**Environmental Impact of the Use of Surfactants and Oxygenates in the Petroleum Industry**

Tomasz Kalak

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68683

#### **Abstract**

The role of surfactants and hydrophilic additives in gasoline fuel was demonstrated. The impact of anionic surfactant sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT) and hydrophilic oxygen containing additives, such as alcohols (methanol, ethanol, propan‐2‐ol, butanol, 2‐methylpropanol) and methyl t‐butyl ether (MTBE) on solubility of water, electrolytic con‐ ductivity in gasoline and interfacial tension in the water/gasoline system was studied. Small amounts of amphiphilic components improve the solubility of water in gasoline as a result of the occurrence of association phenomena with the formation of reverse micelles. The formation of surfactant aggregates and droplet clusters results in an increase in the solubil‐ ity of water in gasoline, electrolytic conductivity, and a decrease in interfacial tension. The changes depend on concentration of the surfactant and type of applied biocomponents. Gasoline fuel in the form of microemulsion has a positive impact on the natural environ‐ ment. The presence of water causes the almost complete combustion of hydrocarbons to the low toxic gases and the absence of carbon black among combustion products reduces fuel consumption, enhances engine power and decreases its temperature, reduces emissions of volatile organic compounds (VOCs), NO*x*, SO2 , CO, and particulate matter. The alternative fuel may have a potential use in spark‐ignition engines in the future.

**Keywords:** environmental protection, surfactants, fuel oxygenates, exhaust emissions, solubility of water, association phenomena, electrolytic conductivity, interfacial tension

## **1. Introduction**

Energy consumption and the standard of living of a society are interrelated constantly growing. Nowadays, there are various sources of energy, such as solar, wind, geothermal, hydrogen, tidal, wave, hydroelectric, biomass, nuclear power, and fossil fuels (coal, oil, and natural gas). Among all the sources, crude oil still plays an important role in providing the

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energy supply of the whole world. It is the most readily available source of energy to human‐ ity, but also a rich source of raw materials for a lot of chemical industries of any kind. The field of surface chemistry is linked to technological processes of crude oil, including the drilling, petroleum refining, and petrochemical processing, and also other related applications and industries. All the processes are associated with interfacial phenomena and surface chemical interactions, as well as have an impact on the environment.

Crude oil is processed into many products (**Figure 1**) and most of them are fuels used for transportation (**Figure 2**). Among the sources of energy, gasoline is the most commonly used fuel in the transportation industry (**Figure 3**). The global production of the fuel presents an upward trend and, in 2012, amounted to approximately 22,377,200 barrels per day [bbl/d]. Taking the regional production into account, the largest amount of gasoline is manufactured in North America (10,017,000 bbl/d in 2012). The next regions are Asia, Europe, South America and Africa. In Europe, gasoline production had continually increased up to 2006 (4,742,000 bbl/d), after the time it started to slowly decrease (**Figure 4**). The largest producer in the world is the United States, with a production of about 9,058,630 bbl/d. Other most productive coun‐ tries are China, Japan, Russia, Canada, India, Germany, and others (**Figure 5**).

Gasoline is a petroleum‐derived liquid that consists of mostly of organic hydrocarbons obtained by the fractional distillation of crude oil, such as paraffins (saturated and unsatu‐ rated), naphthenics, aromatics, and their derivatives. The fuel composition also includes other additives that help attain valuable physicochemical properties [1, 2]. The composition is continually improved by producers in order to achieve better performance and meet the requirements of today's advanced engine technology and environmental institutions. Vapor pressure, distillation curves, or octane rating are features closely associated with the fuel com‐ position and the characteristics of its components. The appropriate additives should ensure antidetonation combustion, good and quick evaporation, high octane number, chemical

**Figure 1.** Global refined petroleum products attributable to one barrel [94]. *Source*: United States Energy Information Admini stration, 2017.

Environmental Impact of the Use of Surfactants and Oxygenates in the Petroleum Industry http://dx.doi.org/10.5772/intechopen.68683 5

energy supply of the whole world. It is the most readily available source of energy to human‐ ity, but also a rich source of raw materials for a lot of chemical industries of any kind. The field of surface chemistry is linked to technological processes of crude oil, including the drilling, petroleum refining, and petrochemical processing, and also other related applications and industries. All the processes are associated with interfacial phenomena and surface chemical

Crude oil is processed into many products (**Figure 1**) and most of them are fuels used for transportation (**Figure 2**). Among the sources of energy, gasoline is the most commonly used fuel in the transportation industry (**Figure 3**). The global production of the fuel presents an upward trend and, in 2012, amounted to approximately 22,377,200 barrels per day [bbl/d]. Taking the regional production into account, the largest amount of gasoline is manufactured in North America (10,017,000 bbl/d in 2012). The next regions are Asia, Europe, South America and Africa. In Europe, gasoline production had continually increased up to 2006 (4,742,000 bbl/d), after the time it started to slowly decrease (**Figure 4**). The largest producer in the world is the United States, with a production of about 9,058,630 bbl/d. Other most productive coun‐

Gasoline is a petroleum‐derived liquid that consists of mostly of organic hydrocarbons obtained by the fractional distillation of crude oil, such as paraffins (saturated and unsatu‐ rated), naphthenics, aromatics, and their derivatives. The fuel composition also includes other additives that help attain valuable physicochemical properties [1, 2]. The composition is continually improved by producers in order to achieve better performance and meet the requirements of today's advanced engine technology and environmental institutions. Vapor pressure, distillation curves, or octane rating are features closely associated with the fuel com‐ position and the characteristics of its components. The appropriate additives should ensure antidetonation combustion, good and quick evaporation, high octane number, chemical

**Figure 1.** Global refined petroleum products attributable to one barrel [94]. *Source*: United States Energy Information

Admini stration, 2017.

tries are China, Japan, Russia, Canada, India, Germany, and others (**Figure 5**).

interactions, as well as have an impact on the environment.

4 Application and Characterization of Surfactants

**Figure 2.** The global application of petroleum products attributable to one barrel [95]. *Source*: United States Energy Infor mation Administration, 2017.

**Figure 3.** Various fuels used for global transportation in 2012 [96]. *Source*: U.S. Energy Information Administration, International Transportation Energy Demand Determinants (ITEDD‐2015) model estimates.

stability, reducing emissions. Furthermore, fuel cannot be corrosive to metals and should not make deposits that interfere with the engine operation [3]. Most of additives belong to a few main functional groups, such as oxygenates (alcohols, ethers, esters, ketones, and others), complex binders, metalorganic compounds, heterorganic compounds, oxidizing organic com‐ pounds, petroleum fractions (aromatics, and light and heavy aliphatic hydrocarbons), surfac‐ tants, and polymers. Among the various types of components, high octane oxygen‐containing

**Figure 4.** Global motor gasoline production by year [97]. *Source*: United States Energy Information Administration, 2016.

**Figure 5.** Motor gasoline production by country in 2012 [98]. *Source*: United States Energy Information Administration, 2016.

compounds, such as alcohols and ethers, are able to reduce pollutants from vehicle exhaust gases, increase the octane number [4], have antiknock properties, they can be obtained from renewable agricultural raw materials instead of fossil sources, they reduce carbon monoxide (CO), volatile organic compounds (VOCs), and unburned hydrocarbons emission [5–8].

A very important property of gasoline fuels is their hygroscopicity, which has a considerable effect on the reliability of vehicles and equipments. Operational experience showed that the permanent addition of a small amount of water to hydrocarbon fuel has a positive effect on a combustion process, provided that water in gasoline is in the form of an emulsion. Therefore, studies to determine physicochemical properties of microemulsions have significant practi‐ cal importance. The emulsion, with the addition of 5 and 10% water, insignificantly increases engine torque, but a mixture with an addition of 15% water decreases engine torque. Water‐ in‐gasoline emulsions (WiGEs) cause an increase in brake‐specific fuel consumption (BSFC) and a decrease in exhaust temperature. Compared to basic gasoline, the WiGE fuel reduces NO*x* and CO and enhances O2 emissions [9].

Owing to specific chemical structure, surfactants are commonly used as fuel additives with a range of various functions, such as reduction of surface tension [10], prevention of particle for‐ mation, removing deposits, dispersion of water, formation of protective layers on surfaces, and an increase in electric conductivity. The addition of surfactants into gasoline leads to a reduction of the amount of deposits formed in the injectors, intake valves, and combustion chambers of gasoline engines. Deposits cause various performance and emissions problems, so their con‐ tinuous removal is needed. Surfactants dissolved in nonpolar solvents may undergo association with the formation of reverse micelles. In the aggregates, polar (hydrophilic) groups are directed to the center of the micelle and hydrocarbon chains toward the apolar (hydrophobic) phase [11].

Water‐in‐gasoline emulsion (WiGE) fuel with an addition of hydrophilic high‐octane oxygen components has become the best alternative fuel to substitute gasoline fuel in spark‐ignition engines. The growing interest to this type of fuel is due to simultaneous reduction of unburned hydrocarbons and CO, reduction of the formation of atmospheric ozone resulting from gasoline emissions, reduction of emissions of exhaust pollutants, such as volatile organic compounds (VOCs), NO*<sup>x</sup>* , and particulate matters. This occurs as a result of the reduction in peak cylinder temperature and secondary atomization by a further breakup of gasoline spray due to microex‐ plosion. Experimental investigation about the effect of various surfactants present in the WiGE fuel on engine performance and pollutant formation has not been fully known. Studies con‐ ducted in this field may constitute the basis for investigation the effects of blends of emulsified fuel with various surfactants and hydrophilic oxygen compounds on the combustion charac‐ teristics, emission formation processes, and engine behaviors also to determine the pollution formation suppression capability of the emulsified fuels by in‐depth combustion characteristics analysis. It is also equally important to select the suitable emulsification method, optimized speed, agitation time, and suitable chemical stabilizers in order to achieve stable emulsions. It is reasonable to conduct intense studies in order to know the effect of water content on the engine combustion characteristics and to determine an optimum percentage of water content in the WiGE fuel. Systematic studies of the optimization of water content in the emulsion for best engine performance and emission by both experimental and numerical investigations are necessary so that it can give the best recommendations for the commercialization of the WiGE fuel as an alternative source of energy for the future spark‐ignition engines.

The aim of this work is to study the effect of chosen hydrophilic additives (alcohols MeOH, EtOH, BuOH, IPA, IBA, and MTBE, 3% v/v) and the anionic surfactant (sulfosuccinic acid bis[2‐ethylhexyl] ester (AOT) at various concentrations) upon the solubility of water in basic gasoline, electrolytic conductivity, and interfacial tension isotherms at water/gasoline interfaces.

## **2. Oxygenates used in gasoline fuel**

compounds, such as alcohols and ethers, are able to reduce pollutants from vehicle exhaust gases, increase the octane number [4], have antiknock properties, they can be obtained from renewable agricultural raw materials instead of fossil sources, they reduce carbon monoxide (CO), volatile organic compounds (VOCs), and unburned hydrocarbons emission [5–8].

**Figure 5.** Motor gasoline production by country in 2012 [98]. *Source*: United States Energy Information Administration,

**Figure 4.** Global motor gasoline production by year [97]. *Source*: United States Energy Information Administration, 2016.

A very important property of gasoline fuels is their hygroscopicity, which has a considerable effect on the reliability of vehicles and equipments. Operational experience showed that the permanent addition of a small amount of water to hydrocarbon fuel has a positive effect on a combustion process, provided that water in gasoline is in the form of an emulsion. Therefore, studies to determine physicochemical properties of microemulsions have significant practi‐ cal importance. The emulsion, with the addition of 5 and 10% water, insignificantly increases engine torque, but a mixture with an addition of 15% water decreases engine torque. Water‐ in‐gasoline emulsions (WiGEs) cause an increase in brake‐specific fuel consumption (BSFC) and a decrease in exhaust temperature. Compared to basic gasoline, the WiGE fuel reduces

emissions [9].

NO*x* and CO and enhances O2

6 Application and Characterization of Surfactants

2016.

Oxygenates are chemical substances that contain oxygen in their structure. There are several oxygenates that can be added into gasoline (**Table 2**) and they can be divided into several groups based on their functions in fuel. Antistatic additives are responsible for reducing the potential for static build up by improving electrical conductivity and charge dissipation. The electrolytic conductivity of basic gasoline is very low (25 pS/m). Static electricity can build up during pumping, filtering, and splash transfer operations within refineries and also at filling stations, so it can be a reason of static discharges presenting an obvious fire hazard due to low conductivity of gasoline. Grounding and bonding during liquid transfer is a need to protect against static discharge. In a container flow discharging back to the walls may happen, thus the rate at which it can discharge depends on the gasoline composition and properties. In the case of walls being conductive, the electric field achieved by the flow can induce a charge on the walls. The external part of the walls can achieve a charge equal to the charge of gasoline, and the internal part can achieve a charge that will be equal and opposite to that of fuel (**Figure 6**). In order to eliminate the possibility of an electrostatic discharge, various antistatic compounds are used for this purpose [15]. Metal deactivators' task is to extend the durability of fuel by reducing the effect of the catalytic metal to its oxidation. The inhibitory action of these addi‐ tives involves the creation of inactive compounds with metal ions present in fuel. Metal ions bound in this way cannot catalyze the oxidation reaction any longer. The most active catalysts are copper and brass [16]. By the contact of hydrocarbons with oxygen at an elevated tempera‐ ture, their oxidation to organic acids, resins, and other compounds usually occurs (**Figure 7**) [17]. Antioxidants interrupt the chain reaction of the oxidation at a stage of peroxides, delay‐ ing aging changes in fuel. The mechanism of action of antioxidants consists in inhibition or interruption of the chain oxidation process by decomposition of peroxides formed in radical reactions of the process. Antioxidants can also react with free radicals to give the stable com‐ pounds breaking chain reactions [18]. Anticorrosion additives protect metal from corrosion mostly caused by the acidic products of fuel oxidation. Due to the physical adsorption or chemical reaction, metal protective layers are formed (passivation). These layers are chemically

**Figure 6.** Fuel tank charging diagram [1, 14].

**Figure 7.** A scheme of hydrocarbons oxidation [17].

stable and resistant to damage caused by friction. Examples of anticorrosive compounds are the following: zinc dialkyldithiophosphates, dialkyldithiocarbamates, zinc alkylsuccinic acids and their monoesters, alkylsulfoamide acids, zinc and calcium salts, organic phosphorus com‐ pounds (phosphoric acid esters), and organic sulfur (sulfides) and amine compounds [19]. Dampness of gasoline at a temperature lower than 4°C causes the formation of ice crystals. Ice crystals are formed by the rapid evaporation of gasoline and are deposited on the surface of the shutter valve and its periphery, making it difficult for proper operation of the valve. While an engine is heated to a high temperature, ice melts, and water dripping from the surface of the aperture contributes to the restricted air flow. In order to eliminate crystallization of water, anticrystallization (anti‐icing) substances are added that increase the solubility of water in fuel and reduce the temperature of crystallization of the aqueous solutions of the additive released from fuel. For this purpose, propan‐2‐ol, butanol, butan‐2‐ol, dimethylformamide (DMF) are used. Surface‐active compounds are another group. They are adsorbed on the surface of ice‐ nucleating agents that prevent their growth and connection into deposited agglomerates [18]. The examples of components used in gasoline fuel are presented in **Table 1**.

during pumping, filtering, and splash transfer operations within refineries and also at filling stations, so it can be a reason of static discharges presenting an obvious fire hazard due to low conductivity of gasoline. Grounding and bonding during liquid transfer is a need to protect against static discharge. In a container flow discharging back to the walls may happen, thus the rate at which it can discharge depends on the gasoline composition and properties. In the case of walls being conductive, the electric field achieved by the flow can induce a charge on the walls. The external part of the walls can achieve a charge equal to the charge of gasoline, and the internal part can achieve a charge that will be equal and opposite to that of fuel (**Figure 6**). In order to eliminate the possibility of an electrostatic discharge, various antistatic compounds are used for this purpose [15]. Metal deactivators' task is to extend the durability of fuel by reducing the effect of the catalytic metal to its oxidation. The inhibitory action of these addi‐ tives involves the creation of inactive compounds with metal ions present in fuel. Metal ions bound in this way cannot catalyze the oxidation reaction any longer. The most active catalysts are copper and brass [16]. By the contact of hydrocarbons with oxygen at an elevated tempera‐ ture, their oxidation to organic acids, resins, and other compounds usually occurs (**Figure 7**) [17]. Antioxidants interrupt the chain reaction of the oxidation at a stage of peroxides, delay‐ ing aging changes in fuel. The mechanism of action of antioxidants consists in inhibition or interruption of the chain oxidation process by decomposition of peroxides formed in radical reactions of the process. Antioxidants can also react with free radicals to give the stable com‐ pounds breaking chain reactions [18]. Anticorrosion additives protect metal from corrosion mostly caused by the acidic products of fuel oxidation. Due to the physical adsorption or chemical reaction, metal protective layers are formed (passivation). These layers are chemically

**Figure 6.** Fuel tank charging diagram [1, 14].

8 Application and Characterization of Surfactants

**Figure 7.** A scheme of hydrocarbons oxidation [17].

Permanent removal of lead from gasoline was a cause of looking for other applicable substances, which are able to improve fuel properties. Therefore, refining technologies have been modernized in order to generate high‐octane hydrocarbon (HC) compounds. In order to face the increasing



**Table 1.** Examples of additives used in gasoline fuel.

demands of environmental protection, oxygen‐containing compounds, organic oxygen‐contain‐ ing compounds started to be used. The Environmental Protection Agency (EPA) allowed the addi‐ tion of detergents to all types of motor gasoline in the United States in 1995 [28]. The minimum content of MTBE in gasoline is about 11% (v/v) in the United States, while in Europe the content is about 2.5% (v/v). In the European Union, according to Directive 2003/30/EC requirements, it is obliged to promote biofuels among the EU members and to recommend replacing conventional fuels by renewable energy sources (biofuels, etc.). The regulation initiated the reduction of green‐ house gas (GHG) emissions inter alia by allowing the use of ETBE and bioethanol. Based on the Directive 98/70/EC and the Directive 2009/30/EC, the limits of content of oxygenates are presented in **Table 2**.


**Table 2.** Requirements for gasoline used in vehicles equipped with spark‐ignition engines [12, 13].

## **3. The role of detergents in gasoline fuel**

demands of environmental protection, oxygen‐containing compounds, organic oxygen‐contain‐ ing compounds started to be used. The Environmental Protection Agency (EPA) allowed the addi‐ tion of detergents to all types of motor gasoline in the United States in 1995 [28]. The minimum content of MTBE in gasoline is about 11% (v/v) in the United States, while in Europe the content

**Group of chemicals Chemical compound CAS no. Ref.** Alcohols 2‐Butoxy ethanol 111‐76‐2 [21]

10 Application and Characterization of Surfactants

Ethers Methyl tert‐butyl ether (MTBE) 1634‐04‐4 [1]

Ester Ethyl acetate 141‐78‐6 [24]

Neutral organics 1,1‐Diethoxyethane 105‐57‐7 [24]

Undesignated Dimethylformamide 68‐12‐2 [1]

1‐Propene, 2‐methyl‐homopolymer, hydroformylation

products, reaction products with ammonia

Ester‐acid 1,2‐Bis(2‐ethylhexyloxycarbonyl) ethanesulphonate potassium salt

**Table 1.** Examples of additives used in gasoline fuel.

2‐Ethyl 1‐hexanol 104‐76‐7 [21] 3‐Methyl 1‐butanol 123‐41‐3 [24] 2‐Methyl 1‐butanol 137‐32‐6 [24] Isobutyl alcohol 78‐83‐1 [24] Tert‐butyl alcohol 75‐65‐0 [1] 2‐Propanol 67‐63‐0 [21] 1‐Propanol 71‐23‐8 [24] Ethanol 64‐17‐5 [24] Methanol 67‐56‐1 [24] 2‐Methoxyethanol 109‐86‐4 [1] 2‐Ethoxyethanol 110‐80‐5 [1] Tetrahydrofurfuryl alcohol 97‐99‐4 [1] Tert‐amyl alcohol 75‐85‐4 [1]

Ethyl tert‐butyl ether (ETBE) 637‐92‐3 [1] Tert‐amyl methyl ether (TAME) 994‐05‐8 [1] Tert‐amyl ethyl ether (TAEE) 919‐94‐8 [25] Diisopropyl ether (DIPE) 108‐20‐3 [1] Tert‐hexyl methyl ether (THEME) 38772‐53‐1 [26]

2‐Ethylhexyl nitrate 27247‐96‐7 [21] Tetrapropylenebutanedioic acid 27859‐58‐1 [21]

(Z)‐4‐Oxo‐4‐(tridecylamino)‐2‐butenoic acid 84583‐68‐6 [21] Polyolefin Mannich base ̶ [21]

Di‐sec‐butyl‐p‐phenylenediamine 101‐96‐2 [27]

7491‐09‐0 [21]

68891‐84‐9 [21]

During combustion processes, fuel forms deposits in the combustion chamber, valves, piston rings, parts injectors, etc. Carbon deposits accumulating on valves can be a cause of their suspension on walls of the combustion chamber and piston head. They change the condi‐ tions of heat exchange and carbon deposits in the injector worsen the quality of fuel atomi‐ zation. Deposits in the grooves of the volute on a piston may lead to their immobilization. This phenomenon deteriorates the conditions of air compression, facilitates the penetration of lubricating oil into a combustion chamber, may even lead to damage to the ring. The addition of detergents soluble in fuel reduces surface tension, but mainly removes all dirt and deposits from engine elements. Their function is to maintain engine cleanliness by counteracting the formation of sludge in the above‐mentioned engine elements [17].

