**Applications**

determination of thermodynamic properties of RTILS (Qingshan), the connection between friction and the nanospace behavior of RTILs (Takaya), and the very important determina‐

The use of RTILs in various types of separations continues to grow as well. In addition to the earlier chapter on the electrochemical purification of rare earth metals, the use of RTILs as membranes for metal separation (Michiaki), in solid/liquid separations (Lavinia), as mul‐

Finally, two interesting chapters focused on supported RTILs (important due to the high cost of most RTILs and thus the economic drive for recycling) are included: siloxane-based system (Tyoshiro) and magnetic nanoparticle-immobilized systems as catalysts (Masoud). In short, the potential for RTILs continues to be as endless and unlimited as the proposed number of RTILs. While they may not be reaching the market for commercial applications as quickly as hoped, their future remains as exciting as ever. I hope that you enjoy this volume

**Prof. Scott Handy**

USA

Middle Tennessee State University

tiphase systems (Lenore), and as extraction media (Saiful) is presented.

as much as I have and that it serves to inspire your creativity and imagination.

tion of RTIL toxicity using *Vibrio fischeri* (Ibrahim).

X Preface

**Chapter 1**

Provisional chapter

**Are Ionic Liquids Suitable as New Components in**

Are Ionic Liquids Suitable as New Components in

El-Shaimaa Abumandour, Fabrice Mutelet and

El-Shaimaa Abumandour, Fabrice Mutelet

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Dominique Alonso

Abstract

AHT.

1. Introduction

to 100°C on a daily basis [1].

and Dominique Alonso

http://dx.doi.org/10.5772/65756

**Working Mixtures for Absorption Heat Transformers?**

The working mixture almost exclusively used to operate absorption heat transformers (AHT) is {H2O + LiBr} ({H2O + NH3} can also be used). Unfortunately, both working pairs present some drawbacks: corrosivity, toxicity, crystallization or high working pressure. Ionic liquids (ILs) possess very interesting properties (thermal stability, possible miscibility with water, negligible vapor pressure) that make them good candidates to be used as absorbents in AHT. This paper aims at providing an overview of available thermodynamic data concerning {H2O + IL} mixtures that could be used to operate an

Keywords: absorption heat pump, waste heat, ionic liquids, absorption heat trans-

Most of the industrial and domestic activities require large amounts of thermal energy to generate steam or heat by burning fossil fuel. After being used and degraded, low temperature heat is released to the environment as low grade waste heat. Large quantities of low temperature thermal waste heat streams from many industrial facilities such as power plants are discharged as thermal pollutants to the air and to the water at temperatures ranging from 60

Among heat-driven devices are the absorption cycles. They can be divided into three classes: absorption heat pump (AHP), absorption chiller (AC), and absorption heat transformer (AHT). Absorption cycles become of great interest since electrical energy is replaced with low grade or waste heat allowing both primary energy savings and energetic efficiency improvements [2]. Consequently, absorption cycles enhance the atmospheric conditions by reducing the

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

© 2017 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.

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.

formers, thermodynamic properties, coefficient of performance

Working Mixtures for Absorption Heat Transformers?

#### **Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?** Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

El-Shaimaa Abumandour, Fabrice Mutelet and Dominique Alonso El-Shaimaa Abumandour, Fabrice Mutelet and Dominique Alonso

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

http://dx.doi.org/10.5772/65756

#### Abstract

The working mixture almost exclusively used to operate absorption heat transformers (AHT) is {H2O + LiBr} ({H2O + NH3} can also be used). Unfortunately, both working pairs present some drawbacks: corrosivity, toxicity, crystallization or high working pressure. Ionic liquids (ILs) possess very interesting properties (thermal stability, possible miscibility with water, negligible vapor pressure) that make them good candidates to be used as absorbents in AHT. This paper aims at providing an overview of available thermodynamic data concerning {H2O + IL} mixtures that could be used to operate an AHT.

Keywords: absorption heat pump, waste heat, ionic liquids, absorption heat transformers, thermodynamic properties, coefficient of performance

### 1. Introduction

Most of the industrial and domestic activities require large amounts of thermal energy to generate steam or heat by burning fossil fuel. After being used and degraded, low temperature heat is released to the environment as low grade waste heat. Large quantities of low temperature thermal waste heat streams from many industrial facilities such as power plants are discharged as thermal pollutants to the air and to the water at temperatures ranging from 60 to 100°C on a daily basis [1].

Among heat-driven devices are the absorption cycles. They can be divided into three classes: absorption heat pump (AHP), absorption chiller (AC), and absorption heat transformer (AHT). Absorption cycles become of great interest since electrical energy is replaced with low grade or waste heat allowing both primary energy savings and energetic efficiency improvements [2]. Consequently, absorption cycles enhance the atmospheric conditions by reducing the

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

emissions of greenhouse gases. Environmental impacts of absorption cycles can even be reduced by the adoption of environmental friendly working mixtures [3, 4].

cold heat source of absorption heat pumps) allows overcoming the crystallization problem of the {water + LiBr} solution that can occur under some conditions [2]. Moreover, aqueous solution of ionic liquids seems to be less corrosive than the {water + LiBr} solutions. Finally, many ionic liquids show a high miscibility with water, which is a recommended refrigerant for absorption cycles (high latent heat, low viscosity, nontoxic, etc.). Consequently, the analysis of

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

http://dx.doi.org/10.5772/65756

5

Few papers were published concerning working fluids containing ILs and a refrigerant such as NH3, water, ethanol, or halogenated hydrocarbon series. Although many working fluids are proposed in the literature, there is not a complete review with comparison of their properties and performances. Studies mainly focus on the evaluation of {H2O + IL} systems for their potential use in absorption heat cycles [11, 12]. Khamooshi et al. [2] studied the performance of different working fluids containing ILs and different types of refrigerants in order to define the most suitable binary system. The coefficient of performance obtained on binary systems {H2O + ILs} in absorption cooling cycle is lower than {H2O + LiBr}. Nevertheless, the COP of

In 2014, Zheng et al. [13] presented a compilation of thermodynamic properties of binary systems containing {H2O or NH3 or HFCs or alcohols + ILs}. The simulation of IL working fluids for single effect absorption cooling cycles showed that several binary systems have a real

Publications concerning the development of working fluids and absorption cycles almost exclusively focus on air conditioning and refrigeration. It seems that no previous review has comprehensively summarized the studies and applications of absorption heat transformers using {H2O + ILs} as the working fluid. Recently, IoLiTec has developed working mixtures composed of {H2O or NH3 + ILs} for absorption cycles and filled the first patent application in this field in 2004, which is now owned by BASF. Meanwhile, other companies, such as DuPont and Degussa, are interested in using ILs in absorption cycles. This ensures the great potential of ILs in this application. Encouraging the use of ILs in this technology in combination with upgrading industrial and other waste heat could enhance the conservation of fossil fuels and

This work mainly focuses on the study of binary systems {H2O + ILs} in absorption heat transformers. The first part of the paper briefly describes the absorption cycle. Then, the thermodynamic and physical properties of the binary systems {H2O + IL} are summarized and the influences of the IL structure on these properties are presented. In this section, results found in the literature about the performance of absorption cycles using {H2O + IL} as working mixtures are also presented. The last section of this paper is devoted to the calculation of the

Absorption cycles perform heat exchange between several heat sources or sinks. In the simplest case, there are three heat reservoirs characterized by their relative temperature level

coefficient of performance of an AHT operated with each binary system {H2O + IL}.

binary systems composed of {ILs + water} for this application has to be explored [10].

these systems is higher than 0.7.

2. Absorption heat cycles

(high, medium, and low) as shown in Figure 1.

hence decrease the emissions of greenhouse gases [14].

potential.

One of the key points to the performance of an absorption cycle is the working fluid used. Nowadays, the most used binary systems in the absorption heat cycle are {water + lithium bromide (H2O + LiBr)} and {ammonia + water (NH3 + H2O)}. The aqueous solution of LiBr is the most successful working mixture in absorption cycles and widely spread all over the world [2, 3]. Nevertheless, LiBr aqueous solution has some main drawbacks as follows:


Due to these disadvantages, which have not been solved properly, the absorption technology has known a very limited expansion [7, 8]. That is why heat pump and absorption chiller technologies suffer from lack of suitable working pairs. Hence, searching for new beneficial and reliable binary systems (to overcome these technical limitations) has become of great importance lately.

Limited numbers of critical reviews have been published in the literature on the subject of absorption technologies. In 2001, Srikhirin et al. [6] reviewed different configurations and types of absorption refrigeration cycles and working pairs. Performance development and enhancement of absorption cycles were evaluated. They concluded that double-stage absorption refrigeration cycle based on {H2O + LiBr} has the highest coefficient of performance (COP) if compared to other systems in the market. In addition, they stated that multistage absorption cycles have a promising future.

In 2012, Sun et al. [3] have shown that {H2O + LiBr} and {NH3 + H2O} mixtures can be improved by the use of additives. They also stated that working pairs dedicated to specific applications such as solar or geothermal energy should use hydrofluorocarbons (HFCs) as a refrigerant.

Ionic liquids (ILs) are environmentally friendly solvents, which have attracted considerable attention recently. Ionic liquids are salts in liquid state having melting point below some arbitrary temperature, such as 100 °C (373 K). These solvents consist of ions (an asymmetric, large organic cation, and organic or inorganic anion). A great advantage of ILs is that their physical properties such as melting points, density, and hydrophobicity can be adjusted to design different types of ILs that can be used for various applications.

It is now well established that ILs exhibit interesting physicochemical properties allowing their use for various industrial applications [1–10].

ILs could be used as alternative working mixtures in absorption heat pump cycles. Hence, the possibility to have ionic liquids with a low melting point (lower than the temperature of the cold heat source of absorption heat pumps) allows overcoming the crystallization problem of the {water + LiBr} solution that can occur under some conditions [2]. Moreover, aqueous solution of ionic liquids seems to be less corrosive than the {water + LiBr} solutions. Finally, many ionic liquids show a high miscibility with water, which is a recommended refrigerant for absorption cycles (high latent heat, low viscosity, nontoxic, etc.). Consequently, the analysis of binary systems composed of {ILs + water} for this application has to be explored [10].

Few papers were published concerning working fluids containing ILs and a refrigerant such as NH3, water, ethanol, or halogenated hydrocarbon series. Although many working fluids are proposed in the literature, there is not a complete review with comparison of their properties and performances. Studies mainly focus on the evaluation of {H2O + IL} systems for their potential use in absorption heat cycles [11, 12]. Khamooshi et al. [2] studied the performance of different working fluids containing ILs and different types of refrigerants in order to define the most suitable binary system. The coefficient of performance obtained on binary systems {H2O + ILs} in absorption cooling cycle is lower than {H2O + LiBr}. Nevertheless, the COP of these systems is higher than 0.7.

In 2014, Zheng et al. [13] presented a compilation of thermodynamic properties of binary systems containing {H2O or NH3 or HFCs or alcohols + ILs}. The simulation of IL working fluids for single effect absorption cooling cycles showed that several binary systems have a real potential.

Publications concerning the development of working fluids and absorption cycles almost exclusively focus on air conditioning and refrigeration. It seems that no previous review has comprehensively summarized the studies and applications of absorption heat transformers using {H2O + ILs} as the working fluid. Recently, IoLiTec has developed working mixtures composed of {H2O or NH3 + ILs} for absorption cycles and filled the first patent application in this field in 2004, which is now owned by BASF. Meanwhile, other companies, such as DuPont and Degussa, are interested in using ILs in absorption cycles. This ensures the great potential of ILs in this application. Encouraging the use of ILs in this technology in combination with upgrading industrial and other waste heat could enhance the conservation of fossil fuels and hence decrease the emissions of greenhouse gases [14].

This work mainly focuses on the study of binary systems {H2O + ILs} in absorption heat transformers. The first part of the paper briefly describes the absorption cycle. Then, the thermodynamic and physical properties of the binary systems {H2O + IL} are summarized and the influences of the IL structure on these properties are presented. In this section, results found in the literature about the performance of absorption cycles using {H2O + IL} as working mixtures are also presented. The last section of this paper is devoted to the calculation of the coefficient of performance of an AHT operated with each binary system {H2O + IL}.

### 2. Absorption heat cycles

emissions of greenhouse gases. Environmental impacts of absorption cycles can even be

One of the key points to the performance of an absorption cycle is the working fluid used. Nowadays, the most used binary systems in the absorption heat cycle are {water + lithium bromide (H2O + LiBr)} and {ammonia + water (NH3 + H2O)}. The aqueous solution of LiBr is the most successful working mixture in absorption cycles and widely spread all over the world

• Absorption heat pumps cannot operate at an evaporation temperature below 0°C because of the use of water as a refrigerant, which makes it unusable for subfreezing refrigeration or heating/domestic hot water (DHW) supplementation in cold regions. Crystallization of {H2O + LiBr} at high concentrations is a common problem. High vacuum conditions should be preserved in the system for suitable operation of the {H2O + LiBr} system; otherwise, the performance of the absorption cycle would be greatly reduced [5]. {H2O + LiBr} is corrosive

Due to these disadvantages, which have not been solved properly, the absorption technology has known a very limited expansion [7, 8]. That is why heat pump and absorption chiller technologies suffer from lack of suitable working pairs. Hence, searching for new beneficial and reliable binary systems (to overcome these technical limitations) has become of great

Limited numbers of critical reviews have been published in the literature on the subject of absorption technologies. In 2001, Srikhirin et al. [6] reviewed different configurations and types of absorption refrigeration cycles and working pairs. Performance development and enhancement of absorption cycles were evaluated. They concluded that double-stage absorption refrigeration cycle based on {H2O + LiBr} has the highest coefficient of performance (COP) if compared to other systems in the market. In addition, they stated that multistage absorption

In 2012, Sun et al. [3] have shown that {H2O + LiBr} and {NH3 + H2O} mixtures can be improved by the use of additives. They also stated that working pairs dedicated to specific applications such as solar or geothermal energy should use hydrofluorocarbons (HFCs) as a refrigerant.

Ionic liquids (ILs) are environmentally friendly solvents, which have attracted considerable attention recently. Ionic liquids are salts in liquid state having melting point below some arbitrary temperature, such as 100 °C (373 K). These solvents consist of ions (an asymmetric, large organic cation, and organic or inorganic anion). A great advantage of ILs is that their physical properties such as melting points, density, and hydrophobicity can be adjusted to

It is now well established that ILs exhibit interesting physicochemical properties allowing their

ILs could be used as alternative working mixtures in absorption heat pump cycles. Hence, the possibility to have ionic liquids with a low melting point (lower than the temperature of the

design different types of ILs that can be used for various applications.

use for various industrial applications [1–10].

reduced by the adoption of environmental friendly working mixtures [3, 4].

[2, 3]. Nevertheless, LiBr aqueous solution has some main drawbacks as follows:

• {NH3 + H2O} requires high working pressure and ammonia is toxic.

to metals [2–6].

4 Progress and Developments in Ionic Liquids

importance lately.

cycles have a promising future.

Absorption cycles perform heat exchange between several heat sources or sinks. In the simplest case, there are three heat reservoirs characterized by their relative temperature level (high, medium, and low) as shown in Figure 1.

Figure 1. Simple absorption cycle.

Absorption systems can operate according to different modes differing by the nature of the driving heat and by the desired useful effect. These modes are as follows:


Absorption cycles are composed of five main components: evaporator, condenser, generator, absorber, and solution heat exchanger (economizer) (Figure 2). They generally use a binary working mixture composed of a low boiling component called the refrigerant and a high boiling component called the absorbent.

This operation releases high temperature useful heat. The resulting weak solution is throttled through a valve and sent back to the generator. The solution heat exchanger allows preheating the strong solution entering the absorber by exchange with the weak solution leaving the absorber [15]. In refrigeration or heat pump absorption cycles (Figure 2), pressure levels are reversed (high pressure in the generator and in the condenser and low in the evaporator and the absorber), high temperature heat is provided to the generator, low temperature heat is provided to the evaporator and medium temperature heat is rejected at the condenser and absorber.

Figure 2. Schematic diagram of an absorption refrigeration cycle; A: absorber, C: condenser, E: evaporator, G: generator,

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

http://dx.doi.org/10.5772/65756

7

The working mixture properties will directly affect the absorption cycle performance. The

i. The presence of absorbent in the refrigerant must increase as high as possible the boiling

following criteria can be followed to properly choose a working mixture [3]:

point of the solution.

SHE: heat exchanger.

In an absorption heat transformer (Figure 3), the driving heat (medium temperature waste heat) is provided to the mixture of an absorbent and a refrigerant (weak solution) in the generator at a low pressure producing two streams: a pure refrigerant vapor stream and a liquid mixture stream (strong solution).

This vapor is condensed in the condenser releasing heat to the low temperature heat sink. The condensate is increased to high pressure through a pump and vaporized in the evaporator, thanks to medium temperature heat. The strong solution passes through a pump to high pressure and is sent to the absorber where it absorbs the vapor produced in the evaporator. Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 7

Absorption systems can operate according to different modes differing by the nature of the

• Refrigerator: The driving heat is provided to the absorption cycle by a high temperature heat source. The low temperature source also provides heat to the cycle producing the cooling effect (useful effect). Heat is released to the medium temperature heat sink (gener-

• Heat pump: As well as in a refrigerator, the cycle is driven by the heat provided by the high temperature source. The cycle also receives heat from the low temperature source. The useful heat is released to the medium temperature sink (generally a building or process

• Absorption heat transformer: Compared to both previous modes, sink and sources are reversed. The driving heat is a medium temperature heat (generally a waste heat). The upgraded useful heat is rejected to the high temperature sink and degraded heat is rejected

Absorption cycles are composed of five main components: evaporator, condenser, generator, absorber, and solution heat exchanger (economizer) (Figure 2). They generally use a binary working mixture composed of a low boiling component called the refrigerant and a high

In an absorption heat transformer (Figure 3), the driving heat (medium temperature waste heat) is provided to the mixture of an absorbent and a refrigerant (weak solution) in the generator at a low pressure producing two streams: a pure refrigerant vapor stream and a

This vapor is condensed in the condenser releasing heat to the low temperature heat sink. The condensate is increased to high pressure through a pump and vaporized in the evaporator, thanks to medium temperature heat. The strong solution passes through a pump to high pressure and is sent to the absorber where it absorbs the vapor produced in the evaporator.

driving heat and by the desired useful effect. These modes are as follows:

to the low temperature sink (generally the environment).

ally the environment).

Figure 1. Simple absorption cycle.

6 Progress and Developments in Ionic Liquids

that requires to be heated).

boiling component called the absorbent.

liquid mixture stream (strong solution).

Figure 2. Schematic diagram of an absorption refrigeration cycle; A: absorber, C: condenser, E: evaporator, G: generator, SHE: heat exchanger.

This operation releases high temperature useful heat. The resulting weak solution is throttled through a valve and sent back to the generator. The solution heat exchanger allows preheating the strong solution entering the absorber by exchange with the weak solution leaving the absorber [15]. In refrigeration or heat pump absorption cycles (Figure 2), pressure levels are reversed (high pressure in the generator and in the condenser and low in the evaporator and the absorber), high temperature heat is provided to the generator, low temperature heat is provided to the evaporator and medium temperature heat is rejected at the condenser and absorber.

The working mixture properties will directly affect the absorption cycle performance. The following criteria can be followed to properly choose a working mixture [3]:

i. The presence of absorbent in the refrigerant must increase as high as possible the boiling point of the solution.

ii. In order to reduce refrigerant flow rate, its vaporization latent heat has to be as high as possible.

COP <sup>¼</sup> <sup>H</sup>: temp: heat flow exchanged at the absorber

there is still a lack of thermodynamic data for such mixtures.

3.1. {H2O + ILs} binary systems in the literature

in different absorption cycles [7–26].

3. Working fluids containing {water + ILs} for absorption cycles

M: temp: heat flows exchanged at the generator and evaporator þ Mechanical pumping power

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

Water can be considered as a green refrigerant, nontoxic, having high latent heat and excellent thermal characteristics. ILs used in the working fluids {H2O + ILs} have to be hygroscopic and stable in aqueous solution. Numerous articles have studied the behavior of ILs with water, but

Recently, the performances of binary mixtures {H2O + IL} have been evaluated as alternative binary systems in the absorption cycle [16–18, 19]. Numerous articles present thermodynamic studies of binary systems containing water and IL. Table 1 lists the most studied ILs in the literature and the thermophysical properties available. Alkylsulfate- and alkylphosphastebased ILs are well known, and their performance in working fluids {H2O + IL} was evaluated

The binary systems {H2O + dialkylimidazolium alkylphosphate}, 1-dimethylimidazolium dimethylphosphate, and 1-ethyl-3-methylimidazolium dimethylphosphate were extensively studied in numerous papers, where not only the data of vapor-liquid equilibria (VLE) but also density, viscosity, heat capacity, and excess enthalpy are available [7–25, 27–29]. These data make it possible to simulate the performance of these binary mixtures as working fluids in the absorption refrigeration cycle [7–25, 27, 30]. The simulation results show that the cycle performance of both systems is lower but close to the value obtained with the conventional working pair {H2O + LiBr}. Yokozeki and Shiflett [31] also examined the feasibility of different binary systems {H2O + IL} in an absorption cooling cycle and they found out that the best system is {H2O + [EMIM][DMP]}.

The performance of {H2O + LiBr} is still higher with a COP of 0.78 and a solution flow rate 53% smaller than [EMIM][DMP]. Nevertheless, the use of {H2O + [DMIM][DMP]} as the working fluid enables to work in a large range of temperatures and to stop crystallization and corrosion caused by {H2O + LiBr}. Several papers were focused on the measurements of thermodynamic properties of the binary system {H2O + [EMIM][EtSO4]} [16–25, 27, 30, 32], {H2O + [EMIM][Ac]} [33, 34], or {H2O + [HOEtMIM][Cl]} [35]. Even if these systems have some interesting properties such as low density and heat capacity or strong negative deviations from Raoult's law, no

The knowledge of thermodynamic properties, phase behavior, and safety/environmental hazards of {H2O + IL} is required for the evaluation of this system in an AHT. The following section presents the behavior of ILs in the presence of water and the influence of their structure

simulation of the performance of these working fluids in an AHT was presented.

4. Thermodynamic properties of {H2O + IL}

on thermodynamic properties.

(2)

9

http://dx.doi.org/10.5772/65756


Figure 3. Schematic diagram of an absorption heat transformer; A: absorber, C: condenser, E: evaporator, G: generator, SHE: heat exchanger, P1 and P2: pump, v: valve.

To assess the energetic performance level of an absorption cycle, a criterion was defined: the coefficient of performance. Its expression depends on the kind of absorption cycle (refrigeration, heat pump, or AHT). Nevertheless, it is possible to define it as the ratio of the useful heat flow exchanged to the costly heat flow consumed [1]. For example, in a refrigeration cycle the expression of COP becomes [7–16]:

$$\text{COP} = \frac{\text{Low temperature heat flow exchanged at the evaporation}}{\text{High temperature heat flow provided at the generator} + \text{Mechanical pumping power}} \tag{1}$$

For an absorption heat transformer the COP expression is [8]

COP <sup>¼</sup> <sup>H</sup>: temp: heat flow exchanged at the absorber M: temp: heat flows exchanged at the generator and evaporator þ Mechanical pumping power (2)

### 3. Working fluids containing {water + ILs} for absorption cycles

Water can be considered as a green refrigerant, nontoxic, having high latent heat and excellent thermal characteristics. ILs used in the working fluids {H2O + ILs} have to be hygroscopic and stable in aqueous solution. Numerous articles have studied the behavior of ILs with water, but there is still a lack of thermodynamic data for such mixtures.

### 3.1. {H2O + ILs} binary systems in the literature

ii. In order to reduce refrigerant flow rate, its vaporization latent heat has to be as high as

i. In order to reduce exchange areas, pressure decreases and more generally the size and cost of equipment, viscosity of the solutions have to be the lowest as possible whereas

iv. The environmental impact of the working mixture should be the lowest as possible,

To assess the energetic performance level of an absorption cycle, a criterion was defined: the coefficient of performance. Its expression depends on the kind of absorption cycle (refrigeration, heat pump, or AHT). Nevertheless, it is possible to define it as the ratio of the useful heat flow exchanged to the costly heat flow consumed [1]. For example, in a refrigeration cycle the

Figure 3. Schematic diagram of an absorption heat transformer; A: absorber, C: condenser, E: evaporator, G: generator,

Hig htemperature heat flow provided at the generator þ Mechanical pumping power

(1)

COP <sup>¼</sup> Low temperature heat flow exchanged at the evaporator

For an absorption heat transformer the COP expression is [8]

thermal conductivity and diffusion coefficient have to be the highest as possible.

ii. Components of the working mixture should not be too expensive. iii. Components of the mixtures should be noncorrosive and nontoxic.

especially in terms of GWP and ODP.

expression of COP becomes [7–16]:

SHE: heat exchanger, P1 and P2: pump, v: valve.

possible.

8 Progress and Developments in Ionic Liquids

Recently, the performances of binary mixtures {H2O + IL} have been evaluated as alternative binary systems in the absorption cycle [16–18, 19]. Numerous articles present thermodynamic studies of binary systems containing water and IL. Table 1 lists the most studied ILs in the literature and the thermophysical properties available. Alkylsulfate- and alkylphosphastebased ILs are well known, and their performance in working fluids {H2O + IL} was evaluated in different absorption cycles [7–26].

The binary systems {H2O + dialkylimidazolium alkylphosphate}, 1-dimethylimidazolium dimethylphosphate, and 1-ethyl-3-methylimidazolium dimethylphosphate were extensively studied in numerous papers, where not only the data of vapor-liquid equilibria (VLE) but also density, viscosity, heat capacity, and excess enthalpy are available [7–25, 27–29]. These data make it possible to simulate the performance of these binary mixtures as working fluids in the absorption refrigeration cycle [7–25, 27, 30]. The simulation results show that the cycle performance of both systems is lower but close to the value obtained with the conventional working pair {H2O + LiBr}. Yokozeki and Shiflett [31] also examined the feasibility of different binary systems {H2O + IL} in an absorption cooling cycle and they found out that the best system is {H2O + [EMIM][DMP]}.

The performance of {H2O + LiBr} is still higher with a COP of 0.78 and a solution flow rate 53% smaller than [EMIM][DMP]. Nevertheless, the use of {H2O + [DMIM][DMP]} as the working fluid enables to work in a large range of temperatures and to stop crystallization and corrosion caused by {H2O + LiBr}. Several papers were focused on the measurements of thermodynamic properties of the binary system {H2O + [EMIM][EtSO4]} [16–25, 27, 30, 32], {H2O + [EMIM][Ac]} [33, 34], or {H2O + [HOEtMIM][Cl]} [35]. Even if these systems have some interesting properties such as low density and heat capacity or strong negative deviations from Raoult's law, no simulation of the performance of these working fluids in an AHT was presented.

### 4. Thermodynamic properties of {H2O + IL}

The knowledge of thermodynamic properties, phase behavior, and safety/environmental hazards of {H2O + IL} is required for the evaluation of this system in an AHT. The following section presents the behavior of ILs in the presence of water and the influence of their structure on thermodynamic properties.


4.1. Thermodynamic models for the representation of a binary system {H2O + IL}

Table 1. Thermodynamic parameters of different binary systems composed of {water + ILs}.

gases, and/or liquids [31, 33–35, 37–45].

