with potential energy savings. In addition, as a result of their

Ionic Liquids for Desulphurization: A Review http://dx.doi.org/10.5772/intechopen.79281 115

The use of ILs for selective extraction of sulfur compounds from diesel is first described by Bosmann et al. in 2001 [33]. Based on the initial idea to extract the sulfur compound by chemical interaction, the extraction of DBT with Lewis and Brønsted acidic ILs was majorly investigated. They indicated that such Lewis-acid based interactions enhance the extraction power of ILs that permit complex formation of sulfur compound and ILs. They also identified that extraction of actual diesel is much more complicated due to the complex chemical composition of diesel which includes many different sulfur compounds and other impurities like

As mentioned previously, due to the limited efficiency of HDS towards aromatic sulfur compounds, a number of research have been focused on extracting them, mainly thiophene, BT, DBT and their derivatives. By using various types of ILs through various anion/cation combinations, some researchers have found that extraction process alone could remove up to 86% sulfur in model oil and 30% in actual diesel, which due to the steric hindrance of various sulfur compounds [25]. There are various types of model oil that have been investigated including aliphatics (n-hexane, n-heptane, n-octane, n-dodecane) and aromatics (toluene). In evaluating desulphurization performance, besides removal percentage, sulfur partition coefficient (KN) gives a better insight in terms of explaining the relationship between ILs amount and its structure against desulphurization performance [25, 27]. KN is defined as the ratio of sulfur concentration on weight basis in ILs to sulfur concentration in hydrocarbons, which the higher

Taib and Murugesan [35] in their report said that at ambient condition operation, sulfur compounds with C5 aromatic ring were observed to favorably absorb over C6 aromatics sulfur, while sulfur with non-aromatic structures were poorly absorbed by imidazolium-based ILs. Eβer and co-worker reported in their article that, even though the concept of extraction in desulphurization seemed feasible, but selective extraction of nitrogen-containing compounds and aromatic hydrocarbons still needs further investigation. Although quite a few researchers preferred pyridiniumbased [36] and ammonium based ILs [37] for extractive desulphurization, it seems that the extraction ability is less promising. Some have been noticed to be comparable to imidazolium-

Extractive desulphurization has been performed on model fuel containing up to 25% aromatics. Basically naphthalene, methylnaphthalene, indole, pyridine and tetralin are the most common aromatics used for preparing model fuel. The extraction efficiency is relatively high, and competing removal of aromatics and sulfur compounds was not detected based on model fuel containing n-dodecane/indole/DBT using [BMIm][OSO] as extractant. Further

negligible vapor pressure, they are able to be regenerated for reuse.

KN the better the desulphurization performance of that ILs [33, 34].

based ILs if the anions matchup is just appropriate [38].

3.2. Extractive desulphurization on model fuel

organic nitrogen and oxygen compounds [34].

3.1. Extractive desulphurization on model oil

#### 3. Extractive desulphurization

When a separation by distillation is ineffective or very difficult, liquid–liquid extraction (LLE) is one of the main alternatives to be considered. Close boiling point mixtures or substances that are unstable at the temperature of distillation, even under a vacuum condition, may often be separated by extraction which utilizes the chemical differences instead of vapor pressure differences. One of the major uses of extraction is to separate petroleum products that have different chemical structures, but have about the same boiling range. In liquid–liquid extraction, two phases must be brought into good contact to permit transfer of solute and then be separated [29, 30].

Extraction is a process in which a liquid mixture (of normally two species that contain the solute and the feed carrier) is contacted in a mixer with a third liquid (normally the solvent) that is immiscible or nearly immiscible with the feed carrier component. When the liquids are contacted, the solute is transferred from the feed carrier into the solvent. It is because during mixing process, bonds between solute and feed carrier are broken and possible new bonds are formed between solute and solvent. The energy, which may or may not be required in breaking the bonds between the solute and feed carrier or in forming the bonds between the solute and solvent, depends on the type of interaction.

The combined mixture is then allowed to settle into two phases that are then separated by gravity in a decanter. When a solute transfers from one phase to another, the transfer rate generally decreases with time until the second phase is saturated with the transferred solute, holding as much as it can hold at the prevailing process condition. When the concentrations of the solute in each phase no longer changes with time, the phases are said to be at equilibrium. The effectiveness of any of the separation processes described depends on both how the solute is distributed between the phases at equilibrium and on the rate at which the system approaches equilibrium from its initial state. The extract is the layer of solvent plus extracted solute and the raffinate is the layer from which the solute has been removed from the feed carrier substance [31, 32].