The mechanism of action of detergents includes such physicochemical processes as solubil‐ ity and the stabilizing effect. Solubility is associated with the process of micelle formation, that is, of colloidal particles electrically charged and surrounded by a layer of associated solvent molecules. One theory explaining the mechanism of action of detergents brings to such processes as peptization and neutralization. Peptization is to move the pellet into sol or colloidal state under the influence of surfactants. Dirt particles of size from 10 to 150 nm may be subject to peptization. Larger particles are difficult to peptize. The particle size of the impurities is shown in **Figure 8**. Stages of an impact of surfactants on dirt particles are presented in **Figure 9**. Examples of detergents applied in gasoline fuel are shown in **Table 3**.

**Figure 8.** Schematic diagram of action of detergents [99].

**Figure 9.** Steps of an influence of surfactants on dirt particles: 1, wetting and penetration; 2, adsorption; 3, emulsification and solubilization (rolling‐up), dispersing; 4, emulsion [29].


may be subject to peptization. Larger particles are difficult to peptize. The particle size of the impurities is shown in **Figure 8**. Stages of an impact of surfactants on dirt particles are presented in **Figure 9**. Examples of detergents applied in gasoline fuel are shown in **Table 3**.

**Figure 9.** Steps of an influence of surfactants on dirt particles: 1, wetting and penetration; 2, adsorption; 3, emulsification

**Figure 8.** Schematic diagram of action of detergents [99].

12 Application and Characterization of Surfactants

and solubilization (rolling‐up), dispersing; 4, emulsion [29].



**SN Examples of detergents with potential use in gasoline presented in patents and articles Ref.**

or dodecylphenyl; R' is H, Me, or Et; *x* is an integer from 12 to 28; and *y* is an integer from 1 to 4.

CH<sup>2</sup>

= C1‐3 alkyl; *n* = 5–50)

(alkylene)*m* in which *n* = 0–4, *m* = 1–4,

CHR'O)*X*OOC‐Ph‐CO with the proviso

CHR'O)*X*OOC‐Ph‐CO and R is C2‐16 straight of branched chain alkyl

= C4‐25 alkyl; R<sup>2</sup>

= C1‐3 alkylene) with a bicyclic keto acid derived from a catalyzed

= C1–5‐alkylene, *x* = 1–9). The compounds reduce carburetor deposits by 85%

to produce a product that has good detergent properties in fuels.

(Z = C2‐6 alkylene; *m* = 0–6). The additives prevent or remove combustion

= H or polyolefin (of which ≥1 is a polyolefin); R<sup>2</sup>

= H, C1‐3 alkyl) or its anhydride to form a

NZ)y.Z, wherein

= C1‐8‐alkylene; R<sup>3</sup>

NH)XH (R

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

11 GB 2320719. The gasoline detergents are compounds of the formula Z.NH(CH<sup>2</sup>

12 WO 9736854, US 6053955. Polyoxyalkylene ether amino acid esters A polyalkylene glycol

CHR<sup>2</sup> ) <sup>n</sup>OH (R<sup>1</sup>

1:1 (un)substituted maleate ester, which reacts in molar ratio ≤ (m + 2):1 with a polyamine

13 WO 9215656. Polyalkylenepolyamines, especially polyisbutylene polyamine. The additive has a

14 WO 9213047. Amine‐ and halogen‐free gasoline detergents consisting of polypropylene glycol

15 GB 2247457. (Keto) diacid amides may be used as gasoline detergents being deposit inhibitors‐

16 US 5286265. Novel carbamates having the formula ROC(:O)NXY [X and Y are independently either H, a (hetero‐substituted) hydrocarbyl group, or ZNHC(:O)OR (I), where Z is a divalent

= H, C1‐6‐alkyl; *x* = 0–5]. Gasoline additives for reducing valve sticking and to have detergency and

and R is a (substituted) hydrocarbyl group, provided that if either one of X or Y is I, the other of X

rearrangement of a C6‐10 cyclic alken‐3‐yl carboxylic acid anhydride in the presence of a Bronsted

detergents are prepared by the reaction (at 100–175°C≥) of ≥1 C10–20 fatty acids, ≥1 C12–26‐alkyl or ‐alkenylsuccinic acid or anhydride, and ≥1 polyalkylenepolyamine of formula RNH(R<sup>1</sup>

acid)] are reacted with polyamines (esp. tetraethylenepentamine) to form a product mixture for

BO3 .

primary or secondary amines with formaldehyde and 2‐nitropropane followed by reduction of the

21 US 4505725. Fuel additives (detergents) obtained from borated, acid‐treated mixtures of vegetable oil derived amides and esters. Reaction products of soybean oil with tetraethylenepentamine,

22 US 4639255. Gasoline detergents (e.g., vegetable oil‐polyamine reaction products) (and optionally hydrogenated polybutenes) are mixed with C18–32 paraffin waxes (m. 130–160°CF) or durene, foamed, and pelleted (or encapsulated) to provide deposit‐control additives which float on the gasoline and readily dissolve. The additives, present at approximately 120 lb/1000 bbl unleaded gasoline, are sol. at extreme temperatures, do not change the gasoline octane rating, and do not

23 US 4400178. Polyamine carburetor dispersants. They are prepared by the Mannich reaction of

17 US 4729769. Reaction products of fatty acid esters and amines. Reaction products of C6–20 fatty acid ester with a mono‐ or di‐(hydroxyhydrocarbyl)amine may be used as carburetor detergents.

18 US 4624682. Gasoline detergents are prepared by reacting an N‐alkylalkylenediamine of formula

acid catalyst (e.g., Nafion H‐501). The aim of the compounds is removing deposits.

20 US 4508541. Vegetable oils [(esp. soybean oil, tall oil acids, or alkyl acids (esp. phenylstearic

19 EP 186473 A2. Lubricating oil detergents and fuel (especially gasoline) deposit inhibitors‐

Z is N, C8‐16 straight of branched chain alkanoyl or RO‐(CH<sup>2</sup>

C6 H4 (OCH<sup>2</sup>

CCR3 :CR4 CO2 H (R<sup>3</sup> , R4

)XNR32 [R1

hydrocarbyl, a substituted hydrocarbyl, or (alkylene)m(NH)*<sup>n</sup>*

or Y is H]. The additives are able to remove deposits.

that at least one Z is RO‐(CH<sup>2</sup>

14 Application and Characterization of Surfactants

mono(alkylphenyl) ether p‐R1

chamber or fuel line deposits.

(NR2

octane requirement reducing additives.

(R = C12‐18 alkyl, R1

deposition‐inhibiting properties.

with hydrophobic end group.

is treated with cis‐HO<sup>2</sup>

NZ(NHZ)mNH<sup>2</sup>

structure R12NR2

RNHR<sup>1</sup> NH<sup>2</sup>

= C1–5‐hydrocarbyl, R1

compared with the base fuel.

subsequent reaction with SO2

The compounds can reduce deposits.

promote gum formation or corrosion.

nitro group.

sulfonated lubricating oil bright stock, and H<sup>3</sup>

H2



**Table 3.** The role of chosen detergents applied in gasoline fuel: a review.

**SN Examples of detergents with potential use in gasoline presented in patents and articles Ref.**

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

[76]

[77]

[78]

mono(alkylphenyl) (2‐aminoethyl)carbamates and poly(oxypropylene)‐poly(oxybutylene) mono‐ Bu ether (2‐aminoethyl)carbamate. Deposit‐inhibiting dispersants (carburetor and intake‐valve

> O2 ).

H6

H) with dodecylbenzenesulfonic acid. The

NHR (R = C20‐1000 alkylhydroxybenzyl; *m* = 2 or 3; R1 = Me, Et; *b* = 1–5).

to give polyisobutenyl

39 US 4191537. Fuel compositions of poly(oxyalkylene) monoether (aminoethyl)carbamates. Poly(oxypropylene) mono‐Bu ether (I) carbamtes, derivs. of H2NCH2CH2NH2 or polyethylenepolyamines, were prepared. Also prepared were poly(oxybutylene)

40 US 4179271. Amine oxide polymers. Gasoline having detergent properties contains 0.05–0.75 wt% of a tertiary amine oxide‐containing polymer(I). Gasoline was prepared by mixing Neodol 25 L methacrylate 54.5, Alfol 1620 methacrylate 16.5, Bu methacrylate 20, and 4‐vinylpyridine 9 wt%,

41 US 4173456. Polyolefin‐acylated poly(alkyleneamine) may be used as a component fuel additive to prevent deposits formation. Detergents containing triamide of tetraethylenepentamine and tall‐oil fatty acids and polypropylene or polyisobutylene (the triamide 13.6, polypropylene (mol. wt. 800) 50, oxyalkylenated alkylphenol 1, corrosion inhibitor 1.1, and xylene (solvent) 34.3 wt% used at 25

42 US 4132531. A detergent (to remove deposits) prepared by condensing 115 g 1‐(2‐aminoethyl) piperazine with 700 g polyisobutenylsuccinic acid‐derived lactones (I) in 700 ml xylene.

43 US 4125382. Polyoxyalkylene ether demulsifiers. Alkylpolyamines as detergents and 5–30 ppm polyoxyalkylenes or their adducts with C8‐18 epoxides as demulsifiers. For example, shaking 32 ml gasoline containing 500 ppm alkylpolyethylenepolyamine, 25 ppm acetal‐coupled 28:72 polyethylene‐polypropylene glycol (mol. wt. 2200), and 4 ppm 13:87 polyethylene‐polypropylene

glycol (mol. wt. 2800) and settling 2 h. The detergents can increase water tolerance.

44 US 4125383. Reaction products of a long‐chain monocarboxylic acid, a polyamine, and a C12‐18

45 US 4105417. Hydrocarbyl‐substituted nitrogenous compounds (e.g., amides, carbamates, or

46 US 4059414. The detergents are prepared by treating long‐chain monocarboxylic acids with trialkanolamines and sulfonic acids. For example, 10 lb of a detergent prepared by treating triethanolamine triisostearate (obtained by the reaction of triethanolamine and isostearic acid

47 US 4054422, US 4121911. Mannich bases containing tertiary amines and fuel compositions containing said Mannich bases. Mannich condensation products have the formula

48 US 4038043. Mixture of monoamine and polyamine (N,N‐bis[2‐hydroksy‐4‐(polipropyleno)

49 US 4038044. A combination of diamine and polyamine Mannich bases. The diamine Mannich base used was based on ethylenediamine with each N atom being substituted by an alkyl‐ and hydroxy‐substituted benzyl group in which the alkyl substituent was derived from polypropylene of 840 mol. wt. The polyamine Mannich base was based on triethylenetetramine with the terminal N atoms each being substituted with one of the alkyl‐and hydroxy‐substituted benzyl groups described above. The multifunctional gasoline additive may be used as a carburetor detergent and

can minimize intake valve deposits and quick‐heat intake manifold deposits.

H4 SO3

A Mannich base was prepared by refluxing (11 h) a mixt. of 300 g of a 75 wt% solution in PhMe of polypropenylphenol (prepared. from PhOH and polypropylene of 840 mol. wt.), 14.5 g 3,3'‐(methylimino)bis(propylamine) and 17 g of a 36 wt% aq. HCHO solution. The carburetor

benzylo]methylamine with triethyltetramine with N atoms and benzyl groups). A multifunctional gasoline additive that may be used as a carburetor detergent and at the same time minimizes

hydrazodicarboxylate (polyisobutenyl av. mol. wt. = 950) in 50 ml C<sup>6</sup>

detergents reduce gum deposits in an unleaded gasoline by 63%.

detergents can reduce gum deposits in unleaded gasoline.

intake valve deposits and quick‐heat intake manifold deposits.

at 135–40°C for 6 h in the presence of p‐MeC6

acid. Improved ashless gasoline detergents decrease the carburetor deposits by 70–80%.

isocyanate. For example, octadecyl isocyanate was reacted with triethylenetetramine and isostearic

ureas) are effective as gasoline detergents (to remove valve deposits) at a concentration of 50–1500 ppm. For example, 0.06 mol diethylenetriamine was added to 0.44 mol polyisobutenyl di‐Et

polymerizing the mixture, and oxidizing the polymer by AcOH and H<sup>2</sup>

detergents) may be used in gasoline.

16 Application and Characterization of Surfactants

lb/1000 bbl).

cyclobiuret (I).

RNH(CH<sup>2</sup> ) *<sup>m</sup>*[NR1 (CH<sup>2</sup> ) *m*]*b*

## **4. Experimental procedure**

### **4.1. Materials**

Basic gasoline, obtained from the petrochemical industry, was used in all experiments. The composition is shown in **Table 4**.

The following compounds were used in experiments:


### **4.2. Advantages and disadvantages of the use of sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)**

Sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT) is an anionic surfactant with a sulfone group directly connected to a hydrophobic group (C20H37NaO7 S, molecular weight 444.56 g/mol, CAS Number 577‐11‐7). Water solubility in hydrocarbon fraction can be increased with an addition of AOT due to the formation of ternary microemulsion system consisting of water, the organic phase, and AOT. Microemulsions can form different types of structures as discontinuous spherical water droplets, interconnected channels of water, and so on. Their size can be controlled by the water content (wo), that is, the molar ratio of water to AOT (wo = [H<sup>2</sup> O]/[AOT]). Microemulsions may be created as "oil‐in‐water" (o/w) or "water‐ in‐oil" (w/o) depending on the nature of the solvent. Due to high interfacial activity and good hydro‐ philic properties, the surfactant AOT is able to form reversed micelles in the hydrocarbon phase, which absorbs large amounts of water. Reverse micelles occur in the situation when wo = [H<sup>2</sup> O]/[AOT] < 10–15. AOT can be hydrolyzed in the presence of an acid or a base, which results in formation of 2‐ethylhexyl alcohol and sulfosuccinate anion (**Figure 10**) [90].

The surfactant AOT contains seven oxygen atoms in its structure, which presence has a posi‐ tive effect. The compound contained in fuel introduces additional oxygen into the system, which plays an important role in combustion processes. Air‐fuel ratio is of great importance and essential measure for antipollution and performance reasons. Air‐fuel ratio is the amount of air needed to burn fuel in the engine and in other words, it is mass ratio of air to fuel pres‐ ent in the combustion chamber. Combustion efficiency depends on the right amount of air, Environmental Impact of the Use of Surfactants and Oxygenates in the Petroleum Industry http://dx.doi.org/10.5772/intechopen.68683 19


**Table 4.** The components of basic gasoline used for the research [1].

**4. Experimental procedure**

18 Application and Characterization of Surfactants

composition is shown in **Table 4**.

The following compounds were used in experiments:

• butanol (BuOH), POCH S.A., p.a., Gliwice, Poland,

• Hydranal Composite 5, Sigma‐Aldrich, Germany,

directly connected to a hydrophobic group (C20H37NaO7

• methanol (MeOH), p.a., POCH S.A., Gliwice, Poland, • ethanol (EtOH), 96%, p.a., Sigma‐Aldrich, Germany,

• propan‐2‐ol (isopropanol, IPA), p.a., POCH S.A., Gliwice, Poland,

• methyl t‐butyl ether (MTBE), 99%, p.a., Sigma‐Aldrich, Germany,

• 2‐methylpropanol (isobutanol, IBA), p.a., POCH S.A., Gliwice, Poland,

• sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT), 99%, p.a., Sigma‐Aldrich, Germany,

**4.2. Advantages and disadvantages of the use of sodium bis‐(2‐ethylhexyl)sulfosuccinate** 

Sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT) is an anionic surfactant with a sulfone group

CAS Number 577‐11‐7). Water solubility in hydrocarbon fraction can be increased with an addition of AOT due to the formation of ternary microemulsion system consisting of water, the organic phase, and AOT. Microemulsions can form different types of structures as discontinuous spherical water droplets, interconnected channels of water, and so on. Their size can be controlled by the water content (wo), that is, the molar ratio of water to AOT

(w/o) depending on the nature of the solvent. Due to high interfacial activity and good hydro‐ philic properties, the surfactant AOT is able to form reversed micelles in the hydrocarbon phase, which absorbs large amounts of water. Reverse micelles occur in the situation when

The surfactant AOT contains seven oxygen atoms in its structure, which presence has a posi‐ tive effect. The compound contained in fuel introduces additional oxygen into the system, which plays an important role in combustion processes. Air‐fuel ratio is of great importance and essential measure for antipollution and performance reasons. Air‐fuel ratio is the amount of air needed to burn fuel in the engine and in other words, it is mass ratio of air to fuel pres‐ ent in the combustion chamber. Combustion efficiency depends on the right amount of air,

results in formation of 2‐ethylhexyl alcohol and sulfosuccinate anion (**Figure 10**) [90].

O]/[AOT]). Microemulsions may be created as "oil‐in‐water" (o/w) or "water‐ in‐oil"

O]/[AOT] < 10–15. AOT can be hydrolyzed in the presence of an acid or a base, which

S, molecular weight 444.56 g/mol,

Basic gasoline, obtained from the petrochemical industry, was used in all experiments. The

**4.1. Materials**

• deionized water.

**(AOT)**

(wo = [H<sup>2</sup>

wo = [H<sup>2</sup> which is reflected on the engine power. Air contains about 21% oxygen, 79% nitrogen, and smaller amounts of other elements. When fuel burns in the presence of O2 and N2 , it is con‐ verted to carbon dioxide, water, nitrogen, and heat according to Eq. (1):

$$\text{CH}\_4 + 2\text{O}\_2 + 7.53\text{N}\_2 \rightarrow \text{CO}\_2 + 2\text{H}\_2\text{O} + 7.53\text{N}\_2 + \Lambda\text{H}\tag{1}$$

The exhaust gases from internal combustion engines mainly consist of the products of complete combustion, small amounts of the oxidation products of sulfur and nitrogen, and components derived from the fuel and various lubricants. The composition of gases is shown in **Table 5** [91].

**Figure 10.** Alkaline hydrolysis of AOT (R—ethylhexyl group) [90].


**Table 5.** Components of internal combustion engine exhaust gases [91].

The disadvantage of the surfactant AOT is the presence of one sulfur atom in its struc‐ ture. There is a tendency to eliminate sulfur from fuel composition in order to reduce its content in emitted gases after combustion processes in engines. In the air, SO<sup>2</sup> is pres‐ ent in the largest quantities, but other sulfur oxides (SO*<sup>x</sup>* ) are found in the atmosphere at much lower concentrations. SO2 influences human health when it is breathed in, at concentrations above 1000 μg/m3 , measured as a 10‐min average. The gas irritates the nose, throat, and airways to cause wheezing, coughing, shortness of breath, and a tight feeling around the chest. The large amounts of SO*<sup>x</sup>* in the atmosphere can harm all types of plants by damaging foliage and decreasing growth. Sulfur oxides are responsible for contributing to acid, which can harm sensitive ecosystems. Therefore, the concentration of sulfur oxides in air is constantly monitored in order to react appropriately in the case of a high concentration.

#### **4.3. Apparatus and procedures**

Samples of basic gasoline (25 ml) containing 3% (v/v) of a hydrophilic additive were mechan‐ ically shaken with 1% of deionized water for 2 h at 4000 revolutions per minute and left to phase separation for 24 h. The content of water in saturated gasoline samples was determined using the Karl Fischer method. The potentiometer 702 SM Titrino (Metrohm, Switzerland) was used for titration using Hydranal Composite 5 (Sigma‐Aldrich, Germany). The basic gasoline was modified with the anionic surfactant AOT at various concentrations and hydro‐ philic additives. After saturation with deionized water, the content of water was determined.

Electrolytic conductivity of modified gasoline samples was determined using pH/ conductivity meter CPC‐551. The K12 tensiometer with a platinum ring (Krüss, Germany) was used to measure the interfacial tension (water/gasoline). After preparation of the systems (15 ml of water and 9 ml of modified gasoline) for measurements, interfacial tension was measured using the Du Noüy ring method at room temperature. All experiments mentioned above were made in triplicate for each method.