4.2.1. Vapor-liquid equilibrium (VLE)

4.2. Experimental thermodynamic data of {H2O + IL}

ILs Refrigerant

1-butyl-3-methylimidazolium thiocyanate

1-butyl-3-methylimidazolium tosylate [BMIM]

1-butyl-3-methylimidazolium methylsulfate

diethylmethylammonium methanesulfonate

1-butyl-3-methylimidazolium dibutyl

[BMIM][SCN]

[BMIM][MeSO4]

([DEMA][OMs])

phosphate [BMIM][DBP]

1-hexyl-3-methylimidazolium tetrafluoroborate [HMIM][BF4])

[TOS]

A large number of thermodynamic models have been used to represent the phase diagrams of binary systems {H2O + IL}. Some groups show that the NRTL model can be successfully used to represent thermodynamic properties of systems containing ILs [7–25, 27–29, 32–37]. Alevizou et al. [38] also used the UNIFAC model to describe the phase equilibria of solvent/ ionic liquid systems. While the ionic liquids were based on an imidazolium cation and a hexafluorophosphate anion, water was considered to be the refrigerant. Two new main groups, the imidazolium and the hexafluorophosphate groups, were introduced in UNIFAC. SAFT-type equation of state was also used to represent mixtures containing ILs. This equation is a good tool to evaluate the density of pure ILs, solute activity coefficients but also VLE or LLE of binary or ternary mixtures {solute 1 + solute 2 + IL} [39, 40]. Cubic equations of state such as Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) were also used to represent quite accurate VLE, bubble point data or critical points of systems containing ionic liquids,

VLE measuring methods

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

microebulliometer, static method

microebulliometer

Water Labodest app. Cp, ρ, H<sup>E</sup> [56]

Water Ebulliometric method VLE data [36]

Water Dynamic method VLE data [27]

Water ρ [63]

Water Isobaric

Water Isobaric

Thermo.

Para. References

http://dx.doi.org/10.5772/65756

11

VLE data [39]

VLE data [39]

The experimental techniques used for VLE measurements are a boiling point technique [16–25, 30, 35, 37, 38, 46], static apparatus [32], and a quasi-static ebulliometer method [37]. The boiling point technique is the most appropriate to study mixtures containing ILs. Most of the articles related to VLE measurements concerning the binary systems {H2O + IL} are listed in Table 1. Activity coefficient can be calculated from VLE data. This parameter illustrating the deviation from ideality of the mixture can be used to investigate the interaction between H2O and ILs and the hydrophilicity of the IL [16–39]. Preferentially, ILs used in AHT might be hydrophilic


Table 1. Thermodynamic parameters of different binary systems composed of {water + ILs}.

### 4.1. Thermodynamic models for the representation of a binary system {H2O + IL}

A large number of thermodynamic models have been used to represent the phase diagrams of binary systems {H2O + IL}. Some groups show that the NRTL model can be successfully used to represent thermodynamic properties of systems containing ILs [7–25, 27–29, 32–37]. Alevizou et al. [38] also used the UNIFAC model to describe the phase equilibria of solvent/ ionic liquid systems. While the ionic liquids were based on an imidazolium cation and a hexafluorophosphate anion, water was considered to be the refrigerant. Two new main groups, the imidazolium and the hexafluorophosphate groups, were introduced in UNIFAC. SAFT-type equation of state was also used to represent mixtures containing ILs. This equation is a good tool to evaluate the density of pure ILs, solute activity coefficients but also VLE or LLE of binary or ternary mixtures {solute 1 + solute 2 + IL} [39, 40]. Cubic equations of state such as Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) were also used to represent quite accurate VLE, bubble point data or critical points of systems containing ionic liquids, gases, and/or liquids [31, 33–35, 37–45].

#### 4.2. Experimental thermodynamic data of {H2O + IL}

#### 4.2.1. Vapor-liquid equilibrium (VLE)

ILs Refrigerant

1,3-dimethylimidazolium dimethylphosphate

1-(2-hydroxyethyl)-3-methylimidazolium

10 Progress and Developments in Ionic Liquids

1-(2-hydroxyethyl)-3-methylimidazolium trifluoroacetate [HOEtMIM][TFA]

1-ethyl-3-methylimidazolium tetrafluoroborate

(trifluoromethylsulfonyl)imide [EMIM][Tf2N]

1-ethyl-3-methylimidazolium acetate [EMIM]

1-ethyl-3-methylimidazolium ethyl sulfate

1-ethyl-3-methylimidazolium diethyl

trifluoromethanesulfonate [EMIM][TFO]

1-ethyl-3-methylimidazolium trifluoroacetate

trifluoromethanesulfonate [BMIM][CF3SO3]

1-butyl-3-methylimidazolium chloride [BMIM]

1-butyl-3-methylimidazolium acetate [BMIM]

1-butyl-3-methylimidazolium trifluoroacetate

1-butyl-3-methylimidazolium bromide

phosphate [EMIM][DEP]

1-ethyl-3-methylimidazolium

1-ethyl-3-methylimidazolium methanesulfonate [EMIM][MeSO3]

1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]

1-butyl-3-methylimidazolium

1-butyl-3-methylimidazolium methanesulfonate [BMIM][C1SO3]

[DMIM][DMP]

[EMIM][BF4]

or [(CF3SO2)2N]

[EMIM][EtSO4]

[Triflate]

[EMIM][TFA]

[TFO][triflate]

[Cl]

[Ac]

[BMIM][Br]

[BMIM][CF3CO2]

[Ac]

chloride [HOEtMIM][Cl]

1-ethyl-3-methylimidazolium dimethylphosphate [EMIM][DMP]

1-ethyl-3-methylimidazolium bis-

VLE measuring methods

Water B. P. method Cp, ρ, H<sup>E</sup>

apparatus (model)

Water B. P. method Cp, ρ, H<sup>E</sup>

Water Dynamic method Cp, ρ,

method and Fischer Labodest apparatus

Water Circulation still VLE data,

Water H<sup>E</sup> [16]

Water Labodest apparatus VLE data [34, 35, 37–39]

Fischer Labodest apparatus

microebulliometer

microebulliometer

microebulliometer, static method

microebulliometer

Water B. P. method & Isobaric microebulliometer

Water B. P. method and static

Water Static method and

Water Fischer Labodest apparatus

Water B.P. method &Static method

Water Isobaric

Water Isobaric

Water Isobaric

Water Isobaric

Water Fischer Labodest

Water B. P. method VLE, Cp, ρ [35]

Water Static method ρ [63]

Water Static method VLE data [28]

1,3-dimethylimidazolium chloride [DMIM][Cl] Water B. P. method ρ [57]

Thermo.

VLE data, HE

viscosity

Cp, ρ, H<sup>E</sup> , viscosity

Cp, ρ, H<sup>E</sup> [56]

Cp, ρ, H<sup>E</sup> [51]

VLE data [39]

VLE data [39]

VLE data [39]

VLE data [34, 35, 37–39]

Cp, ρ, H<sup>E</sup> , viscosity

HE

, viscosity

Para. References

, [7, 24, 28–30]

[16]

[8, 22, 25–27, 30]

[33, 34]

[16–37]

[57]

Cp, ρ, H<sup>E</sup> [39–45, 47–56]

[26, 28, 29, 31– 45, 47–55]

> The experimental techniques used for VLE measurements are a boiling point technique [16–25, 30, 35, 37, 38, 46], static apparatus [32], and a quasi-static ebulliometer method [37]. The boiling point technique is the most appropriate to study mixtures containing ILs. Most of the articles related to VLE measurements concerning the binary systems {H2O + IL} are listed in Table 1.

> Activity coefficient can be calculated from VLE data. This parameter illustrating the deviation from ideality of the mixture can be used to investigate the interaction between H2O and ILs and the hydrophilicity of the IL [16–39]. Preferentially, ILs used in AHT might be hydrophilic

and completely water soluble. Therefore, good working pairs are those presenting a highly negative deviation from Raoult's law [47].

data of {H2O + ILs} published in the literature using Eq. (3). The mass excess heat capacity, Cp

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

where Cp is the mass heat capacity in kJ kg−<sup>1</sup> K−<sup>1</sup> of the mixture, Cp,i is the mass heat capacity of

The low mass heat capacity values of the binary systems {H2O + ILs} lead to reduce power consumption, beneficial to heat transfer and improves the COP in the absorption cycle [7, 26,

HE is a key parameter for the simulation of the performance of AHT and it also gives an insight into the interactions between the molecules. Few HE data for binary systems {H2O + ILs} can be found in the literature. This leads to make the hypothesis that HE is equaled to zero in simulation. Garcia-Miaja et al. [56] measured H<sup>E</sup> for different binary mixtures containing triflate or alkylsulfate or [BMPyr]-based ILs. All collected data show that the sign of H<sup>E</sup> is mainly related

Kurnia and Coutinho [10] reviewed the behavior of H<sup>E</sup> for binary systems composed of {H2O + ILs}. They found that the conductor-like screening model for real system (COSMO-RS) is a successful estimating method to predict the behavior of the interaction between water and ILs. It is obvious from the literature [10–16] that the positive (endothermic) H<sup>E</sup> of the binary system mainly depends on the hydrogen bonding, water molecules, and hydrophobicity. Weak interaction between the water-IL binary system causes water to use the energy of the system to rearrange their molecules and the process turn to be endothermic and the reverse occurs in the case of the exothermic process. Ficke [16] stated that with the increase of the alkyl chain length

the hydrophobicity increases hence decreasing the negativity of H<sup>E</sup>

We used Eq. (5) to regress all H<sup>E</sup> data found in the literature.

H<sup>E</sup> data found in the literature are regressed using Redlich-Kister polynomials [25]:

<sup>H</sup><sup>E</sup> <sup>¼</sup> xm1xm<sup>2</sup> <sup>∑</sup>

Density is an important property because its knowledge is necessary to evaluate the pumping cost in a process. The density of pure ILs roughly ranges between 1.1 and 1.6 g cm−<sup>3</sup>

density of an IL depends on the type of anion and cation, but the key parameter is the anion. Hydrophobicity of ILs has also an important effect on the density of binary mixtures {H2O + IL}. The hydrophobicity of a dialkylimidazolium-based IL increases with an increase of the

n i¼1 Aixm<sup>i</sup>

<sup>p</sup> ¼ Cp−∑xmi � CP,<sup>i</sup> (4)

.

, Ai is the adjustable parameter, and x mi is the mass

<sup>2</sup> (5)

. The

be calculated from the heat capacities of the mixture and that of the pure compounds: CE

the pure compound and xmi is its mass fraction.

)

28, 29, 31, 33–45, 47–55].

4.2.3. Excess enthalpy (H<sup>E</sup>

to the nature of the anion [56].

where H<sup>E</sup> is the excess enthalpy in kJ kg−<sup>1</sup>

fraction of species i (i = 1, 2).

4.2.4. Density

<sup>E</sup> can

13

http://dx.doi.org/10.5772/65756

Most binary systems {water + ILs} present activity coefficients lower than unity. The deviation from Raoult's law of {H2O + IL} is proportional to the IL content [17–19, 23–33, 36]. With respect to the anion, Ficke [16] has shown that the γ values decrease according to: [(CF3SO2)2N]<sup>−</sup> > [BF4] <sup>−</sup> > [EtSO4] <sup>−</sup> > [lactate]<sup>−</sup> > [CH3SO4] <sup>−</sup> > [glycolate]<sup>−</sup> > [(CH3)2PO4] − .

It is important to note that dialkylimidazolium [(CF3SO2)2N] with water present a miscibility gap [28]. The ability of IL to increase the water boiling temperature can be estimated using a simple relationship based on solvation model's parameters such the hydrogen-bond basicity [48].

Studies on water sorption by imidazolium-based ILs with anions [Cl]<sup>−</sup> , [BF4] − , [Br]<sup>−</sup> , [Tf2N]<sup>−</sup> , and [PF6] <sup>−</sup> show that ILs with the shorter alkyl chain length lead to the highest water sorption capacity [7, 25–37]. Moreover, it was found that imidazolium cation is more efficient than pyridinium cation [49]. Cao et al. [49] also studied the anion effect on water sorption for nine ILs with [BMIM]<sup>+</sup> cation and they found that ILs followed this trend [Ac]<sup>−</sup> > [Cl]<sup>−</sup> > [Br]<sup>−</sup> > [TFA]<sup>−</sup> > [NO3] <sup>−</sup> > [TFO]<sup>−</sup> > [BF4] <sup>−</sup> > [Tf2N]<sup>−</sup> > [CHO]<sup>−</sup> > [PF6] − . The nature of the IL anion plays an important role on the boiling temperatures and its impact is as follows: [CF3SO3] <sup>−</sup> < [SCN]<sup>−</sup> < [CF3CO2] <sup>−</sup> < [TOS]<sup>−</sup> < [Br]<sup>−</sup> < [C1SO3] <sup>−</sup> < [C1CO2] <sup>−</sup> [39]. This work shows clearly that the observed trend is related to the anion and to its capacity to interact with water. Other research groups confirmed that anion has an essential rule that aiming to lower the vapor pressure of water H2O [25, 27, 30, 32, 33]. Seiler et al. [50] have shown that some ILs such as acetate and chloride-based ionic liquids are not suitable for absorptions cycles due to their insufficient stability and/or too high corrosion rates.

All VLE of binary systems {H2O + IL} found in the literature have been correlated using the NRTL model. The average relative deviations on activity coefficient and pressure obtained using the NRTL model range between 0.01 and 3.5%. Deviations of the 35 investigated systems are within ±13%.

#### 4.2.2. Heat capacity

Heat capacity evaluates the heat storage capacity of a fluid [51]. Only one theoretical model based on an artificial neural network is proposed in the literature to predict the heat capacity of binary systems containing ILs [52]. This approach gives good estimate of Cp of mixtures containing ILs with an average absolute relative deviation of about 1.60%.

In general, heat capacity is expressed using a temperature- and composition-dependent polynomial equation [35]:

$$\mathcal{C}\_p = \sum\_{i=0}^{3} (A\_{i+}B\_i T) \text{x} m\_2 \,\,^i \tag{3}$$

where Cp is the mass heat capacity in kJ kg−<sup>1</sup> K−<sup>1</sup> , Ai and Bi are adjustable parameters, T is the absolute temperature in K, and xm<sup>2</sup> is the mass fraction of ILs. We have correlated all heat capacity data of {H2O + ILs} published in the literature using Eq. (3). The mass excess heat capacity, Cp <sup>E</sup> can be calculated from the heat capacities of the mixture and that of the pure compounds:

$$\mathbb{C}\_p^{\triangle} = \mathbb{C}\_p - \Sigma \mathbf{x} m \mathbf{i} \cdot \mathbb{C}\_{P,i} \tag{4}$$

where Cp is the mass heat capacity in kJ kg−<sup>1</sup> K−<sup>1</sup> of the mixture, Cp,i is the mass heat capacity of the pure compound and xmi is its mass fraction.

The low mass heat capacity values of the binary systems {H2O + ILs} lead to reduce power consumption, beneficial to heat transfer and improves the COP in the absorption cycle [7, 26, 28, 29, 31, 33–45, 47–55].

#### 4.2.3. Excess enthalpy (H<sup>E</sup> )

and completely water soluble. Therefore, good working pairs are those presenting a highly

Most binary systems {water + ILs} present activity coefficients lower than unity. The deviation from Raoult's law of {H2O + IL} is proportional to the IL content [17–19, 23–33, 36]. With respect to the anion, Ficke [16] has shown that the γ values decrease according to:

It is important to note that dialkylimidazolium [(CF3SO2)2N] with water present a miscibility gap [28]. The ability of IL to increase the water boiling temperature can be estimated using a simple relationship based on solvation model's parameters such the hydrogen-bond basicity [48].

capacity [7, 25–37]. Moreover, it was found that imidazolium cation is more efficient than pyridinium cation [49]. Cao et al. [49] also studied the anion effect on water sorption for nine ILs with [BMIM]<sup>+</sup> cation and they found that ILs followed this trend [Ac]<sup>−</sup> > [Cl]<sup>−</sup> > [Br]<sup>−</sup> >

<sup>−</sup> > [Tf2N]<sup>−</sup> > [CHO]<sup>−</sup> > [PF6]

<sup>−</sup> < [C1CO2]

observed trend is related to the anion and to its capacity to interact with water. Other research groups confirmed that anion has an essential rule that aiming to lower the vapor pressure of water H2O [25, 27, 30, 32, 33]. Seiler et al. [50] have shown that some ILs such as acetate and chloride-based ionic liquids are not suitable for absorptions cycles due to their insufficient

All VLE of binary systems {H2O + IL} found in the literature have been correlated using the NRTL model. The average relative deviations on activity coefficient and pressure obtained using the NRTL model range between 0.01 and 3.5%. Deviations of the 35 investigated systems

Heat capacity evaluates the heat storage capacity of a fluid [51]. Only one theoretical model based on an artificial neural network is proposed in the literature to predict the heat capacity of binary systems containing ILs [52]. This approach gives good estimate of Cp of mixtures

In general, heat capacity is expressed using a temperature- and composition-dependent poly-

absolute temperature in K, and xm<sup>2</sup> is the mass fraction of ILs. We have correlated all heat capacity

ðAiþBiTÞxm<sup>2</sup>

containing ILs with an average absolute relative deviation of about 1.60%.

Cp ¼ ∑ 3 i¼0

an important role on the boiling temperatures and its impact is as follows: [CF3SO3]

<sup>−</sup> show that ILs with the shorter alkyl chain length lead to the highest water sorption

−

<sup>−</sup> > [glycolate]<sup>−</sup> > [(CH3)2PO4]

, [BF4] − , [Br]<sup>−</sup>

. The nature of the IL anion plays

<sup>−</sup> [39]. This work shows clearly that the

<sup>i</sup> (3)

, Ai and Bi are adjustable parameters, T is the

− .

, [Tf2N]<sup>−</sup> ,

<sup>−</sup> < [SCN]<sup>−</sup>

<sup>−</sup> > [lactate]<sup>−</sup> > [CH3SO4]

Studies on water sorption by imidazolium-based ILs with anions [Cl]<sup>−</sup>

negative deviation from Raoult's law [47].

12 Progress and Developments in Ionic Liquids

<sup>−</sup> > [EtSO4]

<sup>−</sup> > [TFO]<sup>−</sup> > [BF4]

stability and/or too high corrosion rates.

where Cp is the mass heat capacity in kJ kg−<sup>1</sup> K−<sup>1</sup>

<sup>−</sup> < [TOS]<sup>−</sup> < [Br]<sup>−</sup> < [C1SO3]

[(CF3SO2)2N]<sup>−</sup> > [BF4]

and [PF6]

[TFA]<sup>−</sup> > [NO3]

are within ±13%.

4.2.2. Heat capacity

nomial equation [35]:

< [CF3CO2]

HE is a key parameter for the simulation of the performance of AHT and it also gives an insight into the interactions between the molecules. Few HE data for binary systems {H2O + ILs} can be found in the literature. This leads to make the hypothesis that HE is equaled to zero in simulation.

Garcia-Miaja et al. [56] measured H<sup>E</sup> for different binary mixtures containing triflate or alkylsulfate or [BMPyr]-based ILs. All collected data show that the sign of H<sup>E</sup> is mainly related to the nature of the anion [56].

Kurnia and Coutinho [10] reviewed the behavior of H<sup>E</sup> for binary systems composed of {H2O + ILs}. They found that the conductor-like screening model for real system (COSMO-RS) is a successful estimating method to predict the behavior of the interaction between water and ILs. It is obvious from the literature [10–16] that the positive (endothermic) H<sup>E</sup> of the binary system mainly depends on the hydrogen bonding, water molecules, and hydrophobicity. Weak interaction between the water-IL binary system causes water to use the energy of the system to rearrange their molecules and the process turn to be endothermic and the reverse occurs in the case of the exothermic process. Ficke [16] stated that with the increase of the alkyl chain length the hydrophobicity increases hence decreasing the negativity of H<sup>E</sup> .

H<sup>E</sup> data found in the literature are regressed using Redlich-Kister polynomials [25]:

$$H^E = \mathfrak{x}m\_1 \mathfrak{x}m\_2 \sum\_{i=1}^n A\_i \mathfrak{x}m\_2^i \tag{5}$$

where H<sup>E</sup> is the excess enthalpy in kJ kg−<sup>1</sup> , Ai is the adjustable parameter, and x mi is the mass fraction of species i (i = 1, 2).

We used Eq. (5) to regress all H<sup>E</sup> data found in the literature.

#### 4.2.4. Density

Density is an important property because its knowledge is necessary to evaluate the pumping cost in a process. The density of pure ILs roughly ranges between 1.1 and 1.6 g cm−<sup>3</sup> . The density of an IL depends on the type of anion and cation, but the key parameter is the anion. Hydrophobicity of ILs has also an important effect on the density of binary mixtures {H2O + IL}. The hydrophobicity of a dialkylimidazolium-based IL increases with an increase of the alkyl chain length [26, 31, 33–35, 37–45, 47–57]. Consequently, the density of such ILs decreases with the increase of the alkyl chain length.

An increase in water content or temperature causes a decrease in the density in most of the binary systems studied. Hence, physical properties of ILs can be adjusted to fulfill the needs of applications for hydrophilic ILs by adding water or changing the temperature [57].

The density data for the 19 investigated binary systems were fitted [35] using Eq. (6).

$$\rho = \sum\_{i=0}^{3} (a\_i + b\_i T) \mathbf{x}\_2^{\;i} \tag{6}$$

Nevertheless, it must be kept in mind that data taken from TGA will not serve to determine the maximum temperature limit for working without decomposition of the IL because this technique overestimates the decomposition temperature [68–70]. The experimental procedure proposed by Seiler et al. [50] based on a long-time thermal decomposition analysis seems to

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

The decomposition temperatures of ionic liquids containing the [TF2N] anion are higher than others [16–45, 47–50, 53, 54]. Ficke [16] and Seiler et al. [50] have shown that the decomposition

sition temperature mainly depends on two structural parameters: the nature of the anion and the alkyl chain length [46, 50–52, 55–71]. Ficke et al. [55] found that [EMIM][EtSO4] reacts with water to give [EMIM][HSO4] and ethanol. Another negative effect of using ILs is their hydrolysis [72]. Kinetic of hydrolysis is governed by the pH and the temperature of system [73, 74]. [TOS]-, [DMP]-, [BF4]-, and [PF6]-based ILs are known to be unstable in the presence of water

IL Decomposition temperature (°C) References

[EMIM][TFA] 178 [16] [EMIM][EtSO4] 355 [16] [EMIM][HSO4] 359 [16] [EMIM][MeSO4] 362 [16] [EMIM][TFO] 388 [16] [EMIM][MeSO3] 335 [16] [EMIM][SCN] 281 [16] [EMIM][DEP] 273 [16] [EMIM][PF6] 375–348.29 [15] [EMIM][BF4] 412–393 [4] [OHEMIM][TFA] 187 [16] [P2444][DEP] 314 [16]

This work focuses on single effect absorption heat transformers (AHT). The simulations used to evaluate the performance of the AHT were performed with the following assumptions [7–9,


http://dx.doi.org/10.5772/65756

15

be more appropriate.

temperature of [EMIM]<sup>+</sup>

under specific conditions [75].

5. Coefficient of performance (COP)

Table 2. Decomposition temperature for miscible ILs.

5.1. Simulation of the AHT cycle performance

16–76]:

i. Steady-state operation;

ii. Negligible heat loss;

where ρ is the density of the solution in g cm−<sup>3</sup> , T is the absolute temperature in K, x<sup>2</sup> <sup>i</sup> is the molar fraction of the ILs, and ai and bi are adjustable parameters.

Excess molar volume (VE ) is an important parameter for the process design while it gives information on the nonideality of the working fluid. In the case of binary mixtures {H2O + IL}, the sign of excess molar volumes is related to the structure of the IL (anion, cation, and alkyl chain length) [26, 31, 33–35, 37–45, 47–56, 58, 59]. Generally, the anion imposes the sign of VE . Hydrophobic anions ([BF4], [SCN], and [CF3SO4]) leading to repulsive interactions with water have positive excess molar volumes. Strong IL-H2O interactions observed with anions containing oxygen atoms lead to negative VE [26, 46, 60]. Gonzalez et al. [61] stated that VE behavior for their investigated binary systems according to the cation type has the following trend: imidazolium VE > pyridinium VE ≅ pyrrolidinium VE . A full description of excess molar volumes of binary mixtures containing {H2O + IL} can be found in the recent review of Bahadur et al. [62].

#### 4.2.5. Viscosity

It is well known that pure ILs have higher viscosity than other solvents such as water, methanol, and ethanol [26]. This may enlarge the AHT size (exchange area) and increase the power required for the pumping process [63]. Nevertheless, various publications [26, 31, 33– 35, 37–67] stated that viscosity of ILs sharply decreases when temperature increases and/or ILs are mixed with water. Taking into account that AHT has a high generator and absorber temperature (between 80 and 150°C), the viscosity of the {H2O + ILs} should not be a limitation for their use as absorbents in AHT [36]. The viscosity of {H2O + ILs} binary systems decreases because of the weak interaction between the IL anion and cation so the mobility of ions increases and the viscosity decreases [67]. It was noticed that fluorinated anions have lower viscosity than other anions such as alkylsulfates [64].

#### 4.2.6. Thermal decomposition

Thermal decomposition could possibly be one of the most important properties to measure during the initial screening of an IL, especially for the operating temperatures of the processes related to this work. Most of the decomposition temperatures of ILs are measured using weight loss thermogravimetric (TGA) experiments and selected data are given in Table 2.

Nevertheless, it must be kept in mind that data taken from TGA will not serve to determine the maximum temperature limit for working without decomposition of the IL because this technique overestimates the decomposition temperature [68–70]. The experimental procedure proposed by Seiler et al. [50] based on a long-time thermal decomposition analysis seems to be more appropriate.

The decomposition temperatures of ionic liquids containing the [TF2N] anion are higher than others [16–45, 47–50, 53, 54]. Ficke [16] and Seiler et al. [50] have shown that the decomposition temperature of [EMIM]<sup>+</sup> -based ILs ranges between 178°C and 388°C (Table 2). The decomposition temperature mainly depends on two structural parameters: the nature of the anion and the alkyl chain length [46, 50–52, 55–71]. Ficke et al. [55] found that [EMIM][EtSO4] reacts with water to give [EMIM][HSO4] and ethanol. Another negative effect of using ILs is their hydrolysis [72]. Kinetic of hydrolysis is governed by the pH and the temperature of system [73, 74]. [TOS]-, [DMP]-, [BF4]-, and [PF6]-based ILs are known to be unstable in the presence of water under specific conditions [75].


Table 2. Decomposition temperature for miscible ILs.

### 5. Coefficient of performance (COP)

#### 5.1. Simulation of the AHT cycle performance

This work focuses on single effect absorption heat transformers (AHT). The simulations used to evaluate the performance of the AHT were performed with the following assumptions [7–9, 16–76]:


alkyl chain length [26, 31, 33–35, 37–45, 47–57]. Consequently, the density of such ILs decreases

An increase in water content or temperature causes a decrease in the density in most of the binary systems studied. Hence, physical properties of ILs can be adjusted to fulfill the needs of

ðai þ biTÞx<sup>2</sup>

information on the nonideality of the working fluid. In the case of binary mixtures {H2O + IL}, the sign of excess molar volumes is related to the structure of the IL (anion, cation, and alkyl chain length) [26, 31, 33–35, 37–45, 47–56, 58, 59]. Generally, the anion imposes the sign of VE

Hydrophobic anions ([BF4], [SCN], and [CF3SO4]) leading to repulsive interactions with water have positive excess molar volumes. Strong IL-H2O interactions observed with anions containing oxygen atoms lead to negative VE [26, 46, 60]. Gonzalez et al. [61] stated that VE behavior for their investigated binary systems according to the cation type has the following trend: imidazolium

It is well known that pure ILs have higher viscosity than other solvents such as water, methanol, and ethanol [26]. This may enlarge the AHT size (exchange area) and increase the power required for the pumping process [63]. Nevertheless, various publications [26, 31, 33– 35, 37–67] stated that viscosity of ILs sharply decreases when temperature increases and/or ILs are mixed with water. Taking into account that AHT has a high generator and absorber temperature (between 80 and 150°C), the viscosity of the {H2O + ILs} should not be a limitation for their use as absorbents in AHT [36]. The viscosity of {H2O + ILs} binary systems decreases because of the weak interaction between the IL anion and cation so the mobility of ions increases and the viscosity decreases [67]. It was noticed that fluorinated anions have lower

Thermal decomposition could possibly be one of the most important properties to measure during the initial screening of an IL, especially for the operating temperatures of the processes related to this work. Most of the decomposition temperatures of ILs are measured using weight loss thermogravimetric (TGA) experiments and selected data are given in

mixtures containing {H2O + IL} can be found in the recent review of Bahadur et al. [62].