Recently, ILs has been applied in the petrochemical industry especially in catalytic processes, extractive distillation and LLE process for example upgrading heavy oils for desulphurization. Their negligible vapor pressure allows the extracted product to be separated from the ILs through low pressure distillation with potential energy savings. In addition, as a result of their negligible vapor pressure, they are able to be regenerated for reuse.

The use of ILs for selective extraction of sulfur compounds from diesel is first described by Bosmann et al. in 2001 [33]. Based on the initial idea to extract the sulfur compound by chemical interaction, the extraction of DBT with Lewis and Brønsted acidic ILs was majorly investigated. They indicated that such Lewis-acid based interactions enhance the extraction power of ILs that permit complex formation of sulfur compound and ILs. They also identified that extraction of actual diesel is much more complicated due to the complex chemical composition of diesel which includes many different sulfur compounds and other impurities like organic nitrogen and oxygen compounds [34].

#### 3.1. Extractive desulphurization on model oil

[omim]) with [BF4] as anion and seven types of anions ([PF6], [CF3SO3], [BF4], [Cl], [MeSO4], [MeSO3] and [OSO4]) with [bmim] as cation, [omim] and [OSO4] depicted better extractability for DBT removal. It was later proved that the combination of these cation-anion, [omim][OSO4] has high viscosity at ambient conditions. Further work was carried out which indicated that [bmim][OSO4] has the best extractability of some sulfur compounds (Eβer et al.; Nie et al.). Later, Holbrey and co-worker screened out 20 ILs for extracting DBT and revealed that 1-butyldimethylpyridinium bis(trifluoromethylsulfonyl)imide ([bdmpy][NTf2]) yielded the highest DBT removal (83%) from n-dodecane. Recently, [bmim] tricyanomethane ([C(CN)3] has been found to yield higher DBT removal (86%) as compared to previous works [28, 29]. The result of

When a separation by distillation is ineffective or very difficult, liquid–liquid extraction (LLE) is one of the main alternatives to be considered. Close boiling point mixtures or substances that are unstable at the temperature of distillation, even under a vacuum condition, may often be separated by extraction which utilizes the chemical differences instead of vapor pressure differences. One of the major uses of extraction is to separate petroleum products that have different chemical structures, but have about the same boiling range. In liquid–liquid extraction, two phases must be brought into good contact to permit transfer of solute and then be

Extraction is a process in which a liquid mixture (of normally two species that contain the solute and the feed carrier) is contacted in a mixer with a third liquid (normally the solvent) that is immiscible or nearly immiscible with the feed carrier component. When the liquids are contacted, the solute is transferred from the feed carrier into the solvent. It is because during mixing process, bonds between solute and feed carrier are broken and possible new bonds are formed between solute and solvent. The energy, which may or may not be required in breaking the bonds between the solute and feed carrier or in forming the bonds between the solute and

The combined mixture is then allowed to settle into two phases that are then separated by gravity in a decanter. When a solute transfers from one phase to another, the transfer rate generally decreases with time until the second phase is saturated with the transferred solute, holding as much as it can hold at the prevailing process condition. When the concentrations of the solute in each phase no longer changes with time, the phases are said to be at equilibrium. The effectiveness of any of the separation processes described depends on both how the solute is distributed between the phases at equilibrium and on the rate at which the system approaches equilibrium from its initial state. The extract is the layer of solvent plus extracted solute and the raffinate is the layer from which the solute has been removed from the feed

Recently, ILs has been applied in the petrochemical industry especially in catalytic processes, extractive distillation and LLE process for example upgrading heavy oils for desulphurization.

both research studies are summarized in Table 1.

3. Extractive desulphurization

solvent, depends on the type of interaction.

separated [29, 30].

114 Recent Advances in Ionic Liquids

carrier substance [31, 32].

As mentioned previously, due to the limited efficiency of HDS towards aromatic sulfur compounds, a number of research have been focused on extracting them, mainly thiophene, BT, DBT and their derivatives. By using various types of ILs through various anion/cation combinations, some researchers have found that extraction process alone could remove up to 86% sulfur in model oil and 30% in actual diesel, which due to the steric hindrance of various sulfur compounds [25]. There are various types of model oil that have been investigated including aliphatics (n-hexane, n-heptane, n-octane, n-dodecane) and aromatics (toluene). In evaluating desulphurization performance, besides removal percentage, sulfur partition coefficient (KN) gives a better insight in terms of explaining the relationship between ILs amount and its structure against desulphurization performance [25, 27]. KN is defined as the ratio of sulfur concentration on weight basis in ILs to sulfur concentration in hydrocarbons, which the higher KN the better the desulphurization performance of that ILs [33, 34].