## **5. Results and discussion**

The disadvantage of the surfactant AOT is the presence of one sulfur atom in its struc‐ ture. There is a tendency to eliminate sulfur from fuel composition in order to reduce its

nose, throat, and airways to cause wheezing, coughing, shortness of breath, and a tight

of plants by damaging foliage and decreasing growth. Sulfur oxides are responsible for contributing to acid, which can harm sensitive ecosystems. Therefore, the concentration of sulfur oxides in air is constantly monitored in order to react appropriately in the case

Samples of basic gasoline (25 ml) containing 3% (v/v) of a hydrophilic additive were mechan‐ ically shaken with 1% of deionized water for 2 h at 4000 revolutions per minute and left to phase separation for 24 h. The content of water in saturated gasoline samples was determined using the Karl Fischer method. The potentiometer 702 SM Titrino (Metrohm, Switzerland) was used for titration using Hydranal Composite 5 (Sigma‐Aldrich, Germany). The basic gasoline was modified with the anionic surfactant AOT at various concentrations and hydro‐ philic additives. After saturation with deionized water, the content of water was determined. Electrolytic conductivity of modified gasoline samples was determined using pH/ conductivity meter CPC‐551. The K12 tensiometer with a platinum ring (Krüss, Germany) was used to measure the interfacial tension (water/gasoline). After preparation of the systems (15 ml of water and 9 ml of modified gasoline) for measurements, interfacial tension was measured using the Du Noüy ring method at room temperature. All experiments mentioned above were

is pres‐

) are found in the atmosphere

in the atmosphere can harm all types

influences human health when it is breathed in, at

, SO3 (c)

H*m*COOH (c)

H*m*CHO (c)

H*m* (c)

H*m*OH (c)

Carbon monoxide, CO (b)

(b)

Hydrogen, H<sup>2</sup>

Smoke (c)

(c)

, measured as a 10‐min average. The gas irritates the

content in emitted gases after combustion processes in engines. In the air, SO<sup>2</sup>

ent in the largest quantities, but other sulfur oxides (SO*<sup>x</sup>*

**Major components (>1%) Minor components (<1%)**

(c) Aldehydes, C*<sup>n</sup>*

(c) Organic acids, C*<sup>n</sup>*

(a) Hydrocarbons C*<sup>n</sup>*

Carbon monoxide, CO (a) Alcohols, C*<sup>n</sup>*

**Table 5.** Components of internal combustion engine exhaust gases [91].

(a) Spark‐ignition engine, (b) diesel engine, (c) both engines.

O (c) Oxides of sulfur, SO2

(c) Oxides of nitrogen, NO, NO2

feeling around the chest. The large amounts of SO*<sup>x</sup>*

at much lower concentrations. SO2

concentrations above 1000 μg/m3

of a high concentration.

Water, H<sup>2</sup>

Nitrogen, N2

Oxygen, O2

Hydrogen, H<sup>2</sup>

Carbon dioxide, CO2

20 Application and Characterization of Surfactants

**4.3. Apparatus and procedures**

made in triplicate for each method.

#### **5.1. The influence of biocomponents on water solubility in gasoline**

The composition of gasoline, type, and concentration of hydrophilic oxygen‐containing addi‐ tives (i.e., alcohols, ethers), amphiphiles (i.e., surfactants), and other functional components affect the solubility of water in the fuel. Preliminary studies demonstrated that the content of water in basic gasoline saturated with 1% of deionized water was about 0.01% (v/v). The solubility of water in basic gasoline modified with the anionic surfactant AOT and chosen hydrophilic alcohols and ether (3%, v/v) is shown in **Figure 13**. The addition of AOT causes significant changes and depends considerably on its concentration. The multifunctional sur‐ factant increases the solubility of water up to about 1%. The sudden increase is observed at very low AOT concentration equal to 6.25 × 10−4 mol/l. This phenomenon can be explained by the fact that the surfactant AOT initiates structural changes and it is able to increase the solubility of water in the fuel through the formation of reverse micelles. The relative standard deviation of the measurements is presented in **Table 6**.

The experimental data shown in **Figure 13** demonstrate the dependence that the solubility of water in gasoline increases with the growing number of carbon atoms in the alcohol mol‐ ecule. The greatest values of water content are observed in the case of an addition of AOT and mixtures of AOT and MTBE in a range of concentrations from 10−5 to 10−3 mol/l. Samples of fuel containing AOT and MTBE were very cloudy, which can be probably the result of reverse micelles formation. The hydrophilic part of the surfactant AOT creates the micelle cores, which are filled with deionized water and thus, a quick rise of solubility of water is noticed. The association phenomena are dependent on a type of hydrophilic components, which are able to act as cosurfactants, which is shown in the schematic diagram in **Figures 11** and **12**. The components present in reverse micelles lead to an increase in micelle's size and water solubility, and they promote the charge of structure with formation of microemulsion.

**Figure 13** shows the higher surfactant AOT concentration, the higher amount of water in the modified fuel. In the presence of MTBE, the content of water is higher compared to samples including various alcohols (3%, v/v). At first, it may be explained that hydrophilic components enhance polarity of gasoline mixture and cause an increase in the solubility of water. Larger amounts of the additives may not act as cosurfactants and furthermore they can delay asso‐ ciation of surfactant AOT into reverse micelles with water pools. Second, alcohols methanol and ethanol contain short hydrocarbon chains, therefore due to their low molecular weights they cannot join the micelles. As a result of that, water solubility in gasoline consequently achieves low level and the situation is improved only by an increase in the concentration of


**Table 6.** Relative standard deviation values for content of water measurements. Source: own research.

**Figure 11.** Reverse micelles formed in gasoline modified with hydrophilic additives and surfactants [1].

**Figure 12.** A scheme of solubilization site for alcohols [1].

**Figure 13.** Solubility of water in gasoline containing hydrophilic additives (3%, v/v) and AOT.

surfactant AOT. The ether MTBE has a higher molecular weight and a branched hydrocarbon chain; therefore, in the presence of a small amount of AOT it is possible to enhance the water content significantly. In comparison to methanol and ethanol, such alcohols as propan‐2‐ol (IPA), 2‐methylpropanol (IBA), and butanol (BuOH) have ability to be highly included in micelles, because of their higher molecular weights and amphiphilic properties. Solubility of water caused by the tested alcohols is low and very similar to basic gasoline. Only an addition of MTBE significantly improves water solubility (**Figure 13**). The impact of the additives changes in the following order: AOT > AOT/MTBE > AOT/IPA < AOT/IBA > AOT/BuOH > AOT/EtOH > AOT/MeOH. Similar results were obtained in the case of an addition of AOT, MTBE, and the alcohols in the amount of 2% (v/v) in our previous studies [1].

#### **5.2. Conducting properties improvement of modified gasoline**

Initial studies demonstrated that electrolytic conductivity of basic gasoline and basic gasoline previously saturated with an addition of deionized water (1%, v/v) was 0 μS/cm. **Figure 14** shows the presence of only AOT caused a sudden and fast increase in electrolytic conductivity even at a concentration of 2 × 10−4 mol/l. The relative standard deviation of measurements is shown in **Table 7**. Yet, the conductivity achieved the value 0.16 μS/cm and next remained on a constant level above the concentration of 1.88 × 10−3 mol/l. The highest values were obtained after modi‐ fication with MTBE (3%, v/v). While the conductivity achieved the level of 0.19 μS/cm, it did not change with the increasing concentration of AOT. The lowest values were observed in the case of an addition of 3% MeOH (0–0.02 μS/cm) and EtOH (0–0.05 μS/cm). Nonetheless, higher branched alcohols (IPA, IBA, and BuOH) with a higher molecular weight significantly caused an increase in conductivity. The anionic surfactant AOT in the presence of MTBE and water gen‐ erated the highest values due to the formation of reverse micelles. It is reported in the literature that electrolytic conductivity is very sensitive to the microemulsion system structure [92]. The occurrence of conductivity percolation is revealed due to an increase in the droplet size, interac‐ tions and the exchange rate of substances between droplets. The percolation threshold coincides with the formation of the first clusters of droplets [93]. The change of electrolytic conductivity demonstrates the alteration of the reverse micellar microstructure and after that the percolation

**Figure 14.** Conductivity of gasoline containing hydrophilic additives (3%, v/v) and AOT.

surfactant AOT. The ether MTBE has a higher molecular weight and a branched hydrocarbon chain; therefore, in the presence of a small amount of AOT it is possible to enhance the water content significantly. In comparison to methanol and ethanol, such alcohols as propan‐2‐ol (IPA), 2‐methylpropanol (IBA), and butanol (BuOH) have ability to be highly included in micelles, because of their higher molecular weights and amphiphilic properties. Solubility of water caused by the tested alcohols is low and very similar to basic gasoline. Only an addition of MTBE significantly improves water solubility (**Figure 13**). The impact of the additives changes in the following order: AOT > AOT/MTBE > AOT/IPA < AOT/IBA > AOT/BuOH >

**Figure 13.** Solubility of water in gasoline containing hydrophilic additives (3%, v/v) and AOT.

**Figure 11.** Reverse micelles formed in gasoline modified with hydrophilic additives and surfactants [1].

**Figure 12.** A scheme of solubilization site for alcohols [1].

22 Application and Characterization of Surfactants


**Table 7.** Relative standard deviation values for electrolytic conductivity measurements. Source: own research.

transition occurs. It is reported that conductivity is firmly related to droplet diameter, however, a temperature, the presence of external entity, or the composition of the microemulsion system also have an influence on the conducting properties of reverse micelles. Microemulsion is able to transport charges and affects the changes in the electrolytic conductivity [1].

#### **5.3. The effects of additives on interfacial tension**

In preliminary studies, it was indicated that interfacial tension at the interface of basic gaso‐ line/deionized water was 27.16 mN/m. The basic gasoline saturated with 1% of deionized water demonstrated the value equal to 25.12 mN/m. **Figure 15** shows the influence of various additives on interfacial tension at the gasoline/water interface. At the abscissae of **Figure 15**, a common logarithm (log to base 10) of the molar concentration c [mol/l] of the surfactant AOT was used to present the interfacial tension isotherms in a clearer way. The relative standard deviation of measurements is shown in **Table 8**. The decrease in interfacial tension depends

**Figure 15.** Interfacial tension isotherms at the modified gasoline/water interface.


**Table 8.** Relative standard deviation values for interfacial tension measurements. Source: own research.

on concentration of the surfactant AOT and the type of an additive. Interfacial tension iso‐ therms of gasoline samples with AOT (23.2–5.8 mN/m) and with AOT and 3% MTBE (24.5– 4.12 mN/m) have a similar course. Alcohols BuOH (12.7–5.3 mN/m), IPA (15.5–3.0 mN/m), and IBA (15.8–4,4 mN/m) showed the greatest surface activity. The effect of examined gaso‐ line additives can be presented in the following order: AOT/BuOH > AOT/IPA > AOT/IBA > AOT/EtOH > AOT/MeOH > AOT/MTBE > AOT.

## **6. Conclusions**

transition occurs. It is reported that conductivity is firmly related to droplet diameter, however, a temperature, the presence of external entity, or the composition of the microemulsion system also have an influence on the conducting properties of reverse micelles. Microemulsion is able

In preliminary studies, it was indicated that interfacial tension at the interface of basic gaso‐ line/deionized water was 27.16 mN/m. The basic gasoline saturated with 1% of deionized water demonstrated the value equal to 25.12 mN/m. **Figure 15** shows the influence of various additives on interfacial tension at the gasoline/water interface. At the abscissae of **Figure 15**, a common logarithm (log to base 10) of the molar concentration c [mol/l] of the surfactant AOT was used to present the interfacial tension isotherms in a clearer way. The relative standard deviation of measurements is shown in **Table 8**. The decrease in interfacial tension depends

**1.49 × 10−2 7.43 × 10−3 3.72 × 10−3 1.86 × 10−3 9.29 × 10−4 4.65 × 10−4**

2.32 × 10−4 1.16 × 10−4 5.81 × 10−5 2.9 × 10−5 1.45 × 10−5 7.26 × 10−6

0.032 0.037 0.023 0.041 0.015 0.016

**Table 8.** Relative standard deviation values for interfacial tension measurements. Source: own research.

0.011 0.008 0.026 0.028 0.031 0.018

to transport charges and affects the changes in the electrolytic conductivity [1].

**5.3. The effects of additives on interfacial tension**

24 Application and Characterization of Surfactants

**Figure 15.** Interfacial tension isotherms at the modified gasoline/water interface.

**Molar concentration AOT [mol/l]**

Relative standard deviation for AOT (RSD)

Molar concentration AOT [mol/l]

Relative standard deviation for AOT (RSD)

The multifunctional anionic surfactant AOT causes an increase in the solubility of water and electrolytic conductivity in gasoline. The obtained properties are the result of the association phenomenon of the surfactant and formation of reverse micelles comprising water pools in the hydrophilic micelle cores. Alcohols containing higher number of carbon atoms in their molecule lead to an increase in the solubility of water. Yet, the compounds may reduce the positive effect of the surfactant AOT on water solubility in gasoline. The results showed that some examined additives may act as cosurfactants.

Alcohols with highly branched hydrocarbon chains (isopropanol, isobutanol, and butanol) essentially increase the electrolytic conductivity. Modification of gasoline with the surfactant AOT and ether MTBE indicated the highest increase in electrolytic conductivity because of the formation of reverse micelles, which are able to transport charges.

The effect of the addition of AOT is the decrease in the interfacial tension at the water/gasoline interface. The decrease depends on the surfactant concentration and type of hydrophilic addi‐ tives. The lowest values were observed in the presence of butanol, isopropanol, and isobu‐ tanol. The examined components have an influence on the interfacial tension, electrolytic conductivity, and the solubility of water in the same order: butanol > 2‐methylpropanol > propan‐2‐ol > ethanol > methanol > MTBE. The research results demonstrated strong rela‐ tionship between the length of the hydrocarbon chain, the molecular weight of hydrophilic components, and the tested properties of gasoline.

The conducted studies are innovative and can significantly contribute to an increase in knowl‐ edge and research of new water‐in‐gasoline emulsion (WiGE) fuel. The fuel with the addition of hydrophilic oxygen components in the presence of small amounts of surfactants and water may have unique properties. Oxygen compounds have a lot of useful properties, including antiknock properties, enhancing octane number, and they can be produced from renew‐ able agricultural raw materials. Gasoline as an emulsion may have a beneficial effect on the combustion process, and the result is the almost complete combustion of hydrocarbons to the low toxic gases and the absence of carbon black among combustion products. The presence of water in gasoline reduces fuel consumption, increases engine power, decreases the tempera‐ ture of its work, thus reducing emissions of volatile organic compounds, NO*<sup>x</sup>* , SO2 , CO, and particulate matter. The use of water in fuel can be a unique chance for development of global economy in terms of energy production. The research may contribute to the commercializa‐ tion of new environmentally friendly fuel that may provide an alternative source of energy for spark‐ignition engines in the future.

## **Author details**

#### Tomasz Kalak

Address all correspondence to: tomasz.kalak@ue.poznan.pl

Department of Commodity Science and Ecology of Industrial Products, Faculty of Commodity Science, Poznań University of Economics and Business, Poznań, Poland

## **References**


[12] The Regulation of the Minister of Economy of 9 December 2008 on quality requirements for liquid fuels (Journal of Laws No. 221, item. 1441), Poland

**Author details**

26 Application and Characterization of Surfactants

Address all correspondence to: tomasz.kalak@ue.poznan.pl

Science, Poznań University of Economics and Business, Poznań, Poland

Department of Commodity Science and Ecology of Industrial Products, Faculty of Commodity

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## **Recent Advances in Catanionic Mixtures**

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detail.php?id=23832 [Accessed: 2017‐01‐10]

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3846089, US 3782912, US 3912771

32 Application and Characterization of Surfactants

Darija Domazet Jurašin, Suzana Šegota, Vida Čadež, Atiđa Selmani and Maja Dutour Sikirć

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67998

#### **Abstract**

Most surfactant mixtures display synergistic physicochemical properties, which have led to their extensive application in various technologies. Aqueous mixtures of two oppo‐ sitely charged surfactants, so‐called catanionic surfactant mixtures, exhibit the strongest synergistic effect, which is manifested as high surface activity, enhanced adsorption and a low critical aggregation concentration. In addition, catanionic systems display rich phase behavior and a range of nano and microstructures, including small spherical micelles, rod‐like micelles as well as open and closed bilayers (vesicles). The spontaneous forma‐ tion of catanionic vesicles is of special interest due to their various applications in nano‐ technology and pharmaceutical formulations. In this chapter, the properties of catanionic mixtures of amphiphilic molecules with advantageous properties are discussed. Since numerous papers dealing with catanionic mixtures of monomeric surfactants already exist, the aim of this chapter is to summarize recent progress in mixtures of structurally different surfactants. At the end of the chapter, special emphasis is placed on applications of catanionic mixtures.

**Keywords:** surfactants, catanionic mixtures, vesicles, phase behavior, application

## **1. Introduction**

Due to their amphiphilic structure, surfactants exhibit unique physicochemical properties both in solutions and in solid state. Mixtures of two or more different surfactants often show improved properties compared to individual surfactant solutions. As a result, in household and industrial applications, surfactant mixtures are usually used [1, 2]. Aqueous mixtures of two oppositely charged surfactants, that is, catanionic surfactant mixtures, exhibit the stron‐ gest synergistic effect, which is manifested as high surface activity, enhanced adsorption and

© 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.

low critical aggregation concentration [3–7]. In addition, catanionic mixtures display rich phase behavior governed by electrostatic and hydrophobic interactions, steric effects (geo‐ metric packing constraints) and hydrogen bonding. Therefore, such systems offer numerous possibilities in controlling molecular self‐assembly by adjusting bulk properties and using appropriate surfactant molecules. Consequently, they are of special interest not only from fundamental point of view but also because of a wide range of industrial applications.

In this chapter, catanionic mixtures of amphiphilic molecules with advantageous properties are discussed. Since numerous papers dealing with catanionic mixtures of monomeric surfactants already exist, the aim of this chapter is to summarize recent year's progress in mixtures of struc‐ turally different surfactants. At the end of the chapter, special emphasis is placed on applications of catanionic mixtures. It should be pointed out that the field of catanionic mixtures investigation is vast and still expanding, so the present review can be neither fully comprehensive nor final.

## **2. Properties of catanionic mixtures**

Catanionic mixtures can be prepared by mixing a cationic surfactant with an anionic one. Because two oppositely charged surfactants are present in the mixture, catanionic mixtures possess unique features, which can be summarized as follows [2–8]:


$$\rm{CA} + \rm{YA} \rightarrow \rm{CA} + \rm{X}^{\cdot} + \rm{Y}^{\cdot} \tag{1}$$

where CX and YA represent cationic and anionic surfactant, and X<sup>−</sup> and Y+ represent respec‐ tive counterions.

Due to strong electrostatic attractions, addition of ionic surfactant to the solution of other, oppositely charged surfactants results in formation of tight ion pairs and removal of hydration water at the mixed aggregate/solution interface [8]. Thus, oppositely charged surfactant mono‐ mers form ion pairs, which can be described as pseudo double‐tailed zwitterionic surfactants (**Figure 1**) [6].

**Figure 1.** A schematic representation of ion pair formation in catanionic mixtures.

low critical aggregation concentration [3–7]. In addition, catanionic mixtures display rich phase behavior governed by electrostatic and hydrophobic interactions, steric effects (geo‐ metric packing constraints) and hydrogen bonding. Therefore, such systems offer numerous possibilities in controlling molecular self‐assembly by adjusting bulk properties and using appropriate surfactant molecules. Consequently, they are of special interest not only from fundamental point of view but also because of a wide range of industrial applications.

In this chapter, catanionic mixtures of amphiphilic molecules with advantageous properties are discussed. Since numerous papers dealing with catanionic mixtures of monomeric surfactants already exist, the aim of this chapter is to summarize recent year's progress in mixtures of struc‐ turally different surfactants. At the end of the chapter, special emphasis is placed on applications of catanionic mixtures. It should be pointed out that the field of catanionic mixtures investigation is vast and still expanding, so the present review can be neither fully comprehensive nor final.

Catanionic mixtures can be prepared by mixing a cationic surfactant with an anionic one. Because two oppositely charged surfactants are present in the mixture, catanionic mixtures

(1) strong electrostatic attractions between oppositely charged headgroups and ion pairing

(2) pronounced synergism and solution behavior that considerably deviate from ideal mix‐ ing, that is, interfacial and aggregation properties of such systems are enhanced com‐

(3) strong dependence of the physicochemical properties, as well as the phase behavior, on

(4) rich phase behavior and structural diversity where the size of aggregates ranges from the nano to micrometer scale (mixed micelles, vesicles, tubules, liquid crystalline phases, etc.),

(5) spontaneous formation of stable vesicles, including, in some cases, equilibrium vesicles and

CX + YA → CA + X<sup>−</sup> + Y+ (1)

Due to strong electrostatic attractions, addition of ionic surfactant to the solution of other, oppositely charged surfactants results in formation of tight ion pairs and removal of hydration water at the mixed aggregate/solution interface [8]. Thus, oppositely charged surfactant mono‐ mers form ion pairs, which can be described as pseudo double‐tailed zwitterionic surfactants

and Y+

represent respec‐

(6) precipitation of catanionic surfactant (CA) at/or near equimolar bulk composition:

possess unique features, which can be summarized as follows [2–8]:

the molar ratio and total concentration of the components,

where CX and YA represent cationic and anionic surfactant, and X<sup>−</sup>

**2. Properties of catanionic mixtures**

34 Application and Characterization of Surfactants

pared to those of single surfactants,

(**Figure 1**),

tive counterions.