<sup>i</sup> (6)

<sup>i</sup> is the

.

, T is the absolute temperature in K, x<sup>2</sup>

. A full description of excess molar volumes of binary

) is an important parameter for the process design while it gives

applications for hydrophilic ILs by adding water or changing the temperature [57]. The density data for the 19 investigated binary systems were fitted [35] using Eq. (6).

> ρ ¼ ∑ 3 i¼0

with the increase of the alkyl chain length.

14 Progress and Developments in Ionic Liquids

where ρ is the density of the solution in g cm−<sup>3</sup>

VE > pyridinium VE ≅ pyrrolidinium VE

viscosity than other anions such as alkylsulfates [64].

4.2.6. Thermal decomposition

Table 2.

Excess molar volume (VE

4.2.5. Viscosity

molar fraction of the ILs, and ai and bi are adjustable parameters.


The steady-state simulation of such a process is achieved by solving mass and energy balance equations.

The generator can be described by the overall and ionic liquid mass balance and heat balance equations:

$$
\dot{m}\_7 - \dot{m}\_1 - \dot{m}\_8 = 0 \tag{7}
$$

m\_ <sup>4</sup> þ m\_ <sup>10</sup>−m\_ <sup>5</sup> ¼ 0 (17)

m\_ <sup>4</sup>h<sup>4</sup> þ m\_ <sup>10</sup>h10−m\_ <sup>5</sup>h5−QA ¼ 0 (19)

T5ðhotinletÞ−T10ðhotoutletÞ ¼ 5K (21)

m\_ <sup>5</sup>h<sup>5</sup> þ m\_ <sup>9</sup>h9−m\_ <sup>6</sup>h6−m\_ <sup>10</sup>h<sup>10</sup> ¼ 0 (22)

<sup>10</sup> (18)

http://dx.doi.org/10.5772/65756

17

<sup>5</sup> Þ (20)

), specific enthalpy

=1000 (24)

(T) is the saturated vapor

<sup>1</sup>ðTÞ (23)

<sup>1</sup> is the IL mole fraction in the liquid phase, ionic liquid

<sup>2</sup> h<sup>2</sup> þ Δmixh (25)

m\_ <sup>5</sup>xm

State point 5 is described by Eq. (20)

Heat balance on the solution heat exchanger can be written:

where T<sup>5</sup> = TA and p<sup>5</sup> = pA = pE

and cold streams:

where m . <sup>i</sup>, hi, xm

(kJ kg−<sup>1</sup>

tively.

with

relations [25]:

ps

<sup>ð</sup>TÞ ¼ exp

with p is the total pressure, x2, γ<sup>2</sup> and ps

The enthalpy of a liquid mixture is expressed as follows:

<sup>h</sup> <sup>¼</sup> xm

<sup>1</sup> <sup>h</sup><sup>1</sup> <sup>þ</sup> xm

<sup>5</sup> <sup>¼</sup> <sup>m</sup>\_ <sup>10</sup>xm

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

<sup>p</sup><sup>5</sup> <sup>¼</sup> <sup>p</sup>ðT5, xm

Then, the heat exchanger is characterized by the minimal temperature approach between hot

<sup>i</sup> (<sup>i</sup> =1, 2, 3, …, 10) are, respectively, the mass flow rate (kg s−<sup>1</sup>

<sup>p</sup>ðT, <sup>x</sup>Þ ¼ <sup>x</sup>2γ2ps

activity coefficient for the liquid phase, and saturated vapor pressure of the refrigerant, respec-

Usually, when simulating an absorption heat transformer, the temperature level of the waste heat source is known (the medium-temperature level) as well as the temperature of the environment that is used as cold heat sink (the low-temperature level). The objective temperature level of the upgraded heat is also an input in this problem (the high-temperature level). Hence, temperatures TG, TE, TC and TA of the generator, the evaporator, the condenser, and the absorber, respectively, are known and taken as independent variables in the present research.

73:649−7258:2=T−7:3037∗lnðTÞ þ 4:1653∗10−6∗ðT2Þ

pressure of H2O at temperature T and p(T, x) is the saturation pressure of the {H2O + ILs} solution at temperature T with an ionic mole fraction x. They are obtained by the following

), and the mass fraction of an absorbent (IL) of each stream. ps

$$
\dot{m}\_7 \mathbf{x}\_7^m = \dot{m}\_8 \mathbf{x}\_8^m \tag{8}
$$

$$\mathbf{Q}\_{\mathbb{G}} + \dot{m}\_{7}h\_{7} - \dot{m}\_{8}h\_{8} - \dot{m}\_{1}h\_{1} = 0\tag{9}$$

The strong solution at the outlet of the generator is a saturated liquid,

$$p\_8 = p(T\_8, \boldsymbol{x}\_8^m) \tag{10}$$

where T<sup>8</sup> = T<sup>1</sup> =TG and p<sup>8</sup> = P<sup>1</sup> =pG = pc

The condenser can be described by

$$
\dot{m}\_1 = \dot{m}\_2 = \dot{m}\_3 = 1 \text{kg.s}^{-1} \tag{11}
$$

$$
\dot{m}\_1 h\_1 - \dot{m}\_2 h\_2 - Q\_c = 0 \tag{12}
$$

For state point 2 (saturated liquid water at the condenser outlet), we have:

$$p\_2 = p\_c = p^s(T\_c) \tag{13}$$

In the case of the evaporator:

$$
\dot{m}\_3 = \dot{m}\_4 \tag{14}
$$

$$
\dot{m}\_4 h\_4 - \dot{m}\_3 h\_3 - Q\_E = 0 \tag{15}
$$

Vapor is saturated at the outlet of the evaporator, so for point 4, we have:

$$p\_4 = p\_E = p^s(T\_E) \tag{16}$$

Balance equations for the absorber give:

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 17

$$
\dot{m}\_4 + \dot{m}\_{10} - \dot{m}\_5 = 0 \tag{17}
$$

$$
\dot{m}\_5 \mathbf{x}\_5^m = \dot{m}\_{10} \mathbf{x}\_{10}^m \tag{18}
$$

$$
\dot{m}\_4 \dot{h}\_4 + \dot{m}\_{10} \dot{h}\_{10} - \dot{m}\_5 \dot{h}\_5 - Q\_A = 0 \tag{19}
$$

State point 5 is described by Eq. (20)

$$p\_5 = p(T\_5, \boldsymbol{\chi}\_5^m) \tag{20}$$

where T<sup>5</sup> = TA and p<sup>5</sup> = pA = pE

iii. Pressure drops not taken into account;

heat exchanger;

16 Progress and Developments in Ionic Liquids

where T<sup>8</sup> = T<sup>1</sup> =TG and p<sup>8</sup> = P<sup>1</sup> =pG = pc The condenser can be described by

In the case of the evaporator:

Balance equations for the absorber give:

equations.

equations:

iv. Outlets of the generator and the absorber are liquids at their bubble point;

viii. Pumping mechanical power is neglected compared to heat flow exchanged.

vi. Enthalpy of the fluid is conserved through the throttling valve;

v. Liquid and vapor at the outlet of the condenser and the evaporator are saturated;

vii. Minimum temperature difference between strong and weak solutions equal 5°C in the

The steady-state simulation of such a process is achieved by solving mass and energy balance

The generator can be described by the overall and ionic liquid mass balance and heat balance

<sup>7</sup> <sup>¼</sup> <sup>m</sup>\_ <sup>8</sup>xm

<sup>p</sup><sup>8</sup> <sup>¼</sup> <sup>p</sup>ðT8, xm

m\_ <sup>1</sup> ¼ m\_ <sup>2</sup> ¼ m\_ <sup>3</sup> ¼ 1kg:s

<sup>p</sup><sup>2</sup> <sup>¼</sup> pc <sup>¼</sup> ps

<sup>p</sup><sup>4</sup> <sup>¼</sup> pE <sup>¼</sup> ps

m\_ <sup>7</sup>xm

The strong solution at the outlet of the generator is a saturated liquid,

For state point 2 (saturated liquid water at the condenser outlet), we have:

Vapor is saturated at the outlet of the evaporator, so for point 4, we have:

m\_ <sup>7</sup>−m\_ <sup>1</sup>−m\_ <sup>8</sup> ¼ 0 (7)

QG þ m\_ <sup>7</sup>h7−m\_ <sup>8</sup>h8−m\_ <sup>1</sup>h<sup>1</sup> ¼ 0 (9)

m\_ <sup>1</sup>h1−m\_ <sup>2</sup>h2−Qc ¼ 0 (12)

<sup>8</sup> (8)

<sup>8</sup> Þ (10)

�<sup>1</sup> (11)

ðTcÞ (13)

ðTEÞ (16)

m\_ <sup>3</sup> ¼ m\_ <sup>4</sup> (14)

m\_ <sup>4</sup>h4−m\_ <sup>3</sup>h3−QE ¼ 0 (15)

Then, the heat exchanger is characterized by the minimal temperature approach between hot and cold streams:

$$T\_5(\text{hotlet}) - T\_{10}(\text{hotoutlet}) = 5\text{K} \tag{21}$$

Heat balance on the solution heat exchanger can be written:

$$
\dot{m}\_5 \text{h}\_5 + \dot{m}\_9 \text{h}\_9 - \dot{m}\_6 \text{h}\_6 - \dot{m}\_{10} \text{h}\_{10} = 0 \tag{22}
$$

where m . <sup>i</sup>, hi, xm <sup>i</sup> (<sup>i</sup> =1, 2, 3, …, 10) are, respectively, the mass flow rate (kg s−<sup>1</sup> ), specific enthalpy (kJ kg−<sup>1</sup> ), and the mass fraction of an absorbent (IL) of each stream. ps (T) is the saturated vapor pressure of H2O at temperature T and p(T, x) is the saturation pressure of the {H2O + ILs} solution at temperature T with an ionic mole fraction x. They are obtained by the following relations [25]:

$$p(T, \mathbf{x}) = \mathbf{x}\_2 \mathbf{y}\_2 p\_1^s(T) \tag{23}$$

$$p^s(T) = \exp\left(73.649 \text{--} 7258.2/T \text{--} 7.3037 \ast \ln(T) + 4.1653 \ast 10 \text{--} 6 \ast (T \text{2})\right) / 1000 \tag{24}$$

with p is the total pressure, x2, γ<sup>2</sup> and ps <sup>1</sup> is the IL mole fraction in the liquid phase, ionic liquid activity coefficient for the liquid phase, and saturated vapor pressure of the refrigerant, respectively.

Usually, when simulating an absorption heat transformer, the temperature level of the waste heat source is known (the medium-temperature level) as well as the temperature of the environment that is used as cold heat sink (the low-temperature level). The objective temperature level of the upgraded heat is also an input in this problem (the high-temperature level). Hence, temperatures TG, TE, TC and TA of the generator, the evaporator, the condenser, and the absorber, respectively, are known and taken as independent variables in the present research.

The enthalpy of a liquid mixture is expressed as follows:

$$h = \mathbf{x}\_1^m h\_1 + \mathbf{x}\_2^m h\_2 + \Delta\_{\text{mix}} h \tag{25}$$

with

$$h\_1 = h\_{\rm ref} + \frac{1}{\rho\_{1, \rm liq, T\_{\rm ref}}} \* (p - p\_{\rm ref}) + \int\_{T\_{\rm ref}}^{T} \mathbb{C}\_{p, 1} dT \tag{26}$$

<sup>q</sup> <sup>¼</sup> QA m\_ <sup>1</sup>

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

The performance of {H2O + ILs} as working fluids was mainly evaluated for the absorption refrigeration cycle. Zhang and Hu [27] estimated the COP of an absorption chiller using {water + [EMIM][DMP]} and {H2O + LiBr} mixtures under the same operating conditions. {H2O + [EMIM][DMP]} leads to a value of COP higher than 0.7 that is lower than that obtained with

Under precise conditions, Kim et al. [63] found that the COP value for a {H2O + [EMIM][BF4]} based refrigeration cycle can reach 0.91, this good performance is linked to the suitable compatibility of water with [EMIM][BF4] and to the excellent intrinsic properties of water as a

The {H2O + [DMIM][DMP]} mixture has been studied by Dong et al. [7] and was compared to {H2O + LiBr} in a single-effect absorption refrigeration configuration. The ionic liquid-based working mixture leads to close performance to those obtained with the conventional mixture. Nevertheless, {H2O + [DMIM][DMP]} presents the advantage to allow a wider temperature

A VBA dedicated calculation code has been developed to evaluate the performance of {H2O +

Simulation results for both {H2O + IL} mixtures and conventional fluids as working fluids in

IL} mixture as a working fluid in an absorption heat transformer (Figures 4 and 5).

working range as well as avoiding corrosion and crystallization problems.

5.2. COP for the absorption refrigeration cycle

{H2O + LiBr}.

refrigerant.

5.3. Absorption heat transformer

AHT are presented in Table 3.

Figure 4. Thermodynamic cycle in absorption heat transformer (AHT).

(31)

19

http://dx.doi.org/10.5772/65756

$$h\_2 = h\_{\rm ref} + \frac{1}{\rho\_{2, \rm liq, T\_{\rm ref}}} \* (p - p\_{\rm ref}) + \int\_{T\_{\rm ref}}^{T} \mathbb{C}\_{p, 2} dT \tag{27}$$

where h<sup>1</sup> and h<sup>2</sup> are the enthalpy of pure liquid H2O and IL, xm <sup>1</sup> and xm <sup>2</sup> are the mass fraction of H2O and IL, respectively, Δmixh is mixing enthalpy of the system, which can be sometimes neglected. Cp,1 and Cp,2 are the heat capacity of H2O and IL, respectively. The reference state for enthalpy calculations is defined Tref, href, and pref, respectively, its temperature, enthalpy, and pressure. These parameters are chosen arbitrarily as being:

href = 0

Tref = 273.15 K

pref = 101.325 kPa

ρ1,liq,Tref and ρ2,liq,Tref are the densities of pure liquid water and pure IL at reference temperature and pressure, respectively.

The performance of the AHT is evaluated through different criteria. The main one is the coefficient of performance. Its expression is given in Eq. (2) as the ratio of useful heat flow produced at the absorber to the waste heat flows provided to the generator and to the evaporator (pumping work is neglected).

Among other meaningful criteria, Δxm is the difference between ionic liquid mass fractions in the strong and weak solutions.

$$
\Delta \mathbf{x}^{m} = \mathbf{x}\_{8}^{m} - \mathbf{x}\_{5}^{m} = \mathbf{x}\_{s}^{m} - \mathbf{x}\_{w}^{m} \tag{28}
$$

If COP is used to represent the quantitative aspect of heat upgrading, the gross temperature lift Δt, which is the temperature level difference between the upgraded heat and the waste heat, provides a qualitative performance criterion. It is defined as follows:

$$
\Delta t = T\_A - T\_E \tag{29}
$$

Another important criterion is the solution circulation ratio f, which is defined as the ratio of the strong solution mass flow rate to the vapor mass flow rate:

$$f = \frac{\dot{m}\_7}{\dot{m}\_1 = \frac{\chi\_r^m}{(\chi\_s^m - \chi\_w^m)}}\tag{30}$$

This criterion allows knowing if the use of one working mixture leads or not to high solution flow rate which is linked to the capital cost (cost of the required working mixture and pumps) and operating costs (pumping energy cost). Observed values of f for the {H2O + LiBr} and {H2O + NH3} systems are generally low (typically around 10 [2–16]).

Another criterion to assess system compactness is the available heat output per unit mass of refrigerant, q (kJ kg–<sup>1</sup> ):

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 19

$$q = \frac{Q\_A}{\dot{m}\_1} \tag{31}$$

#### 5.2. COP for the absorption refrigeration cycle

h<sup>1</sup> ¼ href þ

h<sup>2</sup> ¼ href þ

where h<sup>1</sup> and h<sup>2</sup> are the enthalpy of pure liquid H2O and IL, xm

pressure. These parameters are chosen arbitrarily as being:

href = 0

Tref = 273.15 K

pref = 101.325 kPa

ture and pressure, respectively.

18 Progress and Developments in Ionic Liquids

the strong and weak solutions.

refrigerant, q (kJ kg–<sup>1</sup>

evaporator (pumping work is neglected).

1 ρ<sup>1</sup>;liq,Tref

1 ρ<sup>2</sup>;liq,Tref

H2O and IL, respectively, Δmixh is mixing enthalpy of the system, which can be sometimes neglected. Cp,1 and Cp,2 are the heat capacity of H2O and IL, respectively. The reference state for enthalpy calculations is defined Tref, href, and pref, respectively, its temperature, enthalpy, and

ρ1,liq,Tref and ρ2,liq,Tref are the densities of pure liquid water and pure IL at reference tempera-

The performance of the AHT is evaluated through different criteria. The main one is the coefficient of performance. Its expression is given in Eq. (2) as the ratio of useful heat flow produced at the absorber to the waste heat flows provided to the generator and to the

Among other meaningful criteria, Δxm is the difference between ionic liquid mass fractions in

<sup>8</sup> −xm <sup>5</sup> <sup>¼</sup> xm <sup>s</sup> −xm

If COP is used to represent the quantitative aspect of heat upgrading, the gross temperature lift Δt, which is the temperature level difference between the upgraded heat and the waste heat,

Another important criterion is the solution circulation ratio f, which is defined as the ratio of

This criterion allows knowing if the use of one working mixture leads or not to high solution flow rate which is linked to the capital cost (cost of the required working mixture and pumps) and operating costs (pumping energy cost). Observed values of f for the {H2O + LiBr} and {H2O

Another criterion to assess system compactness is the available heat output per unit mass of

<sup>f</sup> <sup>¼</sup> <sup>m</sup>\_ <sup>7</sup> <sup>m</sup>\_ <sup>1</sup> <sup>¼</sup> <sup>x</sup><sup>m</sup> s ðxm <sup>s</sup> −xm w Þ

<sup>Δ</sup>xm <sup>¼</sup> <sup>x</sup><sup>m</sup>

provides a qualitative performance criterion. It is defined as follows:

the strong solution mass flow rate to the vapor mass flow rate:

+ NH3} systems are generally low (typically around 10 [2–16]).

):

∗ðp−prefÞ þ ∫

∗ðp−prefÞ þ ∫

T Tref

T Tref

<sup>1</sup> and xm

Cp,1dT (26)

Cp,2dT (27)

<sup>w</sup> (28)

(30)

Δt ¼ TA−TE (29)

<sup>2</sup> are the mass fraction of

The performance of {H2O + ILs} as working fluids was mainly evaluated for the absorption refrigeration cycle. Zhang and Hu [27] estimated the COP of an absorption chiller using {water + [EMIM][DMP]} and {H2O + LiBr} mixtures under the same operating conditions. {H2O + [EMIM][DMP]} leads to a value of COP higher than 0.7 that is lower than that obtained with {H2O + LiBr}.

Under precise conditions, Kim et al. [63] found that the COP value for a {H2O + [EMIM][BF4]} based refrigeration cycle can reach 0.91, this good performance is linked to the suitable compatibility of water with [EMIM][BF4] and to the excellent intrinsic properties of water as a refrigerant.

The {H2O + [DMIM][DMP]} mixture has been studied by Dong et al. [7] and was compared to {H2O + LiBr} in a single-effect absorption refrigeration configuration. The ionic liquid-based working mixture leads to close performance to those obtained with the conventional mixture. Nevertheless, {H2O + [DMIM][DMP]} presents the advantage to allow a wider temperature working range as well as avoiding corrosion and crystallization problems.

#### 5.3. Absorption heat transformer

A VBA dedicated calculation code has been developed to evaluate the performance of {H2O + IL} mixture as a working fluid in an absorption heat transformer (Figures 4 and 5).

Figure 4. Thermodynamic cycle in absorption heat transformer (AHT).

Simulation results for both {H2O + IL} mixtures and conventional fluids as working fluids in AHT are presented in Table 3.

Refrigerant Absorbent COP xm

Water LiBr<sup>1</sup>

TFE E181<sup>2</sup>

Water LiBr<sup>4</sup>

Water [EMIM][DMP]<sup>3</sup>

<sup>s</sup> Δxm f

http://dx.doi.org/10.5772/65756

21

T (°C) TC TE TG TA

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

\* 0.50 0.64 0.07 11.00

\* 0.42 0.90 0.10 9.00

Water [DMIM][DMP] 0.45 0.92 0.09 10.01 Water [EMIM][DMP] 0.44 0.92 0.08 11.26 Water [EMIM][DEP] 0.44 0.92 0.07 13.54 Water [EMIM][AC] 0.44 0.87 0.08 10.54 Water [HOEtMIM][Cl] 0.45 0.94 0.10 9.88 Water [EMIM][EtSO4] 0.41 0.97 0.03 38.21 Water [EMIM][Triflate] 0.42 0.99 0.02 41.34 Water [EMIM][TFA] 0.44 0.97 0.05 19.70 Water [DEMA][Oms] 0.44 0.95 0.08 12.63 Water [BMIM][Triflate] 0.36 0.99 0.01 71.56 Water [BMIM][BF4] 0.43 0.97 0.04 25.01

Water [DMIM][DMP] 0.43 0.93 0.06 15.92 Water [EMIM][DMP] 0.41 0.93 0.05 19.52 Water [EMIM][DEP] 0.42 0.92 0.04 21.06 Water [EMIM][AC] 0.43 0.87 0.05 17.98 Water [HOEtMIM][Cl] 0.43 0.95 0.06 15.58 Water [EMIM][EtSO4] 0.14 0.97 0.01 172.47 Water [EMIM][Triflate] 0.38 0.99 0.01 69.49 Water [EMIM][TFA] 0.42 0.97 0.03 31.99 Water [DEMA][Oms] 0.43 0.96 0.05 21.07 Water [BMIM][Triflate] 0.27 0.99 0.01 140.50 Water [BMIM][BF4] 0.40 0.97 0.02 43.60

Water [DMIM][DMP] 0.44 0.94 0.07 12.90 Water [EMIM][DMP] 0.42 0.94 0.06 15.09 Water [EMIM][AC] 0.43 0.89 0.06 14.16 Water [EMIM][DEP] 0.43 0.88 0.93 17.71 Water [HOEtMIM][Cl] 0.43 0.96 0.07 13.14 Water [EMIM][EtSO4] 0.35 0.98 0.01 74.48

35 90 90 130

\* 0.48 Not mentioned Not mentioned Not mentioned

25 80 80 130

20 80 80 130

\*\* 0.48 Not mentioned Not mentioned 9.51

Figure 5. Flowchart for COP simulation.

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 21


Figure 5. Flowchart for COP simulation.

20 Progress and Developments in Ionic Liquids


\* Data found in the literature [8].\*\* Data found in the literature [60].

Table 3. Calculated COP of {H2O + ILs} binary systems for single-effect absorption heat transformer cycle.

In this work, evaporating temperature TE, condensing temperature TC, absorbing temperature TA, and generator temperature TG are set to 80°C, 20°C, 130°C, and 80°C, respectively (Figures 6 and 7 and Table 3).

> Figure 8 shows that an increase in the condenser temperature leads to a decrease in the COP. This behavior is due to the fact that the low pressure level evolves the same way as the condenser temperature. Hence, when the condenser temperature increases, the strong solution ionic liquid fraction will decrease and f increases. For the investigated working pairs, the COP remains unchanged for TC lower than 30°C. The COP sharply decreases especially for {H2O + LiBr}, {H2O + [BMIM][MeSO4]}, {H2O + [BMIM][Triflate]}, and {H2O + [EMIM][EtSO4]} when

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

http://dx.doi.org/10.5772/65756

23

Figure 8. COP of binary systems {water + ILs} versus (Tc) TL for single-effect absorption heat transformer cycle.

TC is higher than 30°C.

Figure 7. Circulation ratio f of binary systems {water + ILs} presented.

Figure 6. Calculated COP of binary systems {water + ILs} presented.

The NRTL model, Cp, H<sup>E</sup> , and density correlation parameters that were regressed by the authors and used for the simulations. The resulting calculated COP values for 12 binary systems are shown in Table 3.

The influence of the working temperature levels on the COP is shown in Figures 8–10.

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 23

Figure 7. Circulation ratio f of binary systems {water + ILs} presented.

In this work, evaporating temperature TE, condensing temperature TC, absorbing temperature TA, and generator temperature TG are set to 80°C, 20°C, 130°C, and 80°C, respectively (Fig-

Water [EMIM][Triflate] 0.39 0.99 0.02 60.08 Water [EMIM][TFA] 0.42 0.98 0.04 26.58 Water [DEMA][Oms] 0.43 0.97 0.06 17.30 Water [BMIM][Triflate] 0.32 0.99 0.01 108.71 Water [BMIM][BF4] 0.41 0.98 0.03 34.96

Table 3. Calculated COP of {H2O + ILs} binary systems for single-effect absorption heat transformer cycle.

T (°C) TC TE TG TA

<sup>s</sup> Δxm f

, and density correlation parameters that were regressed by the

authors and used for the simulations. The resulting calculated COP values for 12 binary

The influence of the working temperature levels on the COP is shown in Figures 8–10.

ures 6 and 7 and Table 3).

Refrigerant Absorbent COP xm

22 Progress and Developments in Ionic Liquids

\* Data found in the literature [8].\*\* Data found in the literature [60].

The NRTL model, Cp, H<sup>E</sup>

systems are shown in Table 3.

Figure 6. Calculated COP of binary systems {water + ILs} presented.

Figure 8 shows that an increase in the condenser temperature leads to a decrease in the COP. This behavior is due to the fact that the low pressure level evolves the same way as the condenser temperature. Hence, when the condenser temperature increases, the strong solution ionic liquid fraction will decrease and f increases. For the investigated working pairs, the COP remains unchanged for TC lower than 30°C. The COP sharply decreases especially for {H2O + LiBr}, {H2O + [BMIM][MeSO4]}, {H2O + [BMIM][Triflate]}, and {H2O + [EMIM][EtSO4]} when TC is higher than 30°C.

Figure 8. COP of binary systems {water + ILs} versus (Tc) TL for single-effect absorption heat transformer cycle.

Figure 9 shows that an increase of TE or TG leads to an increase of the COP. In fact, the high pressure level of AHT depends on the evaporator temperature. Increasing TE (or TG) leads to a decrease of the weak solution concentration by decreasing the flow ratio. The lower flow ratio results in a higher heat flow released during absorption and consequently in a higher COP. Figure 9 shows that the evolution of the COP values versus the generator temperature is quite similar to {H2O + IL} for all binary systems studied in this work. The evolution of COP values with TG firstly increases, then stabilizes and finally decreases. When TG approaches its minimal value, f tends to reach infinity and so it requires the generation of heat. Consequently, the COP of the cycle tends toward zero. With the increase of generator temperature, f decreases, COP sharply increases and then smoothens.

Figure 10. COP of binary systems {water + ILs} versus (TA) TH for single-effect absorption heat transformer cycle.

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

http://dx.doi.org/10.5772/65756

25

Figure 11. Effect of (TA) TH on x<sup>m</sup>

w .

Figure 9. COP of binary systems {water + ILs} versus (TG, E) TM for single-effect absorption heat transformer cycle.

It can be seen from Figure 10 that the COP of an AHTdecreases at different rates depending on the working mixture when absorber temperature (TA) increases. This behavior can be explained in Figure 11 that illustrates the ionic liquid mass fraction variation of the weak solution xm <sup>w</sup> with TA.

A decrease of xm <sup>w</sup> means that the less refrigerant has been absorbed and consequently less heat is released at the absorber, which leads to lower the COP. The same behavior was observed by Zhang and Hu [8]. Figure 10 shows that the COP of all the binary systems {H2O + IL} as well as {H2O + LiBr} is basically unchanged when the gross temperature lift is lower than 45 °C. Upon increasing the gross temperature lift more than 45°C, the COP of {H2O + LiBr} and {H2O + ILs} sharply decreases.