Taib and Murugesan [35] in their report said that at ambient condition operation, sulfur compounds with C5 aromatic ring were observed to favorably absorb over C6 aromatics sulfur, while sulfur with non-aromatic structures were poorly absorbed by imidazolium-based ILs. Eβer and co-worker reported in their article that, even though the concept of extraction in desulphurization seemed feasible, but selective extraction of nitrogen-containing compounds and aromatic hydrocarbons still needs further investigation. Although quite a few researchers preferred pyridiniumbased [36] and ammonium based ILs [37] for extractive desulphurization, it seems that the extraction ability is less promising. Some have been noticed to be comparable to imidazoliumbased ILs if the anions matchup is just appropriate [38].

#### 3.2. Extractive desulphurization on model fuel

Extractive desulphurization has been performed on model fuel containing up to 25% aromatics. Basically naphthalene, methylnaphthalene, indole, pyridine and tetralin are the most common aromatics used for preparing model fuel. The extraction efficiency is relatively high, and competing removal of aromatics and sulfur compounds was not detected based on model fuel containing n-dodecane/indole/DBT using [BMIm][OSO] as extractant. Further investigations showed that ILs gave higher removal of molecules that have higher density of aromatic π-electrons. Cross-miscibility of the studied aromatics in the ILs produced an unwanted effect, whereby high cross-miscibility will demonstrate a loss of fuel or at least contribute to an increase of process costs [39]. However, the effect of aromatic hydrocarbons such as benzene and xylene needs further research in order to understand the selective extraction process.

Author details

Malaysia

References

Syamsul Bahari Abdullah<sup>1</sup>

Gambang, Pahang, Malaysia

Data. 2003;48:475-479

AlChE Journal. 2002;48:369-385

Research. 2010;49:2916-2925

Engineering Data. 2006;51:2170-2177

neering Chemistry Research. 2005;44:1610-1624

\*Address all correspondence to: syamsul@ump.edu.my

\*, Hanida Abdul Aziz2 and Zakaria Man<sup>3</sup>

Ionic Liquids for Desulphurization: A Review http://dx.doi.org/10.5772/intechopen.79281 117

1 Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang,

2 Faculty of Engineering Technology, Universiti Malaysia Pahang, Gambang, Pahang,

Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh, Perak, Malaysia

3 Centre of Research in Ionic Liquids (CORIL), Chemical Engineering Department, Universiti

[1] Klamt A, Eckert F. COSMO-RS: A novel and efficient method for the a priori prediction of

[2] Diedenhofen M, Eckert F, Klamt A. Prediction of infinite dilution activity coefficients of organic compounds in ionic liquids using COSMO-RS. Journal of Chemical & Engineering

[3] Eckert F, Klamt A. Fast solvent screening via quantum chemistry: COSMO-RS approach.

[4] Banerjee T, Khanna A. Infinite dilution activity coefficients for trihexyltetradecyl phosphonium ionic liquids: Measurements and COSMO-RS prediction. Journal of Chemical &

[5] Palomar J, Torrecilla JS, Ferro VR, Rodriguez F. Development of an a priori ionic liquid design tool: Ionic liquid selection through the prediction of COSMO-RS molecular descriptor by inverse neural network. Industrial & Engineering Chemistry Research. 2009;48:2257-2265

[6] Grensemann H, Gmehling J. Performance of a conductor-like screening model for real solvents model in comparison to classical group contribution methods. Industrial & Engi-

[7] Klamt A. Conductor-like screening model for real solvents: A new approach to the quantitative calculation of solvation phenomena. Journal of Physical Chemistry. 1995;99:2224-2235

[8] Mohanty S, Banerjee T, Mohanty K. Quantum chemical based screening of ionic liquids for the extraction of phenol from aqueous solution. Industrial & Engineering Chemistry

thermophysical data of liquids. Fluid Phase Equilibria. 2000;172:43-72

#### 3.3. Extractive desulphurization on actual diesel

An approach based on extraction for removing sulfur compounds from actual diesel using ILs have been investigated by many researchers [39]. Compared to model oil or model fuel, the extraction from actual diesel is much more complicated due to its complex chemical composition including many different sulfur compounds and other impurities such as nitrogen and oxygen-containing compounds. For example, the removal of sulfur from model oil is 64% but in actual diesel this percentage is drastically reduced to 24.3% when the same ILs is applied. The obvious or most sterically hindered sulfur species would still remain in the actual diesel even though after several extraction steps. However, it has been proven that extractive desulphurization of actual diesel with ILs is still possible, although the operating expenses such as the number of theoretical extraction steps may vary in order to reach ultra-low concentration of sulfur [40].