(**Figure 1**) [6].

The formation of ion pairs has pronounced influence on the adsorption properties and self‐ assembly of catanionic mixtures. Unlike the solution of individual monomeric surfactants, in catanionic mixtures aggregates with minimal curvature, such as open and closed bilayers (vesicles), are spontaneously formed even at the low surfactant concentrations. For that rea‐ son, experimental investigations of dilute catanionic mixtures have made a key contribution to our understanding of the factors governing vesicle formation in surfactant systems [8, 9].

Spontaneous formation of stable vesicles in these systems can be explained by using pack‐ ing parameter (*P*), which is defined with three nominal geometric parameters of surfactant molecules:

$$P = \frac{v\_{hc}}{a\_o l\_{hc}}\tag{2}$$

where *a*<sup>0</sup> , *v*hc and *l* hc are the surface area per headgroup, the volume and fully extended length of the hydrophobic tail of the molecule, respectively [10]. The molecular shape and respective *P* will determine the type of preferred surfactants' aggregate: for *P* = 0.33, spherical micelles; for *P* ≈ 0.33–0.5, cylindrical micelles; for *P* ≈ 0.5–1, bilayer disks and vesicles; for cylinders (**Figure 1**) with *P* ≈ 1 planar bilayers; and for *P* > 1, reverse structures. In other words, from the aspect of geometric constraints, the preferred structure of surfactant's aggregates, for a given hydrophobic tail size, strongly depends on the effective headgroup area. In general, the *a*<sup>0</sup> value depends on two opposite forces: (1) attractive hydrophobic interactions between hydro‐ carbon chains at the hydrocarbon‐water interface and (2) repulsive electrostatic and/or steric interactions [8]. In the case of catanionic ion pairs, the effective headgroup area decreases, compared to the value of each of the surfactants individually, while the volume of the hydro‐ phobic chain increases. As a result, the value of packing parameter approaches unity, which favors structures with low curvature like vesicles and flexible bilayers [10].

However, for better explanation of spontaneous vesicles' formation in catanionic mixtures, in addition to geometric parameters, the curvature free energy and its dependence on bilayer's composition should also be taken into account. In order for bilayer to have non‐zero spon‐ taneous curvature, its two individual leaflets should have equal and opposite spontaneous curvature (**Figure 2**). This is possible only when two leaflets have different compositions. Moreover, the composition should be such that average headgroup area in the outer leaflet is larger than the one in the inner. In catanionic mixtures, this is achieved by having higher molar ratio of the excess surfactant in the outer leaflet, resulting in larger headgroup spacing due to the electrostatic repulsions. In the inner leaflet, the higher fraction of paired surfactants reduces the headgroup area and results in positive curvature [5]. Therefore, spontaneously

**Figure 2.** A schematic representation of catanionic bilayer with equal and opposite curvature of inner (*c*<sup>i</sup> ) and outer (*c*<sup>o</sup> ) leaflet (after [5]).

formed catanionic vesicles owe their stability to the non‐ideal mixing of oppositely charged surfactants as well as to electrostatic effects.

The spontaneous formation of catanionic vesicles is of special interest due to their various applications in nanotechnology and pharmaceutical formulation, as will be discussed later in this chapter. Regarding the vesicle stability, different theoretical models have tried to ratio‐ nalize why vesicles behave as true equilibrium aggregates rather than a dispersed form of a lamellar liquid crystal [11–13]. However, what is important for application purposes is that vesicles do readily form in catanionic mixtures and they appear to pose long‐term stability. In addition, the low‐cost and versatile physicochemical properties make them a good alternative to phospholipid vesicles, that is, liposomes [5].

A typical phase diagram for catanionic mixtures is schematically illustrated in **Figure 3**. However, there are numerous variations in the appearance of the catanionic phase diagram. The concentration regions in which vesicles form are represented by the lobes on both sides of the equimolar line. This indicates that vesicles are stabilized by the presence of excess surfac‐ tants. Catanionic vesicles usually have high degree of polydispersity, and their stability can be tailored by the choice of surfactant molecular structure, that is branched surfactants, and/ or those containing a bulky group in alkyl tail usually form more stable vesicles [6]. Likewise, in asymmetric surfactant mixtures, in terms of different alkyl chain numbers or length as well as different chain morphology, the vesicle phase is often considerably enlarged and found in a broad concentration range [14–16]. The size, surface charge density and permeability of catanionic vesicles can be tailored by varying temperature, concentration and molar ratio, as well as chain length of surfactants [17].

As the molar mixing ratio of the two surfactants or the total surfactant concentration is varied, different phase transitions involving vesicles are found: micelle‐to‐vesicle, vesicle‐to‐lamellar and vesicle‐to‐solid phase transitions (**Figure 3**). At the highest excess of the mixture compo‐ nents, mixed micelles of various sizes and shapes can be found, including globular, elongated (worm‐like) and branched ones [7]. The size and shape of mixed micelles depend on bulk composition and total surfactant concentration as well as geometry of the surfactants, temper‐ ature, salt content, etc. In surfactant mixtures, micelle‐to‐vesicle transition has been broadly found to occur through two pathways [18]. One path involves limited micellar growth and

**Figure 3.** A schematic triangular phase diagram of symmetric catanionic mixture at constant temperature and pressure. The dashed line denotes the equimolar line dividing the diagram into the cationic‐rich and the anionic‐rich region. Close to the charge neutrality line, a solid precipitate (*P*) is usually formed, but excess charge in the system usually leads to vesicle stabilization (denoted as V+ and V<sup>−</sup> ). Mixed micelles (denoted as M+ and M<sup>−</sup> ) are usually formed at the highest excess of the mixture components. Multiphase regions (multi‐Φ) often involve a lamellar phase occurring at higher concentrations (denoted as L+ and L<sup>−</sup> ) (after [9]).

micelle/vesicle coexistence and is more common for systems with symmetric chain lengths [19]. Examples are dodecyltrimethylammonium bromide (C12TAB)/sodium dodecyl sulfate (SDS) (**Figures 4** and **5**) [20], didodecyldimethylammonium bromide (DDAB, **Figure 6**)/SDS [21] mixtures and a few others involving amino acid‐based surfactants [22, 23]. The second path involves strong micellar growth and is typical of highly asymmetric systems [19].

formed catanionic vesicles owe their stability to the non‐ideal mixing of oppositely charged

) and outer (*c*<sup>o</sup>

)

**Figure 2.** A schematic representation of catanionic bilayer with equal and opposite curvature of inner (*c*<sup>i</sup>

The spontaneous formation of catanionic vesicles is of special interest due to their various applications in nanotechnology and pharmaceutical formulation, as will be discussed later in this chapter. Regarding the vesicle stability, different theoretical models have tried to ratio‐ nalize why vesicles behave as true equilibrium aggregates rather than a dispersed form of a lamellar liquid crystal [11–13]. However, what is important for application purposes is that vesicles do readily form in catanionic mixtures and they appear to pose long‐term stability. In addition, the low‐cost and versatile physicochemical properties make them a good alternative

A typical phase diagram for catanionic mixtures is schematically illustrated in **Figure 3**. However, there are numerous variations in the appearance of the catanionic phase diagram. The concentration regions in which vesicles form are represented by the lobes on both sides of the equimolar line. This indicates that vesicles are stabilized by the presence of excess surfac‐ tants. Catanionic vesicles usually have high degree of polydispersity, and their stability can be tailored by the choice of surfactant molecular structure, that is branched surfactants, and/ or those containing a bulky group in alkyl tail usually form more stable vesicles [6]. Likewise, in asymmetric surfactant mixtures, in terms of different alkyl chain numbers or length as well as different chain morphology, the vesicle phase is often considerably enlarged and found in a broad concentration range [14–16]. The size, surface charge density and permeability of catanionic vesicles can be tailored by varying temperature, concentration and molar ratio, as

As the molar mixing ratio of the two surfactants or the total surfactant concentration is varied, different phase transitions involving vesicles are found: micelle‐to‐vesicle, vesicle‐to‐lamellar and vesicle‐to‐solid phase transitions (**Figure 3**). At the highest excess of the mixture compo‐ nents, mixed micelles of various sizes and shapes can be found, including globular, elongated (worm‐like) and branched ones [7]. The size and shape of mixed micelles depend on bulk composition and total surfactant concentration as well as geometry of the surfactants, temper‐ ature, salt content, etc. In surfactant mixtures, micelle‐to‐vesicle transition has been broadly found to occur through two pathways [18]. One path involves limited micellar growth and

surfactants as well as to electrostatic effects.

36 Application and Characterization of Surfactants

leaflet (after [5]).

to phospholipid vesicles, that is, liposomes [5].

well as chain length of surfactants [17].

**Figure 4.** Molecular structures of monomeric cationic surfactants—quaternary alkyl ammonium salts. *m* = number of C atoms in alkyl chains and *s* = number of C atoms in spacer.

**Figure 5.** Molecular structures of various anionic amphiphiles.

In the majority of catanionic mixtures at equimolar concentrations precipitate, a new catan‐ ionic surfactant, which very often possesses lamellar structure, forms [2, 18]. It can form as only phase or in coexistence with (1) coacervates (small droplets in solution rich with surfactants) [6, 24] as well as (2) micelles or (3) lamellar phase, usually in asymmetric mixtures [14, 16, 22–28]. Formed precipitate can be redissolved by increasing concentration of one of the surfac‐ tants, leading to formation of micelles or vesicles. Although catanionic precipitate is generally found only near equimolar compositions or in samples below their Krafft temperature, it is

**Figure 6.** Molecular structures of double‐tailed and dimeric cationic surfactants—quaternary alkyl ammonium salts*. m* = number of C atoms in alkyl chains and *s* = number of C atoms in spacer.

considered to be the main drawback for application of catanionic mixtures [3]. However, with the right selection of mixtures components, precipitation can be circumvented. Increased asym‐ metry in hydrophobic parts of surfactant molecules (different tail length or number, branched tails, rigid ring‐based structures, etc.) weakens hydrophobic attractions among alkyl chains and prevents efficient packing into a crystalline lattice [14–16, 29]. As a result, precipitation does not occur and instead large micelles, vesicles or liquid crystalline phases can be formed.

In the majority of catanionic mixtures at equimolar concentrations precipitate, a new catan‐ ionic surfactant, which very often possesses lamellar structure, forms [2, 18]. It can form as only phase or in coexistence with (1) coacervates (small droplets in solution rich with surfactants) [6, 24] as well as (2) micelles or (3) lamellar phase, usually in asymmetric mixtures [14, 16, 22–28]. Formed precipitate can be redissolved by increasing concentration of one of the surfac‐ tants, leading to formation of micelles or vesicles. Although catanionic precipitate is generally found only near equimolar compositions or in samples below their Krafft temperature, it is

**Figure 5.** Molecular structures of various anionic amphiphiles.

38 Application and Characterization of Surfactants

Numerous studies showed that catanionic mixtures are characterized with solution behavior that considerably deviates from ideal mixing as well as pronounced synergism (high surface activity, enhanced adsorption, low critical micellization concentration (cmc), etc.) compared to other types of surfactant mixtures [1, 3–7]. For example, the cmc values in catanionic mix‐ tures can be several orders of magnitude lower than the ones of single surfactants. It is not surprising considering that molecular interactions between cationic and anionic surfactants are generally dominated by the attractive electrostatic forces. Additionally, driving force for the mixed aggregates formation is large increase in entropy, which is a consequence of coun‐ terion release from both surfactants. On the contrary, in the case of single surfactant aggre‐ gate, the entropy decreases due to the condensation of counterions [2].

Synergism in catanionic mixtures can be quantitatively described using the regular solution theory (RST), which provides a thermodynamical approach to non‐ideal mixing [1, 30–32]. Although frequently criticized on fundamental grounds, the RST still remains a helpful tool for description of the behavior of catanionic mixtures.

According to the RST and the standard state surface tension method, the mixed monolayer composition and the mixed monolayer interaction parameter can be calculated [1, 30–32]. The molar fraction of the cationic surfactant in the mixed monolayer (*X*<sup>1</sup> ) can be calculated accord‐ ing to the following equation: <sup>2</sup> ln(*α*<sup>1</sup> *<sup>c</sup>*1,2 / *<sup>c</sup>*<sup>1</sup> *<sup>X</sup>*1) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (<sup>1</sup> <sup>−</sup> *<sup>X</sup>*1)

$$\frac{X\_i^2 \ln\left(a\_{i,1}c\_{i,2}/c\_i X\_i\right)}{\left(1 - X\_i\right)^2 \ln\left[\left(1 - a\_i\right)c\_{i,2}/(1 - X\_i)\right]c\_2\right]} = 1\tag{3}$$

where *c*<sup>1</sup> , *c*<sup>2</sup> and *c*1,2 are the molar concentrations in the solution phase of surfactant 1, sur‐ factant 2 and their mixture, respectively, at the mole fraction *α*<sup>1</sup> of surfactant 1, required to produce a given surface tension (*γ*) value (see [31]).

The iterative solution of Eq. (3) gives *X*<sup>1</sup> . The molecular interaction parameter in the mixed monolayer (*β*mon) can be calculated according to the following equation:

$$\beta\_{\text{moon}} = \frac{\ln \left( a\_{\text{i}} c\_{\text{i},2} / c\_{\text{i}} X\_{\text{i}} \right)}{\left( 1 - X\_{\text{i}} \right)^2} \tag{4}$$

According to the RST, the deviation of experimentally obtained mixed micelle cmc value (cmc1,2), from that calculated by assuming ideal mixing, can be represented by the molecular interaction parameter in the mixed micelle (*β*mic). The molar fraction of the cationic surfactant in the mixed micelles (*x*<sup>1</sup> ) and *β*mic, can be calculated according the following equations:

$$\begin{aligned} \text{interaction parameter in the mixed micelle } (\boldsymbol{\beta}\_{\text{nie}}). & \text{The molar fraction of the cation surface} \\ \text{in the mixed miemens } (\mathbf{x}\_i) \text{ and } \boldsymbol{\beta}\_{\text{nie}} \text{ can be calculated according the following equations:}\\ & \quad \frac{\mathbf{x}\_i^2 \ln \left( a\_i \mathbf{c} \mathbf{m} \mathbf{c}\_{i2} / \mathbf{c} \mathbf{m} \mathbf{c}\_i \mathbf{x}\_i \right)}{\left( 1 - \mathbf{x}\_i \right)^2 \ln \left[ \left( 1 - a\_i \right) \mathbf{c} \mathbf{m} \mathbf{c}\_{i2} / (1 - \mathbf{x}\_i) \, \mathbf{m} \mathbf{c}\_{i2} \right]} = 1 \end{aligned} \tag{5}$$

$$\left(1 - \mathbf{x}\_{\mathrm{i}}\right)^{2} \ln\left[\left(1 - \alpha\_{\mathrm{i}}\right) \mathsf{cmc}\_{\mathrm{i},2} / \left(1 - \mathbf{x}\_{\mathrm{i}}\right) \mathsf{cmc}\_{\mathrm{i}}\right] \tag{4.7}$$

$$\mathcal{B}\_{\mathrm{mic}} = \frac{\ln\left(\alpha\_{\mathrm{i}} \,\mathsf{cmc}\_{\mathrm{i},2} / \mathsf{cmc}\_{\mathrm{i}} \,\mathsf{x}\_{\mathrm{i}}\right)}{\left(1 - \mathsf{x}\_{\mathrm{i}}\right)^{2}} \tag{6}$$

where cmc1 , cmc2 and cmc1,2 are the critical micelle concentrations of surfactant 1, surfactant 2 and their mixture, respectively, at the mole fraction *α*<sup>1</sup> [31].

The *β* parameters measure attractive net interaction between different surfactants relative to the self‐interaction of the two surfactants under the same conditions before mixing. In other words, the *β* parameters describe the extent of non‐ideal mixing. When parameter *β* is nega‐ tive, the interaction is attractive; however, when it is positive, the interaction between two different surfactants is repulsive. To obtain valid *β* parameters, several conditions must be met as pointed out by Zhou and Rosen [31].

By applying the RST, additional parameters can be calculated for catanionic systems, such as activity coefficients and free energy of mixing, but for the sake of brevity, only the main prin‐ ciples of the theory are mentioned. A detailed discussion of the RST is far beyond the scope of this chapter.

## **3. Phase behavior and physicochemical properties of catanionic mixtures containing structurally different surfactants**

#### **3.1. Catanionic mixtures of oligomeric and monomeric surfactants**

to other types of surfactant mixtures [1, 3–7]. For example, the cmc values in catanionic mix‐ tures can be several orders of magnitude lower than the ones of single surfactants. It is not surprising considering that molecular interactions between cationic and anionic surfactants are generally dominated by the attractive electrostatic forces. Additionally, driving force for the mixed aggregates formation is large increase in entropy, which is a consequence of coun‐ terion release from both surfactants. On the contrary, in the case of single surfactant aggre‐

Synergism in catanionic mixtures can be quantitatively described using the regular solution theory (RST), which provides a thermodynamical approach to non‐ideal mixing [1, 30–32]. Although frequently criticized on fundamental grounds, the RST still remains a helpful tool

According to the RST and the standard state surface tension method, the mixed monolayer composition and the mixed monolayer interaction parameter can be calculated [1, 30–32]. The

ln[(1 − *α*1)*c*1,2 /(1 − *X*<sup>1</sup> ) *c*2]

(1 − *X*1)

) and *β*mic, can be calculated according the following equations:

and cmc1,2 are the critical micelle concentrations of surfactant 1, surfactant 2

[31].

According to the RST, the deviation of experimentally obtained mixed micelle cmc value (cmc1,2), from that calculated by assuming ideal mixing, can be represented by the molecular interaction parameter in the mixed micelle (*β*mic). The molar fraction of the cationic surfactant

<sup>2</sup> ln(*α*<sup>1</sup> cmc1,2 /cmc1 *x*1) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (<sup>1</sup> <sup>−</sup> *<sup>x</sup>*1)

ln[(1 − *α*1)cmc1,2 /(1 − *x*<sup>1</sup> ) cmc2]

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

(1 − *x*1)

The *β* parameters measure attractive net interaction between different surfactants relative to the self‐interaction of the two surfactants under the same conditions before mixing. In other words, the *β* parameters describe the extent of non‐ideal mixing. When parameter *β* is nega‐ tive, the interaction is attractive; however, when it is positive, the interaction between two different surfactants is repulsive. To obtain valid *β* parameters, several conditions must be

and *c*1,2 are the molar concentrations in the solution phase of surfactant 1, sur‐

<sup>2</sup> ln(*α*<sup>1</sup> *<sup>c</sup>*1,2 / *<sup>c</sup>*<sup>1</sup> *<sup>X</sup>*1) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (<sup>1</sup> <sup>−</sup> *<sup>X</sup>*1)

) can be calculated accord‐

of surfactant 1, required to

= 1 (3)

. The molecular interaction parameter in the mixed

<sup>2</sup> (4)

<sup>2</sup> (6)

= 1 (5)

gate, the entropy decreases due to the condensation of counterions [2].

molar fraction of the cationic surfactant in the mixed monolayer (*X*<sup>1</sup>

factant 2 and their mixture, respectively, at the mole fraction *α*<sup>1</sup>

produce a given surface tension (*γ*) value (see [31]).

*<sup>β</sup>*mon <sup>=</sup> ln(*α*<sup>1</sup> *<sup>c</sup>*1,2 / *<sup>c</sup>* \_\_\_\_\_\_\_\_\_\_\_ <sup>1</sup> *<sup>X</sup>*1)

2

monolayer (*β*mon) can be calculated according to the following equation:

2

*<sup>β</sup>*mic <sup>=</sup> ln(*α*<sup>1</sup> cmc1,2 /cmc1 *<sup>x</sup>*1)

and their mixture, respectively, at the mole fraction *α*<sup>1</sup>

met as pointed out by Zhou and Rosen [31].

for description of the behavior of catanionic mixtures.

ing to the following equation:

40 Application and Characterization of Surfactants

where *c*<sup>1</sup>

, *c*<sup>2</sup>

in the mixed micelles (*x*<sup>1</sup>

, cmc2

where cmc1

*<sup>x</sup>*<sup>1</sup>

*<sup>X</sup>*<sup>1</sup>

The iterative solution of Eq. (3) gives *X*<sup>1</sup>

Surfactants, which have attracted considerable interest in last three decades, are oligomeric surfactants. These compounds are made up of two (dimeric or gemini surfactants) or more (higher oligomeric surfactants) amphiphilic moieties covalently linked at the level of the headgroups, or very close to them, by a spacer group (**Figure 7**) [33, 34]. Large interest for the investigation and synthesis of oligomeric surfactants is a consequence of their superior properties in comparison to the conventional ones [33, 34]:


Due to their enhanced properties, catanionic mixtures containing various dimeric surfactants have been subject of numerous papers [25, 35–46]. The most investigated are catanionic mix‐ tures containing bis‐quaternary ammonium salts with the alkyl spacers. This type of surfac‐ tants is usually denoted as *m*–*s*–*m* where *m* represents the number of carbon atoms in the hydrophobic chain and *s* is the number of carbon atoms in the spacer (**Figure 6**). The great

**Figure 7.** Schematic representations of (a) dimeric, (b) trimeric and (c) tetrameric surfactant molecule.

advantages of *m*–*s*–*m* surfactants are relative ease of their synthesis and possibility to tailor surfactant properties by changing spacer and chain length. Despite numerous papers dealing with catanionic mixtures containing dimeric surfactants, very few provide complete picture of the systems phase behavior.