The available heat output per unit mass of refrigerant (q) for the studied binary systems was calculated and compared with Zhang and Hu [8] data under the same conditions. It was found that q for {H2O + [DMIM][DMP]} is 2029 kJ kg<sup>−</sup><sup>1</sup> and for {H2O + [HOEtMIM][Cl]} is less than 2026 kJ kg−<sup>1</sup> . For the other binary systems, q ranges between 2012 and 1527 kJ kg−<sup>1</sup> (Table 4).

These values must be compared with those obtained for {H2O + LiBr}: 2466 kJ kg<sup>−</sup><sup>1</sup> and 311 kJ kg−<sup>1</sup> for {TFE + E181}. Hence, to produce the same amount of useful heat, the refrigerant flow rate is lower when using water-based mixtures (water latent heat of vaporization is much higher than that of E181).

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers? http://dx.doi.org/10.5772/65756 25

Figure 10. COP of binary systems {water + ILs} versus (TA) TH for single-effect absorption heat transformer cycle.

Figure 11. Effect of (TA) TH on x<sup>m</sup> w .

Figure 9 shows that an increase of TE or TG leads to an increase of the COP. In fact, the high pressure level of AHT depends on the evaporator temperature. Increasing TE (or TG) leads to a decrease of the weak solution concentration by decreasing the flow ratio. The lower flow ratio results in a higher heat flow released during absorption and consequently in a higher COP. Figure 9 shows that the evolution of the COP values versus the generator temperature is quite similar to {H2O + IL} for all binary systems studied in this work. The evolution of COP values with TG firstly increases, then stabilizes and finally decreases. When TG approaches its minimal value, f tends to reach infinity and so it requires the generation of heat. Consequently, the COP of the cycle tends toward zero. With the increase of generator temperature, f decreases,

It can be seen from Figure 10 that the COP of an AHTdecreases at different rates depending on the working mixture when absorber temperature (TA) increases. This behavior can be explained in

Figure 9. COP of binary systems {water + ILs} versus (TG, E) TM for single-effect absorption heat transformer cycle.

released at the absorber, which leads to lower the COP. The same behavior was observed by Zhang and Hu [8]. Figure 10 shows that the COP of all the binary systems {H2O + IL} as well as {H2O + LiBr} is basically unchanged when the gross temperature lift is lower than 45 °C. Upon increasing the gross temperature lift more than 45°C, the COP of {H2O + LiBr} and {H2O + ILs} sharply

The available heat output per unit mass of refrigerant (q) for the studied binary systems was calculated and compared with Zhang and Hu [8] data under the same conditions. It was found that q for {H2O + [DMIM][DMP]} is 2029 kJ kg<sup>−</sup><sup>1</sup> and for {H2O + [HOEtMIM][Cl]} is less than

These values must be compared with those obtained for {H2O + LiBr}: 2466 kJ kg<sup>−</sup><sup>1</sup> and 311 kJ kg−<sup>1</sup> for {TFE + E181}. Hence, to produce the same amount of useful heat, the refrigerant flow rate is lower when using water-based mixtures (water latent heat of vaporization is much

. For the other binary systems, q ranges between 2012 and 1527 kJ kg−<sup>1</sup> (Table 4).

<sup>w</sup> means that the less refrigerant has been absorbed and consequently less heat is

<sup>w</sup> with TA.

Figure 11 that illustrates the ionic liquid mass fraction variation of the weak solution xm

COP sharply increases and then smoothens.

24 Progress and Developments in Ionic Liquids

A decrease of xm

decreases.

2026 kJ kg−<sup>1</sup>

higher than that of E181).


In the light of these results, it would be highly recommended to further investigate these

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

http://dx.doi.org/10.5772/65756

27

A large number of binary mixtures {H2O + ILs} have been identified to be used in absorption heat transformers. The resulting performances of these new working fluids were evaluated for

Ionic liquid-based working mixtures lead to slightly low COP than the classical {H2O + LiBr} mixture and larger circulation ratios. Nevertheless, the possibility to find ILs that are significantly less corrosive than LiBr is a condition for reliable operation and a moderate investment cost. Moreover, many ILs are totally miscible with water which avoid crystallization problems. It must be kept in mind that thermal and chemical stability of {H20 + IL} mixtures have to be

Laboratoire Réactions et Génie des Procédés (CNRS UMR 7274), Ecole Nationale Supérieure

[1] Horuz I, Kurt B. Absorption heat transformers and an industrial application. Renewable

[2] Khamooshi M, Parham K, Atikol U. Overview of ionic liquids used as working fluids in absorption cycles. Advances in Mechanical Engineering. 2013;2013:1–7. DOI: org/10.1155/

[3] Sun J, Fu L, Zhang S. A review of working fluids of absorption cycles. Renewable and Sustainable Energy Reviews. 2012;16:1899–1906. DOI: 10.1016/j.rser.2012.01.011

[4] Schaefer LA. Single pressure absorption heat pump analysis [thesis]. Georgia: Georgia

[5] Wu W, Wang B, Shi W, Li X. An overview of ammonia-based absorption chillers and heat pumps. Renewable and Sustainable Energy Reviews. 2014;31:681−707. DOI: org/10.1016/

assessed in order to prove their practical use for industrial applications.

El-Shaimaa Abumandour, Fabrice Mutelet\* and Dominique Alonso

\*Address all correspondence to: fabrice.mutelet@univ-lorraine.fr

des Industries Chimiques, Université de Lorraine, Nancy, France

Energy. 2010;35:2175–81. DOI: 10.1016/j.renene.2010.02.025

binary systems.

6. Conclusion

Author details

References

2013/620592

j.rser.2013.12.021

Institute of Technology; 2000.

single-effect absorption heat transformer cycles.

Table 4. The available heat output per unit mass of refrigerant (q) for AHT cycle.

The concentration (mass fraction) of ILs in the strong solution is exceeding 0.9 for most of the binary systems studied, and is only 0.64 for {H2O + LiBr} (Table 3). This behavior is not in favor of ionic liquid-based working mixtures and will particularly lead to increased pumping costs.

Simulation results show that for {H2O + [DMIM][DMP]}, {H2O + [HOEtMIM][Cl]}, {H2O + [EMIM][Ac]}, {H2O + [EMIM][TFA]}, and {H2O + [DEMA][OMS]} mixtures, COP values are close to, but lower than, that obtained working with {H2O + LiBr}. Nevertheless, these slightly low performances of ionic liquid-based mixtures can be counterbalanced by the ability to reach higher gross temperature lifts and to potentially avoid corrosion issues.

Simulations for evaporator temperature TE, condenser temperature TC, absorber temperature TA, and generator temperature TG are set to 80, 20, 130, and 80 °C, respectively, which shows that the COP values for the studied binary systems have the following behavior:

[DMIM][DMP] > [HOEtMIM][Cl] > [EMIM][Ac] > [DEMA][OMS] > [EMIM][TFA] > [EMIM] [DMP] > [BMIM][BF4] > [EMIM][Triflate] > [EMIM][EtSO4] > [BMIM][Triflate]. We can conclude that ionic liquids with a short-alkyl chain lead to higher COP values and a lower circulation ratio f.

Simulations indicate that binary systems {H2O + acetate or chloride-based ILs} have high COP. Nonetheless, these families of ILs are not sufficiently stable and they present high corrosion rates [50]. It was noticed that binary system composed of {H2O + [BMIM][MeSO4]} exhibits smaller Δt than the other investigated 11 systems. Its largest observed Δt is about 40 K and the lowest is 30 K. Simulation of the binary system consisting of {H2O + [BMIM][MeSO4]} is not promising due to low solubility of IL in water or to stability of the IL [16].

Thirty-three binary systems out of 39 are found to have only VLE data available, the literature lacking other thermodynamic properties. Using NRTL, the VLE data of these 33 binary systems were correlated, and f was determined. Simulations for these binary systems performed with evaporator temperature TE, condenser temperature TC, absorber temperature TA, and generator temperature TG are set to 80°C, 20°C, 130°C, and 80°C, respectively. Simulation results showed that there are promising binary systems exhibiting low f values such as [BMIM][C1SO3], [BMIM][I], [BMPyr][DCA], which are 19.892, 16.958, and 17.247, respectively. In the light of these results, it would be highly recommended to further investigate these binary systems.

### 6. Conclusion

A large number of binary mixtures {H2O + ILs} have been identified to be used in absorption heat transformers. The resulting performances of these new working fluids were evaluated for single-effect absorption heat transformer cycles.

Ionic liquid-based working mixtures lead to slightly low COP than the classical {H2O + LiBr} mixture and larger circulation ratios. Nevertheless, the possibility to find ILs that are significantly less corrosive than LiBr is a condition for reliable operation and a moderate investment cost. Moreover, many ILs are totally miscible with water which avoid crystallization problems.

It must be kept in mind that thermal and chemical stability of {H20 + IL} mixtures have to be assessed in order to prove their practical use for industrial applications.

### Author details

The concentration (mass fraction) of ILs in the strong solution is exceeding 0.9 for most of the binary systems studied, and is only 0.64 for {H2O + LiBr} (Table 3). This behavior is not in favor of ionic liquid-based working mixtures and will particularly lead to increased pumping costs.

TC TG TE TA

20 80 80 130

{H2O + [DMIM][DMP]} 1982.31 {H2O + [EMIM][TFA]} 1871.63 {H2O + [EMIM][DMP]} 1867.90 {H2O + [HOEtMIM][Cl]} 1974.05 {H2O + [EMIM][EtSO4]} 1391.97 {H2O + [BMIM][Triflate]} 1198.78 {H2O + [EMIM][AC]} 1953.77 {H2O + [BMIM][BF4]} 1777.21 {H2O + [EMIM][Triflate]} 1636.28 {H2O + [DEMA][OMs]} 1932.00

) Binary system q (kJ kg−<sup>1</sup>

)

Simulation results show that for {H2O + [DMIM][DMP]}, {H2O + [HOEtMIM][Cl]}, {H2O + [EMIM][Ac]}, {H2O + [EMIM][TFA]}, and {H2O + [DEMA][OMS]} mixtures, COP values are close to, but lower than, that obtained working with {H2O + LiBr}. Nevertheless, these slightly low performances of ionic liquid-based mixtures can be counterbalanced by the ability to reach

Simulations for evaporator temperature TE, condenser temperature TC, absorber temperature TA, and generator temperature TG are set to 80, 20, 130, and 80 °C, respectively, which shows

[DMIM][DMP] > [HOEtMIM][Cl] > [EMIM][Ac] > [DEMA][OMS] > [EMIM][TFA] > [EMIM] [DMP] > [BMIM][BF4] > [EMIM][Triflate] > [EMIM][EtSO4] > [BMIM][Triflate]. We can conclude that ionic liquids with a short-alkyl chain lead to higher COP values and a lower

Simulations indicate that binary systems {H2O + acetate or chloride-based ILs} have high COP. Nonetheless, these families of ILs are not sufficiently stable and they present high corrosion rates [50]. It was noticed that binary system composed of {H2O + [BMIM][MeSO4]} exhibits smaller Δt than the other investigated 11 systems. Its largest observed Δt is about 40 K and the lowest is 30 K. Simulation of the binary system consisting of {H2O + [BMIM][MeSO4]} is not

Thirty-three binary systems out of 39 are found to have only VLE data available, the literature lacking other thermodynamic properties. Using NRTL, the VLE data of these 33 binary systems were correlated, and f was determined. Simulations for these binary systems performed with evaporator temperature TE, condenser temperature TC, absorber temperature TA, and generator temperature TG are set to 80°C, 20°C, 130°C, and 80°C, respectively. Simulation results showed that there are promising binary systems exhibiting low f values such as [BMIM][C1SO3], [BMIM][I], [BMPyr][DCA], which are 19.892, 16.958, and 17.247, respectively.

higher gross temperature lifts and to potentially avoid corrosion issues.

Table 4. The available heat output per unit mass of refrigerant (q) for AHT cycle.

Binary system q (kJ kg−<sup>1</sup>

26 Progress and Developments in Ionic Liquids

circulation ratio f.

that the COP values for the studied binary systems have the following behavior:

promising due to low solubility of IL in water or to stability of the IL [16].

El-Shaimaa Abumandour, Fabrice Mutelet\* and Dominique Alonso

\*Address all correspondence to: fabrice.mutelet@univ-lorraine.fr

Laboratoire Réactions et Génie des Procédés (CNRS UMR 7274), Ecole Nationale Supérieure des Industries Chimiques, Université de Lorraine, Nancy, France

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[72] Freire MG, Neves CMSS, Marrucho IM, Coutinho JAP, Fernandes A M. Hydrolysis of tetrafluoroborate and hexafluorophosphate counter ions in imidazolium-based ionic liquids. Journal of Physical Chemistry A. 2010;114:3744–3749. DOI: 10.1021/jp903292n [73] Islam MM, Ohsako T. Roles of ion pairing on electroreduction of dioxygen in imidazolium-cation-based room-temperature ionic liquid. Journal of Physical Chemistry

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32 Progress and Developments in Ionic Liquids


**Chapter 2**

**Provisional chapter**

**Gas Sensing Ionic Liquids on Quartz Crystal**

**Gas Sensing Ionic Liquids on Quartz Crystal** 

© 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,

© 2017 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.

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

Recent advances in "designer solvents" have facilitated the development of ultrasensi‐ tive gas sensing ionic liquids (SILs) based on quartz crystal microbalance (QCM) that can real‐time detect and discriminate volatile molecules. The amalgamation of tailored‐made SILs and label‐free QCM resulted in a new class of qualitative and semi‐quantitative gas sensing device, which represents a model system of electronic nose. Because a myriad of human‐made or naturally occurring volatile organic compounds (VOCs) are of great interest in many areas, several functional SILs have been designed to detect gaseous alde‐ hyde, ketone, amine and azide molecules chemoselectively in our laboratory. The versa‐ tility of this platform lies in the selective capture of volatile compounds by thin‐coated reactive SILs on QCM at room temperature. Notably, the detection limit of the proto‐ type system can be as low as single‐digit parts‐per‐billion. This chapter briefly introduces some conventional gas sensing approaches and collates recent research results in the inte‐ gration of SILs and QCM and finally gives an account of the state‐of‐the‐art gas sensing

**Keywords:** chemoselective gas sensing, ionic liquid, label‐free detection, quartz crystal

Real‐time detection and monitoring of naturally occurring or human‐made volatile com‐ pounds are of paramount importance in many areas such as (1) disease diagnosis (e.g., breath VOCs); (2) manufacturing industry (e.g., flammable and toxic gases); (3) environmental protec‐ tion (e.g., automobile emissions and greenhouse gases); (4) indoor air quality monitoring (e.g., asphyxiant and hazardous gases); (5) homeland security (e.g., chemical and biological warfare agents). Mammalian olfaction has been used as tools in many settings to detect or measure

**Microbalance**

**Microbalance**

Yi-Pin Chang and Yen-Ho Chu

Yi-Pin Chang and Yen-Ho Chu

http://dx.doi.org/10.5772/65793

**Abstract**

technology.

**1. Introduction**

Additional information is available at the end of the chapter

microbalance and volatile organic compound

Additional information is available at the end of the chapter

### **Gas Sensing Ionic Liquids on Quartz Crystal Microbalance Gas Sensing Ionic Liquids on Quartz Crystal Microbalance**

Yi-Pin Chang and Yen-Ho Chu Yi-Pin Chang and Yen-Ho Chu

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

http://dx.doi.org/10.5772/65793

#### **Abstract**

Recent advances in "designer solvents" have facilitated the development of ultrasensi‐ tive gas sensing ionic liquids (SILs) based on quartz crystal microbalance (QCM) that can real‐time detect and discriminate volatile molecules. The amalgamation of tailored‐made SILs and label‐free QCM resulted in a new class of qualitative and semi‐quantitative gas sensing device, which represents a model system of electronic nose. Because a myriad of human‐made or naturally occurring volatile organic compounds (VOCs) are of great interest in many areas, several functional SILs have been designed to detect gaseous alde‐ hyde, ketone, amine and azide molecules chemoselectively in our laboratory. The versa‐ tility of this platform lies in the selective capture of volatile compounds by thin‐coated reactive SILs on QCM at room temperature. Notably, the detection limit of the proto‐ type system can be as low as single‐digit parts‐per‐billion. This chapter briefly introduces some conventional gas sensing approaches and collates recent research results in the inte‐ gration of SILs and QCM and finally gives an account of the state‐of‐the‐art gas sensing technology.

**Keywords:** chemoselective gas sensing, ionic liquid, label‐free detection, quartz crystal microbalance and volatile organic compound

### **1. Introduction**

Real‐time detection and monitoring of naturally occurring or human‐made volatile com‐ pounds are of paramount importance in many areas such as (1) disease diagnosis (e.g., breath VOCs); (2) manufacturing industry (e.g., flammable and toxic gases); (3) environmental protec‐ tion (e.g., automobile emissions and greenhouse gases); (4) indoor air quality monitoring (e.g., asphyxiant and hazardous gases); (5) homeland security (e.g., chemical and biological warfare agents). Mammalian olfaction has been used as tools in many settings to detect or measure

© 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. © 2017 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.

volatile molecules in drinks, food, perfumes as well as explosives and illegal drugs. Perfumers, flavorists and sniffer dogs are trained professionals and experts in aroma; however, they can‐ not work 24/7 and their sensory can be extremely subjective, regardless of other factors such as sensitivity, toxicity and when sites are beyond reach. Inspired by the mammalian olfactory sys‐ tem, artificial olfaction or electronic noses have been developed to precisely analyze smells or odorants [1]. In this chapter, the development of chemoselective SIL‐based QCM gas analysis system for the detection of VOCs in our laboratory is described. **Figure 1** illustrates the side‐ by‐side comparison of the human olfactory system and the SIL on QCM gas sensing system.

olfaction, in which the sensing material plays a pivotal role in the recognition or capture of volatile molecules. The gas sample passes through the sensor chip and induces physical or chemical changes in the sensing material, which are transduced into electrical signals or pat‐ terns and then processed by a computer system. The sensing material should be sensitive enough to recognize the presence of target gas, which is the counterpart of the peripheral system in the human olfactory system. The first event of molecular recognition underscores the importance of sensing material that sensitivity and selectivity are inherited in the gas

Gas Sensing Ionic Liquids on Quartz Crystal Microbalance

http://dx.doi.org/10.5772/65793

37

Ionic liquids are room‐temperature molten salts that have been increasingly used in elec‐ trochemical devices, such as batteries, fuel cells and biosensors. Their intrinsic unique physiochemical properties by design have facilitated the birth of a variety of novel sensing technologies in recent years. Zhang et al. [2] reported fabrication of polymeric ionic liquid/ graphene nanocomposite for glucose oxidase immobilization and direct electrochemistry. Liu et al. described a hydrophobic ionic liquid was used as an entrapping agent to facilitate the electron transfer of horseradish peroxidase on a glassy carbon electrode [3]. Ratel et al. devel‐ oped imidazolium‐based ionic liquid self‐assembled monolayers for binding streptavidin to promote affinity biosensing [4]. Abdelhamid et al. designed UV‐light absorbing ionic liquid matrices for matrix‐assisted laser desorption/ionization mass spectrometry (MS) [5]. Arkan et al. demonstrated an impedimetric immunosensor based on a gold nanoparticle/multiwall carbon nanotube‐ionic liquid electrode for the determination of human epidermal growth fac‐ tor receptor 2 [6]. As shown above, the scope of the applications of ionic liquid‐based sensors

Conventional gas sensing methods can be generally categorized into conductivity sensors (e.g., metal‐oxide semiconductors and conducting organic polymers), piezoelectric sen‐ sors (e.g., quartz crystal microbalance (QCM) and surface acoustic waves (SAW) and spec‐ troscopic instruments. These gas sensing systems require a set of reactive materials that recognize gaseous molecules and transduce the volatile compounds into electrical signals. A number of nanomaterials have been developed for gas sensing, such as field effect transis‐ tors (FETs) based on single‐walled carbon nanotubes (CNTs) [7, 8]. The design of sensing materials generally utilizes the "lock‐and‐key" method, which has theoretical high sensitiv‐ ity and selectivity. In some cases, it may not be practical for the analysis of complex sam‐ ples due to the fact that most sensing materials exhibit some cross‐reactivity to structurally similar compounds. In addition, higher selectivity may come at the price of irreversibility, lengthy recovery times, memory effects and lower sensitivity (detection limits down to hun‐

QCM is a highly sensitive instrument that measures the mass difference per unit area by recording the change in resonant frequency of a build‐in quartz crystal [9]. The transduced signal (ΔF) represents the measured frequency change (Hz), which is based on a physical phenomenon called the converse piezoelectric effect. Piezoelectricity is generated on opposite surfaces of a crystalline material upon mechanical deformation (e.g., pressure or torsion) of the crystal along a given direction. Among the many types of crystals exhibit piezoelectric‐ ity, quartz exceptionally possesses the desired chemical, electrical, mechanical and thermal

is abundant, but its use in gas sensing is in the ascendant.

sensing system.

dreds part‐per billion).

**Figure 1.** Side‐by‐side comparison of the human olfactory system and the SIL on QCM gas sensing system.

Gas is one of the fundamental states of matter that is considered between the liquid and plasma states. Unlike other states of matter, what distinguishes gas from liquid and solid is the distinct separation of the individual gas molecules, which makes them travel fast and freely, and is usually invisible to the human naked eyes. A pure gas can be composed of single atoms (e.g., noble gas), one type of atom (e.g., oxygen), or organic molecules made from a combination of atoms (e.g., acetone). The question is how to selectively detect and precisely measure a single gas out of a mixture of other gases? In the human olfactory system, vola‐ tile compounds are inhaled into nasal cavity and then diffuse through mucus to epithelium receptor cells. The peripheral system then senses the external stimulus and the central system encodes it as an electric signal in neurons, where all signals are integrated and processed in the brain to give us the sense of a smell. The design of a gas sensing device is similar to human olfaction, in which the sensing material plays a pivotal role in the recognition or capture of volatile molecules. The gas sample passes through the sensor chip and induces physical or chemical changes in the sensing material, which are transduced into electrical signals or pat‐ terns and then processed by a computer system. The sensing material should be sensitive enough to recognize the presence of target gas, which is the counterpart of the peripheral system in the human olfactory system. The first event of molecular recognition underscores the importance of sensing material that sensitivity and selectivity are inherited in the gas sensing system.

volatile molecules in drinks, food, perfumes as well as explosives and illegal drugs. Perfumers, flavorists and sniffer dogs are trained professionals and experts in aroma; however, they can‐ not work 24/7 and their sensory can be extremely subjective, regardless of other factors such as sensitivity, toxicity and when sites are beyond reach. Inspired by the mammalian olfactory sys‐ tem, artificial olfaction or electronic noses have been developed to precisely analyze smells or odorants [1]. In this chapter, the development of chemoselective SIL‐based QCM gas analysis system for the detection of VOCs in our laboratory is described. **Figure 1** illustrates the side‐ by‐side comparison of the human olfactory system and the SIL on QCM gas sensing system.

36 Progress and Developments in Ionic Liquids

Gas is one of the fundamental states of matter that is considered between the liquid and plasma states. Unlike other states of matter, what distinguishes gas from liquid and solid is the distinct separation of the individual gas molecules, which makes them travel fast and freely, and is usually invisible to the human naked eyes. A pure gas can be composed of single atoms (e.g., noble gas), one type of atom (e.g., oxygen), or organic molecules made from a combination of atoms (e.g., acetone). The question is how to selectively detect and precisely measure a single gas out of a mixture of other gases? In the human olfactory system, vola‐ tile compounds are inhaled into nasal cavity and then diffuse through mucus to epithelium receptor cells. The peripheral system then senses the external stimulus and the central system encodes it as an electric signal in neurons, where all signals are integrated and processed in the brain to give us the sense of a smell. The design of a gas sensing device is similar to human

**Figure 1.** Side‐by‐side comparison of the human olfactory system and the SIL on QCM gas sensing system.

Ionic liquids are room‐temperature molten salts that have been increasingly used in elec‐ trochemical devices, such as batteries, fuel cells and biosensors. Their intrinsic unique physiochemical properties by design have facilitated the birth of a variety of novel sensing technologies in recent years. Zhang et al. [2] reported fabrication of polymeric ionic liquid/ graphene nanocomposite for glucose oxidase immobilization and direct electrochemistry. Liu et al. described a hydrophobic ionic liquid was used as an entrapping agent to facilitate the electron transfer of horseradish peroxidase on a glassy carbon electrode [3]. Ratel et al. devel‐ oped imidazolium‐based ionic liquid self‐assembled monolayers for binding streptavidin to promote affinity biosensing [4]. Abdelhamid et al. designed UV‐light absorbing ionic liquid matrices for matrix‐assisted laser desorption/ionization mass spectrometry (MS) [5]. Arkan et al. demonstrated an impedimetric immunosensor based on a gold nanoparticle/multiwall carbon nanotube‐ionic liquid electrode for the determination of human epidermal growth fac‐ tor receptor 2 [6]. As shown above, the scope of the applications of ionic liquid‐based sensors is abundant, but its use in gas sensing is in the ascendant.

Conventional gas sensing methods can be generally categorized into conductivity sensors (e.g., metal‐oxide semiconductors and conducting organic polymers), piezoelectric sen‐ sors (e.g., quartz crystal microbalance (QCM) and surface acoustic waves (SAW) and spec‐ troscopic instruments. These gas sensing systems require a set of reactive materials that recognize gaseous molecules and transduce the volatile compounds into electrical signals. A number of nanomaterials have been developed for gas sensing, such as field effect transis‐ tors (FETs) based on single‐walled carbon nanotubes (CNTs) [7, 8]. The design of sensing materials generally utilizes the "lock‐and‐key" method, which has theoretical high sensitiv‐ ity and selectivity. In some cases, it may not be practical for the analysis of complex sam‐ ples due to the fact that most sensing materials exhibit some cross‐reactivity to structurally similar compounds. In addition, higher selectivity may come at the price of irreversibility, lengthy recovery times, memory effects and lower sensitivity (detection limits down to hun‐ dreds part‐per billion).

QCM is a highly sensitive instrument that measures the mass difference per unit area by recording the change in resonant frequency of a build‐in quartz crystal [9]. The transduced signal (ΔF) represents the measured frequency change (Hz), which is based on a physical phenomenon called the converse piezoelectric effect. Piezoelectricity is generated on opposite surfaces of a crystalline material upon mechanical deformation (e.g., pressure or torsion) of the crystal along a given direction. Among the many types of crystals exhibit piezoelectric‐ ity, quartz exceptionally possesses the desired chemical, electrical, mechanical and thermal properties and is thus used as the crystal in QCM systems. In order to make the best use of QCM, exquisite design on the chip is needed to functionalize the electrode with a variety of surface chemistries and modifications for molecular recognition [10]. Our laboratory has a long‐standing interest in QCM, where a 9‐MHz QCM apparatus was used to develop the integrated system equipped with ionic liquids tailored for reaction‐based gas sensing. This chapter will give a brief introduction on current gas sensing technologies and latest advances in the field of gas SILs based on QCM.

from such analysis is the average or accumulated level rather than spatial variations over the

and TiO2

common sensing materials due to its low cost and good sensitivity [15, 16]. The principle of detection is through redox reactions between the oxide surface and the target gas, where the electronic variation on the oxide surface is transduced into an electrical resistance vari‐ ation. Depending on the transducer, the difference of resistance can be determined by the change of capacitance, mass, optical characteristics, reaction energy or work function. Metal oxides have been used to detect combustible, oxidizing, or reducing gases such as carbon monoxide, hydrogen, liquid petroleum gas, methane and nitrogen oxide [17]. Despite the fact that some metal‐oxide semiconductors have good sensitivity, they may also suffer from poor response linearity and selectivity due to the interference of other gases. In addition, most metal‐oxide gas sensors require high operating temperature (up to 500 °C) to reach the optimal reaction temperature for the target gas [18]. The sensing material has to be preheated to enhance the adsorption of gas molecules on the sensing surface, which has limited the application of metal‐oxide gas sensors. Another major issue is the long recovery time that may make it unpractical for the development of electronic noses. In general, metal‐oxide gas sen‐ sors exhibit drastically greater sensitivity to inorganic gases and a few VOCs such as ethanol and formaldehyde. However, it has been demonstrated that the indiscriminate response of

in gas sensing using metal‐oxide semiconductor devices, that is, selectivity and response time. In addition, many other VOCs that result in health problems are not able to be detected by

On the contrary, conducting polymer‐based gas sensors are frequently used to detect a wide range of gases such as VOCs, aromatic volatiles and halogenated compounds. The organic gas sensing polymer composite may be spray‐, spin‐, or dip‐coated onto the sensor, which typically has two electrodes that are fabricated on an insulating polymer. Upon exposure to a gas, the physical properties of the insulating substrate changes due to the absorption of volatile molecules. The signal transduction mechanism can be described by London disper‐ sion, dipole/induced dipole interactions, dipole/dipole interactions and hydrogen bonds, in which responses are normally measured as the relative differential resistance. Polyaniline, polypyrrole, polythiophene and their derivatives are typical organic conducting polymers that have been investigated for gas sensing, in which doping process is required to increase conductivity by redox reactions or protonation [20]. Polymer‐based gas sensors have several advantages for gas detection, including high sensitivity and short response time. Moreover, while operation temperatures of metal‐oxide gas sensors are usually more demanding, poly‐ mer‐based sensors operate at room temperature. However, polymer composites are also sen‐ sitive to temperature fluctuations that may result in variation of sensor responses and thus

) semiconductors are one of the most

Gas Sensing Ionic Liquids on Quartz Crystal Microbalance

http://dx.doi.org/10.5772/65793

39

films, which reflects the major challenges

sampling time period.