#### 4. Regeneration of spent ILs

Besides being efficient for extraction process, regeneration or recycibility of spent ILs is equally important since ILs has been recognized as environmentally benign solvent. Since ILs is quite expensive as compared to some conventional organic solvents, finding an alternative way to recycle spent ILs is the key for cost effectiveness in order to ensure the feasibility of using ILs at a larger scale application.

Undoubtedly, regeneration has become a fundamental issue from economic point of view. However, this is not only limited to the operating cost, but also concerning environmental issues such as disposal, biodegradable and toxicity. In general, ILs has a higher density compared to organic solvents or water; therefore, many ILs form separate phases when mixed with organic or aqueous solution. This behavior makes ILs as feasible for regeneration, which in turn presents potential economic viability of desulphurization process using ILs. In addition, the process is considered as being environmental benign since no waste is generated [33–40].

In conclusion, extractive desulphurization process using selective ILs as the extractant is still in need of further research, starting from screening of suitable ILs for desulphurization, synthesis of ILs, physical property analysis of ILs, single batch extraction study encompassing process optimization up to actual diesel application, and including regeneration of spent ILs.

### Author details

investigations showed that ILs gave higher removal of molecules that have higher density of aromatic π-electrons. Cross-miscibility of the studied aromatics in the ILs produced an unwanted effect, whereby high cross-miscibility will demonstrate a loss of fuel or at least contribute to an increase of process costs [39]. However, the effect of aromatic hydrocarbons such as benzene and xylene needs further research in order to understand the selective extrac-

An approach based on extraction for removing sulfur compounds from actual diesel using ILs have been investigated by many researchers [39]. Compared to model oil or model fuel, the extraction from actual diesel is much more complicated due to its complex chemical composition including many different sulfur compounds and other impurities such as nitrogen and oxygen-containing compounds. For example, the removal of sulfur from model oil is 64% but in actual diesel this percentage is drastically reduced to 24.3% when the same ILs is applied. The obvious or most sterically hindered sulfur species would still remain in the actual diesel even though after several extraction steps. However, it has been proven that extractive desulphurization of actual diesel with ILs is still possible, although the operating expenses such as the number of theoretical extraction steps may vary in order to reach ultra-low

Besides being efficient for extraction process, regeneration or recycibility of spent ILs is equally important since ILs has been recognized as environmentally benign solvent. Since ILs is quite expensive as compared to some conventional organic solvents, finding an alternative way to recycle spent ILs is the key for cost effectiveness in order to ensure the feasibility of using ILs at

Undoubtedly, regeneration has become a fundamental issue from economic point of view. However, this is not only limited to the operating cost, but also concerning environmental issues such as disposal, biodegradable and toxicity. In general, ILs has a higher density compared to organic solvents or water; therefore, many ILs form separate phases when mixed with organic or aqueous solution. This behavior makes ILs as feasible for regeneration, which in turn presents potential economic viability of desulphurization process using ILs. In addition, the process is

In conclusion, extractive desulphurization process using selective ILs as the extractant is still in need of further research, starting from screening of suitable ILs for desulphurization, synthesis of ILs, physical property analysis of ILs, single batch extraction study encompassing process

optimization up to actual diesel application, and including regeneration of spent ILs.

considered as being environmental benign since no waste is generated [33–40].

tion process.

116 Recent Advances in Ionic Liquids

concentration of sulfur [40].

a larger scale application.

4. Regeneration of spent ILs

3.3. Extractive desulphurization on actual diesel

Syamsul Bahari Abdullah<sup>1</sup> \*, Hanida Abdul Aziz2 and Zakaria Man<sup>3</sup>

\*Address all correspondence to: syamsul@ump.edu.my

1 Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Gambang, Pahang, Malaysia

2 Faculty of Engineering Technology, Universiti Malaysia Pahang, Gambang, Pahang, Malaysia

3 Centre of Research in Ionic Liquids (CORIL), Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh, Perak, Malaysia

### References


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**Section 4**

**State of the Art Applications**

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