Shang et al. determined the phase diagram for aqueous mixtures of 12–3–12 and SDS using freeze etching and negative staining on transmission electron microscope (TEM) [25]. Constructed phase diagram shows different phase regions in the majority of which coex‐ istence of vesicles and micelles was found. As expected, the ratio of vesicles to micelles in diluted mixtures varies with bulk composition and total surfactant concentration. At higher surfactant concentrations, 12–3–12/SDS mixtures displayed very rich phase behavior, that is, regions of anisotropic phase, aqueous two‐phase system (ATPS) as well as rod‐like micelles and cylindrical clusters were detected. In order to corroborate experimental results, the authors used dissipative particle dynamics simulations. It was found that due to the finite size of the simulation box, results were somewhat different from that obtained by experi‐ ments [25].

Our group employed a variety of techniques: imaging by various microscopy techniques (light microscopy, confocal laser scanning microscopy (CLSM) and TEM) as well as dynamic (DLS) and electrophoretic light scattering (ELS) to determine phase diagram for 12–2–12/SDS system at the water‐rich corner [35]. It was found that depending on bulk composition and total surfactant concentration in 12–2–12/SDS mixtures, various mixed nano and microaggregates form. The sequence of phases in the clear region in the SDS‐ rich side of the phase diagram is vesicles → the narrow coexistence region of vesicles and mixed micelles at the cmcSDS → small mixed micelles. On the other hand, in the clear region in the 12–2–12‐rich side, the sequence of phases is fragments of planar bilayers/ lamellar sheets and vesicles → worm‐like mixed micelles → transformation from worm‐ like to small mixed micelles above the cmc12–2–12. In the precipitation region, two types of aggregates were detected, the tubules as prevailing aggregates on the 12–2–12‐rich side and vesicles as prevailing aggregates on the SDS‐rich side. The formation of tubules was ascribed to mutual influence of (1) specific molecular structure of 12–2–12 surfactant and (2) electrostatic interactions at the catanionic bilayer/solution interface. The microscopic observations indicated that tubular structures grow from rolled‐up stacked catanionic bilayers [35].

Cheon et al. studied phase behavior in a very similar catanionic system, 12–2–12/sodium lau‐ ryl ether sulfate (SLES, **Figure 5**) by means of differential scanning calorimetry (DSC), UV‐VIS spectroscopy, DLS, ELS and TEM [36]. These mixtures display less complex phase behav‐ ior compared to the 12–2–12/SDS system. In the phase diagram, isotropic molecular solution region, region of mixed micelles and vesicles formation as well as region of their coexistence were detected. Spontaneous vesicles formation has been attributed to electrostatic attractions and geometric packing constraints, that is formation of ion pair with "cuplike" structure that favors bilayer formation [36].

The phase behavior of 12–10–12/SDS system in diluted SDS‐rich region using Langmuir trough, isothermal titration microcalorimetry (ITC), cryo‐TEM and conductivity measurements has been investigated by Bai et al. [37]. The phase diagram shows three regions with a single type of aggregate (spherical and non‐spherical micelles and vesicles), separated by two regions where two types of aggregates coexist (spherical/non‐spherical micelles and non‐spherical micelles/vesicles) and finally one multiphase region. Authors have concluded that observed phase transitions are consequences of asymmetric and uneven distributions of oppositely charged surfactants in vesicles' bilayers and non‐spherical micelles, respectively [37].

advantages of *m*–*s*–*m* surfactants are relative ease of their synthesis and possibility to tailor surfactant properties by changing spacer and chain length. Despite numerous papers dealing with catanionic mixtures containing dimeric surfactants, very few provide complete picture

Shang et al. determined the phase diagram for aqueous mixtures of 12–3–12 and SDS using freeze etching and negative staining on transmission electron microscope (TEM) [25]. Constructed phase diagram shows different phase regions in the majority of which coex‐ istence of vesicles and micelles was found. As expected, the ratio of vesicles to micelles in diluted mixtures varies with bulk composition and total surfactant concentration. At higher surfactant concentrations, 12–3–12/SDS mixtures displayed very rich phase behavior, that is, regions of anisotropic phase, aqueous two‐phase system (ATPS) as well as rod‐like micelles and cylindrical clusters were detected. In order to corroborate experimental results, the authors used dissipative particle dynamics simulations. It was found that due to the finite size of the simulation box, results were somewhat different from that obtained by experi‐

Our group employed a variety of techniques: imaging by various microscopy techniques (light microscopy, confocal laser scanning microscopy (CLSM) and TEM) as well as dynamic (DLS) and electrophoretic light scattering (ELS) to determine phase diagram for 12–2–12/SDS system at the water‐rich corner [35]. It was found that depending on bulk composition and total surfactant concentration in 12–2–12/SDS mixtures, various mixed nano and microaggregates form. The sequence of phases in the clear region in the SDS‐ rich side of the phase diagram is vesicles → the narrow coexistence region of vesicles and mixed micelles at the cmcSDS → small mixed micelles. On the other hand, in the clear region in the 12–2–12‐rich side, the sequence of phases is fragments of planar bilayers/ lamellar sheets and vesicles → worm‐like mixed micelles → transformation from worm‐ like to small mixed micelles above the cmc12–2–12. In the precipitation region, two types of aggregates were detected, the tubules as prevailing aggregates on the 12–2–12‐rich side and vesicles as prevailing aggregates on the SDS‐rich side. The formation of tubules was ascribed to mutual influence of (1) specific molecular structure of 12–2–12 surfactant and (2) electrostatic interactions at the catanionic bilayer/solution interface. The microscopic observations indicated that tubular structures grow from rolled‐up stacked catanionic

Cheon et al. studied phase behavior in a very similar catanionic system, 12–2–12/sodium lau‐ ryl ether sulfate (SLES, **Figure 5**) by means of differential scanning calorimetry (DSC), UV‐VIS spectroscopy, DLS, ELS and TEM [36]. These mixtures display less complex phase behav‐ ior compared to the 12–2–12/SDS system. In the phase diagram, isotropic molecular solution region, region of mixed micelles and vesicles formation as well as region of their coexistence were detected. Spontaneous vesicles formation has been attributed to electrostatic attractions and geometric packing constraints, that is formation of ion pair with "cuplike" structure that

The phase behavior of 12–10–12/SDS system in diluted SDS‐rich region using Langmuir trough, isothermal titration microcalorimetry (ITC), cryo‐TEM and conductivity measurements has

of the systems phase behavior.

42 Application and Characterization of Surfactants

ments [25].

bilayers [35].

favors bilayer formation [36].

Wang et al. determined phase diagram for 12–6–12/SDS system at the water‐rich corner by employing turbidity measurements, ITC and TEM [38]. At constant total surfactant con‐ centration, as the molar fraction of SDS increased, the morphology of mixed aggregates gradually changed from 12–6–12‐rich micelles, through multiphase regions containing a precipitate (catanionic surfactant) and vesicle region, to SDS‐rich micelles. Both TEM and ITC allowed identification of stable vesicles' region in the SDS‐rich side of the phase dia‐ gram. Authors have argued that spontaneously formed vesicles in investigated mixture are consequences of (1) non‐ideal mixing of cationic and anionic surfactant in bilayers as well as (2) a mechanism which involves an entropic stabilization in cases where the spontane‐ ous curvature is not favorable but the bending penalty is not too high (soft bilayers) [38].

The same group of authors investigated monolayers formed in mixtures of *m*–2–*m* (*m* = 12, 14, 16 and 18) surfactants with SDS using the Langmuir trough technique [39], as well as micellization in mixtures of 12–*s*–12 (*s* = 2, 6 and 10) with several common anionic surfactants (SDS, sodium taurodeoxycholate (NaTDC, **Figure 8**) and sodium dodecanoate (SD, **Figure 5**)) by conductivity [40].

In *m*–2–*m*/SDS systems, it was found, from pressure‐area, pressure‐temperature and compres‐ sion‐expansion curves, that all the equivalent mixtures form highly stable monolayers with rich phase behavior and different desorption mechanisms [39]. Furthermore, it was established that if excess of cationic dimeric surfactants is present in 12–2–12/SDS and 14–2–14/SDS mix‐ tures, the molecules in excess desorb from the monolayer so that the electroneutral composi‐ tion of adsorbed film is maintained.

Results obtained by conductivity method revealed that all investigated systems containing 12–*s*–12 dimers and anionic surfactants show synergistic effects and have negative values of the molecular interaction parameter [40]. For the mixtures with 12–2–12, the strength of interaction increases in the order SD > SDS > NaTDC, while for 12–6–12, the order was SD ≈ SDS > NaTDC. Additionally, for the same anionic surfactant, the interaction with 12–2–12 is always stronger than that with 12–6–12. It is known that short spacers (*s* < 10) tend to lie flat at the water‐hydrocarbon interface, which can lead to unfavorable packing constraints at the mixed micelles. Since 12–2–12 has a shorter spacer than 12–6–12, the packing constraints are slightly weaker and, together with a higher charge density of the headgroup region, lead to more favorable attractive interactions with anionic surfactants. Results reported for mixtures with 12–10–12 suggest that a catanionic solid is largely stabilized compared to mixed micelles when the alkyl spacer is long and flexible enough [40].

Aggregation behavior in mixtures of cationic dimeric surfactants derived from dodecyltri‐ methylammonium chloride (C12TACl, **Figure 4**) and SDS by means of small‐angle neutron

**Figure 8.** Molecular structures of bile salts.

scattering (SANS) and small‐angle X‐ray scattering (SAXS) was investigated by Prévost et al. [41]. Dimeric surfactants with spacers of different nature and geometry were used: *m*‐, *p*‐ and *o*‐xylylene (aromatic spacer), diethyl ether (ethoxy spacer) and trans‐1,4‐buten‐2‐ylene (ethylene spacer) (**Figure 6**). Authors concluded that among five spacers, due to their weak geometrical constraints and the ambivalent, hydrophilic and non‐extensive lipophobic nature, ethoxy spacer is the most suitable for formation of vesicles in aqueous mixtures. On the contrary, the aromatic spacers with their low flexibility and higher apolarity, compared to ethoxy spacer, generally led to precipitation in investigated mixtures. Furthermore, it was established that all mentioned dimeric surfactants form colloidally more stable mixtures with SDS than their monomeric coun‐ terpart C12TACl [41].

Ji et al. studied temperature induced phase transitions in aqueous mixtures of cationic dimeric sur‐ factant, 1,4‐bis(dodecyl‐*N,N*‐dimethylammonium bromide)‐2,3‐butane‐diol (C12C4 (OH)2 C12Br2 , **Figure 6**), and anionic amino acid surfactant, *N*‐dodecanoylglutamic acid (C12Glu, **Figure 5**) at pH = 10.0 [42]. At 25 °C small spherical micelles, vesicles and entangled worm‐like micelles were detected in the system. The main controlling factor for the aggregates transition at constant total surfactant concentration and varying molar ratio is strong electrostatic binding between oppositely charged surfactants which significantly reduces the headgroup area. Because both C12C4 (OH)2 C12Br2 and C12Glu carry two charges, strong electrostatic interactions in these mix‐ tures are not surprising. At higher temperatures, mixed aggregates formed at 25 °C experience different transitions, that is, the following phase transitions occur: (1) small spherical micelles → large vesicles, (2) large vesicles → solid spherical aggregates → larger irregular aggregates and (3) entangled worm‐like micelles → branched worm‐like micelles. The larger irregular aggregates and branched micelles ultimately lead to precipitation and clouding phenomenon, respectively. All described transitions are thermally reversible and transition temperatures can be tuned by varying the molar ratio and/or the total surfactant concentration [42].

Aghdastinat et al. investigated self‐assembly in cation‐rich mixtures of ester‐containing cat‐ ionic dimeric surfactants, named dodecyl esterquat and dodecyl betainate (**Figure 6**), with SDS in the presence of salt, KCl [43]. Obtained results show that the position of ester bonds in sur‐ factants' tail plays an important role in physicochemical properties and aggregation behavior in their mixtures with SDS. After mixing with SDS morphology of dodecyl esterquat, aggre‐ gates change from cubic nanoparticles (cubosomes) to cylindrical nanoparticles which coexist with cubosomes. On the contrary, upon mixing with SDS, no significant structural change can be observed in dodecyl betainate aggregates, that is, vesicles are formed in both cases. Authors explained observed changes in morphology of mixed aggregates using RST [43].

Investigation of higher oligomeric surfactants and their mixtures is hindered by the more complex synthesis and purification compared to the dimeric molecules [33, 34]. Very few reports can be found on mixtures containing trimeric or tetrameric quaternary ammonium surfactants [47–49].

Chen et al. studied self‐assembly in mixtures of trimeric cationic surfactants, tri‐(*N‐* dodecyldimethylhydroxypropylammonium chloride) phosphate (PTA, **Figure 9**) and double‐tailed anionic surfactant, bis(2‐ethylhexyl) sulfosuccinate (AOT, **Figure 5**) by means of DLS and TEM [47]. Obtained results demonstrated that PTA/AOT vesicles are stable and can be found in a broad concentration range. The TEM micrographs revealed that at high surfactant concentrations, tubular microstructures, vesicle fusion and vesi‐ cle‐tubular microstructure transition occurred. In addition, it was found that formation of tubular structures is more pronounced in aged samples. Authors have discussed the mechanism of vesicles and tubules formation from the viewpoint of molecular geometry and electrostatic interaction between oppositely charged surfactants [47].

scattering (SANS) and small‐angle X‐ray scattering (SAXS) was investigated by Prévost et al. [41]. Dimeric surfactants with spacers of different nature and geometry were used: *m*‐, *p*‐ and *o*‐xylylene (aromatic spacer), diethyl ether (ethoxy spacer) and trans‐1,4‐buten‐2‐ylene (ethylene spacer) (**Figure 6**). Authors concluded that among five spacers, due to their weak geometrical constraints and the ambivalent, hydrophilic and non‐extensive lipophobic nature, ethoxy spacer is the most suitable for formation of vesicles in aqueous mixtures. On the contrary, the aromatic

**Figure 8.** Molecular structures of bile salts.

44 Application and Characterization of Surfactants

**Figure 9.** Molecular structures of higher oligomeric surfactants—quaternary alkyl ammonium salts. *m* = number of C atoms in alkyl chains and *s* = number of C atoms in spacer.

Yoshimura et al. investigated mixtures of trimeric cationic surfactants, *m*–2–*m*–2–*m* (*m* = 8, 10 and 12, **Figure 9**), and sodium *n*‐octyl sulfate (SOS, **Figure 5**), employing several techniques such as static surface tension, fluorescence spectroscopy and DLS [48]. As expected, *m*–2–*m*–2–*m*/SOS mixtures show stronger micellization ability and lower cmc values compared with pure trimeric surfactants. In addition, the chain length of trimeric surfactants significantly influenced mixtures' properties at the air/water interface and in the solution. For example, *m*–2–*m*–2–*m*/SOS mixtures show a linear decrease in the cmc values with increasing alkyl chain length. Furthermore, the 8–2–8–2–8/SOS system exhibited a smaller surface area occupied by a surfactant molecule (*a*min) compared to 10–2–10–2–10/SOS and 12–2–12–2–12/SOS mixtures [48].

Our group studied a series of quaternary ammonium bromide oligomers (from dimer to tetra‐ mer, **Figures 6** and **9**) with dodecyl chains connected at the level of headgroups by a short eth‐ ylene spacer and their mixtures with SDS [49]. In high excess of SDS (cationic surfactant: SDS = 1:9), negatively charged vesicles form in all mixtures regardless of the number of dodecyl chains in cationic surfactant. Contrary to vesicles, the mixed monolayer is enriched with cat‐ ionic surfactant. Moreover, the increase in the number of dodecyl chains decreases the molar fraction of SDS in the mixed monolayer. Observed results can be explained by strong electro‐ static headgroup interactions modulated by packing constraints imposed by the geometry of oligomeric surfactants [49].

Since anionic dimeric surfactants attract much less attention than cationic, there are only few reports describing catanionic mixtures containing dimeric surfactants as anionic compo‐ nents [44–46]. Back in 1996, Zana et al. investigated mixtures of disodium1,11‐didecyl‐3,6,9‐ trioxaundecane‐1, 11‐disulfate (**Figure 5**) and C16TAB, in the presence of NaBr, employing conductivity, spectrofluorometry, time‐resolved fluorescence quenching and cryo‐TEM [44]. Obtained results proved that the aggregation numbers of the mixed micelles are larger than those of pure C16TAB micelles even at very low molar ratio of dimeric surfactants. Apart from micelles, TEM micrographs revealed the presence of vesicles and very large aggregates which looked like distorted multi‐bilayered vesicles with many defects [44].

Luo et al. and Zhao et al. studied interactions in catanionic mixtures containing anionic dimeric surfactant: O,O‐bis(sodium 2‐lauricate)‐*p*‐benzenediol (C11pPHCNa, **Figure 5**) [45, 46]. It was found that large spherical aggregates form in C12TAB/C11pPHCNa mixtures and transform into branched and worm‐like micelles with increasing NaBr concentration. In addi‐ tion, authors established that due to the changes in morphology of mixed aggregates, vis‐ cosity of the C12TAB/C11pPHCNa mixtures gradually increases. Furthermore, it was reported that adsorption behavior in mixtures of two dimeric surfactants, C11pPHCNa and (oligoona) alkanediyl‐*α*,*ω*‐bis(dimethyldodecylammonium bromide) (C12‐2‐E*<sup>x</sup>* ‐C12, **Figure 6**), strongly depends on the molar ratio, that is, strong adsorption at the air/water interface is present in excess of cationic surfactant while in excess of anionic surfactant premicellization occurs.

In addition to properties which can be found in traditional catanionic systems, common fea‐ tures that can be drawn for catanionic mixtures of oligomeric and monomeric surfactants are:

(1) both physicochemical properties and phase behavior strongly depend on the length of the spacer (*s*) and alkyl chains (*m*) as well as the nature of the spacer (aromatic, hydrophilic, hydrophobic, etc.) in the oligomeric molecule,

Yoshimura et al. investigated mixtures of trimeric cationic surfactants, *m*–2–*m*–2–*m* (*m* = 8, 10 and 12, **Figure 9**), and sodium *n*‐octyl sulfate (SOS, **Figure 5**), employing several techniques such as static surface tension, fluorescence spectroscopy and DLS [48]. As expected, *m*–2–*m*–2–*m*/SOS mixtures show stronger micellization ability and lower cmc values compared with pure trimeric surfactants. In addition, the chain length of trimeric surfactants significantly influenced mixtures' properties at the air/water interface and in the solution. For example, *m*–2–*m*–2–*m*/SOS mixtures show a linear decrease in the cmc values with increasing alkyl chain length. Furthermore, the 8–2–8–2–8/SOS system exhibited a smaller surface area occupied by a surfactant molecule (*a*min)

**Figure 9.** Molecular structures of higher oligomeric surfactants—quaternary alkyl ammonium salts. *m* = number of C

Our group studied a series of quaternary ammonium bromide oligomers (from dimer to tetra‐ mer, **Figures 6** and **9**) with dodecyl chains connected at the level of headgroups by a short eth‐ ylene spacer and their mixtures with SDS [49]. In high excess of SDS (cationic surfactant: SDS = 1:9), negatively charged vesicles form in all mixtures regardless of the number of dodecyl chains in cationic surfactant. Contrary to vesicles, the mixed monolayer is enriched with cat‐ ionic surfactant. Moreover, the increase in the number of dodecyl chains decreases the molar fraction of SDS in the mixed monolayer. Observed results can be explained by strong electro‐ static headgroup interactions modulated by packing constraints imposed by the geometry of

Since anionic dimeric surfactants attract much less attention than cationic, there are only few reports describing catanionic mixtures containing dimeric surfactants as anionic compo‐ nents [44–46]. Back in 1996, Zana et al. investigated mixtures of disodium1,11‐didecyl‐3,6,9‐ trioxaundecane‐1, 11‐disulfate (**Figure 5**) and C16TAB, in the presence of NaBr, employing conductivity, spectrofluorometry, time‐resolved fluorescence quenching and cryo‐TEM [44]. Obtained results proved that the aggregation numbers of the mixed micelles are larger than

compared to 10–2–10–2–10/SOS and 12–2–12–2–12/SOS mixtures [48].

atoms in alkyl chains and *s* = number of C atoms in spacer.

46 Application and Characterization of Surfactants

oligomeric surfactants [49].