Metal‐oxide (SnO2

**2.2. Conductivity gas sensors**

, CuO, Cr2

methyl, ethyl, isopropyl and butyl alcohols on SnO2

metal‐oxide gas sensors effectively [19].

output errors in the system.

O3 , V2 O5 , WO3

### **2. Gas sensing methods**

#### **2.1. Spectroscopic gas sensors**

Spectroscopic instruments are mainly based on absorption and emission spectrometry. The principal of absorption spectrometry is the Beer Lambert law that differential optical absorption spectroscopy, Raman light detection and ranging, tunable diode laser absorp‐ tion spectroscopy, and so on, have been developed. One of the most commonly used on‐ site methods for continuous monitoring of airborne VOCs is differential optical absorption spectroscopy [11]. It has the advantages of fast response time and low limit of detection, but also has the disadvantage of optical interference from oxygen, ozone, and several hydrocarbons. The theory of emission spectrometry is that excited atoms emit photons and then return to its ground state that laser‐induced breakdown spectroscopy is one example. Interestingly, Fourier transform infrared spectroscopy can be used in either absorption or emission spectrometry such as non‐dispersive infrared and quantum‐cascade lasers gas sensors for the latter [12].

Analytical instruments have been utilized for gas detection such as mass spectrometry (MS) and gas chromatography (GC). Mass spectrometry via direct injection is frequently used for the detection of VOCs. To enhance the sensitivity required for the identification of trace levels of VOCs, tandem mass analysis is typically employed. Ions of a particular mass to charge ratio are selected first and then subject to the next stage for further fragmentation. The fragmented daughter ions are analyzed without interference of large amount of unrelated parent frag‐ ments and thus beneficial for the detection of trace gases in complex mixtures. For example, Proton‐transfer reaction mass spectrometry (PTR‐MS) is among the techniques that have been used extensively for on‐line analysis of VOCs [13]. The PTR‐MS technique offers rapid and accurate measurement of VOCs with a very low limit of detection. However, isomeric and isobaric compounds are not able to be separated and measured individually by PTR‐MS instruments. On the other hand, gas chromatography (GC) in conjunction with flame ioniza‐ tion detection, mass spectrometry or photoionization has been utilized for VOC detection such as in the food industry [14]. GC is used for analyte separation, while the coupled detec‐ tor is for the measurement of separated analyte. These GC‐related methods normally utilize batch detection that involves analyte sampling, transportation, pre‐concentration and finally separation via chromatography before data analysis. These methods are useful for trace VOC detection, but they are time‐ and labor‐consuming. In addition, the concentration detected from such analysis is the average or accumulated level rather than spatial variations over the sampling time period.

#### **2.2. Conductivity gas sensors**

properties and is thus used as the crystal in QCM systems. In order to make the best use of QCM, exquisite design on the chip is needed to functionalize the electrode with a variety of surface chemistries and modifications for molecular recognition [10]. Our laboratory has a long‐standing interest in QCM, where a 9‐MHz QCM apparatus was used to develop the integrated system equipped with ionic liquids tailored for reaction‐based gas sensing. This chapter will give a brief introduction on current gas sensing technologies and latest advances

Spectroscopic instruments are mainly based on absorption and emission spectrometry. The principal of absorption spectrometry is the Beer Lambert law that differential optical absorption spectroscopy, Raman light detection and ranging, tunable diode laser absorp‐ tion spectroscopy, and so on, have been developed. One of the most commonly used on‐ site methods for continuous monitoring of airborne VOCs is differential optical absorption spectroscopy [11]. It has the advantages of fast response time and low limit of detection, but also has the disadvantage of optical interference from oxygen, ozone, and several hydrocarbons. The theory of emission spectrometry is that excited atoms emit photons and then return to its ground state that laser‐induced breakdown spectroscopy is one example. Interestingly, Fourier transform infrared spectroscopy can be used in either absorption or emission spectrometry such as non‐dispersive infrared and quantum‐cascade lasers gas

Analytical instruments have been utilized for gas detection such as mass spectrometry (MS) and gas chromatography (GC). Mass spectrometry via direct injection is frequently used for the detection of VOCs. To enhance the sensitivity required for the identification of trace levels of VOCs, tandem mass analysis is typically employed. Ions of a particular mass to charge ratio are selected first and then subject to the next stage for further fragmentation. The fragmented daughter ions are analyzed without interference of large amount of unrelated parent frag‐ ments and thus beneficial for the detection of trace gases in complex mixtures. For example, Proton‐transfer reaction mass spectrometry (PTR‐MS) is among the techniques that have been used extensively for on‐line analysis of VOCs [13]. The PTR‐MS technique offers rapid and accurate measurement of VOCs with a very low limit of detection. However, isomeric and isobaric compounds are not able to be separated and measured individually by PTR‐MS instruments. On the other hand, gas chromatography (GC) in conjunction with flame ioniza‐ tion detection, mass spectrometry or photoionization has been utilized for VOC detection such as in the food industry [14]. GC is used for analyte separation, while the coupled detec‐ tor is for the measurement of separated analyte. These GC‐related methods normally utilize batch detection that involves analyte sampling, transportation, pre‐concentration and finally separation via chromatography before data analysis. These methods are useful for trace VOC detection, but they are time‐ and labor‐consuming. In addition, the concentration detected

in the field of gas SILs based on QCM.

**2. Gas sensing methods**

38 Progress and Developments in Ionic Liquids

**2.1. Spectroscopic gas sensors**

sensors for the latter [12].

Metal‐oxide (SnO2 , CuO, Cr2 O3 , V2 O5 , WO3 and TiO2 ) semiconductors are one of the most common sensing materials due to its low cost and good sensitivity [15, 16]. The principle of detection is through redox reactions between the oxide surface and the target gas, where the electronic variation on the oxide surface is transduced into an electrical resistance vari‐ ation. Depending on the transducer, the difference of resistance can be determined by the change of capacitance, mass, optical characteristics, reaction energy or work function. Metal oxides have been used to detect combustible, oxidizing, or reducing gases such as carbon monoxide, hydrogen, liquid petroleum gas, methane and nitrogen oxide [17]. Despite the fact that some metal‐oxide semiconductors have good sensitivity, they may also suffer from poor response linearity and selectivity due to the interference of other gases. In addition, most metal‐oxide gas sensors require high operating temperature (up to 500 °C) to reach the optimal reaction temperature for the target gas [18]. The sensing material has to be preheated to enhance the adsorption of gas molecules on the sensing surface, which has limited the application of metal‐oxide gas sensors. Another major issue is the long recovery time that may make it unpractical for the development of electronic noses. In general, metal‐oxide gas sen‐ sors exhibit drastically greater sensitivity to inorganic gases and a few VOCs such as ethanol and formaldehyde. However, it has been demonstrated that the indiscriminate response of methyl, ethyl, isopropyl and butyl alcohols on SnO2 films, which reflects the major challenges in gas sensing using metal‐oxide semiconductor devices, that is, selectivity and response time. In addition, many other VOCs that result in health problems are not able to be detected by metal‐oxide gas sensors effectively [19].

On the contrary, conducting polymer‐based gas sensors are frequently used to detect a wide range of gases such as VOCs, aromatic volatiles and halogenated compounds. The organic gas sensing polymer composite may be spray‐, spin‐, or dip‐coated onto the sensor, which typically has two electrodes that are fabricated on an insulating polymer. Upon exposure to a gas, the physical properties of the insulating substrate changes due to the absorption of volatile molecules. The signal transduction mechanism can be described by London disper‐ sion, dipole/induced dipole interactions, dipole/dipole interactions and hydrogen bonds, in which responses are normally measured as the relative differential resistance. Polyaniline, polypyrrole, polythiophene and their derivatives are typical organic conducting polymers that have been investigated for gas sensing, in which doping process is required to increase conductivity by redox reactions or protonation [20]. Polymer‐based gas sensors have several advantages for gas detection, including high sensitivity and short response time. Moreover, while operation temperatures of metal‐oxide gas sensors are usually more demanding, poly‐ mer‐based sensors operate at room temperature. However, polymer composites are also sen‐ sitive to temperature fluctuations that may result in variation of sensor responses and thus output errors in the system.

#### **2.3. Piezoelectric sensors**

A general piezoelectric gas sensor is composed of a substrate of quartz that is cut at a crys‐ talline angle to support a pressure‐ or mass‐sensitive material that is coated on the quartz surface. QCM and surface acoustic wave (SAW) devices are two typical microbalance sensors that the former employs a bulk acoustic wave sensor while the latter uses a surface acoustic wave sensor. Sensing materials such as non‐conducting polymers can be coated on QCM and SAW sensors to detect the analytes of interest. When the sensing material adsorbs spe‐ cific molecules, the mass of the coated material increases and causes the acoustic waves to travel slower. The piezoelectric quartz converts acoustic waves to electric signals. This subtle change in mass can be detected by the sensor microelectronics once the acoustic wave is con‐ verted to an electric signal. The signal response varies in physisorption and chemisorptions. A few materials such as carbon nanotubes [21], ionic liquids [22] and molecular imprinted polymers [23] have been used to coat on QCM and have enabled the detection of a variety of pollutants and the sensing of VOCs. Temperature and humidity control are the major issues for accurate detection, as the resonant frequency is affected by those factors in this type of gas sensors. Therefore, modifications in coating materials have been the focus to improve the sensitivity and specificity in gas sensing. Some commercial QCM sensor systems are available for moisture and inorganic gas detection, but the detection for VOCs is rare and sensitivity is typically in the range of 10–103 ppm, which is not good enough for trace level detection [24].

to the high surface area‐to‐volume ratio of the nanofibrous membrane structures. The pre‐ liminary results showed that the sensitivities of electrospun nanofibrous membranes to detect

at room temperature and shows high selectivity, good reproducibility and long‐term stabil‐ ity in a dry atmosphere [30]. A room temperature ionic liquid was used as a solvent for the detection of highly toxic methylamine and hydrogen chloride on Pt screen‐printed electrodes. The achieved limit of detections were lower than the current Occupational Safety and Health Administration Permissible Exposure Limit, suggesting that Pt screen‐printed electrodes can successfully be combined with ionic liquids as cheap alternatives for amperometric gas sens‐ ing [31]. Most recently, a gas sensing approach based on differential capacitance of electrified ionic liquid electrode interfaces in the presence and absence of adsorbed gas molecules was developed. The observed change of differential capacitance has a local maximum at a certain potential that is unique for each type of gas, and is concentration‐dependent. Characterization

detection was completed at ppb levels with less than 1.8% signal from other interfering

A new series of reaction‐based SIL gas analysis system on QCM have been continuously devel‐ oped in our laboratory. This SIL‐on‐QCM chip system not only is a cost‐effective approach but also shows a great potential to detect a wide range of VOCs with high efficiency and speci‐ ficity. In combination, the tunable chemical reactivity, negligible volatility, and good thermal stability of ionic liquids with high sensitivity of QCM sensor chips make this integrated plat‐ form highly attractive for chemoselective gas sensing. The negligible vapor pressure of ionic

) [32]. The aforementioned studies pave the way of utilizing ionic liquids for the

 are 24‐ to 120‐fold higher than those of the thin film‐based sensors. The response times of the sensing reagents were short, and the signal changes were also fully reversible. In addi‐ tion, the stability of the employed matrix materials was excellent as there was no significant drift in signal intensity after stored in the ambient air for months [27]. The effects of conduc‐ tive polymer oxidation states and structures on the design and development of ionic liquid/ conductive polymer composite films for gas sensing have also been systematically charac‐ terized. Polyvinyl ferrocene films were tested for their sensing properties (e.g., sensitivity, selectivity, response time, linearity, and dynamic range against various gas analytes such as dichloromethane, ethanol, natural gas, methane, formaldehyde and benzene) utilizing QCM. The highest sensitivity film immobilized with ionic liquids allowed the development of a ionic liquid composite‐based sensor array to analyze complex mixtures utilizing structural differences and the extent of intermolecular interactions [28]. An electrochemical ethylene sensor was reported by employing a thin layer of ionic liquid as electrolyte. Ionic liquids served as an alternative electrolyte for many electrochemical gas sensors generally relied on a strongly acidic electrolyte. A detection limit of 760 ppb and a linear response up to 10 ppm were achieved in this work [29]. Next, an ionic liquid‐mediated electrochemiluminescent sen‐ sor for the detection of sulfur dioxide has been developed. The portable system is based on

on the electrochemiluminescent of the coreactant system

electrochemiluminescent sensor can be operated

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41

and cyclohexane, tested at the same concentra‐

CO2

of SO2

species (i.e., CO2

tion as SO2

the strong quenching effect of SO2

, O2 , NO2

development of gas sensing devices.

, NO, SO2

**3.2. Reaction‐based gas sensing ionic liquids on QCM**

, H2 O, H2

in the ionic liquid film. This proposed SO2

### **3. Gas sensing ionic liquids and quartz crystal microbalance**

#### **3.1. Ionic liquids for gas sensing**

A sensor array using room‐temperature ionic liquids as sensing materials and a QCM as a transducer was developed for the detection of ethanol, dichloromethane, benzene and hep‐ tane at ambient and elevated temperatures [25]. These ionic liquids responded proportionately and reversibly to the volatile compounds at room and elevated temperatures but deviated from this linear relationship at high concentrations for the highly volatile dichloromethane. The different response intensity of the gas sensor to the volatile compounds depends on the solubilities of organic vapors in ionic liquids and interactions between each organic vapor and ionic liquid. In addition, the study of a diverse set of ionic liquid showed structural differences resulted in selective responses. Consequently, a sensor array of ionic liquids is promising to effectively differentiate different volatile compounds in pattern recognition in room or high temperatures. A room‐temperature ionic liquid has also been developed for the sensing of ammonia gas. The work function responses of the cast films with and without IL were analyzed by "stepwise" changes of ammonia gas concentration from 0.5 to 694 ppm in air. The camphorsulfonic acid‐doped polyaniline layers showed enhanced sensitivities, lower detection limits and shorter response times. Experimental evidence suggested that polyan‐ iline forms a charge‐transfer complex with imidazolium cation [26]. The first use of ionic liquid‐doped electrospun nanofibrous materials as highly responsive fluorescence quench‐ ing‐based optical CO2 sensors was reported. The sensor slides have high sensitivities due to the high surface area‐to‐volume ratio of the nanofibrous membrane structures. The pre‐ liminary results showed that the sensitivities of electrospun nanofibrous membranes to detect CO2 are 24‐ to 120‐fold higher than those of the thin film‐based sensors. The response times of the sensing reagents were short, and the signal changes were also fully reversible. In addi‐ tion, the stability of the employed matrix materials was excellent as there was no significant drift in signal intensity after stored in the ambient air for months [27]. The effects of conduc‐ tive polymer oxidation states and structures on the design and development of ionic liquid/ conductive polymer composite films for gas sensing have also been systematically charac‐ terized. Polyvinyl ferrocene films were tested for their sensing properties (e.g., sensitivity, selectivity, response time, linearity, and dynamic range against various gas analytes such as dichloromethane, ethanol, natural gas, methane, formaldehyde and benzene) utilizing QCM. The highest sensitivity film immobilized with ionic liquids allowed the development of a ionic liquid composite‐based sensor array to analyze complex mixtures utilizing structural differences and the extent of intermolecular interactions [28]. An electrochemical ethylene sensor was reported by employing a thin layer of ionic liquid as electrolyte. Ionic liquids served as an alternative electrolyte for many electrochemical gas sensors generally relied on a strongly acidic electrolyte. A detection limit of 760 ppb and a linear response up to 10 ppm were achieved in this work [29]. Next, an ionic liquid‐mediated electrochemiluminescent sen‐ sor for the detection of sulfur dioxide has been developed. The portable system is based on the strong quenching effect of SO2 on the electrochemiluminescent of the coreactant system in the ionic liquid film. This proposed SO2 electrochemiluminescent sensor can be operated at room temperature and shows high selectivity, good reproducibility and long‐term stabil‐ ity in a dry atmosphere [30]. A room temperature ionic liquid was used as a solvent for the detection of highly toxic methylamine and hydrogen chloride on Pt screen‐printed electrodes. The achieved limit of detections were lower than the current Occupational Safety and Health Administration Permissible Exposure Limit, suggesting that Pt screen‐printed electrodes can successfully be combined with ionic liquids as cheap alternatives for amperometric gas sens‐ ing [31]. Most recently, a gas sensing approach based on differential capacitance of electrified ionic liquid electrode interfaces in the presence and absence of adsorbed gas molecules was developed. The observed change of differential capacitance has a local maximum at a certain potential that is unique for each type of gas, and is concentration‐dependent. Characterization of SO2 detection was completed at ppb levels with less than 1.8% signal from other interfering species (i.e., CO2 , O2 , NO2 , NO, SO2 , H2 O, H2 and cyclohexane, tested at the same concentra‐ tion as SO2 ) [32]. The aforementioned studies pave the way of utilizing ionic liquids for the development of gas sensing devices.

#### **3.2. Reaction‐based gas sensing ionic liquids on QCM**

**2.3. Piezoelectric sensors**

40 Progress and Developments in Ionic Liquids

detection [24].

**3.1. Ionic liquids for gas sensing**

ing‐based optical CO2

A general piezoelectric gas sensor is composed of a substrate of quartz that is cut at a crys‐ talline angle to support a pressure‐ or mass‐sensitive material that is coated on the quartz surface. QCM and surface acoustic wave (SAW) devices are two typical microbalance sensors that the former employs a bulk acoustic wave sensor while the latter uses a surface acoustic wave sensor. Sensing materials such as non‐conducting polymers can be coated on QCM and SAW sensors to detect the analytes of interest. When the sensing material adsorbs spe‐ cific molecules, the mass of the coated material increases and causes the acoustic waves to travel slower. The piezoelectric quartz converts acoustic waves to electric signals. This subtle change in mass can be detected by the sensor microelectronics once the acoustic wave is con‐ verted to an electric signal. The signal response varies in physisorption and chemisorptions. A few materials such as carbon nanotubes [21], ionic liquids [22] and molecular imprinted polymers [23] have been used to coat on QCM and have enabled the detection of a variety of pollutants and the sensing of VOCs. Temperature and humidity control are the major issues for accurate detection, as the resonant frequency is affected by those factors in this type of gas sensors. Therefore, modifications in coating materials have been the focus to improve the sensitivity and specificity in gas sensing. Some commercial QCM sensor systems are available for moisture and inorganic gas detection, but the detection for VOCs is rare and sensitivity is typically in the range of 10–103 ppm, which is not good enough for trace level

**3. Gas sensing ionic liquids and quartz crystal microbalance**

A sensor array using room‐temperature ionic liquids as sensing materials and a QCM as a transducer was developed for the detection of ethanol, dichloromethane, benzene and hep‐ tane at ambient and elevated temperatures [25]. These ionic liquids responded proportionately and reversibly to the volatile compounds at room and elevated temperatures but deviated from this linear relationship at high concentrations for the highly volatile dichloromethane. The different response intensity of the gas sensor to the volatile compounds depends on the solubilities of organic vapors in ionic liquids and interactions between each organic vapor and ionic liquid. In addition, the study of a diverse set of ionic liquid showed structural differences resulted in selective responses. Consequently, a sensor array of ionic liquids is promising to effectively differentiate different volatile compounds in pattern recognition in room or high temperatures. A room‐temperature ionic liquid has also been developed for the sensing of ammonia gas. The work function responses of the cast films with and without IL were analyzed by "stepwise" changes of ammonia gas concentration from 0.5 to 694 ppm in air. The camphorsulfonic acid‐doped polyaniline layers showed enhanced sensitivities, lower detection limits and shorter response times. Experimental evidence suggested that polyan‐ iline forms a charge‐transfer complex with imidazolium cation [26]. The first use of ionic liquid‐doped electrospun nanofibrous materials as highly responsive fluorescence quench‐

sensors was reported. The sensor slides have high sensitivities due

A new series of reaction‐based SIL gas analysis system on QCM have been continuously devel‐ oped in our laboratory. This SIL‐on‐QCM chip system not only is a cost‐effective approach but also shows a great potential to detect a wide range of VOCs with high efficiency and speci‐ ficity. In combination, the tunable chemical reactivity, negligible volatility, and good thermal stability of ionic liquids with high sensitivity of QCM sensor chips make this integrated plat‐ form highly attractive for chemoselective gas sensing. The negligible vapor pressure of ionic liquids ensures that the sensors do not "dry out" on QCM chips and show free of leakage and the loss of loading during the measurement. As illustrated in **Figure 2**, when gases rapidly diffuse into the SIL thin film on QCM chips and specific chemical reactions for selective gases in ionic liquids occur under appropriate experimental conditions. The mass changes on QCM chips during the chemical reactions of a gas analyte and the tailored ionic liquid are read‐ ily obtained and ultimately transduced to generate an analytical signal. The thin coatings (200–300 nm thickness) of ionic liquids on the surface of the QCM chip (9 MHz) are achieved by depositing the diluted methanol containing SILs. The used SIL layer on QCM chip could be easily washed away by methanol and further replaced with a new SIL. This regeneratable SIL‐on‐QCM chip system can be performed at room temperature, and dried ambient air is used as carrier gas.

measurements of aldehyde and ketone sensing by both SILs suggesting a non‐equilibrium formation of Schiff bases. Notably, this SIL‐on‐QCM chip system was totally insensitive to common VOCs such as methanol, ethanol, ethyl acetate, hexane and most importantly, moisture (water) (ΔF ~ 0 Hz); that is, any water present in the gas stream would not be in any direct competition with target gases. The results of sensing aldehyde and ketone sens‐ ing prompted us to synthesize **SIL 3** for the detection of amine gases. The chemical reac‐ tion between **SIL 3** and amine gases was based on the transimination reaction. Although the model amine gas (propylamine) was detectable at low concentration (28.5 ppb), the minimal QCM response (~0.5 Hz) and seemly reversible in its signal were noticed. From a quick search of the literature, we realized that Lewis acids could notably facilitate the transimination reaction as well as imine and hydrazone forming reactions in conventional

transimination reaction to produce the largest and irreversible QCM response (ΔF = 20 Hz). The sensitivity of detection was also significantly improved about 11.4‐fold for the model amine gas (28.5 ppb → 2.5 ppb). Remarkably, even the smallest molecular weight amine gas, ammonia, the detection limit could be achieved approximately 3.9 ppb (ΔF ~ 1.0 Hz). With this in mind, we could expect to develop an ultrasensitive SIL for detection of ketone

produce twofold increase and irreversible QCM response. With the addition of metal tri‐ flate, the detecting sensitivity of **SIL 2** was significantly down to 0.6 ppb for cyclohexanone and 1.1 ppb for acetone, respectively. Surprisingly, even the masked ketone gases such as

could catalyze the

also could promote hydrazone formation and

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43

molecular solvents. We found that **SIL 3** with 1 mol% hint of Sc(OTf)3

2,2‐dimethoxypropane was also detectable at a level of 34 ppb.

gases. Indeed, **SIL 2** with 2 mol% of Sc(OTf)3

**Figure 3.** Chemical structures of SIL 1‐6.

**Figure 2.** Schematic representation of a SIL‐on‐QCM gas analysis system for reaction‐based gas sensing.

As shown in **Figure 3**, a series of chemoselective SILs for the detections of aldehyde, ketone, amine and azide gases have been prepared. This section is intended as a comprehensive update to our previous and recent works on the developments of SILs. **SIL 1** was first synthesized for the detection of aldehyde and ketone gases [33]. Interestingly, the results showed that **SIL 1** was more sensitive and selective to capturing aldehyde than ketone gases. To improve the sensitivity of ketone gas sensing, **SIL 2** was synthesized subsequently [34]. As the sensing reactions take place, **SIL 1** and **SIL 2** formed imine and hydrazone adducts with aldehydes and ketones, respectively (**Figure 4**). **SIL 1** displayed a similar reac‐ tion rate to aliphatic and aromatic aldehydes while **SIL 2** reacted efficiently with acyclic and cyclic ketone gases. It is noted that the irreversible nature of the frequency drops from QCM measurements of aldehyde and ketone sensing by both SILs suggesting a non‐equilibrium formation of Schiff bases. Notably, this SIL‐on‐QCM chip system was totally insensitive to common VOCs such as methanol, ethanol, ethyl acetate, hexane and most importantly, moisture (water) (ΔF ~ 0 Hz); that is, any water present in the gas stream would not be in any direct competition with target gases. The results of sensing aldehyde and ketone sens‐ ing prompted us to synthesize **SIL 3** for the detection of amine gases. The chemical reac‐ tion between **SIL 3** and amine gases was based on the transimination reaction. Although the model amine gas (propylamine) was detectable at low concentration (28.5 ppb), the minimal QCM response (~0.5 Hz) and seemly reversible in its signal were noticed. From a quick search of the literature, we realized that Lewis acids could notably facilitate the transimination reaction as well as imine and hydrazone forming reactions in conventional molecular solvents. We found that **SIL 3** with 1 mol% hint of Sc(OTf)3 could catalyze the transimination reaction to produce the largest and irreversible QCM response (ΔF = 20 Hz). The sensitivity of detection was also significantly improved about 11.4‐fold for the model amine gas (28.5 ppb → 2.5 ppb). Remarkably, even the smallest molecular weight amine gas, ammonia, the detection limit could be achieved approximately 3.9 ppb (ΔF ~ 1.0 Hz). With this in mind, we could expect to develop an ultrasensitive SIL for detection of ketone gases. Indeed, **SIL 2** with 2 mol% of Sc(OTf)3 also could promote hydrazone formation and produce twofold increase and irreversible QCM response. With the addition of metal tri‐ flate, the detecting sensitivity of **SIL 2** was significantly down to 0.6 ppb for cyclohexanone and 1.1 ppb for acetone, respectively. Surprisingly, even the masked ketone gases such as 2,2‐dimethoxypropane was also detectable at a level of 34 ppb.

**Figure 3.** Chemical structures of SIL 1‐6.

liquids ensures that the sensors do not "dry out" on QCM chips and show free of leakage and the loss of loading during the measurement. As illustrated in **Figure 2**, when gases rapidly diffuse into the SIL thin film on QCM chips and specific chemical reactions for selective gases in ionic liquids occur under appropriate experimental conditions. The mass changes on QCM chips during the chemical reactions of a gas analyte and the tailored ionic liquid are read‐ ily obtained and ultimately transduced to generate an analytical signal. The thin coatings (200–300 nm thickness) of ionic liquids on the surface of the QCM chip (9 MHz) are achieved by depositing the diluted methanol containing SILs. The used SIL layer on QCM chip could be easily washed away by methanol and further replaced with a new SIL. This regeneratable SIL‐on‐QCM chip system can be performed at room temperature, and dried ambient air is

As shown in **Figure 3**, a series of chemoselective SILs for the detections of aldehyde, ketone, amine and azide gases have been prepared. This section is intended as a comprehensive update to our previous and recent works on the developments of SILs. **SIL 1** was first synthesized for the detection of aldehyde and ketone gases [33]. Interestingly, the results showed that **SIL 1** was more sensitive and selective to capturing aldehyde than ketone gases. To improve the sensitivity of ketone gas sensing, **SIL 2** was synthesized subsequently [34]. As the sensing reactions take place, **SIL 1** and **SIL 2** formed imine and hydrazone adducts with aldehydes and ketones, respectively (**Figure 4**). **SIL 1** displayed a similar reac‐ tion rate to aliphatic and aromatic aldehydes while **SIL 2** reacted efficiently with acyclic and cyclic ketone gases. It is noted that the irreversible nature of the frequency drops from QCM

**Figure 2.** Schematic representation of a SIL‐on‐QCM gas analysis system for reaction‐based gas sensing.

used as carrier gas.

42 Progress and Developments in Ionic Liquids

could be applied to detect azide gases with dual functional groups such as 2‐azioethyl amine. Most remarkably, **SIL 4**, which carries a reactive alkyne dienophile group, can also readily capture cyclopentadiene gas at low ppb (65.5 ppb) through the Diels‐Alder [4+2] cycloaddi‐ tion reaction [35]. Namely, **SIL 4‐**based upon cycloaddition reactions is well‐suited to detect

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Recently, transition metal‐containing ionic liquids have received significant research atten‐ tion. Due to the strong affinities between transition metal ions and neutral alkylamines, transition metal‐containing ionic liquids can be easily prepared under convenient reaction conditions (e.g., aqueous solution and room temperature) with high efficiency. Furthermore, there is no tedious organic synthesis steps involved but only simply sample mixing followed by straightforward extraction workups. Thus, we synthesized a new transition metal‐con‐ taining ionic liquids, **SIL 6**, for detecting exclusive for aldehyde gases from an inexpensive and commercially available alkylamine, 1, 2‐bis(2‐aminoethoxy)ethane as the ligand for silver (I) [37]. Unlike the synthesis of imidazolium‐based **SIL 1** that required four synthetic steps with a low yield (37%), the preparation of **SIL 6** could be achieved by only straightfor‐ ward mixing of silver and amine reagents with a moderate high yield (66%). **SIL 6** was totally insensitive to the ketone gases. Notably, with the same concentration of model aldehyde gas (propionaldehyde, 100 ppb), **SIL 6** displayed a stronger QCM response (ΔF = ‐40 Hz) than **SIL 1** (ΔF = ‐19 Hz) (**Figure 6**). Despite silver ionic liquids having the apparent but inherent drawback that they are less stable toward light, they process many advantages such as only minute amounts of SILs (10–15 nL per quartz chip) are consumed. In addition, no chemical immobilization on quartz chips is needed, plus they can be readily regenerated by simply

**Figure 5.** Chemoselective detection of benzyl azide gas (146 ppb) by 9 MHz QCM thin coated with **SIL 4** and **SIL 5** (3.3 nL each, 300 nm thickness). Nitrogen was used as the carrier gas with a flow rate of 3 ml/min, and gas samples were

injected at 1000 s. The resonance frequency drop (ΔF) is the QCM response on the quartz chip surface.

both azide and diene gases with a high sensitivity.

**Figure 4.** Chemoselective detection of acetone and propionaldehyde gases (98 ppb each) of identical molecular weight (C3 H6 O) by 9 MHz QCM thin coated with (A) **SIL 1** and (B) **SIL 2** (3.3 nL each, 300 nm thickness). Air was used as the carrier gas with a flow rate of 3 mL/min, and gas samples were injected at 300 s. The resonance frequency drop (ΔF) is the QCM response on the quartz chip surface.

Next, on the basis of the recent advances in click chemistry, the Huisgen 1, 3‐dipolar azide and alkyne [3+2] cycloaddition, we synthesized **SIL 4** and **5** for the chemoselective detec‐ tion of organic azide gases [35]. Compared to the unstrained **SIL 5**, we can expect that the strained **SIL 4** should possess much greater enhancement in reactivity toward organic azides. Indeed, **SIL 4** showed high sensitivities toward both aliphatic and aryl azide gases, but **SIL 5** was totally inert toward azide gas sensing (**Figure 5**). Among all azide gases investigated, the sensitivity of detection was 5 ppb for benzyl azide and 35 ppb for butyl azide, respec‐ tively. It is noted that the reactivity order of benzyl azides > phenyl azides > allyl azides toward **SIL 4** could be understood by the reported activation energy [36]. In addition, **SIL 4**

could be applied to detect azide gases with dual functional groups such as 2‐azioethyl amine. Most remarkably, **SIL 4**, which carries a reactive alkyne dienophile group, can also readily capture cyclopentadiene gas at low ppb (65.5 ppb) through the Diels‐Alder [4+2] cycloaddi‐ tion reaction [35]. Namely, **SIL 4‐**based upon cycloaddition reactions is well‐suited to detect both azide and diene gases with a high sensitivity.

**Figure 5.** Chemoselective detection of benzyl azide gas (146 ppb) by 9 MHz QCM thin coated with **SIL 4** and **SIL 5** (3.3 nL each, 300 nm thickness). Nitrogen was used as the carrier gas with a flow rate of 3 ml/min, and gas samples were injected at 1000 s. The resonance frequency drop (ΔF) is the QCM response on the quartz chip surface.

Recently, transition metal‐containing ionic liquids have received significant research atten‐ tion. Due to the strong affinities between transition metal ions and neutral alkylamines, transition metal‐containing ionic liquids can be easily prepared under convenient reaction conditions (e.g., aqueous solution and room temperature) with high efficiency. Furthermore, there is no tedious organic synthesis steps involved but only simply sample mixing followed by straightforward extraction workups. Thus, we synthesized a new transition metal‐con‐ taining ionic liquids, **SIL 6**, for detecting exclusive for aldehyde gases from an inexpensive and commercially available alkylamine, 1, 2‐bis(2‐aminoethoxy)ethane as the ligand for silver (I) [37]. Unlike the synthesis of imidazolium‐based **SIL 1** that required four synthetic steps with a low yield (37%), the preparation of **SIL 6** could be achieved by only straightfor‐ ward mixing of silver and amine reagents with a moderate high yield (66%). **SIL 6** was totally insensitive to the ketone gases. Notably, with the same concentration of model aldehyde gas (propionaldehyde, 100 ppb), **SIL 6** displayed a stronger QCM response (ΔF = ‐40 Hz) than **SIL 1** (ΔF = ‐19 Hz) (**Figure 6**). Despite silver ionic liquids having the apparent but inherent drawback that they are less stable toward light, they process many advantages such as only minute amounts of SILs (10–15 nL per quartz chip) are consumed. In addition, no chemical immobilization on quartz chips is needed, plus they can be readily regenerated by simply

Next, on the basis of the recent advances in click chemistry, the Huisgen 1, 3‐dipolar azide and alkyne [3+2] cycloaddition, we synthesized **SIL 4** and **5** for the chemoselective detec‐ tion of organic azide gases [35]. Compared to the unstrained **SIL 5**, we can expect that the strained **SIL 4** should possess much greater enhancement in reactivity toward organic azides. Indeed, **SIL 4** showed high sensitivities toward both aliphatic and aryl azide gases, but **SIL 5** was totally inert toward azide gas sensing (**Figure 5**). Among all azide gases investigated, the sensitivity of detection was 5 ppb for benzyl azide and 35 ppb for butyl azide, respec‐ tively. It is noted that the reactivity order of benzyl azides > phenyl azides > allyl azides toward **SIL 4** could be understood by the reported activation energy [36]. In addition, **SIL 4**

**Figure 4.** Chemoselective detection of acetone and propionaldehyde gases (98 ppb each) of identical molecular weight

O) by 9 MHz QCM thin coated with (A) **SIL 1** and (B) **SIL 2** (3.3 nL each, 300 nm thickness). Air was used as the carrier gas with a flow rate of 3 mL/min, and gas samples were injected at 300 s. The resonance frequency drop (ΔF) is

(C3 H6

the QCM response on the quartz chip surface.

44 Progress and Developments in Ionic Liquids

washing them away. Finally, the SIL platform developed in this work is highly chemoselec‐ tive (**SIL 1** and **SIL 6**: specific to aldehyde, **SIL 2**: sensitive to ketone, **SIL 3**: specific to amine, and **SIL 4**: selective to azide gases, respectively) with superior gas reactivity for **SIL 6** than the imidazolium‐based **SIL 1** and, most significantly, totally insensitive to moisture.

the properties of ionic liquid. Gas sensing systems based on different principles have been developed for real‐time detection of human‐made or naturally occurring VOCs including QCM. In light of the potential problems of many gas sensors, the electromechanical device, QCM represents an excellent platform if sensitive, selective and versatile sensing mate‐ rials were available. To this end, we have developed a series of ultrasensitive SILs that are capable of detecting VOCs selectively. SILs on QCM detect VOCs by sensing normally neglect changes in weight on a nanogram level. Target analytes are captured by SILs and the accumulated weights are transduced into frequency shifts on QCM. An integrated multi‐ channel system could be crafted to efficiently detect and optimally exploit the advantages of various SILs for various VOCs sensing simultaneously. We thus anticipate the design of a pattern recognition library of chemical sensor arrays in the future. Finally, the ultimate goal would be SILs on QCM electronic nose system to "smell" as good as mammalian olfaction

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47

We thank Jerry Lo and Dr. I‐Nan Chang of the ANT Technology Co. for longstanding support and assistance. This work was supported in part by a grant (MOST103‐2113‐M‐194‐002‐MY3)

2 Department of Chemistry and Biochemistry, National Chung Cheng University, Minhsiung,

[1] Gutierrez J, Horrillo MC. Advances in artificial olfaction: sensors and applications.

[2] Zhang Q, Wu S, Zhang L, Lu J, Verproot F, Liu Y, et al. Fabrication of polymeric ionic liquid/graphene nanocomposite for glucose oxidase immobilization and direct electro‐

[3] Liu X, Feng H, Zhang J, Zhao R, Liu X, Wong DK. Hydrogen peroxide detec‐ tion at a horseradish peroxidase biosensor with a Au nanoparticle‐dotted titanate nanotube|hydrophobic ionic liquid scaffold. Biosens Bioelectron. 2012;32(1):188‐94.

from the Ministry of Science and Technology of Taiwan, Republic of China.

\*

chemistry. Biosens Bioelectron. 2011;26(5):2632‐7.

or better.

**Acknowledgments**

**Author details**

Chiayi, Taiwan, ROC

and Yen‐Ho Chu2

1 The Forsyth Institute, Cambridge, MA, USA

Talanta. 2014;124:95‐105.

\*Address all correspondence to: cheyhc@ccu.edu.tw

Yi‐Pin Chang1

**References**

**Figure 6.** Chemoselective detection of water, ethyl acetate, hexane, methanol, acetone, acetonitrile and propionaldehyde gases (100 ppb each) all by a multichannel QCM thin coated with (A) **SIL 6** and (B) **SIL 1** (33 nmol each, 200–300 nm thickness). The QCM sensograms for water, ethyl acetate, hexane, methanol, acetone and acetonitrile gases were vertically shifted (10 Hz in between) for clarity. Nitrogen was used as the carrier gas with a flow rate of 3 mL/min, and gaseous samples were injected at 100 s. The resonance frequency drop (ΔF, in Hz) is the QCM response on the quartz chip surface.

#### **4. Conclusion**

Ionic liquids are commonly defined as molten organic salts, which have been used in ana‐ lytical sciences and biosensing technologies by harnessing the transformation in chemical structure and hence fine‐tuning their physiochemical properties. A myriad of assays can be performed in ionic liquids and a plethora of composite materials based on carbon nano‐ tubes, graphene, graphite, metal nanomaterials, polymers and sol–gels have demonstrated their usefulness in biosensors. However, there are few examples of gas sensors exploiting the properties of ionic liquid. Gas sensing systems based on different principles have been developed for real‐time detection of human‐made or naturally occurring VOCs including QCM. In light of the potential problems of many gas sensors, the electromechanical device, QCM represents an excellent platform if sensitive, selective and versatile sensing mate‐ rials were available. To this end, we have developed a series of ultrasensitive SILs that are capable of detecting VOCs selectively. SILs on QCM detect VOCs by sensing normally neglect changes in weight on a nanogram level. Target analytes are captured by SILs and the accumulated weights are transduced into frequency shifts on QCM. An integrated multi‐ channel system could be crafted to efficiently detect and optimally exploit the advantages of various SILs for various VOCs sensing simultaneously. We thus anticipate the design of a pattern recognition library of chemical sensor arrays in the future. Finally, the ultimate goal would be SILs on QCM electronic nose system to "smell" as good as mammalian olfaction or better.

### **Acknowledgments**

washing them away. Finally, the SIL platform developed in this work is highly chemoselec‐ tive (**SIL 1** and **SIL 6**: specific to aldehyde, **SIL 2**: sensitive to ketone, **SIL 3**: specific to amine, and **SIL 4**: selective to azide gases, respectively) with superior gas reactivity for **SIL 6** than

Ionic liquids are commonly defined as molten organic salts, which have been used in ana‐ lytical sciences and biosensing technologies by harnessing the transformation in chemical structure and hence fine‐tuning their physiochemical properties. A myriad of assays can be performed in ionic liquids and a plethora of composite materials based on carbon nano‐ tubes, graphene, graphite, metal nanomaterials, polymers and sol–gels have demonstrated their usefulness in biosensors. However, there are few examples of gas sensors exploiting

**Figure 6.** Chemoselective detection of water, ethyl acetate, hexane, methanol, acetone, acetonitrile and propionaldehyde gases (100 ppb each) all by a multichannel QCM thin coated with (A) **SIL 6** and (B) **SIL 1** (33 nmol each, 200–300 nm thickness). The QCM sensograms for water, ethyl acetate, hexane, methanol, acetone and acetonitrile gases were vertically shifted (10 Hz in between) for clarity. Nitrogen was used as the carrier gas with a flow rate of 3 mL/min, and gaseous samples were injected at 100 s. The resonance frequency drop (ΔF, in Hz) is the QCM response on the quartz chip surface.

the imidazolium‐based **SIL 1** and, most significantly, totally insensitive to moisture.

**4. Conclusion**

46 Progress and Developments in Ionic Liquids

We thank Jerry Lo and Dr. I‐Nan Chang of the ANT Technology Co. for longstanding support and assistance. This work was supported in part by a grant (MOST103‐2113‐M‐194‐002‐MY3) from the Ministry of Science and Technology of Taiwan, Republic of China.

### **Author details**

Yi‐Pin Chang1 and Yen‐Ho Chu2 \*

\*Address all correspondence to: cheyhc@ccu.edu.tw

1 The Forsyth Institute, Cambridge, MA, USA

2 Department of Chemistry and Biochemistry, National Chung Cheng University, Minhsiung, Chiayi, Taiwan, ROC

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48 Progress and Developments in Ionic Liquids


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**Chapter 3**

**Application of Ionic Liquids in Paper Properties and**

For centuries, paper has been an important medium of information. Currently, the basic risk to the paper collection is "acidic paper" and the action of enzymes secreted by microorganisms on them. In order to 'prolong life' of these materials, in recent years, various chemical compounds have been used. In this chapter, ionic liquids (IL) are explored as substances for deacidification of paper and its conservation, including antifungal activity. The use of these substances in the manufacturing of paper is possible, but the ingredients play an important role. Imidazolium IL cause an increase in the pH (deacidification) of historical papers and do not cause worsening of their strength properties, but these compound can cause a colour change. Benzalkonium DL‐ lactate and didecyldimethylammonium DL‐lactate and derivatives of 1,2,4‐triazole are used as effective inhibitors of growth of moulds on paper. The best antifungal activity in these ionic liquids is observed in the paper pine at a concentration of 5% and weakest in the samples from the pulp after chemical‐thermomechanical treatment. New paper impregnated with ionic liquids is characterised by an increase in tear resistance,

reduction of breaking length and a favourable influence on the paper colour.

and artworks of an inestimable historical value are recorded on paper.

**Keywords:** ionic liquids, paper deacidification, antifungal activity, paper properties

Since the time of its invention, paper has been an essential carrier of historical, cultural, economic and scientific information. However, for the past several decades, paper has been facing competition with electronic media. Nevertheless, the majority of human knowledge

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

© 2017 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,

Koziróg Anna and Wysocka‐Robak Agnieszka

Additional information is available at the end of the chapter

**Preservation**

http://dx.doi.org/10.5772/65860

**Abstract**

**1. Introduction**


## **Application of Ionic Liquids in Paper Properties and Preservation**

Koziróg Anna and Wysocka‐Robak Agnieszka

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65860

#### **Abstract**

[33] Tseng MC, Chu YH. Chemoselective gas sensing ionic liquids. Chem Commun (Camb).

[34] Liu YL, Tseng MC, Chu YH. Sensing ionic liquids for chemoselective detection of acyclic

[35] Tseng MC, Chu YH. Reaction‐based azide gas sensing with tailored ionic liquids mea‐

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[37] Li HY, Hsu TH, Chen CY, Tseng MC, Chu YH. Exploring silver ionic liquids for reac‐ tion‐based gas sensing on a quartz crystal microbalance. Analyst. 2015;140(18):6245‐9.

and cyclic ketone gases. Chem Commun (Camb). 2013;49(25):2560‐2.

sured by quartz crystal microbalance. Anal Chem. 2014;86(4):1949‐52.

tool to predict reaction kinetics. Eur J Org Chem. 2013;18:3712–20.

2010;46(17):2983‐5.

50 Progress and Developments in Ionic Liquids

For centuries, paper has been an important medium of information. Currently, the basic risk to the paper collection is "acidic paper" and the action of enzymes secreted by microorganisms on them. In order to 'prolong life' of these materials, in recent years, various chemical compounds have been used. In this chapter, ionic liquids (IL) are explored as substances for deacidification of paper and its conservation, including antifungal activity. The use of these substances in the manufacturing of paper is possible, but the ingredients play an important role. Imidazolium IL cause an increase in the pH (deacidification) of historical papers and do not cause worsening of their strength properties, but these compound can cause a colour change. Benzalkonium DL‐ lactate and didecyldimethylammonium DL‐lactate and derivatives of 1,2,4‐triazole are used as effective inhibitors of growth of moulds on paper. The best antifungal activity in these ionic liquids is observed in the paper pine at a concentration of 5% and weakest in the samples from the pulp after chemical‐thermomechanical treatment. New paper impregnated with ionic liquids is characterised by an increase in tear resistance, reduction of breaking length and a favourable influence on the paper colour.

**Keywords:** ionic liquids, paper deacidification, antifungal activity, paper properties

### **1. Introduction**

Since the time of its invention, paper has been an essential carrier of historical, cultural, economic and scientific information. However, for the past several decades, paper has been facing competition with electronic media. Nevertheless, the majority of human knowledge and artworks of an inestimable historical value are recorded on paper.

© 2017 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.

Until the nineteenth century, paper was manufactured by craftwork in an alkaline environ‐ ment, using pulped rags with highly polymerised cellulose. The increasing demand for paper was the reason for its production on an industrial scale, using wood fibres, commenced in the nineteenth century. Paper obtained in this manner is characterised by a lower degree of polymerisation and acidic pH (pH = 4.5–5.5), which additionally increases the rate of cellu‐ lose depolymerisation. This process is also assisted by microorganisms, in particular, moulds producing cellulolytic enzymes. All these factors adversely affect the durability and quality of the paper, resulting in reduced ageing resistance [1, 2].

**2. Ionic liquids as neutralisers of acid paper**

of polymerisation, cellulose content, lignin content)

ed by protons and active oxides, respectively [12, 15–21].

structural stiffness and the paper becomes brittle.

input mass of fibrous raw materials)

eth century [11–14].

sition, and the factors are:

following two phenomena:

The reason for paper instability can be sought in two improvements that were introduced in the nineteenth century: a new method of paper sizing and a change in raw material. Paper sizing, necessary to obtain a writable surface, was improved by means of an adhesive added to the pulp before forming a paper sheet or band. Sizing with a resin adhesive added to the pulp required an additive such as aluminium sulphate as a coagulating agent. In an aqueous environment, the compound undergoes hydrolysis; as a result, sulphuric acid is produced, leading to an acidic reaction of the paper‐pulp‐water suspension. In addition, this causes continuous cellulose depolymerisation in the finished product. With the development of wood‐based method of manufacturing, the quality of the manufactured paper product began to worsen compared to that of the paper that was until then manufactured using long, fibrous rag pulp. One of the problems faced today is disintegration of pages in books and docu‐ ments that were printed in the nineteenth and twentieth century; however, the incoming flow of such materials to libraries and archives was fortunately stopped by new, less expensive and acid‐free methods of paper manufacturing, which developed towards the end of the twenti‐

Application of Ionic Liquids in Paper Properties and Preservation

http://dx.doi.org/10.5772/65860

53

The internal factor causing deterioration of paper properties with time is its chemical compo‐

**•** type of the fibrous pulp used to produce paper (wood or wood‐less pulp, fibre length, degree

**•** ancillary agents used during paper production (the amount of aluminium sulphate as an additive for paper sizing in pulp with resin adhesives should not exceed 5% in terms of the

Symptoms of paper ageing mostly include yellowing and structure weakening (brittleness), resulting in some extreme cases in a complete lack of mechanical resistance. Paper ageing is a very complex process, because of the non‐homogeneous composition of this fine material. According to the recent research, paper degradation and ageing are believed to mostly attribute to autocatalytic reactions of acid hydrolysis and oxidisation, i.e. processes accelerat‐

During acid hydrolysis, cellulose chains are torn apart into smaller pieces, which results in the

**•** on both ends of the torn polymer chain, there are active groups which easily attach to the adjacent cellulose chains resulting in improved structural cross‐linking, which enhances

Cut cellulose fragments may also undergo oxidation to form carboxylic acids, such as formic acid and acetic acid. These organic acids reduce paper pH and accelerate acidic hydrolysis reactions, providing fuel to other reactions and causing autocatalytic degradation in paper.

**•** shorter average chain length, and as such—reduced paper tearing resistance;

In recent years, paper manufacturers have replaced the applied technologies with acid‐free methods. However, the problem of accumulated vast collections of acidic paper in archives and libraries remains. Due to the extent of this problem, many million tonnes of paper collections are to be protected. Intensive measures are being taken on a global scale to preserve our heritage. A range of methods has been developed for deacidification on a large scale and for the partial preservation of the archive collections, but none of the methods devised thus far meet the expectations.

The factors causing paper degradation can generally be divided into endogenous—acidity, metal ions and lignin; and exogenous—UV radiation, humidity, pollutants and microor‐ ganisms. As previously mentioned, moulds dominate among the latter. Additionally, the quality of paper materials can be deteriorated by bacteria; however, bacteria, as compared with moulds, require more humidity for growth. Environmental conditions typically present in libraries, archives and museums are more suitable for the growth of moulds than bacteria.

Various chemical compounds are used in order to prevent the above‐described factors from creating favourable conditions for paper degradation. They are paper disinfection, deacidifi‐ cation and coating. The substances used during the processes should provide a good chemi‐ cal stability and ensure cost efficiency. They cannot be toxic to humans and the environment. Also, the effect on the material is important as it cannot undergo any negative changes. As antimicrobial agents, they should be characterised by a wide spectrum of action at low concentrations and in a short period of time [3, 4].

While selecting an agent for paper protection in the broad sense, one must bear in mind that the agent must possess marketing authorisation. Regulation (EU) No. 528/2012 of the European Parliament and of the Council of 22 May 2012, concerning the making available in the market and use of biocidal products has been in force since 1 September, 2013 [5].

In recent years, attention has been paid to chemical compounds generally known as ionic liquids (ILs), many of which show promising properties with the potential for use in the paper industry [6]. These are chemically, electrochemically and thermally stable compounds, which do not decompose at high temperatures. Due to low volatility, they are also recognised as environmentally sound. Moreover, they are characterised by incombustibility, antimicrobial activity or pH buffering capability [6–10].

### **2. Ionic liquids as neutralisers of acid paper**

Until the nineteenth century, paper was manufactured by craftwork in an alkaline environ‐ ment, using pulped rags with highly polymerised cellulose. The increasing demand for paper was the reason for its production on an industrial scale, using wood fibres, commenced in the nineteenth century. Paper obtained in this manner is characterised by a lower degree of polymerisation and acidic pH (pH = 4.5–5.5), which additionally increases the rate of cellu‐ lose depolymerisation. This process is also assisted by microorganisms, in particular, moulds producing cellulolytic enzymes. All these factors adversely affect the durability and quality of

In recent years, paper manufacturers have replaced the applied technologies with acid‐free methods. However, the problem of accumulated vast collections of acidic paper in archives and libraries remains. Due to the extent of this problem, many million tonnes of paper collections are to be protected. Intensive measures are being taken on a global scale to preserve our heritage. A range of methods has been developed for deacidification on a large scale and for the partial preservation of the archive collections, but none of the methods devised thus

The factors causing paper degradation can generally be divided into endogenous—acidity, metal ions and lignin; and exogenous—UV radiation, humidity, pollutants and microor‐ ganisms. As previously mentioned, moulds dominate among the latter. Additionally, the quality of paper materials can be deteriorated by bacteria; however, bacteria, as compared with moulds, require more humidity for growth. Environmental conditions typically present in libraries, archives and museums are more suitable for the growth of moulds

Various chemical compounds are used in order to prevent the above‐described factors from creating favourable conditions for paper degradation. They are paper disinfection, deacidifi‐ cation and coating. The substances used during the processes should provide a good chemi‐ cal stability and ensure cost efficiency. They cannot be toxic to humans and the environment. Also, the effect on the material is important as it cannot undergo any negative changes. As antimicrobial agents, they should be characterised by a wide spectrum of action at low

While selecting an agent for paper protection in the broad sense, one must bear in mind that the agent must possess marketing authorisation. Regulation (EU) No. 528/2012 of the European Parliament and of the Council of 22 May 2012, concerning the making available in the market

In recent years, attention has been paid to chemical compounds generally known as ionic liquids (ILs), many of which show promising properties with the potential for use in the paper industry [6]. These are chemically, electrochemically and thermally stable compounds, which do not decompose at high temperatures. Due to low volatility, they are also recognised as environmentally sound. Moreover, they are characterised by incombustibility, antimicrobial

and use of biocidal products has been in force since 1 September, 2013 [5].

the paper, resulting in reduced ageing resistance [1, 2].

concentrations and in a short period of time [3, 4].

activity or pH buffering capability [6–10].

far meet the expectations.

52 Progress and Developments in Ionic Liquids

than bacteria.

The reason for paper instability can be sought in two improvements that were introduced in the nineteenth century: a new method of paper sizing and a change in raw material. Paper sizing, necessary to obtain a writable surface, was improved by means of an adhesive added to the pulp before forming a paper sheet or band. Sizing with a resin adhesive added to the pulp required an additive such as aluminium sulphate as a coagulating agent. In an aqueous environment, the compound undergoes hydrolysis; as a result, sulphuric acid is produced, leading to an acidic reaction of the paper‐pulp‐water suspension. In addition, this causes continuous cellulose depolymerisation in the finished product. With the development of wood‐based method of manufacturing, the quality of the manufactured paper product began to worsen compared to that of the paper that was until then manufactured using long, fibrous rag pulp. One of the problems faced today is disintegration of pages in books and docu‐ ments that were printed in the nineteenth and twentieth century; however, the incoming flow of such materials to libraries and archives was fortunately stopped by new, less expensive and acid‐free methods of paper manufacturing, which developed towards the end of the twenti‐ eth century [11–14].