#### **3.2. Catanionic mixtures of surface active ionic liquids (SAILs) and surfactants**

In recent years, surface active ionic liquids (SAILs) have emerged as fascinating compounds due to their dual nature as well as unique and tunable physicochemical properties [50]. With combined properties of ionic liquids (ILs) and amphiphiles, SAILs represent a novel class of surfactants. The term ionic liquids refers to a class of substances formed by a poorly coor‐ dinated large organic cation with delocalized charge and either a small anion, such as Br<sup>−</sup> , or relatively large one, such as [(CF3 SO2 )N2 ] − [51]. Consequently, ILs possess melting points under 100 o C, often even lower than room temperature. Due to their unique characteristics such as high thermal stability, negligible vapor pressure, high conductivity and great ability to dissolve inorganic/organic compounds, ILs have attracted much interest for a variety of applications [52, 53].

Among different classes of SAILs, imidazolium‐based compounds composed of the 1‐alkyl‐3‐ methylimidazolium cation ([C*<sup>n</sup>* mim]+ , where *n* = number of carbon atoms in the hydropho‐ bic chain, **Figure 10**) and their mixtures have been most extensively studied. Compared to the conventional alkyltrimethylammonium surfactants, imidazolium‐based SAILs exhibit a stronger tendency to self‐assemble and slightly better surface activity [50, 54]. Imidazol ring, which can be found, for example, in amino acid histidine, makes them also biologically interesting. Furthermore, it is known that SAILs exhibit low toxicity so their use as drug‐ delivery agents can represent a step forward in medicinal chemistry [50]. Recently, Sharma and Mahajan published a comprehensive review summarizing influence of various additives, including surfactants, on the physicochemical properties of imidazolium‐based ILs [50]. Due to the large amount of published data, the aim of this section is to discuss only aqueous cat‐ anionic mixtures containing SAIL, although numerous reports of systems in which ILs acted as self‐assembly media exist.

Zhao et al. reported the phase diagram of catanionic system composed by cationic SAILs, [C16mim]Cl and SDS [55]. Results from rheology and polarized optical microscopy observa‐ tions demonstrated that a gel phase with quite high water content is formed in the [C16mim] Cl‐rich side of the phase diagram. On the contrary, in the SDS‐rich side, lamellar phases were detected. The [C16mim]Cl/SDS gel phase showed low ordering and similar rheological prop‐ erties to vesicles usually formed in traditional catanionic systems [55].

Formation of gel phase was observed in a very similar system, [C14mim]Cl/SDS mixtures, by Zhao et al. as well [56]. The SEM micrographs showed that gel phase is structured as a complex three‐dimensional network. Authors argued that hydrophobic and electrostatic interactions present in the system are essential for gel‐phase formation. In order to prove this thesis, mix‐ tures in which [C14mim]Cl was replaced with [C4 mim]Cl and SDS with SOS were also studied. In both of these systems, gel phase was not found. Performed control experiments demonstrated a key role of hydrophobic interactions in gel formation. In additional control experiments, 1‐ dodecanol was used instead of SDS to confirm the crucial role of electrostatic interactions in gel formation. In this case, the gel phase was also not found [56].

**Figure 10.** Molecular structures of cations in surface active ionic liquids (SAILs). *n* = number of C atoms in alkyl chains.

The group of the same author also studied phase behavior in mixtures of *N*‐dodecyl‐*N*‐meth‐ ylpyrrolidinium bromide ([C12MP]Br, **Figure 10**) and SDS by employing TEM, conductivity and rheological measurements [57]. It was found that at constant [C12MP]Br concentration, as the molar fraction of SDS increases, the morphology of mixed aggregates changes as follows: mixed micelles → vesicles → coexistence of catanionic precipitate and vesicles → coexistence of catanionic precipitate and mixed micelles. Spontaneous vesicles formation was discussed in terms of packing parameter [57].

under 100 o

applications [52, 53].

methylimidazolium cation ([C*<sup>n</sup>*

48 Application and Characterization of Surfactants

as self‐assembly media exist.

C, often even lower than room temperature. Due to their unique characteristics

, where *n* = number of carbon atoms in the hydropho‐

mim]Cl and SDS with SOS were also studied.

such as high thermal stability, negligible vapor pressure, high conductivity and great ability to dissolve inorganic/organic compounds, ILs have attracted much interest for a variety of

Among different classes of SAILs, imidazolium‐based compounds composed of the 1‐alkyl‐3‐

bic chain, **Figure 10**) and their mixtures have been most extensively studied. Compared to the conventional alkyltrimethylammonium surfactants, imidazolium‐based SAILs exhibit a stronger tendency to self‐assemble and slightly better surface activity [50, 54]. Imidazol ring, which can be found, for example, in amino acid histidine, makes them also biologically interesting. Furthermore, it is known that SAILs exhibit low toxicity so their use as drug‐ delivery agents can represent a step forward in medicinal chemistry [50]. Recently, Sharma and Mahajan published a comprehensive review summarizing influence of various additives, including surfactants, on the physicochemical properties of imidazolium‐based ILs [50]. Due to the large amount of published data, the aim of this section is to discuss only aqueous cat‐ anionic mixtures containing SAIL, although numerous reports of systems in which ILs acted

Zhao et al. reported the phase diagram of catanionic system composed by cationic SAILs, [C16mim]Cl and SDS [55]. Results from rheology and polarized optical microscopy observa‐ tions demonstrated that a gel phase with quite high water content is formed in the [C16mim] Cl‐rich side of the phase diagram. On the contrary, in the SDS‐rich side, lamellar phases were detected. The [C16mim]Cl/SDS gel phase showed low ordering and similar rheological prop‐

Formation of gel phase was observed in a very similar system, [C14mim]Cl/SDS mixtures, by Zhao et al. as well [56]. The SEM micrographs showed that gel phase is structured as a complex three‐dimensional network. Authors argued that hydrophobic and electrostatic interactions present in the system are essential for gel‐phase formation. In order to prove this thesis, mix‐

In both of these systems, gel phase was not found. Performed control experiments demonstrated a key role of hydrophobic interactions in gel formation. In additional control experiments, 1‐ dodecanol was used instead of SDS to confirm the crucial role of electrostatic interactions in gel

**Figure 10.** Molecular structures of cations in surface active ionic liquids (SAILs). *n* = number of C atoms in alkyl chains.

mim]+

erties to vesicles usually formed in traditional catanionic systems [55].

tures in which [C14mim]Cl was replaced with [C4

formation. In this case, the gel phase was also not found [56].

Micelle‐to‐vesicle transition induced by *β*‐cyclodextrin (*β*‐CD) in mixtures of [C16mim]Cl and sodium oleate (NaOle, **Figure 5**) has been investigated by Dai et al. [58]. Cyclodextrins are structurally related cyclic oligosaccharides formed during bacterial digestion of cellulose [59]. It is known that the interior environment of *β*‐CD is hydrophobic and its outer surface is hydrophilic. Authors established that in [C16mim]Cl‐rich mixtures, micelle‐to‐vesicle transi‐ tion can be triggered by addition of sufficiently high *β*‐CD concentration. The main factor governing this phase transition is formation of inclusion complexes in the system [58].

Chabba et al. employed various techniques such as tensiometry, steady‐state fluorescence, DLS and SANS to study interactions of cationic SAILs, [C*<sup>n</sup>* mim]Cl (*n* = 8, 10 and 12), with sodium dodecylbenzenesulfonate (SDBS, **Figure 5**) [60]. Results obtained by tensiometric and steady‐ state fluorescence measurements revealed strong synergism in the system. As well as in the classic catanionic mixtures, strong synergism between the cationic SAIL and anionic SDBS can be attributed to the strong electrostatic interaction between oppositely charged headgroups along with the hydrophobic interactions between the alkyl chains. However, authors argued that in addition to these forces, *π*–*π* and cation–*π* interactions between the imidazolium cation and the benzene ring of SDBS, as well as hydrogen bonding, between most acidic proton of imidazolium ring and sulfonate group of SDBS, also come into play. In general, ionic liquid cations frequently contain multiple donor sites able to participate in hydrogen bonding, result‐ ing in H‐bonds of varying strength and type [61]. Within an imidazolium cation, the H‐bond donor is the C─H unit, the C2─H proton being the most acidic, followed by the other two hydrogens on the aromatic ring (C4─H and C5─H) and alkyl chain methyl hydrogens. The H atoms on imidazolium ring all participate in the formation of H‐bonds with water molecules and in the ubiquitous H‐bonds among the highly hydrated imidazolium cations even in very diluted IL solutions [62]. Similar increase in synergistic behavior due to additional non‐cova‐ lent interaction was found in [C12mim]Br/AOT, [C12mim]Cl/ibuprofen (**Figure 11**) and [C8 mim] Br/SDBS mixtures [16, 63, 64]. Furthermore, as observed from DLS and SANS, the [C*<sup>n</sup>* mim]Cl/ SDBS mixtures exhibit micelle‐to‐vesicle transition dependent on the alkyl chain length and the molar ratio of the surfactants. It was found that vesicle region prevails in a broad range of concentrations and that mixtures show high stability towards precipitation [60].

Gehlot et al. studied mixtures of [C8 mim]Br and SDBS by employing a variety of techniques such as tensiometry, conductometry, UV‐VIS spectroscopy, cryo‐TEM, AFM, DLS, ELS, ITC, steady‐state fluorescence and <sup>1</sup> H NMR measurements [64]. Based on the results obtained from these various physicochemical and imaging techniques, authors have concluded that: (1) spontaneously formed and differently shaped [C<sup>8</sup> mim]Br/SDBS vesicles (sphere, tubes and ribbons) exist in a broad range of concentrations, (2) a negative value of interaction param‐ eter and lower experimental cmc values, compared to the theoretically determined values,

indicate high synergism in the system, (3) major forces responsible for synergism are electro‐ static and hydrophobic interactions as well as *π*‐*π* stacking of aromatic rings and (4) Br<sup>−</sup> as a counterion in palisade layer assists in compact packing of ions, which leads to the formation of vesicles [64].

Our group studied phase transitions in mixtures of [C12mim]Br and AOT using a multi‐tech‐ nique approach [16]. Depending on the bulk composition and total surfactant concentration, mixed micelles, coacervates, lamellar and inverse bicontinuous cubic liquid crystalline phase were observed. At stoichiometric conditions, coexistence of coacervates and vesicles was found at lower and bicontinuous cubic phase and vesicles at higher total surfactant concentrations. A mechanism was proposed in which phase transitions from a dispersed lamellar to inverse cubic bicontinuous phase occur as a consequence of charge shielding and closer packing of oppositely charged headgroups followed by a change in bilayers curvature. Additionally, along with electrostatic attractions and geometric packing constraints, additional non‐cova‐ lent interactions in the system (hydrogen bonding, *π‐π* stacking) enhanced attractive inter‐ actions and stabilized low curvature aggregates [16]. In the ternary diagram of the system similar to [C12mim]/AOT, 1‐butyl‐3‐methylimidazolium tetrafluoborate ([bmim]BF<sup>4</sup> )/AOT, different phase behavior was observed. In the water‐rich corner of the phase diagram, regions of isotropic fluid and lamellar phase were found [65]. In addition, substitution of BF4 anion with Br<sup>−</sup> causes the collapse of lamellar phase [66]. Murgia and co‐workers employed a variety of techniques such as conductivity, optical microscopy, SAXS and NMR self‐diffusion experi‐ ments to detect modifications on macro‐ and micro‐scales within the systems upon the substi‐ tution of ILs counterion BF4 with Br<sup>−</sup> . Thus, the remarkable differences observed between the two systems appear to be mainly due to a specific counterion effect [65, 66].

Singh et al. reported structural changes induced by composition and dilution in aqueous catanionic mixtures containing SAILs ([C12mim]Br and [C14mim]Br) and a drug, diclofenac sodium (DFNa, **Figure 11**), as the anionic component [29]. The observed phase transitions were probed by SANS, DLS and ELS. The SAIL/DFNa systems display rich phase behavior and structural diversity of mixed aggregates, that is, depending on the bulk composition and total surfactant concentration, spherical and small micelles with prolate ellipsoidal shape as well as rod‐shaped micelles and vesicles were detected. The 1 H NMR measurements revealed that DFNa intercalated into SAIL micelles via cation‐*π* and *π*‐*π* interaction in addition to hydrophobic interaction. It was found that increase in DFNa molar ratio increases aggregates curvature. Unlike conventional linear chain surfactants, a specific structure of DFNa does not allow effective packing of cationic‐anionic pairs which prevented precipitation in equimolar mixtures [29].

Catanionic systems also containing a drug, ibuprofen and [C12mim]Cl have been investigated by Sanan et al. [63]. Various techniques such as surface tension, steady‐state fluorescence, UV‐ VIS spectroscopy, DLS and 1 H NMR measurements were used to provide a comprehensive knowledge about [C12mim]Cl‐ibuprofen interactions. The interactions between the SAIL and drug molecules are found to be highly synergistic both in the mixed micelles and in the mixed monolayer. The formation of highly surface active catanionic complexes of 1:1 stoichiometry ([C12mim]Cl+ ibuprofen<sup>−</sup> ), stabilized largely by a combination of electrostatic, hydropho‐ bic, cation‐*π* and *π*‐*π* interactions, was established through spectroscopic investigations. Depending on the bulk composition and total surfactant concentration of mixed micelles, unilamellar and multi‐lamellar vesicles were detected in the system [63].

Vashishat et al. investigated mixtures of bile salts (sodium cholate (NaC) and sodium deoxy‐ cholate ((NaDC), **Figure 8**) with [C12mim]Br [67]. From a biochemical point of view, bile salts

indicate high synergism in the system, (3) major forces responsible for synergism are electro‐ static and hydrophobic interactions as well as *π*‐*π* stacking of aromatic rings and (4) Br<sup>−</sup>

counterion in palisade layer assists in compact packing of ions, which leads to the formation

of vesicles [64].

**Figure 11.** Molecular structures of amphiphilic drugs.

50 Application and Characterization of Surfactants

as a

play a vital role in many physiological processes, and more will be said about their catan‐ ionic mixtures in the next section. In order to obtain detailed information about interactions between [C12mim]Br and bile salts in the mixed monolayer and in the mixed micelles, surface tension and steady‐state fluorescence measurements were conducted. Various micellar and interfacial parameters, including cmc, *β*mic, *a*min, surface excess concentration (*π*max) and surface pressure at cmc (*π*cmc), were estimated. It was found that investigated mixtures exhibit pro‐ nounced synergism in mixed monolayer formation as well as micellization. Due to the more hydrophobic nature of NaDC, which allows its molecules to get deeply intercalated in the mixed micelles compared to NaC, mixture with NaDC showed stronger synergistic effect. In addition, this study aimed to determine the solubilization capacity of the poorly soluble drug, phenothiazine, in micellar media. It was found that solubility of phenothiazine is dependent on the hydrophobicity and the size of the micelles, with solubility increasing in the order: NaC < NaDC < [C12mim]Br/NaC < [C12mim]Br/NaDC [67].

Along with the properties of traditional catanionic mixtures, in systems containing SAIL, the following characteristics can be found:


#### **3.3. Catanionic mixtures of biologically active molecules and surfactants**

Due to their important roles in various physiological processes and pharmaceutical appli‐ cations amphiphilic biologically active molecules, as well as their catanionic mixtures, have been the subject of numerous papers. Historically, the most‐studied catanionic mixtures of this type as biologically active molecules contain either (1) amphiphilic drug or (2) bile salt. Therefore, aqueous catanionic mixtures of such molecules are mostly summarized and dis‐ cussed in this section.

In addition to [C*<sup>n</sup>* mim]Br/DFNa and [C12mim]Cl/ibuprofen systems [29, 63], mentioned in the previous section, Mahajan's group recently reported on: (1) interactions between the cat‐ ionic drug, trifluoperazine dihydrochloride (TFP, **Figure 11**) and anionic surfactants, SDS and AOT [68] as well as (2) interactions prevailing in catanionic mixtures containing cationic drug, tetracaine hydrochloride (TC, **Figure 11**) and anionic surfactants, SDBS and sodium lauroyl sarcosinate (SLS, **Figure 5**) [69]. In all investigated mixtures, various micellar and interfacial parameters were determined from surface tension measurements and by applying the RST. Obtained results revealed strong synergism in systems' bulk and surface properties such as high surface activity and low cmcs. As expected, it was established that TC interacts more strongly with SDBS and that TC‐SDBS complex possesses higher binding constant, as com‐ pared to TC/SLS mixture, due to the additional non‐covalent interactions in the system (*π*‐*π* stacking). Furthermore, Jiang et al. investigated aggregation behavior of vesicles formed by TC and AOT using conductivity, turbidity measurements, TEM, DLS and ELS [70]. The TC/ AOT aggregates exhibited different morphology, charge properties, interaction enthalpies and drug release behaviors depending on the mixtures' bulk composition. Obtained drug release profiles indicated that investigated drug‐containing vesicles have promising applica‐ tions in drug delivery systems.

play a vital role in many physiological processes, and more will be said about their catan‐ ionic mixtures in the next section. In order to obtain detailed information about interactions between [C12mim]Br and bile salts in the mixed monolayer and in the mixed micelles, surface tension and steady‐state fluorescence measurements were conducted. Various micellar and interfacial parameters, including cmc, *β*mic, *a*min, surface excess concentration (*π*max) and surface pressure at cmc (*π*cmc), were estimated. It was found that investigated mixtures exhibit pro‐ nounced synergism in mixed monolayer formation as well as micellization. Due to the more hydrophobic nature of NaDC, which allows its molecules to get deeply intercalated in the mixed micelles compared to NaC, mixture with NaDC showed stronger synergistic effect. In addition, this study aimed to determine the solubilization capacity of the poorly soluble drug, phenothiazine, in micellar media. It was found that solubility of phenothiazine is dependent on the hydrophobicity and the size of the micelles, with solubility increasing in the order:

Along with the properties of traditional catanionic mixtures, in systems containing SAIL, the

(1) variation in the alkyl chain length of both SAIL (*n*) and surfactant (*m*) causes a significant

(2) the magnitude of SAIL‐surfactant interactions is larger for surfactants with aromatic moi‐ ety in their structure, for example, SDBS. The reason behind this is the involvement of cation‐*π* and *π*‐*π* interactions due to the *π*‐electron cloud of the benzene ring in SDBS and

(3) along with electrostatic and hydrophobic interactions, additional non‐covalent interac‐ tions (hydrogen bonding, *π*‐*π* stacking) (1) enhance attractive interactions and (2) in‐ crease synergistic effect as well as (3) stabilize low curvature structures in SAIL‐sur‐

(4) apart from self‐assembled aggregates commonly found in catanionic mixtures, formation

Due to their important roles in various physiological processes and pharmaceutical appli‐ cations amphiphilic biologically active molecules, as well as their catanionic mixtures, have been the subject of numerous papers. Historically, the most‐studied catanionic mixtures of this type as biologically active molecules contain either (1) amphiphilic drug or (2) bile salt. Therefore, aqueous catanionic mixtures of such molecules are mostly summarized and dis‐

the previous section, Mahajan's group recently reported on: (1) interactions between the cat‐ ionic drug, trifluoperazine dihydrochloride (TFP, **Figure 11**) and anionic surfactants, SDS and AOT [68] as well as (2) interactions prevailing in catanionic mixtures containing cationic drug,

mim]Br/DFNa and [C12mim]Cl/ibuprofen systems [29, 63], mentioned in

change in the physicochemical properties and phase behavior of the systems,

cation,

mim]+

of gel phase was observed in some SAIL‐surfactant systems.

**3.3. Catanionic mixtures of biologically active molecules and surfactants**

NaC < NaDC < [C12mim]Br/NaC < [C12mim]Br/NaDC [67].

following characteristics can be found:

52 Application and Characterization of Surfactants

the imidazolium ring in [C*<sup>n</sup>*

factant systems and

cussed in this section.

In addition to [C*<sup>n</sup>*

Zhao et al. determined various physicochemical parameters (cmc, *Г*max, *a*min, surface tension at the cmc (*γ*cmc), degree of counterion binding, etc.) from the surface tension and electrical conductivity measurements in mixtures of DFNa and DDAB [71]. The cmc and *γ*cmc of mixed DDAB/DFNa systems were found to have values between that of pure DFNa and DDAB solutions. In addition, (1) the in vitro release results demonstrated that DDAB/DFNa vesicles exhibit good sustained drug release properties while (2) the hemolytic toxicity studies show that vesicles in mixtures with high DFNa molar ratio are safe for intravenous administration within the effective concentration [71].

Bile salts are well known and important biologically active surfactants, produced in the liver from cholesterol, which play an important role in emulsification of lipids, fats, fat soluble vitamins, etc. [67]. Due to their great importance in metabolism of insoluble molecules, such as phospholipids and monoglycerides, they have been extensively studied. They possess a unique molecular structure when compared with typical surfactant molecules, like the ste‐ roids, they have a nucleus composed of four fused rings, three cyclohexane rings and one cyclopentane ring as well as hydrophilic hydroxyl groups (**Figure 8**). Bile acids are favorable compounds for construction of supramolecular structures because of their biocompatibility, high structural rigidity, amphiphilicity and chirality [72].