The internal factor causing deterioration of paper properties with time is its chemical compo‐ sition, and the factors are:


Symptoms of paper ageing mostly include yellowing and structure weakening (brittleness), resulting in some extreme cases in a complete lack of mechanical resistance. Paper ageing is a very complex process, because of the non‐homogeneous composition of this fine material. According to the recent research, paper degradation and ageing are believed to mostly attribute to autocatalytic reactions of acid hydrolysis and oxidisation, i.e. processes accelerat‐ ed by protons and active oxides, respectively [12, 15–21].

During acid hydrolysis, cellulose chains are torn apart into smaller pieces, which results in the following two phenomena:


Cut cellulose fragments may also undergo oxidation to form carboxylic acids, such as formic acid and acetic acid. These organic acids reduce paper pH and accelerate acidic hydrolysis reactions, providing fuel to other reactions and causing autocatalytic degradation in paper. Acidic hydrolysis is one of the most dangerous degradation reactions, which happens in libraries and archives worldwide [20–24].

For test purposes, paper materials from old books were used (designated as A1 and A2 in the following text, **Table 1**). While selecting the materials for tests, the following criteria were applied: age of the book—publication year before 1970, i.e. age > 45 years; pH < 7 and the raw

Solutions of the tested ionic liquids were prepared by dilution of 50% alcoholic solutions of

The paper samples were impregnated using two methods. The first one was carried out on petri dishes for the liquids [C4mim] [Bt] (1‐butyl‐3‐methylimidazolium benzotriazol) and [C4mim] [Tr] (1‐butyl‐3‐methylimidazolium 1,2,4‐triazolate). The ionic liquid was applied on the paper with a pipette (**Table 2**). Then, the paper was put aside for 24 h. As this method failed to be effective for the liquid [DDA][DL‐lactate] and the impregnation was not complete, another impregnation method was employed using a closed container instead of a petri dish (**Table 3**). These samples were also put aside for 24 h (the container provided constant temperature and relative humidity during impregnation). After 24 h of impregna‐ tion, the samples were transferred onto petri dishes for another 24 h to remove the solvent (IPA) that evaporated naturally. This procedure enabled to obtain paper samples impreg‐

**G [g/m2**

[C4mim] [Bt] and [C4mim] [Tr] 60 0.50 120 0.25

**Table 2.** The consumption of ionic liquid solutions per 1 g impregnated paper (petri dish).

**A1 A2**

**/g] G [g/m2**

Application of Ionic Liquids in Paper Properties and Preservation

http://dx.doi.org/10.5772/65860

55

60 1.50 120 0.75

60 2.50 120 1.25

60 3.50 120 1.75

60 4.00 120 2.00

**] Z [cm3**

**/g]**

**] Z [cm3**

; *Z*, consumption of ionic liquid solutions per 1 g impregnated paper.

material composition—different content values of groundwood and cellulosic pulp.

**Paper Age pH [‐] Fibrous composition**

A2 56 4.75 ± 0.27 100% cellulose pulp

**Table 1.** Samples' fibrous composition and pH.

nated only with ionic liquids.

*Note: G*, paper grammage, g/m2

**Ionic liquids Paper**

A1 48 2.83 ± 0.16 30% cellulose pulp and 70% groundwood

ionic liquids using an appropriate amount of isopropyl alcohol (IPA).

There are millions of materials printed on unstable paper lying in libraries, archives and museums. In order to save and protect them from damage and disintegration into pieces, some methods have been developed which slow down cellulose degradation, namely deacidifica‐ tion, which involves introduction of excess alkali into the paper structure to prevent decom‐ position—the so‐called alkaline reserve. In addition, to extend paper lifetime, objects undergoing deacidification are protected from acid‐forming air components, and deacidifica‐ tion is combined with reinforcement of partially degraded paper (filling up material losses, patching, lamination) [12].

Features of a perfect deacidification method [11, 25] are as follows:


There are many deacidification methods in the world; however, none of them can meet all of the mentioned requirements simultaneously. The application of ionic liquids in paper preservation increases great hopes. The discussed group of compounds appears to be promising in terms of their ability to change acidic pH of the paper as well as disinfection (removal of microorganisms) and disinsectisation (removal of insects and rodents).

The presented tests verified whether the selected ionic liquids had a deacidification effect on 'acidic paper', whether they could change pH from acidic (pH < 7) to alkaline (pH > 7).

For test purposes, paper materials from old books were used (designated as A1 and A2 in the following text, **Table 1**). While selecting the materials for tests, the following criteria were applied: age of the book—publication year before 1970, i.e. age > 45 years; pH < 7 and the raw material composition—different content values of groundwood and cellulosic pulp.


**Table 1.** Samples' fibrous composition and pH.

Acidic hydrolysis is one of the most dangerous degradation reactions, which happens in

There are millions of materials printed on unstable paper lying in libraries, archives and museums. In order to save and protect them from damage and disintegration into pieces, some methods have been developed which slow down cellulose degradation, namely deacidifica‐ tion, which involves introduction of excess alkali into the paper structure to prevent decom‐ position—the so‐called alkaline reserve. In addition, to extend paper lifetime, objects undergoing deacidification are protected from acid‐forming air components, and deacidifica‐ tion is combined with reinforcement of partially degraded paper (filling up material losses,

**•** it should be effective, ensuring that the deacidification substance can penetrate into the book

**•** it should not adversely affect any material in the book (paper, adhesives, printing inks,

**•** it should be suitable for deacidification of any object regardless of the type of paper, its

**•** applied deacidification agents cannot sensitise paper to light or be allergic for humans;

**•** it should be efficient in use, inexpensive and available at the site where the collections at

**•** the deacidification agent should not promote flammability, hygroscopicity or paper

There are many deacidification methods in the world; however, none of them can meet all of the mentioned requirements simultaneously. The application of ionic liquids in paper preservation increases great hopes. The discussed group of compounds appears to be promising in terms of their ability to change acidic pH of the paper as well as disinfection

The presented tests verified whether the selected ionic liquids had a deacidification effect on 'acidic paper', whether they could change pH from acidic (pH < 7) to alkaline (pH > 7).

(removal of microorganisms) and disinsectisation (removal of insects and rodents).

**•** it should leave permanent alkaline reserve in the paper to provide pH close to 8.5;

**•** it should not cause any formation of deposits on the surface left after deacidification;

Features of a perfect deacidification method [11, 25] are as follows:

**•** paper deacidification should take place within the entire thickness;

writing inks, illustration paints, leather and leather‐like elements);

**•** it should not leave any visible or sensible sign after the treatment;

**•** it should not cause any permanently unpleasant odour;

**•** it should result in paper purification and strengthening.

format and the degree of degradation;

susceptibility to microorganisms; and

**•** it should not adversely affect the environment;

libraries and archives worldwide [20–24].

54 Progress and Developments in Ionic Liquids

patching, lamination) [12].

or document;

risk are stored;

Solutions of the tested ionic liquids were prepared by dilution of 50% alcoholic solutions of ionic liquids using an appropriate amount of isopropyl alcohol (IPA).

The paper samples were impregnated using two methods. The first one was carried out on petri dishes for the liquids [C4mim] [Bt] (1‐butyl‐3‐methylimidazolium benzotriazol) and [C4mim] [Tr] (1‐butyl‐3‐methylimidazolium 1,2,4‐triazolate). The ionic liquid was applied on the paper with a pipette (**Table 2**). Then, the paper was put aside for 24 h. As this method failed to be effective for the liquid [DDA][DL‐lactate] and the impregnation was not complete, another impregnation method was employed using a closed container instead of a petri dish (**Table 3**). These samples were also put aside for 24 h (the container provided constant temperature and relative humidity during impregnation). After 24 h of impregna‐ tion, the samples were transferred onto petri dishes for another 24 h to remove the solvent (IPA) that evaporated naturally. This procedure enabled to obtain paper samples impreg‐ nated only with ionic liquids.


*Note: G*, paper grammage, g/m2 ; *Z*, consumption of ionic liquid solutions per 1 g impregnated paper.

**Table 2.** The consumption of ionic liquid solutions per 1 g impregnated paper (petri dish).


liquid [C4mim] [Tr] proved to be a slightly less effective in deacidification. As a result of impregnation of the A1 and A2 paper samples with this liquid at 5% concentration, their pH values increased to more than 7. However, when [C4mim] [Tr] at a concentration over 5% was used, pH of the A1 and A2 samples after implementation practically did not change and

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Different results were obtained for [DDA] [DL‐ lactate], which was applied at a concentra‐ tion of 10–20%, increasing pH of the A1 paper after impregnation from 2.83 to 4.05–4.34, respectively. For the A2 paper, its pH after impregnation practically did not change. For both the A1 and A2 papers, the pH value after impregnation remained at a practically constant level,

Due to the fact that neutralisation of acid paper should not leave any visible or detectable sign, once the treatment was completed, the effect of the ionic liquids on colouring substances as

Paper impregnated with solutions of the ionic liquids [C4mim] [Bt] and [C4mim] [Tr] was not affected in terms of print, odour and texture—no textural changes were noticed in the paper. Paper was not deformed either (no creasing or rolling occurred). Unfortunately, the liquids reduced non‐transparency of the paper during impregnation; however, once the solvent evaporated, transparency was restored to the initial state. In addition, printing inks from illustrations spilled out and dyed the paper. While observing the paper after impregnation in a closed container, however, it was found that [DDA] [DL‐lactate did not damage the print and the paper experienced no deformation during impregnation. As an effect of the solution of the liquid discussed above, the impregnated paper permanently lost its non‐transparency.

Based on the results, it was concluded that, among the tested ionic liquids, [C4mim] [Tr] and [C4mim] [Bt] turned out to be effective in paper deacidification. The compounds changed paper

Paper deacidification is just preventive because paper degradation can be only stopped, not reversed. Unfortunately, basic deacidification methods fail to meet all the parameters which determine whether the method is effective or not. Nevertheless, ionic liquids create a new

The application of chemical compounds to paper is only possible provided its original parameters can be maintained. Any change in properties of this organic material can contrib‐ ute, for instance, to quality deterioration of particularly valuable antique works created on paper. Therefore, the effect of ionic liquids on selected optical and strength properties of the material was checked. For this purpose, hand sheets made of bleached pine pulp, recycled pulp and CTMP were used. The pine pulp beating was carried out in a valley beater accord‐

were made under

pH to alkaline when solutions at a concentration exceeding 3% were used.

ing to ISO 5264‐1:1979 [26]. Paper test sheets of approximately 70 g/m2

independently of the applied concentration of the ionic liquid solution.

well as appearance and odour of the paper were checked.

**3. Effect of ionic liquids on paper properties**

remained to be approximately 8.

perspective in this area.

*Note: G*, paper grammage, g/m<sup>2</sup> ; *Z*, consumption of ionic liquid solutions per 1 g impregnated paper.

**Table 3.** The consumption of ionic liquid solutions per 1 g impregnated paper (closed container).

After impregnation of the A1 and A2 paper samples, the pH values were changed using the ionic liquids [C4mim] [Bt] and [C4mim] [Tr] at concentrations of 1–8%, and the solutions [DDA] [DL‐lactate] (didecylodimethylammonium DL‐lactate) at concentrations of 10–20%, as present‐ ed in **Table 4**.


**Table 4.** pH values of samples after impregnation of solutions of [C4mim] [Bt]; [C4mim] [Tr] and [DDA][DL‐lactate].

Based on the results of A1 and A2 paper impregnation, it was found that the ionic liquid [C4mim] [Bt] showed high capability of deacidification of acidic paper. In the case of both A1 and A2 paper samples, the 3% solution of the compound used for impregnation increased pH of the objects undergoing deacidification to more than 7, from 2.83 (A1 paper) and 4.85 (A2 paper), respectively. Moreover, it was observed that the increase in the concentration of the [C4mim] [Bt] solution resulted in a noticeable rise in paper pH after impregnation. The

liquid [C4mim] [Tr] proved to be a slightly less effective in deacidification. As a result of impregnation of the A1 and A2 paper samples with this liquid at 5% concentration, their pH values increased to more than 7. However, when [C4mim] [Tr] at a concentration over 5% was used, pH of the A1 and A2 samples after implementation practically did not change and remained to be approximately 8.

**Ionic liquids Paper**

56 Progress and Developments in Ionic Liquids

*Note: G*, paper grammage, g/m<sup>2</sup>

ed in **Table 4**.

**Concentration of the solution %** 

**A1 A2**

**] Z [ml/g] G [g/m2**

; *Z*, consumption of ionic liquid solutions per 1 g impregnated paper.

After impregnation of the A1 and A2 paper samples, the pH values were changed using the ionic liquids [C4mim] [Bt] and [C4mim] [Tr] at concentrations of 1–8%, and the solutions [DDA] [DL‐lactate] (didecylodimethylammonium DL‐lactate) at concentrations of 10–20%, as present‐

**pH 24 h after impregnation pH 24 h after impregnation** 

**lactate]** 

**Table 4.** pH values of samples after impregnation of solutions of [C4mim] [Bt]; [C4mim] [Tr] and [DDA][DL‐lactate].

Based on the results of A1 and A2 paper impregnation, it was found that the ionic liquid [C4mim] [Bt] showed high capability of deacidification of acidic paper. In the case of both A1 and A2 paper samples, the 3% solution of the compound used for impregnation increased pH of the objects undergoing deacidification to more than 7, from 2.83 (A1 paper) and 4.85 (A2 paper), respectively. Moreover, it was observed that the increase in the concentration of the [C4mim] [Bt] solution resulted in a noticeable rise in paper pH after impregnation. The

**Paper A1 Paper A2** 

 4.96 4.39 – 5.99 5.66 – 7.55 5.76 – 7.24 7.59 – **7.56** 7.93 – **8.09** 7.83 – 9.01 8.02 – 8.45 7.92 – 9.60 **8.13** – 9.22 **7.98** – – – 4.05 – – 4.61 – – 4.11 – – 4.60 – – 4.14 – – 4.62 – – 4,16 – – 4.65 – – 4.24 – – 4.68 – – 4.34 – – 4.59

**[C4mim] [Bt] [C4mim] [Tr] [DDA] [DL‐**

 6.00 120 3.00 7.00 120 3.50 8.00 120 4.00 9.00 120 4.50

**] Z [ml/g]**

**[C4mim] [Bt] [C4mim] [Tr] [DDA] [DL‐**

**lactate]** 

**G [g/m2**

**Table 3.** The consumption of ionic liquid solutions per 1 g impregnated paper (closed container).

[DDA] [DL‐lactate] 60 5.00 120 2.50

Different results were obtained for [DDA] [DL‐ lactate], which was applied at a concentra‐ tion of 10–20%, increasing pH of the A1 paper after impregnation from 2.83 to 4.05–4.34, respectively. For the A2 paper, its pH after impregnation practically did not change. For both the A1 and A2 papers, the pH value after impregnation remained at a practically constant level, independently of the applied concentration of the ionic liquid solution.

Due to the fact that neutralisation of acid paper should not leave any visible or detectable sign, once the treatment was completed, the effect of the ionic liquids on colouring substances as well as appearance and odour of the paper were checked.

Paper impregnated with solutions of the ionic liquids [C4mim] [Bt] and [C4mim] [Tr] was not affected in terms of print, odour and texture—no textural changes were noticed in the paper. Paper was not deformed either (no creasing or rolling occurred). Unfortunately, the liquids reduced non‐transparency of the paper during impregnation; however, once the solvent evaporated, transparency was restored to the initial state. In addition, printing inks from illustrations spilled out and dyed the paper. While observing the paper after impregnation in a closed container, however, it was found that [DDA] [DL‐lactate did not damage the print and the paper experienced no deformation during impregnation. As an effect of the solution of the liquid discussed above, the impregnated paper permanently lost its non‐transparency.

Based on the results, it was concluded that, among the tested ionic liquids, [C4mim] [Tr] and [C4mim] [Bt] turned out to be effective in paper deacidification. The compounds changed paper pH to alkaline when solutions at a concentration exceeding 3% were used.

Paper deacidification is just preventive because paper degradation can be only stopped, not reversed. Unfortunately, basic deacidification methods fail to meet all the parameters which determine whether the method is effective or not. Nevertheless, ionic liquids create a new perspective in this area.

### **3. Effect of ionic liquids on paper properties**

The application of chemical compounds to paper is only possible provided its original parameters can be maintained. Any change in properties of this organic material can contrib‐ ute, for instance, to quality deterioration of particularly valuable antique works created on paper. Therefore, the effect of ionic liquids on selected optical and strength properties of the material was checked. For this purpose, hand sheets made of bleached pine pulp, recycled pulp and CTMP were used. The pine pulp beating was carried out in a valley beater accord‐ ing to ISO 5264‐1:1979 [26]. Paper test sheets of approximately 70 g/m2 were made under laboratory conditions using the Rapid‐Köthen apparatus in accordance with ISO 5269‐2:2004 [27]. The test sheets were conditioned in accordance with ISO 187:1990(E) [28].

**3.2. Change in paper opacity after impregnation with ionic liquids**

three tested compounds.

**Table 5.** The opacity tested papers.

sample decreases.

**Ionic liquid Concentration Opacity [%]**

Paper opacity is an important performance parameter of publishing and packaging paper materials; it determines the resistance of paper against light penetration. This parameter is reciprocal to visible light penetration. It characterises non‐transparent paper as completely impenetrable to visible light. The parameter was determined according to ISO 2471:2008 [30].

It is required that printing papers should feature the highest possible opacity because this enhances print legibility and aesthetics. Opacity of all tested papers—bleached pine pulp, CTMP and wastepaper—which were impregnated with ionic liquids, remains at a similar level (**Table 5**). A small 3–5% increase in opacity was observed for the pine pulp paper for all

8% 84.3 99.4 99.2

8% 84.3 97.9 99.1

8% 86.4 98.4 98.6

Another very important paper parameter is its breaking length. It should be emphasised that this parameter is not measured but it is calculated from tensile force at break, according to ISO1924‐1:1992 [31]. Nevertheless, in papermaking field, the breaking length, not the tensile force, is used as one of the key parameters for characterising end‐use properties of paper. Breaking length is generally used in the paper trade to characterise the inherent strength of paper. The breaking length is the paper strip length at which the sample would break by its own weight, if suspended vertically from one end. It affords an excellent basis for compar‐ ing the strength of papers made from different furnishes and having different basis weight.

All the three ionic liquids—benzalkonium nitrate [BA][NO3]; benzalkonium lactate [BA][DL‐ lactate] and didecylodimethylammonium DL‐lactate [DDA][DL‐lactate]—reduced breaking length, compared to non‐impregnated samples (**Figure 2**). Also, concentrations of the compounds play a significant role. As the concentration rises, breaking length of the paper

However, it must be emphasised that the average breaking length of publishing paper materials reaches approximately 2000 m, and hence impregnation with 3% compounds does

Blank test – 81.3 98.9 77.1 [BA][DL‐lactate] 3% 84.8 99.1 100.0

[BA][NO3] 3% 84.6 99.3 100.0

[DDA][DL‐lactate] 3% 84.5 98.2 98.6

**3.3. Effect of paper impregnation with ionic liquids on breaking length**

**Bleached pine pulp CTMP Wastepaper**

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Then, the prepared paper was treated with ionic liquids: benzalkonium nitrate [BA][NO3]; benzalkonium [BA][DL‐lactate] and didecylodimethylammonium DL‐lactate at a concentra‐ tion of 3% and 8%. The paper samples underwent impregnation on petri dishes by analogy to the deacidification process.

#### **3.1. Paper impregnation with ionic liquids versus paper brightness**

Paper brightness is determined as a percentage ratio of the light reflected from the paper surface to the diffused light reflected from the surface of the masterpiece, the brightness of which is assumed to be 100%. Brightness was determined using SpectroEye supplied by GretagMacbeth, in accordance with ISO 11475:2004 [29].

**Figure 1** shows a small increase in paper brightness as the effect of ionic liquids. In case of the pine pulp paper, lactates proved to be the most effective, although the effect depended on their concentration.

**Figure 1.** The brightness of paper impregnated with ionic liquids vs. blank test.

For [BA][DL‐lactate], the highest brightness was obtained when the concentration was 8%, and for [DDA][DL‐lactate]—a lower concentration was effective. The CTMP paper showed a regular increase in brightness as a result of each of the three applied compounds. In turn, the recy‐ cled paper obtained the highest increase in the parameter when [DDA][DL‐lactate] was used.

#### **3.2. Change in paper opacity after impregnation with ionic liquids**

Paper opacity is an important performance parameter of publishing and packaging paper materials; it determines the resistance of paper against light penetration. This parameter is reciprocal to visible light penetration. It characterises non‐transparent paper as completely impenetrable to visible light. The parameter was determined according to ISO 2471:2008 [30].

It is required that printing papers should feature the highest possible opacity because this enhances print legibility and aesthetics. Opacity of all tested papers—bleached pine pulp, CTMP and wastepaper—which were impregnated with ionic liquids, remains at a similar level (**Table 5**). A small 3–5% increase in opacity was observed for the pine pulp paper for all three tested compounds.


**Table 5.** The opacity tested papers.

laboratory conditions using the Rapid‐Köthen apparatus in accordance with ISO 5269‐2:2004

Then, the prepared paper was treated with ionic liquids: benzalkonium nitrate [BA][NO3]; benzalkonium [BA][DL‐lactate] and didecylodimethylammonium DL‐lactate at a concentra‐ tion of 3% and 8%. The paper samples underwent impregnation on petri dishes by analogy to

Paper brightness is determined as a percentage ratio of the light reflected from the paper surface to the diffused light reflected from the surface of the masterpiece, the brightness of which is assumed to be 100%. Brightness was determined using SpectroEye supplied by

**Figure 1** shows a small increase in paper brightness as the effect of ionic liquids. In case of the pine pulp paper, lactates proved to be the most effective, although the effect depended on their

For [BA][DL‐lactate], the highest brightness was obtained when the concentration was 8%, and for [DDA][DL‐lactate]—a lower concentration was effective. The CTMP paper showed a regular increase in brightness as a result of each of the three applied compounds. In turn, the recy‐ cled paper obtained the highest increase in the parameter when [DDA][DL‐lactate] was used.

[27]. The test sheets were conditioned in accordance with ISO 187:1990(E) [28].

**3.1. Paper impregnation with ionic liquids versus paper brightness**

GretagMacbeth, in accordance with ISO 11475:2004 [29].

**Figure 1.** The brightness of paper impregnated with ionic liquids vs. blank test.

the deacidification process.

58 Progress and Developments in Ionic Liquids

concentration.

#### **3.3. Effect of paper impregnation with ionic liquids on breaking length**

Another very important paper parameter is its breaking length. It should be emphasised that this parameter is not measured but it is calculated from tensile force at break, according to ISO1924‐1:1992 [31]. Nevertheless, in papermaking field, the breaking length, not the tensile force, is used as one of the key parameters for characterising end‐use properties of paper. Breaking length is generally used in the paper trade to characterise the inherent strength of paper. The breaking length is the paper strip length at which the sample would break by its own weight, if suspended vertically from one end. It affords an excellent basis for compar‐ ing the strength of papers made from different furnishes and having different basis weight.

All the three ionic liquids—benzalkonium nitrate [BA][NO3]; benzalkonium lactate [BA][DL‐ lactate] and didecylodimethylammonium DL‐lactate [DDA][DL‐lactate]—reduced breaking length, compared to non‐impregnated samples (**Figure 2**). Also, concentrations of the compounds play a significant role. As the concentration rises, breaking length of the paper sample decreases.

However, it must be emphasised that the average breaking length of publishing paper materials reaches approximately 2000 m, and hence impregnation with 3% compounds does not disqualify any paper, except CTMP. While soaking with ionic liquids, the material structure was slackened, so that breaking length measurement could not be taken.

composition of the tested papers plays a significant role. The highest brightness was encoun‐ tered in the case of the pine pulp paper, whereas the highest opacity was observed for the CTMP paper. The paper samples impregnated with ionic liquids were characterised by worse

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static properties; however, dynamic strength properties were improved.

**Figure 3.** The tearing resistance of paper impregnated with ionic liquids vs. blank test.

**microorganisms**

The ionic liquids used in the research cause a high reduction in breaking length, which clearly depends on the concentration. Higher concentrations result in lower breaking length. It may be supposed that the change in this property by the effect of ionic liquids results from slackening of the structure and breaking bonds between fibres, which reduces paper strength. In the case of paper impregnated with ionic liquids, an increase in tear resistance was observed. The best results were obtained for the pine pulp paper samples impregnated with [BA][NO3].

**4. Application of ionic liquids in protecting paper from the growth of**

Paper is an easily biodegradable material. It is particularly susceptible to microbiological decomposition as its main component—cellulose—is a polymer decomposed by many microorganisms, among which moulds are the most active group. They participate in oxygenic paper decomposition through the production of extracellular cellulases. Initially, moulds from the *Aspergillus*, *Penicillium*, *Trichoderma* and *Fusarium* genera, which require a less humid substrate, take part in the process. As the source of nutrients, they use components present in paper and, simultaneously, they prepare the substrate for other fungi—*Alternaria*, *Chaetomi‐*

**Figure 2.** The breaking length of paper impregnated with ionic liquids vs. blank test.

#### **3.4. Changes in tear resistance of the paper impregnated with ionic liquids**

Tear resistance is the force required to tear a notched paper sample. This property depends on the length of fibres and their longitudinal or transversal arrangement. The result of tear resistance after application of ionic liquids on the paper samples was determined according to ISO 1974:1990 [32] as presented in **Figure 3**.

By the effect of [BA][NO3], tear resistance of pine and wastepaper pulp (impregnation with a 3% solution) increases; for CTMP, compared with samples without ionic liquids, the value decreases. Tear resistance of the pine pulp paper, impregnated with [BA][DL‐lactate] at a higher concentration, is also higher, compared to the non‐impregnated paper. For the wastepaper pulp, it was found that tear resistance increased as concentrations of all three ionic liquids decreased.

#### **3.5. Summary**

Benzalkonium nitrate [BA][NO3]; benzalkonium lactate [BA][DL‐lactate] and didecylodime‐ thylammonium DL‐lactate [DDA][DL‐lactate], as representatives of ionic liquids, showed a beneficial effect on paper optical properties. Depending on the ionic liquid concentration, paper brightness changes; higher concentration results in higher brightness. Also, raw material

composition of the tested papers plays a significant role. The highest brightness was encoun‐ tered in the case of the pine pulp paper, whereas the highest opacity was observed for the CTMP paper. The paper samples impregnated with ionic liquids were characterised by worse static properties; however, dynamic strength properties were improved.

not disqualify any paper, except CTMP. While soaking with ionic liquids, the material

structure was slackened, so that breaking length measurement could not be taken.

**Figure 2.** The breaking length of paper impregnated with ionic liquids vs. blank test.

to ISO 1974:1990 [32] as presented in **Figure 3**.

60 Progress and Developments in Ionic Liquids

decreased.

**3.5. Summary**

**3.4. Changes in tear resistance of the paper impregnated with ionic liquids**

Tear resistance is the force required to tear a notched paper sample. This property depends on the length of fibres and their longitudinal or transversal arrangement. The result of tear resistance after application of ionic liquids on the paper samples was determined according

By the effect of [BA][NO3], tear resistance of pine and wastepaper pulp (impregnation with a 3% solution) increases; for CTMP, compared with samples without ionic liquids, the value decreases. Tear resistance of the pine pulp paper, impregnated with [BA][DL‐lactate] at a higher concentration, is also higher, compared to the non‐impregnated paper. For the wastepaper pulp, it was found that tear resistance increased as concentrations of all three ionic liquids

Benzalkonium nitrate [BA][NO3]; benzalkonium lactate [BA][DL‐lactate] and didecylodime‐ thylammonium DL‐lactate [DDA][DL‐lactate], as representatives of ionic liquids, showed a beneficial effect on paper optical properties. Depending on the ionic liquid concentration, paper brightness changes; higher concentration results in higher brightness. Also, raw material

**Figure 3.** The tearing resistance of paper impregnated with ionic liquids vs. blank test.

The ionic liquids used in the research cause a high reduction in breaking length, which clearly depends on the concentration. Higher concentrations result in lower breaking length. It may be supposed that the change in this property by the effect of ionic liquids results from slackening of the structure and breaking bonds between fibres, which reduces paper strength.

In the case of paper impregnated with ionic liquids, an increase in tear resistance was observed. The best results were obtained for the pine pulp paper samples impregnated with [BA][NO3].

### **4. Application of ionic liquids in protecting paper from the growth of microorganisms**

Paper is an easily biodegradable material. It is particularly susceptible to microbiological decomposition as its main component—cellulose—is a polymer decomposed by many microorganisms, among which moulds are the most active group. They participate in oxygenic paper decomposition through the production of extracellular cellulases. Initially, moulds from the *Aspergillus*, *Penicillium*, *Trichoderma* and *Fusarium* genera, which require a less humid substrate, take part in the process. As the source of nutrients, they use components present in paper and, simultaneously, they prepare the substrate for other fungi—*Alternaria*, *Chaetomi‐*

*um* and *Stachybotrys*—which, in turn, require higher humidity, but are capable of hydrolysis of resistant cellulose fibres. Cellulose decomposition can completely disqualify finished paper products since the degree of cellulose polymerisation is decreased by the activity of cellulo‐ lytic enzymes. An impairment of structure of the cellulosic fibre results in its reduced strength, and hence in the complete decomposition [33–35]. However, one must bear in mind that there are plenty of various types of paper in the world, and not every type is susceptible to micro‐ organisms to the same extent. Paper biodegradation is affected by technological parameters of the material, the manufacturing process and environmental factors [36–38].

**Ionic liquids Mould strains** 

**I GROUP** 

**II GROUP**

atoms on the cation.

*A. niger* **ATCC 16404** 

**Table 6.** Value of minimal inhibitory concentration [ppm] [45].

*A. terreus* **ATCC 10020** 

[BA] [DC‐lactate] 78.2 39.1 39.1 78.2 78.2 [DDA] [DC‐lactate] 78.2 39.1 39.1 39.1 19.5 [BA][NO3] 156.3 39.1 39.1 78.2 39.1

[C12mim][Bt] 107.9 107.9 107.9 215.8 107.9 [C4mim][Bt] 3439.5 3439.5 3439.5 6879 6879 [C4mim][Tr] 6145 6145 6145 >6145 6145

Taking the strains of the investigated moulds into account, it was *Aspergillus niger* that was found least susceptible to the action of ionic liquids in Group 1. For imidazoline ionic liquids in Group 2, the highest MIC values were recorded for two strains of the genus *Penicillium*.

The ionic liquids used in the research show good antifungal activity, but their effectiveness essentially depend on the molecular structure. The lowest MIC values were obtained for didecylodimethylammonium DL‐lactate and 1‐dodecyl‐3‐methylimidazolium benzotriazole. These are compounds, the structures of which contain long alkyl chains responsible for the antimicrobial activity of the compounds. The results are confirmed by the work conducted by Demberelnyamba et al., who managed to determine MIC values of quaternary 1‐alkyl‐3‐ methylimidazolium compounds against bacteria, yeasts and algae. The lowest values were obtained for compounds containing 12–14 carbon atoms in the alkyl chain. The relationship between MIC and the length of the alkyl chain in ionic liquids was also repeatedly as proven byRefs. [9, 41]. Overall, it was found thatthe highest antimicrobial activity across allthe groups of ionic liquids is obtained for the compounds with an alkyl chain substituent of 12 carbon

**4.2. Evaluation of the antifungal activity of paper modified with ionic liquids**

A series of testing methods have been developed for the evaluation of antimicrobial proper‐ ties of paper; the differences lie in the intended use of the tested paper materials, time of exposure to microorganisms, and their physical properties. Selecting adequate microorgan‐ isms, depending on the chosen testing method is also an important element of the tests.

The methods to evaluate the bioactive effect on paper products can be divided into quantita‐ tive and qualitative. They are described in various standards developed by, forinstance, ASTM International (ASTM E723, ASTM E875, ASTM E1839, ASTM D2020) and the Technical Association of the Pulp and Paper Industry (TAPPI T449, TAPPI T487) [4]. For paper testing,

*A. versicolor* **ATCC 9577**  *P. aurantiogriseum* **ATCC 18382** 

Application of Ionic Liquids in Paper Properties and Preservation

*P. chrysogenum* **ATCC 60739** 

63

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In recent years, biocides have been used more and more often and to a greater extent. They are used not only for disinfection, but also for finishing processes in cellulosic materials, and thus the latter may become less susceptible to the destructive action of microorganisms. Also, antimicrobial agents are used to protect different paper forms (books, photographs, paint‐ ings) stored in libraries, archives and museums [2, 15, 36, 39].

The aim of the presented study is to determine the antifungal activity of ionic liquids which, once added, protect paper from the growth of moulds.

#### **4.1. Minimal concentrations of ionic liquids inhibiting the growth of moulds**

Two of basic parameters determined during the control of the antimicrobial activity of various chemical compounds or mixtures are minimal inhibitory concentration (MIC) and minimal bactericidal/fungicidal concentration (MBC/MFC). The former describes the lowest com‐ pound concentration that inhibits the growth of microorganisms in the sample. Values of the latter parameter–MBC/MFC–refer to the lowest concentration of the antimicrobial agents required to reduce the viability of 99% microorganisms. In research on the antimicrobial activity of ionic liquids, these two parameters are determined most frequently, as reported in references [8, 10, 40–43]. Primary research methods used to determine MIC/MBC/MFC values include dilution tests and agar diffusion tests [10, 44].

To date, most of the research has focused on bacteria and yeast, while the biodegradation of paper is most frequently caused by moulds. For that reason, in course of authors' own research [45], the effect of the following ionic liquids on moulds was verified: benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammonium DL‐lactate [DDA][DL‐lactate], benzal‐ konium nitrate [BA][NO3], 1‐butyl‐3‐methylimidazolium benzotriazole [C4mim][Bt], 1‐ dodecyl‐3‐methylimidazolium benzotriazole [C12mim][Bt], and 1‐butyl‐3‐ methylimidazolium 1,2,4‐triazolate [C4mim][Tr].

Based on the results, it has been concluded that there are three compounds inhibiting the growth of moulds at concentration up to 100 ppm: lactates and nitrates (**Table 6**). Among them, the best antimicrobial properties were demonstrated by [DDA][DL‐lactate], the MIC values of which were in the range of 19.5–78.2 ppm. Within the group of imidazole‐based ionic liquids, the lowest MIC values were obtained for 1‐dodecyl‐3‐methylimidazolium benzotriazole, which was effective at 30‐ to 60‐fold lower concentrations compared to two other compounds —[C4mim][Bt] and [C4mim][Tr].


**Table 6.** Value of minimal inhibitory concentration [ppm] [45].

*um* and *Stachybotrys*—which, in turn, require higher humidity, but are capable of hydrolysis of resistant cellulose fibres. Cellulose decomposition can completely disqualify finished paper products since the degree of cellulose polymerisation is decreased by the activity of cellulo‐ lytic enzymes. An impairment of structure of the cellulosic fibre results in its reduced strength, and hence in the complete decomposition [33–35]. However, one must bear in mind that there are plenty of various types of paper in the world, and not every type is susceptible to micro‐ organisms to the same extent. Paper biodegradation is affected by technological parameters

In recent years, biocides have been used more and more often and to a greater extent. They are used not only for disinfection, but also for finishing processes in cellulosic materials, and thus the latter may become less susceptible to the destructive action of microorganisms. Also, antimicrobial agents are used to protect different paper forms (books, photographs, paint‐

The aim of the presented study is to determine the antifungal activity of ionic liquids which,

Two of basic parameters determined during the control of the antimicrobial activity of various chemical compounds or mixtures are minimal inhibitory concentration (MIC) and minimal bactericidal/fungicidal concentration (MBC/MFC). The former describes the lowest com‐ pound concentration that inhibits the growth of microorganisms in the sample. Values of the latter parameter–MBC/MFC–refer to the lowest concentration of the antimicrobial agents required to reduce the viability of 99% microorganisms. In research on the antimicrobial activity of ionic liquids, these two parameters are determined most frequently, as reported in references [8, 10, 40–43]. Primary research methods used to determine MIC/MBC/MFC values

To date, most of the research has focused on bacteria and yeast, while the biodegradation of paper is most frequently caused by moulds. For that reason, in course of authors' own research [45], the effect of the following ionic liquids on moulds was verified: benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammonium DL‐lactate [DDA][DL‐lactate], benzal‐ konium nitrate [BA][NO3], 1‐butyl‐3‐methylimidazolium benzotriazole [C4mim][Bt], 1‐ dodecyl‐3‐methylimidazolium benzotriazole [C12mim][Bt], and 1‐butyl‐3‐

Based on the results, it has been concluded that there are three compounds inhibiting the growth of moulds at concentration up to 100 ppm: lactates and nitrates (**Table 6**). Among them, the best antimicrobial properties were demonstrated by [DDA][DL‐lactate], the MIC values of which were in the range of 19.5–78.2 ppm. Within the group of imidazole‐based ionic liquids, the lowest MIC values were obtained for 1‐dodecyl‐3‐methylimidazolium benzotriazole, which was effective at 30‐ to 60‐fold lower concentrations compared to two other compounds

of the material, the manufacturing process and environmental factors [36–38].

**4.1. Minimal concentrations of ionic liquids inhibiting the growth of moulds**

ings) stored in libraries, archives and museums [2, 15, 36, 39].

once added, protect paper from the growth of moulds.

62 Progress and Developments in Ionic Liquids

include dilution tests and agar diffusion tests [10, 44].

methylimidazolium 1,2,4‐triazolate [C4mim][Tr].

—[C4mim][Bt] and [C4mim][Tr].

Taking the strains of the investigated moulds into account, it was *Aspergillus niger* that was found least susceptible to the action of ionic liquids in Group 1. For imidazoline ionic liquids in Group 2, the highest MIC values were recorded for two strains of the genus *Penicillium*.

The ionic liquids used in the research show good antifungal activity, but their effectiveness essentially depend on the molecular structure. The lowest MIC values were obtained for didecylodimethylammonium DL‐lactate and 1‐dodecyl‐3‐methylimidazolium benzotriazole. These are compounds, the structures of which contain long alkyl chains responsible for the antimicrobial activity of the compounds. The results are confirmed by the work conducted by Demberelnyamba et al., who managed to determine MIC values of quaternary 1‐alkyl‐3‐ methylimidazolium compounds against bacteria, yeasts and algae. The lowest values were obtained for compounds containing 12–14 carbon atoms in the alkyl chain. The relationship between MIC and the length of the alkyl chain in ionic liquids was also repeatedly as proven byRefs. [9, 41]. Overall, it was found thatthe highest antimicrobial activity across allthe groups of ionic liquids is obtained for the compounds with an alkyl chain substituent of 12 carbon atoms on the cation.

#### **4.2. Evaluation of the antifungal activity of paper modified with ionic liquids**

A series of testing methods have been developed for the evaluation of antimicrobial proper‐ ties of paper; the differences lie in the intended use of the tested paper materials, time of exposure to microorganisms, and their physical properties. Selecting adequate microorgan‐ isms, depending on the chosen testing method is also an important element of the tests.

The methods to evaluate the bioactive effect on paper products can be divided into quantita‐ tive and qualitative. They are described in various standards developed by, forinstance, ASTM International (ASTM E723, ASTM E875, ASTM E1839, ASTM D2020) and the Technical Association of the Pulp and Paper Industry (TAPPI T449, TAPPI T487) [4]. For paper testing,

the standards developed by the Association of Textile, Apparel & Materials Professionals (AATCC 100, AATCC 147) are followed as well.

macroscopic observation. Despite the fact that they are less labour consuming, qualitative methods should not be the final tests aimed to obtain result at the antimicrobial evaluation of

> **[BA] [DL‐ lactate]**

**[DDA] [DL‐ lactate]** 

Wastepaper 1 2 2 3 2 3 1 2 CTMP 0 2 0 2 0 2 0 1

Wastepaper 6 8 5 6 7 9 12 14 CTMP 4 6 3 4 5 7 10 10

Wastepaper 7 8 4 5 7 9 5 8 CTMP 4 7 3 5 7 10 2 4

Wastepaper 1 3 3 3 4 4 3 4 CTMP 0 3 0 2 2 3 2 3

Wastepaper 4 5 2 3 4 5 4 5 CTMP 3 4 2 3 3 5 3 4

**Table 7.** Growth inhibition zones [mm] observed for moulds under the influence of ionic liquids contained in paper

In the next stage of the study, the changed amount of conidia on paper samples with ionic liquids within 24 h was determined. For the test purposes, *A. niger* was selected as a strain which is least susceptible to ionic liquids, and the CTMP paper and pine BKP paper con‐ taining benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammonium DL‐lactate [DDA][DL‐lactate], benzalkonium nitrate [BA][NO3] at concentrations of 3 and 5% were used. Papers without biocide constituted reference samples. On each paper sample, 0.1 mL

**3 5 3 5 3 5 3 5** 

Application of Ionic Liquids in Paper Properties and Preservation

1 2 3 5 3 4 1 2

6 9 4 6 8 10 13 16

7 10 4 6 10 11 5 7

3 4 4 4 3 5 5 6

4 5 3 4 5 6 4 6

**[BA][NO3] [C12mim] [Bt]** 

http://dx.doi.org/10.5772/65860

65

**Mould strains Kind of paper Kind of ionic liquid** 

pulp

pulp

pulp

Bleached pine pulp

Bleached pine pulp

*4.2.2. Antifungal activity of paper: quantitative method*

selected materials.

*A. niger* **ATCC 16404** Bleached pine

*A. terreus* **ATCC 10020** Bleached pine

*A. versicolor* **ATCC 9577** Bleached pine

*P. aurantiogriseum* **ATCC**

*P.chrysogenum* **ATCC**

**18382**

**60739**

samples [35].

#### *4.2.1. Antifungal activity of paper: qualitative method*

The qualitative methods are used to determine the bioactivity of paper materials containing biocides and make it possible to evaluate the bacteriostatic and fungistatic properties. The methods essentially consist in placing a paper sample on an agar substrate with cultivated microorganisms with the appropriate density of inoculum. If the tested paper shows antimi‐ crobial properties, a zone where the growth of microorganisms under the sample and around it has been inhibited, will be present. The usual good effect of the biostatic action of paper for this method is indicated by the lack of growth of microorganisms in the sample.

In course of authors' own research [35], paper samples were soaked with four of the most effective ionic liquids: benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammoni‐ um DL‐lactate [DDA][DL‐lactate], benzalkonium nitrate [BA][NO3] and 1‐dodecyl‐3‐methyli‐ midazolium benzotriazole [C12mim] [Bt]—at concentration of 3% and 5%. Next, the spore suspension (106 conidia/mL) was placed on petri dishes containing malt extract agar (Merck). Prepared paper stripes with ionic liquids were then placed there one by one. The dishes were incubated at 28°C for 24–72 h. Once the experiment was completed, the efficiency of ionic liquids action on the paper against the moulds was evaluated. The growth of microorgan‐ isms between the medium and the tested sample was checked for this purpose. In addition, the inhibition zones around the sample relative to the reference sample without ionic liquids were determined. The results are presented in **Table 7**.

It was found that the ionic liquids in the paper at concentration of 3% do not show potent antifungal properties against the tested species. A very good antifungal activity relative to the studied strains was not obtained until compounds at concentration of 5% were used. The best antifungal activity among the four biocides was demonstrated by 1‐dodecyl‐3‐methylimida‐ zolium benzotriazole. For the *Aspergillus terreus* strain—the most sensitive one among the studied strains—growth inhibition zones on pine paper are more than 4‐fold higher com‐ pared with the *Aspergillus niger* strain The tested microorganisms can be ordered according to their decreasing sensitivity as follows: *Aspergillus niger* > *Penicillium aurantiogriseum* > *Penicillium chrysogenum* > *Aspergillus versicolor* > *Aspergillus terreus*.

Taking the type of paper into account, the largest inhibition zones were observed for pine paper, whereas the smallest for CTMP paper. CTMP paper, obtained using cellulosic mass after chemical‐thermo‐mechanical treatment with the tested compounds at concentration of 3%, showed limited antimicrobial activity against *A. niger* and *P.aurantiogriseum.* For pine paper, the growth inhibition zones of the mentioned strains were twice as large compared with the CTMP paper.

The interpretation of the results in qualitative methods is based on an analysis of the growth inhibition zone, which can indicate not only high antimicrobial activity of the tested paper materials, but also the weak binding of the biocide with the surface of the material in ques‐ tion. Also in the TAPPI T487 test, the evaluation of paper resistance to moulds involves macroscopic observation. Despite the fact that they are less labour consuming, qualitative methods should not be the final tests aimed to obtain result at the antimicrobial evaluation of selected materials.

the standards developed by the Association of Textile, Apparel & Materials Professionals

The qualitative methods are used to determine the bioactivity of paper materials containing biocides and make it possible to evaluate the bacteriostatic and fungistatic properties. The methods essentially consist in placing a paper sample on an agar substrate with cultivated microorganisms with the appropriate density of inoculum. If the tested paper shows antimi‐ crobial properties, a zone where the growth of microorganisms under the sample and around it has been inhibited, will be present. The usual good effect of the biostatic action of paper for

In course of authors' own research [35], paper samples were soaked with four of the most effective ionic liquids: benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammoni‐ um DL‐lactate [DDA][DL‐lactate], benzalkonium nitrate [BA][NO3] and 1‐dodecyl‐3‐methyli‐ midazolium benzotriazole [C12mim] [Bt]—at concentration of 3% and 5%. Next, the spore

Prepared paper stripes with ionic liquids were then placed there one by one. The dishes were incubated at 28°C for 24–72 h. Once the experiment was completed, the efficiency of ionic liquids action on the paper against the moulds was evaluated. The growth of microorgan‐ isms between the medium and the tested sample was checked for this purpose. In addition, the inhibition zones around the sample relative to the reference sample without ionic liquids

It was found that the ionic liquids in the paper at concentration of 3% do not show potent antifungal properties against the tested species. A very good antifungal activity relative to the studied strains was not obtained until compounds at concentration of 5% were used. The best antifungal activity among the four biocides was demonstrated by 1‐dodecyl‐3‐methylimida‐ zolium benzotriazole. For the *Aspergillus terreus* strain—the most sensitive one among the studied strains—growth inhibition zones on pine paper are more than 4‐fold higher com‐ pared with the *Aspergillus niger* strain The tested microorganisms can be ordered according to their decreasing sensitivity as follows: *Aspergillus niger* > *Penicillium aurantiogriseum* >

Taking the type of paper into account, the largest inhibition zones were observed for pine paper, whereas the smallest for CTMP paper. CTMP paper, obtained using cellulosic mass after chemical‐thermo‐mechanical treatment with the tested compounds at concentration of 3%, showed limited antimicrobial activity against *A. niger* and *P.aurantiogriseum.* For pine paper, the growth inhibition zones of the mentioned strains were twice as large compared with

The interpretation of the results in qualitative methods is based on an analysis of the growth inhibition zone, which can indicate not only high antimicrobial activity of the tested paper materials, but also the weak binding of the biocide with the surface of the material in ques‐ tion. Also in the TAPPI T487 test, the evaluation of paper resistance to moulds involves

conidia/mL) was placed on petri dishes containing malt extract agar (Merck).

this method is indicated by the lack of growth of microorganisms in the sample.

(AATCC 100, AATCC 147) are followed as well.

64 Progress and Developments in Ionic Liquids

*4.2.1. Antifungal activity of paper: qualitative method*

were determined. The results are presented in **Table 7**.

*Penicillium chrysogenum* > *Aspergillus versicolor* > *Aspergillus terreus*.

suspension (106

the CTMP paper.


**Table 7.** Growth inhibition zones [mm] observed for moulds under the influence of ionic liquids contained in paper samples [35].

#### *4.2.2. Antifungal activity of paper: quantitative method*

In the next stage of the study, the changed amount of conidia on paper samples with ionic liquids within 24 h was determined. For the test purposes, *A. niger* was selected as a strain which is least susceptible to ionic liquids, and the CTMP paper and pine BKP paper con‐ taining benzalkonium DL‐lactate [BA][DL‐lactate], didecylodimethylammonium DL‐lactate [DDA][DL‐lactate], benzalkonium nitrate [BA][NO3] at concentrations of 3 and 5% were used. Papers without biocide constituted reference samples. On each paper sample, 0.1 mL of conidia suspension (107 conidia/mL) was placed in petri dishes. The dishes with the samples were incubated for 24 h at 28°C and the relative humidity RH of 80%. After 0, 3, 6, 12 and 24 h, the samples were shaken in a saline solution with a neutraliser in order to leach microorganisms from the test material. Next, the suspension was diluted and trans‐ ferred onto petri dishes and the MEA (Merck) medium was poured them. The dishes were incubated at 28°C, RH 80%, for 72 h. Taking the dilutions into account, it was possible to calculate the conidia that survived on the paper surface. The results were presented as log conidia per paper cm2 in **Figures 4** and **5**.

lactate amount of conidia was reduced by 4.5 log and equalled 2.5 log. The reduction of the number of conidia by 4–5 log as the disinfecting effect is high. However, even conidia at such

mycelium and cause material destruction. For this reason, ionic liquids were added to other

**Figure 5.** Changes the number of conidia on the surface of the tested paper stripes (pine bleached kraft pulp) modified

A higher concentration proved to be much more effective. After 24 h, no presence of conidia

The ionic liquids in the pine BKP paper were more reactive compared to the CTMP paper; the same conclusion was drawn during the already presented qualitative tests. For paper materials with 3% [BA][NO3] or [DDA][DL‐lactate], no growth of conidia was observed after 24 h. An exception was presented by [BA][DL‐lactate], for which the number of conidia decreased by 6

was found on any paper, except the reference sample (the one without biocide).

with ionic liquids. Concentration of ionic liquids in the samples: (a) 3% and (b) 5% [35].

conidia/sample) in paper with higher humidity can develop into

Application of Ionic Liquids in Paper Properties and Preservation

http://dx.doi.org/10.5772/65860

67

a low concentration (102

samples in an amount of 5%.

**Figure 4.** Changes the number of conidia on the surface of the tested paper stripes with CTMP modified with ionic liquids. Concentration of ionic liquids in the samples: (a) 5% and (b) 3% [35].

Active conidia (see **Figure 4**) were still observed after 24 h on the paper made of CTMP pulp with an addition of each of the three ionic liquids at concentration of 3%. This number decreased by more than 5 log in samples with benzalkonium nitrate and didecylodimethy‐ lammonium DL‐lactate. On paper materials modified with didecylodimethylammonium, DL‐

lactate amount of conidia was reduced by 4.5 log and equalled 2.5 log. The reduction of the number of conidia by 4–5 log as the disinfecting effect is high. However, even conidia at such a low concentration (102 conidia/sample) in paper with higher humidity can develop into mycelium and cause material destruction. For this reason, ionic liquids were added to other samples in an amount of 5%.

of conidia suspension (107

66 Progress and Developments in Ionic Liquids

conidia per paper cm2

conidia/mL) was placed in petri dishes. The dishes with the

samples were incubated for 24 h at 28°C and the relative humidity RH of 80%. After 0, 3, 6, 12 and 24 h, the samples were shaken in a saline solution with a neutraliser in order to leach microorganisms from the test material. Next, the suspension was diluted and trans‐ ferred onto petri dishes and the MEA (Merck) medium was poured them. The dishes were incubated at 28°C, RH 80%, for 72 h. Taking the dilutions into account, it was possible to calculate the conidia that survived on the paper surface. The results were presented as log

**Figure 4.** Changes the number of conidia on the surface of the tested paper stripes with CTMP modified with ionic

Active conidia (see **Figure 4**) were still observed after 24 h on the paper made of CTMP pulp with an addition of each of the three ionic liquids at concentration of 3%. This number decreased by more than 5 log in samples with benzalkonium nitrate and didecylodimethy‐ lammonium DL‐lactate. On paper materials modified with didecylodimethylammonium, DL‐

liquids. Concentration of ionic liquids in the samples: (a) 5% and (b) 3% [35].

in **Figures 4** and **5**.

**Figure 5.** Changes the number of conidia on the surface of the tested paper stripes (pine bleached kraft pulp) modified with ionic liquids. Concentration of ionic liquids in the samples: (a) 3% and (b) 5% [35].

A higher concentration proved to be much more effective. After 24 h, no presence of conidia was found on any paper, except the reference sample (the one without biocide).

The ionic liquids in the pine BKP paper were more reactive compared to the CTMP paper; the same conclusion was drawn during the already presented qualitative tests. For paper materials with 3% [BA][NO3] or [DDA][DL‐lactate], no growth of conidia was observed after 24 h. An exception was presented by [BA][DL‐lactate], for which the number of conidia decreased by 6

log compared with the initial concentration, but active forms were still present. The increase in concentration by 2% contributed to a quicker reduction of conidia. After 24 h, no growth of moulds was recorded on any tested paper. As soon as after 12 h, the reduction of active conidia amounted to 100% for the samples with [BA][NO3] and [DDA][DL‐lactate]. The number of spores within the period reached 2.4 log for paper materials with the additive of [BA][DL‐ lactate].

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### **5. Conclusion**

In recent years, ionic liquid have enjoyed more and more interest, both in research and in practical applications. Their various aspects are discussed, such as antimicrobial properties [8, 10, 42, 46] used in histopathologic diagnostics [47], in paper protection [6] and in wood preservation [48]. In the presented research, ionic liquids were used as an additive to paper to neutralise acid paper and as antifungal compounds. Ionic liquids create an alternative to currently used compounds; a great number of combinations (1018) due to their ionic struc‐ ture is the reason why this type of liquids seems to be an almost inexhaustible resource. Thus, newer and newer compounds with improved properties can be designed, and such com‐ pounds can be applied for purposes of paper protection, too.

### **Acknowledgements**

The study was supported by KBN grant 'The research of paper conservation with the use of ionic liquids' N N209 199238.

#### **Author details**

Koziróg Anna1\* and Wysocka‐Robak Agnieszka<sup>2</sup>

\*Address all correspondence to: anna.kozirog@p.lodz.pl

1 Institute of Fermentation Technology and Microbiology, Faculty of Biotechnology and Food Sciences, Lodz University of Technology, Lodz, Poland

2 Institute of Papermaking and Printing, Lodz University of Technology, Lodz, Poland

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log compared with the initial concentration, but active forms were still present. The increase in concentration by 2% contributed to a quicker reduction of conidia. After 24 h, no growth of moulds was recorded on any tested paper. As soon as after 12 h, the reduction of active conidia amounted to 100% for the samples with [BA][NO3] and [DDA][DL‐lactate]. The number of spores within the period reached 2.4 log for paper materials with the additive of [BA][DL‐

In recent years, ionic liquid have enjoyed more and more interest, both in research and in practical applications. Their various aspects are discussed, such as antimicrobial properties [8, 10, 42, 46] used in histopathologic diagnostics [47], in paper protection [6] and in wood preservation [48]. In the presented research, ionic liquids were used as an additive to paper to neutralise acid paper and as antifungal compounds. Ionic liquids create an alternative to currently used compounds; a great number of combinations (1018) due to their ionic struc‐ ture is the reason why this type of liquids seems to be an almost inexhaustible resource. Thus, newer and newer compounds with improved properties can be designed, and such com‐

The study was supported by KBN grant 'The research of paper conservation with the use of

1 Institute of Fermentation Technology and Microbiology, Faculty of Biotechnology and Food

2 Institute of Papermaking and Printing, Lodz University of Technology, Lodz, Poland

pounds can be applied for purposes of paper protection, too.

lactate].

**5. Conclusion**

68 Progress and Developments in Ionic Liquids

**Acknowledgements**

ionic liquids' N N209 199238.

Koziróg Anna1\* and Wysocka‐Robak Agnieszka<sup>2</sup>

\*Address all correspondence to: anna.kozirog@p.lodz.pl

Sciences, Lodz University of Technology, Lodz, Poland

**Author details**


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**Section 2**

**Biological**


**Section 2**