Our group investigated 12–6–12/NaC and C*m*TACl (*m* = 12, 14 and 16)/NaC mixtures employing a combination of techniques such as surface tension, conductometry, light microscopy, DLS and ELS [24, 73]. In all investigated systems, synergism in micelliza‐ tion and adsorption was observed. With increasing total surfactant concentration, in equivalent 12–6–12/NaC mixtures, morphology of mixed aggregates changes as follows: complexes → flexible cylindrical mixed micelles → coexistence of vesicles, coacervates and solid crystalline phase. In the high excess of cationic surfactant, the small 12–6–12 micelles are prevailing structures while with an increasing content of NaC, the long flex‐ ible mixed micelles are dominant. Obtained results demonstrated that interplay between (1) electrostatic effects, (2) geometry of molecules as well as (3) dissimilar separation of the hydrophobic and hydrophilic moieties in the surfactants dictates phase behavior of these systems [24].

The most interesting discovery in C*m*TACl/NaC systems was that catanionic surfactants, pre‐ cipitated in/or close to equimolar region, show a variety of morphologies including twisted ribbons and crystalline tubules, which are not commonly found in this kind of systems [73]. The three‐dimensional structures that are yielded by the self‐assembly of lipids and surfac‐ tants have recently drawn much research interest due to their applications in nanotechnology [74]. Formation of tubules in C*m*TACl/NaC mixtures can be attributed to several factors: (1) chiral packing of NaC molecule in a bilayer, (2) strong attractive interactions between oppo‐ sitely charged headgroups at the bilayer/solution interface and (3) hydrogen bonding at the bilayer surfaces, which enhance formation of multilayer sheets and their twisting and/or roll‐ ing up [73].

Long, fiber‐like tubular structures instead of crystalline tubules were observed in the cat‐ ionic‐rich dilute region of DDAB/NaTDC system by Marques and Khan [75]. Authors sug‐ gested that formation of long tubular structures is a consequence of specific NaTDC's rigid ring‐based structure with hydroxyl groups. Previously, Marques's group studied the phase behavior of the same catanionic pair but in the bile salt‐rich area [26]. It was reported that the system displays coacervation instead of precipitation at equimolarity, consisting of a viscous isotropic solution in equilibrium with a very dilute solution. Formation of tubular structures was not detected in this part of the phase diagram.

Liu et al. reported that in lithocholic acid (LCA, **Figure 8**)/tetradecyltrimethylammonium hydroxide (C14TAOH, **Figure 4**) system transition from vesicles to tubules was observed, while in mixtures of LCA with cetyltrimethylammonium hydroxide (C16TAOH), transition from vesicles to helical ribbons occurred [72]. Thus, despite a difference of only two methy‐ lene groups in the alkyl chain of C14TAOH and C16TAOH, morphology of mixed aggregates in these two systems is largely different. In addition, it was found that time required for the phase transition depends on alkyl chain length as well. In the C14TAOH/LCA systems, the transition from vesicles to tubules was completed within several hours, while in the C16TAOH/ LCA system, the vesicles were converted to helical ribbons after more than 4 days, depending on the concentration and temperature [72].

Motivated by their numerous potential applications in nanotechnology, Manghisi et al. pre‐ pared and characterized tubules in mixtures of anionic (ACD) and cationic (CCD) derivatives of NaC (**Figure 8**) [74]. It was found that charge of synthesized CCD/ACD tubules ranges from negative to positive values depending on the surfactant molar ratio in the mixtures. Analysis of the TEM micrographs revealed a correlation between the diameter and the com‐ position of the tubules [74].

Bhattacharjee et al. investigated mixtures of cetylpyridinium chloride (C16PC, **Figure 4**) and NaDC using DLS, SANS and SAXS [76]. It was shown that phase separation, i.e. coacervate phase, occurs near the equimolar composition at low surfactant concentrations and, contrary to expectations, disappears at higher concentrations. This associative phase separation has been explained on the basis of competition between electrostatic attraction and entropy of the components mixing. Additionally, based on the obtained results, authors suggested that structural features of bile salts are not favorable for formation of catanionic vesicles when combined with C16PC. However, stable mixed micelles of widely differing morphologies were formed in a broad concentration range [76].

The most interesting discovery in C*m*TACl/NaC systems was that catanionic surfactants, pre‐ cipitated in/or close to equimolar region, show a variety of morphologies including twisted ribbons and crystalline tubules, which are not commonly found in this kind of systems [73]. The three‐dimensional structures that are yielded by the self‐assembly of lipids and surfac‐ tants have recently drawn much research interest due to their applications in nanotechnology [74]. Formation of tubules in C*m*TACl/NaC mixtures can be attributed to several factors: (1) chiral packing of NaC molecule in a bilayer, (2) strong attractive interactions between oppo‐ sitely charged headgroups at the bilayer/solution interface and (3) hydrogen bonding at the bilayer surfaces, which enhance formation of multilayer sheets and their twisting and/or roll‐

Long, fiber‐like tubular structures instead of crystalline tubules were observed in the cat‐ ionic‐rich dilute region of DDAB/NaTDC system by Marques and Khan [75]. Authors sug‐ gested that formation of long tubular structures is a consequence of specific NaTDC's rigid ring‐based structure with hydroxyl groups. Previously, Marques's group studied the phase behavior of the same catanionic pair but in the bile salt‐rich area [26]. It was reported that the system displays coacervation instead of precipitation at equimolarity, consisting of a viscous isotropic solution in equilibrium with a very dilute solution. Formation of tubular structures

Liu et al. reported that in lithocholic acid (LCA, **Figure 8**)/tetradecyltrimethylammonium hydroxide (C14TAOH, **Figure 4**) system transition from vesicles to tubules was observed, while in mixtures of LCA with cetyltrimethylammonium hydroxide (C16TAOH), transition from vesicles to helical ribbons occurred [72]. Thus, despite a difference of only two methy‐ lene groups in the alkyl chain of C14TAOH and C16TAOH, morphology of mixed aggregates in these two systems is largely different. In addition, it was found that time required for the phase transition depends on alkyl chain length as well. In the C14TAOH/LCA systems, the transition from vesicles to tubules was completed within several hours, while in the C16TAOH/ LCA system, the vesicles were converted to helical ribbons after more than 4 days, depending

Motivated by their numerous potential applications in nanotechnology, Manghisi et al. pre‐ pared and characterized tubules in mixtures of anionic (ACD) and cationic (CCD) derivatives of NaC (**Figure 8**) [74]. It was found that charge of synthesized CCD/ACD tubules ranges from negative to positive values depending on the surfactant molar ratio in the mixtures. Analysis of the TEM micrographs revealed a correlation between the diameter and the com‐

Bhattacharjee et al. investigated mixtures of cetylpyridinium chloride (C16PC, **Figure 4**) and NaDC using DLS, SANS and SAXS [76]. It was shown that phase separation, i.e. coacervate phase, occurs near the equimolar composition at low surfactant concentrations and, contrary to expectations, disappears at higher concentrations. This associative phase separation has been explained on the basis of competition between electrostatic attraction and entropy of the components mixing. Additionally, based on the obtained results, authors suggested that structural features of bile salts are not favorable for formation of catanionic vesicles when

ing up [73].

54 Application and Characterization of Surfactants

was not detected in this part of the phase diagram.

on the concentration and temperature [72].

position of the tubules [74].

Fernández‐Leyes et al. reported on physicochemical properties and phase behavior of DDAB/ sodium dehydrocholate (NaDHC, **Figure 8**) and DDAB/NaDC mixtures using surface tension measurements, conductivity, DLS, ELS and TEM [77, 78]. The RST was applied for evaluating the non‐ideal interactions between molecules in adsorbed monolayer and mixed micelles. All systems exhibited synergism in mixed monolayer formation as well as micellization. The obtained p*C*20 values, negative logarithms of the surfactant concentrations at which the sur‐ face tension of water is reduced by 20 mN m−1, demonstrated that both mixed systems have analogous adsorption efficiencies, which are similar to the pure DDAB solutions and supe‐ rior to that obtained for both bile salts. Nevertheless, difference in their adsorption effective‐ ness was observed: NaDC causes an increase of surface excess concentration, while NaDHC produces the opposite effect. The lower *Γ*max values obtained for DDAB/NaDHC system are related to the deep penetration of the hydrophobic steroid backbone of NaDHC molecules that cause a great disturbance of DDAB hydrocarbon tails, that is larger *a*min [77]. Furthermore, it was found that mixed aggregates in DDAB/NaDHC system are mainly composed of DDAB, regardless of the NaDHC solution molar fraction. Nevertheless, the gradual inclusion of NaDHC molecules leads to structural transformations in the system. The incorporation of NaDHC into DDAB bilayers had two effects: (1) the DHC− and DDA+ ions form ion pairs that are much less hydrated than separate ion headgroups, which consequently reduce the effective headgroup area and (2) the intercalation of the rigid ring‐based structure of bile salts between DDAB chains causes an increase of chain repulsion due to steric effects [78].

Pereyra et al. analyzed C16TAB/NaDHC system with two procedures: (1) the RST and (2) the EOMMM (Equation Oriented Mixed Micellization Modeling) [79]. Investigated system showed a non‐ideal and asymmetric behavior with attractive interaction between the compo‐ nents, as reflected by the obtained interaction parameters. Moreover, it was established that the affinity of DHC− ions for C16TAB micelles is stronger than that of C16TA+ ions for NaDHC ones [79].

Apart from the properties which can be found in traditional catanionic systems, common features of catanionic mixtures with biologically active molecules are:


(4) frequent occurrence of tubules in catanionic mixtures containing bile salts can be attrib‐ uted to several factors: (1) chiral packing of bile salt molecule in a bilayer, (2) strong attrac‐ tive interactions between oppositely charged headgroups at the bilayer/solution interface and (3) hydrogen bonding at the bilayer surfaces, which enhance formation of multilayer sheets and their twisting and/or rolling up.

## **4. Applications of catanionic mixtures**

In the past decades, a large number of systems for the controlled and targeted delivery of pharmaceutical compounds have been designed based on various self‐assembled aggregates such as micelles, vesicles, liquid crystalline phases, tubules, etc. [80–82]. Catanionic systems, due to their rich phase behavior and numerous possibilities in mediating molecular self‐ assembly, by adjusting the mixing molar ratio and using appropriate geometry of surfactant molecules, offer considerable advantages in delivering biomolecules. For example, catanionic mixtures easily and spontaneously form vesicles at non‐stoichiometric molar ratios. Vesicles are not only significant because they mimic biological membranes, but also due to their utility as drug carriers and targeted drug delivery systems.

As already mentioned, precipitation is considered to be the main drawback for application of catanionic mixtures [3]. However, as it can be seen from preceding sections, this draw‐ back can be circumvented by using surfactants of largely different molecular structures. In addition, biologically active molecules, such as amphiphilic drugs, can be used as one of the mixtures' components, which provide a whole range of possibilities for designing novel drug delivery systems.

In addition to drug delivery applications, catanionic mixtures are drawing attention in the synthesis of novel materials, development of novel analytical methods and corrosion protec‐ tion. As in drug delivery systems, vesicles play the most prominent role in these applications. However, other types of catanionic aggregates are becoming increasingly of more interest as structure‐directing templates.

## **4.1. Pharmaceutical applications: drug delivery systems**

Among different types of self‐assembled drug delivery systems, vesicles remain one of the most common strategies for the delivery of drugs and genetic material in the human body [80, 83]. In general, vesicles can adsorb considerable amount of species needed to be transferred and efficiently bind to the cells. Also, it is possible to tune their physical state (gel, liquid, liq‐ uid crystalline) and in that way additionally control the release [83]. By far, most used vesicles are those composed of natural polar lipids—so‐called liposomes. Liposomes possess excellent biocompatibility and biodegradability but often exhibit low stability, as they are susceptible to chemical degradation by hydrolysis and peroxidation. This is the key reason why catan‐ ionic vesicles, with their relative ease of preparation and long‐term stability, attract attention as possible alternatives. In addition, catanionic vesicles can be made of biocompatible surfac‐ tants as well, such as amino acid‐derived surfactants [19, 22, 23, 84].

(4) frequent occurrence of tubules in catanionic mixtures containing bile salts can be attrib‐ uted to several factors: (1) chiral packing of bile salt molecule in a bilayer, (2) strong attrac‐ tive interactions between oppositely charged headgroups at the bilayer/solution interface and (3) hydrogen bonding at the bilayer surfaces, which enhance formation of multilayer

In the past decades, a large number of systems for the controlled and targeted delivery of pharmaceutical compounds have been designed based on various self‐assembled aggregates such as micelles, vesicles, liquid crystalline phases, tubules, etc. [80–82]. Catanionic systems, due to their rich phase behavior and numerous possibilities in mediating molecular self‐ assembly, by adjusting the mixing molar ratio and using appropriate geometry of surfactant molecules, offer considerable advantages in delivering biomolecules. For example, catanionic mixtures easily and spontaneously form vesicles at non‐stoichiometric molar ratios. Vesicles are not only significant because they mimic biological membranes, but also due to their utility

As already mentioned, precipitation is considered to be the main drawback for application of catanionic mixtures [3]. However, as it can be seen from preceding sections, this draw‐ back can be circumvented by using surfactants of largely different molecular structures. In addition, biologically active molecules, such as amphiphilic drugs, can be used as one of the mixtures' components, which provide a whole range of possibilities for designing novel drug

In addition to drug delivery applications, catanionic mixtures are drawing attention in the synthesis of novel materials, development of novel analytical methods and corrosion protec‐ tion. As in drug delivery systems, vesicles play the most prominent role in these applications. However, other types of catanionic aggregates are becoming increasingly of more interest as

Among different types of self‐assembled drug delivery systems, vesicles remain one of the most common strategies for the delivery of drugs and genetic material in the human body [80, 83]. In general, vesicles can adsorb considerable amount of species needed to be transferred and efficiently bind to the cells. Also, it is possible to tune their physical state (gel, liquid, liq‐ uid crystalline) and in that way additionally control the release [83]. By far, most used vesicles are those composed of natural polar lipids—so‐called liposomes. Liposomes possess excellent biocompatibility and biodegradability but often exhibit low stability, as they are susceptible to chemical degradation by hydrolysis and peroxidation. This is the key reason why catan‐ ionic vesicles, with their relative ease of preparation and long‐term stability, attract attention

sheets and their twisting and/or rolling up.

56 Application and Characterization of Surfactants

**4. Applications of catanionic mixtures**

as drug carriers and targeted drug delivery systems.

**4.1. Pharmaceutical applications: drug delivery systems**

delivery systems.

structure‐directing templates.

However, despite the positive outlook for catanionic vesicles, the first studies have shown that they display number of problems such as (1) low encapsulation efficiency, both initial and long‐term efficiency were not as high as for liposomes and (2) permeability, that is, occur‐ rence of leakage due to the poor bilayers tightness [5, 7, 85]. Kaler et al. were the first to report spontaneous vesicles' formation from mixed cationic and anionic single‐chain surfactants, that is, cetyltrimethylammonium tosylate (CTAT, **Figure 4**) and SDBS, as well as their poten‐ tial to load glucose [86], while Caillet et al. investigated the encapsulation of anionic dye carboxyfluorescein (CF), riboflavine and glucose in C16TAB/SOS vesicles [87]. These studies have shown that the permeability of vesicle membranes can be tailored by choosing appro‐ priate surfactants' tail length. Surfactants with short alkyl chain enable higher permeabil‐ ity of amphiphilic films which in turn enable rapid and complete release, while longer tails increase vesicles' stability. These studies have also shown that expected specific interactions of ionic compounds with the surface of the vesicles can improve the entrapment efficiency [7]. Additionally, Wang et al. reported that CF can be encapsulated in the inner water pool as well as electrostatically adsorbed to the oppositely charged bilayers of CTAT‐rich vesicles formed in CTAT/SDBS mixtures [88]. Moreover, achieved loading capacity was 10 times greater com‐ pared to phosphatidylcholine liposomes. However, no entrapment of CF was observed in SDBS‐rich vesicles.

One way to overcome drawbacks of catanionic vesicles as drug delivery systems is prepara‐ tion of mixtures in which one of the components is amphiphilic drug. For example, problem with permeability, that is integrity and tightness of the vesicles, is then reduced since drug molecule is incorporated into the catanionic bilayers. This approach also enables usage of the mixed micelles as self‐assembled delivery systems [7].

As already discussed in the previous section, regarding the physicochemical properties and phase behavior of drug‐surfactant mixtures, most of the recent research was done by Mahajan's group [29, 63, 68, 69]. Catanionic systems containing (1) anti‐inflammatory drug for pain con‐ trol and treatment of rheumatic diseases, diclofenac sodium [29, 71], (2) non‐steroidal anti‐ inflammatory drug, ibuprofen [63], (3) antidepressant and antipsychotic drug, trifluoperazine dihydrochloride [68], as well as (4) tetracaine hydrochloride [69, 70], an anesthetic used in topical ophthalmic solutions, were investigated by a number of groups. In addition, Liu et al. established that amphiphilic anticancer drug, cytarabine hydrochloride (CH, **Figure 11**), and AOT can self‐assemble into vesicles in the aqueous solution [89]. The parallel artificial mem‐ brane permeability assay (PAMPA) and hemolytic toxicity studies were carried out to evaluate the potential use of CH/AOT vesicles in drug delivery. The results indicate that catanionic vesicles can improve the permeability of CH about 160 times in PAMPA model and markedly decrease the hemolytic toxicity of both CH and AOT compared with their respective solutions. In addition, in vitro drug release behavior results for both CH/AOT vesicles and CH/AOT vesicles incorporated into the thermosensitive PLGA‐PEG‐PLGA hydrogel revealed them as good sustained drug release systems [89].

In most cases, two main strategies to improve release properties of catanionic vesicles are employed which are (1) incorporation of vesicles into the gels and (2) preparation of environ‐ ment sensitive vesicles. Catanionic aggregates formed from drug and oppositely charged sur‐ factant and then incorporated into the gel have been extensively studied by Edsman's group with the objective to utilize them for prolonged release [90–96]:


Regarding the preparation of environment‐sensitive vesicles, Ghosh et al. [97] investigated pH‐induced release of model drug (calcein, fluorescent dye) as well as hemocompatibility and cytotoxicity of catanionic vesicles containing anionic amino acid‐based carboxylate sur‐ factants, sodium *N*‐alkanoyl‐l‐sarcosinate with varying chain length (**Figure 5**) and C12TAOH or C16TAOH. Obtained results demonstrated that with pH decrease (pH ≤ 5), vesicles are transformed into small mixed micelles. It can be concluded that investigated vesicles are sen‐ sitive to pH change of the environment and interesting as drug delivery systems in which drug release is triggered by pH change. The hemocompatibility and cytotoxicity evaluation revealed that vesicles are hemocompatible and nontoxic.

In most cases, two main strategies to improve release properties of catanionic vesicles are employed which are (1) incorporation of vesicles into the gels and (2) preparation of environ‐ ment sensitive vesicles. Catanionic aggregates formed from drug and oppositely charged sur‐ factant and then incorporated into the gel have been extensively studied by Edsman's group

(1) Catanionic aggregates containing various drug compounds, diphenhydramine, lidocaine, ibuprofen, naproxen, alprenolol, propranolol or orphenadrine (**Figure 11**), and ionic sur‐ factants, SDS, C14TACl, C12PC or benzalkonium chloride (C*m*BzCl, **Figure 4**) incorporated in Carbopol® 940 or agar‐agar gels, were studied. Obtained results demonstrated that both micelles and vesicles from the three systems examined in the release studies (lido‐ cain/SDS, orphenadrine/SDS, ibuprofen/C14TACl) helped to prolong the release between

(2) Constructed phase diagrams of the mixtures of three different cationic drug compounds, diphenhydramine, tetracaine and amitriptyline (**Figure 11**), with SDS, showed that al‐ though the diagrams may differ in some parts, vesicles and branched micelles are present in all three cases on the SDS‐rich side. Drug release from Carbopol® 940 and agar gels re‐ vealed that sustained drug release may be accomplished by incorporation of investigated

(3) Investigation of pH and ionic strength influence on the phase behavior of diphenhy‐ dramine/SDS and tetracaine/SDS mixtures, as well as study of drug release from drug/ surfactant aggregates in Carbopol® gels, demonstrated that drug release in both systems was somewhat affected by changes in both pH and ionic strength but remained in all

(4) A study of controlled release of charged drugs from five different types of gels by adding surfactants (SDS, Brij 58, C12BzBr) that can interact with the drug and polymer matrix demonstrated that interactions between the surfactant aggregates and the polymer can

(5) When drug/SDS vesicles, drug substance being alprenolol or tetracaine, were mixed with polymers, one bearing hydrophobic modifications, one positively charged and one posi‐ tively charged bearing hydrophobic modification, gels were form only in the case when negatively charge catanionic vesicles were mixed with positively charged polymer‐bear‐ ing hydrophobic modification. In addition, the release of drug substance from these sys‐ tems, where the vesicles are not trapped within the gel but constitute a founding part of it, could be significantly prolonged. The release rate was affected to a greater extent by variation of vesicles' concentrations than by variation in polymer concentration [94]. (6) Release profiles of (1) alprenolol/SDS aggregates incorporated into the SoftCAT and Car‐ bopol® gels [95] and (2) tetracaine/SDS or capric acid aggregates incorporated into the SoftCAT and carbomer gels [96] have shown that prolonged drug release from this system

cases significantly prolonged compared to the release of the free drug [92].

10 and 100 times compared to the release of the pure drug from the gel [90].

with the objective to utilize them for prolonged release [90–96]:

58 Application and Characterization of Surfactants

catanionic vesicles and micelles into the gels [91].

be used to further modify the drug release [93].

enables prolonged skin penetration.

Motivated with known antibacterial activity of anionic and cationic surfactants, Chaouat et al. prepared three component vesicles consisting of *N*‐dodecyldiethanolamine, decanoic acid and azelaic acid (**Figures 4** and **5**) and evaluated their antimicrobial activity against differ‐ ent strains of bacteria [98]. Obtained results revealed that antimicrobial activity of catanionic vesicles displays synergistic effect compared with the activity of individual components.

Not only catanionic vesicles are considered of interest for designing drug delivery systems. The 1D structures, such as tubules, that are yielded by the self‐assembly of lipids and sur‐ factants are of particular interest for their applications in nanotechnology and pharmaceuti‐ cal applications [74]. Lin et al. prepared multi‐walled nanotubes using two anticancer drug amphiphiles in which drug camptothecin (CPT, **Figure 11**) was loaded [99]. Used amphiphi‐ les contained one, two or four hydrophobic CPTs conjugated to a β‐sheet‐forming peptide sequence through a reducible disulfylbutyrate linker. The authors proposed that nanotubules were formed by combination of three occurrences: (1) 1D elongation, (2) formation of multi‐ layers and (3) bilayer extension from helical ribbons due to mixing of oppositely charged drug amphiphiles.

The interaction between amphiphiles and DNA was studied over a long period of time in the area of gene therapy [7]. Likewise, due to their features, catanionic vesicles are of potential interest as non‐viral gene carriers. Interactions of DNA and cationic vesicles result in com‐ plexes in which DNA molecule adopts more compact conformation and has reduced charge, facilitating its uptake through cell membranes. The fundamental framework for DNA/catan‐ ionic vesicles application has been established by Lindman's group [100–103]. In a number of studies, they have shown that:


(3) Interactions between DNA and positively charged vesicles are strong. Formed complexes withstand dilution or addition of excess surfactant or DNA and do not dissolve. Their structure resembled to other systems previously described, that is DNA molecules were packed between surfactant bilayers [103].

La Mesa's group [104] has shown that interaction with C16TAB/SDS vesicles can protect a sen‐ sitive molecule, exogeneous RNA, from RNase, resulting in efficient delivery of RNA across the cell membrane. The efficiency of delivery increases when vesicles are formed in the pres‐ ence of RNA. In a recent study [105] of DDAB/8‐hexadecyl sulfate (8‐SHS, **Figure 5**) vesicles interaction with calf thymus DNA, it was shown that strongly associating complexes are formed. Results revealed that their structure depended on DNA content. At low concentra‐ tion, formed complexes resemble to bare vesicles, while at higher concentrations, multi‐lamel‐ lar entities are formed in which adsorbed amount of DNA increases with its concentration. Further increasing DNA concentration leads first to formation of large clusters of vesicles and then to precipitation. DNA molecules undergo compaction process, which facilitates penetra‐ tion into cell and at the same time protects it from nucelases action. The compaction process is reversible as addition of anionic surfactant induces DNA release [105].

#### **4.2. Synthesis of advanced materials**

Surfactants' role in the synthesis of nanomaterials renewed interest of research community for applying surfactants and self‐assembled aggregates in the preparation of new materials. Surfactants have been used in the synthesis of inorganic materials, either as soft templates or in the surfactant‐mediated synthesis [106]. Despite wide‐ranging structural diversity of sur‐ factants' aggregates, vesicles, and thus catanionic vesicles, are still frequently the template of choice. Due to the special structure of vesicles, inorganic material can be formed in different reaction environments: (1) the "bulk" solution outside the vesicles, (2) the inner chamber, (3) the outside surface or (4) the hydrophobic palisade layer of the vesicles [107, 108]. Different reaction environments enable formation of material of vastly different morphologies.

Recently, vesicles formed by SAIL, [C12MP]Br (**Figure 10**), and a divalent metal surfactant, cop‐ per dodecyl sulfate (Cu(DS)2 ·4H2 O), were used for preparation of leaf‐like CuO nanosheets [108].

Using vesicles composed of imidazolium‐based SAIL, [C12mim]Br (**Figure 10**) and SDS as structure‐directing templates, Yuan et al. synthesized silica hollow spheres [107]. Silica hol‐ low spheres of controlled size were previously synthesized in C12TAB/SDBS mixtures by Kepczynski et al. [109]. Furthermore, catanionic vesicles formed in C16TAOH/Mg(DS)2 mix‐ ture were used for preparation of Mg(OH)2 hollow nanospheres [110]. Interestingly, it was observed that encapsulation of Mg(OH)2 particles, followed by crystal fusion, can induce the size and shape change of catanionic vesicles under non‐equilibrium conditions. This phenom‐ enon facilitated the direct observation of hydrophobic membrane fusion by means of TEM microscopy [110].

Not only inorganic hollowspheres were synthesized in the presence of catanionic vesicles. Morgan et al. developed the method for preparation of polymeric spheres by introduc‐ ing polymerizable monomer into the vesicle's bilayer [111]. This method was later used for synthesis of polydisperse hollow polystyrene spheres in CTAT/SDBS and C16TAB/SOS mixtures [112]. Additionally, a lot of research work employing the different preparation methods with polymerized ion pair amphiphile vesicles was done by Chung's group [113–117]. Recently, hol‐ low microspheres of poly(3,4‐ethylenedioxythiophene (PEDOT), ranging from 0.5 to 10 mm, were synthesized by oxidative polymerization in the presence of C16TAB/SDBS vesicles. It was established that formation and size of microspheres were influenced by surfactant molar ratio. Moreover, it was shown that SDBS was incorporated in the polymer chain as dopant [118].

(3) Interactions between DNA and positively charged vesicles are strong. Formed complexes withstand dilution or addition of excess surfactant or DNA and do not dissolve. Their structure resembled to other systems previously described, that is DNA molecules were

La Mesa's group [104] has shown that interaction with C16TAB/SDS vesicles can protect a sen‐ sitive molecule, exogeneous RNA, from RNase, resulting in efficient delivery of RNA across the cell membrane. The efficiency of delivery increases when vesicles are formed in the pres‐ ence of RNA. In a recent study [105] of DDAB/8‐hexadecyl sulfate (8‐SHS, **Figure 5**) vesicles interaction with calf thymus DNA, it was shown that strongly associating complexes are formed. Results revealed that their structure depended on DNA content. At low concentra‐ tion, formed complexes resemble to bare vesicles, while at higher concentrations, multi‐lamel‐ lar entities are formed in which adsorbed amount of DNA increases with its concentration. Further increasing DNA concentration leads first to formation of large clusters of vesicles and then to precipitation. DNA molecules undergo compaction process, which facilitates penetra‐ tion into cell and at the same time protects it from nucelases action. The compaction process

Surfactants' role in the synthesis of nanomaterials renewed interest of research community for applying surfactants and self‐assembled aggregates in the preparation of new materials. Surfactants have been used in the synthesis of inorganic materials, either as soft templates or in the surfactant‐mediated synthesis [106]. Despite wide‐ranging structural diversity of sur‐ factants' aggregates, vesicles, and thus catanionic vesicles, are still frequently the template of choice. Due to the special structure of vesicles, inorganic material can be formed in different reaction environments: (1) the "bulk" solution outside the vesicles, (2) the inner chamber, (3) the outside surface or (4) the hydrophobic palisade layer of the vesicles [107, 108]. Different

reaction environments enable formation of material of vastly different morphologies.

·4H2

ture were used for preparation of Mg(OH)2

observed that encapsulation of Mg(OH)2

Recently, vesicles formed by SAIL, [C12MP]Br (**Figure 10**), and a divalent metal surfactant, cop‐

Using vesicles composed of imidazolium‐based SAIL, [C12mim]Br (**Figure 10**) and SDS as structure‐directing templates, Yuan et al. synthesized silica hollow spheres [107]. Silica hol‐ low spheres of controlled size were previously synthesized in C12TAB/SDBS mixtures by Kepczynski et al. [109]. Furthermore, catanionic vesicles formed in C16TAOH/Mg(DS)2

size and shape change of catanionic vesicles under non‐equilibrium conditions. This phenom‐ enon facilitated the direct observation of hydrophobic membrane fusion by means of TEM

Not only inorganic hollowspheres were synthesized in the presence of catanionic vesicles. Morgan et al. developed the method for preparation of polymeric spheres by introduc‐ ing polymerizable monomer into the vesicle's bilayer [111]. This method was later used for

O), were used for preparation of leaf‐like CuO nanosheets [108].

hollow nanospheres [110]. Interestingly, it was

particles, followed by crystal fusion, can induce the

mix‐

is reversible as addition of anionic surfactant induces DNA release [105].

packed between surfactant bilayers [103].

60 Application and Characterization of Surfactants

**4.2. Synthesis of advanced materials**

per dodecyl sulfate (Cu(DS)2

microscopy [110].

In addition to vesicles, catanionic micelles can be also effective templates for preparation of nanoparticles (NPs). For example, C16TAB/SDS micelles were used in synthesis of mesopo‐ rous *γ*‐Al2 O3 NPs [119]. Authors demonstrated that the choice of the surfactant is important for the synthesis of organized mesoporous aluminas with a well‐defined porosity, although it is unclear how the presence of micellar aggregates affects the final architecture in cationic‐ anionic double hydrolysis method [119].

Short‐chain catanionic mixtures composed of C10TAB and SOS were used in synthesis of highly ordered supermicroporous silica [120]. Pore size in the range 1–2 nm had hexagonal structure which was strongly dependent on the surfactants molar ratio. Previously, Ohkubo et al. reported synthesis of silica particles in which precise control of both, the pore size and the structure of pores, was achieved by changing C16TAB/SOS mixing ratio [121]. Moreover, use of the cationic surfactant with longer alkyl chain, C18TAB, shifted the point of phase transition from hexagonal phase to lamellar phase to lower concentration of SOS. Lind et al. reported on vesicle‐like patterned, mesoscopically ordered silica synthesized in C16TAB/decanoic acid mixtures with toluene used as the swelling agent [122]. Obtained results demonstrated that lower interfacial charge density of the mixed aggregates stabilizes structures of lower interfa‐ cial curvature and therefore facilitates a more controlled solubilization of toluene. In addition, it was shown that the pore size of the hexagonal phase could be controlled by changing the C16TAB/decanoic acid and the C16TAB/toluene molar ratios [122].

Using surfactants' aggregates as structure‐directing templates in the synthesis of new mate‐ rials is essentially a biomimetic approach [106]. Hard tissues in organisms, such as bones and teeth, are formed in the processes in which organic matrix (composed of surface active proteins, lipids, etc.) has a role of the template which determines morphology, size and ori‐ entation of inorganic phase. Therefore, it is not surprising that several attempts of biomineral synthesis in the presence of catanionic mixtures have been reported.

Prelote and Zemb used catanionic aggregates with hexagonal structures formed in mixtures of polyoxyethyleneoleyl ether phosphate (POEPO4 ) and C14TAB as structure‐directing tem‐ plates for synthesis of mesoporous hydroxapatite (HAP) with high surface area [123]. HAP is thermodynamically the most stable calcium phosphate phase which attracts attention due to its similarity to bone mineral. It is widely used as biomaterial for bone and dental tis‐ sue regeneration in the form of different ceramics formulations and as coating. In that sense, mesoporous HAP is of special interest as a 3D scaffold. Hexagonal network of cylindrical micelles formed in the C14TAB/POEPO4 mixture was preserved during the synthesis of HAP, which enabled formation of the precipitates with the structural characteristic of the hexagonal network. However, the repetition distance was low and obtained precipitates were not truly mesoporous material. In addition, the precipitates were not able to withstand calcification.

Tari et al. have shown that the morphology of HAP NPs in C16TAB/SDS solution depends on surfactant molar ratio [124]. In the SDS‐rich region, rod‐like HAP NPs were obtained, while in C16TAB‐rich region, HAP nanosheets were formed.

Control of polymorphism and crystal morphology is not only important in the biomineral synthesis but also for fundamental understanding of biomineralization processes in vivo. Chen and Nang have shown that surfactants molar ratio in C16TAB/SDS mixtures can be used to control both the morphology and polymorphism of CaCO3 crystals [125]. Furthermore, Dong et al. obtained brick‐like (dodecahedrons) and star‐like (icositetrahedrons) calcium oxa‐ late monohydrate (CaC2 O4 ·H2 O) crystals, not observed before, in mixtures of calcium dodecyl sulfate and C14TAB with excess CaBr2 [126].

## **4.3. Novel analytical methods**

Several research groups investigated the use of catanionic aggregates in the development of new analytical and detection methods.

CTAT/SDBS vesicles, both positively and negatively charged, were used for highly efficient electrostatic sequestration of small molecules of similar weight but opposite charge, that is CF, lucifer yellow, sulforhodamine 101, doxorubicin and rhodamine 6G [127]. Authors have established that charge‐dependent effect enables use of CTAT/SDBS vesicles for selective cap‐ ture and separation of oppositely charged solute from a mixture of solutes.

Kahe et al. used C16TAB/SDS mixtures in propanol water as a novel microextraction system for the preconcentration and determination of trace amounts of lead in (1) saline solutions and (2) food samples [128]. Since only small amount of propanol in water was used, both hydrophilic and hydrophobic sites in extraction solvent were available for interaction with analytes of various polarities enabling good efficacy. Obtained results confirm that the cat‐ anionic aggregate dispersive microextraction method can be used as a simple, safe, fast and low‐cost technique for the microextraction of various organic and inorganic compounds from real samples [128].

Chen et al. employed coacervates formed by addition of hexafluoroisopropanol (HFIP) to C12TAB/SDS mixtures for extraction of strongly polar sulphonamides (SAs) from environmen‐ tal water samples [129]. Results demonstrated that even small amount of HFIP can induce coacervation and two‐phase separation in a broad concentration range in C12TAB/SDS system. In addition, analysis of real water samples confirmed that investigated method can be effi‐ ciently used for the preconcentration and determination of SAs traces.

With an aim to improve methods for Au(III) extraction, Wang et al. used C12C3 (OH)C12Cl2 (**Figure 6**)/NaDC vesicles [130]. Through stepwise extraction and ligand‐modified vesicles system, separation of Au (III), Cu (II) and Fe (III) from mixed solution was successfully achieved. The results collected in this study revealed great potential of catanionic aggregates in development of environmental friendly Au recovery method [130].

Gao et al. proposed a new method for determination of anionic surfactants based on in situ formation of catanionic aggregates in the presence of amphiphilic 2‐(2‐hydroxyphenyl) benzothiazolefluorogen probe [131]. Described approach enables quantitative determina‐ tion of low anionic surfactant concentrations and can be extended to wash‐free imaging of bacteria.

The role that carbohydrate‐protein interactions have in biological processes and difficulties in their evaluation motivates development of novel analytical methods. Pond et al. applied CTAT/SDBS vesicles with incorporated glycans in the outer surface to form glycan array for investigating carbohydrate‐lecitin interactions. The method proved to be facile and opens possibilities for characterizing unknown lecitins [132].

#### **4.4. Corrosion protection**

network. However, the repetition distance was low and obtained precipitates were not truly mesoporous material. In addition, the precipitates were not able to withstand calcification.

Tari et al. have shown that the morphology of HAP NPs in C16TAB/SDS solution depends on surfactant molar ratio [124]. In the SDS‐rich region, rod‐like HAP NPs were obtained, while

Control of polymorphism and crystal morphology is not only important in the biomineral synthesis but also for fundamental understanding of biomineralization processes in vivo. Chen and Nang have shown that surfactants molar ratio in C16TAB/SDS mixtures can be used

Dong et al. obtained brick‐like (dodecahedrons) and star‐like (icositetrahedrons) calcium oxa‐

Several research groups investigated the use of catanionic aggregates in the development of

CTAT/SDBS vesicles, both positively and negatively charged, were used for highly efficient electrostatic sequestration of small molecules of similar weight but opposite charge, that is CF, lucifer yellow, sulforhodamine 101, doxorubicin and rhodamine 6G [127]. Authors have established that charge‐dependent effect enables use of CTAT/SDBS vesicles for selective cap‐

Kahe et al. used C16TAB/SDS mixtures in propanol water as a novel microextraction system for the preconcentration and determination of trace amounts of lead in (1) saline solutions and (2) food samples [128]. Since only small amount of propanol in water was used, both hydrophilic and hydrophobic sites in extraction solvent were available for interaction with analytes of various polarities enabling good efficacy. Obtained results confirm that the cat‐ anionic aggregate dispersive microextraction method can be used as a simple, safe, fast and low‐cost technique for the microextraction of various organic and inorganic compounds from

Chen et al. employed coacervates formed by addition of hexafluoroisopropanol (HFIP) to C12TAB/SDS mixtures for extraction of strongly polar sulphonamides (SAs) from environmen‐ tal water samples [129]. Results demonstrated that even small amount of HFIP can induce coacervation and two‐phase separation in a broad concentration range in C12TAB/SDS system. In addition, analysis of real water samples confirmed that investigated method can be effi‐

(**Figure 6**)/NaDC vesicles [130]. Through stepwise extraction and ligand‐modified vesicles system, separation of Au (III), Cu (II) and Fe (III) from mixed solution was successfully achieved. The results collected in this study revealed great potential of catanionic aggregates

[126].

ture and separation of oppositely charged solute from a mixture of solutes.

ciently used for the preconcentration and determination of SAs traces.

in development of environmental friendly Au recovery method [130].

With an aim to improve methods for Au(III) extraction, Wang et al. used C12C3

O) crystals, not observed before, in mixtures of calcium dodecyl

crystals [125]. Furthermore,

(OH)C12Cl2

in C16TAB‐rich region, HAP nanosheets were formed.

O4 ·H2

sulfate and C14TAB with excess CaBr2

62 Application and Characterization of Surfactants

new analytical and detection methods.

**4.3. Novel analytical methods**

late monohydrate (CaC2

real samples [128].

to control both the morphology and polymorphism of CaCO3

Catanionic mixtures also proved to be efficient in corrosion protection of mild steel. C16TAB/ SDS mixtures demonstrated better protective efficiency than the individual surfactants. This was explained by strong adsorption on the metal surface and formation of protective surfac‐ tant film. The strong adsorption was evidenced by more negative values of the adsorption free energy of C16TAB/SDS mixtures compared to the individual surfactants [133].

## **Author details**

Darija Domazet Jurašin\*, Suzana Šegota, Vida Čadež, Atiđa Selmani and Maja Dutour Sikirć

\*Address all correspondence to: djurasin@irb.hr

Laboratory for Biocolloids and Surface Chemistry, Division of Physical Chemistry, Ruđer Bošković Institute, Zagreb, Croatia

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## **Chapter 3**

## **Hydrophobic Polymers Flooding**

Abdelaziz N. El-hoshoudy, Saad M. Desouky,

Mohamed H. Betiha and Ahmed M. Alsabagh

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69645

#### **Abstract**

Crude oil and other petroleum products are crucial to the global economy today due to increasing energy demand approximately (~1.5%) per year and significant oil remaining after primary and secondary oil recovery (~45-55% of original oil in place, OOIP), which accelerates the development of enhanced oil recovery (EOR) technologies. Polymer flooding through hydrophobically associated polyacrylamides (HAPAM) is a widely implemented EOR-technique, so they attracted much attention on both academic and industrial scales. Hydrophobically associating polyacrylamide (HAPAM) prepared by free radical emulsion polymerization of acrylamide (AM) monomer, divinyl sulfone as hydrophobic crosslinked moiety and surfmers, to chemically anchor a surfmer and hydrophobic crosslinker moiety onto the back bone of acrylamide chain. After that, polymeric nanocomposite was prepared through copolymerization of prepared HAPAM with different molar ratios of silica nanoparticles through one shot synthesis. Rheological properties for the prepared composites were evaluated. Wettability evaluation carried through quantitative and qualitative techniques where the results indicate novel polymers ability to alter rock wettability from oil-wet to water- wet.

**Keywords:** hydrophobic polymers, wettability alteration, enhanced oil recovery, polymerization, surfmers

© 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.

## **1. Introduction**

Crude oil is the most critical energy source in the world, especially for transportation, provision of heat and light as there has not been a sufficient energy source to replace crude oil has broadly integrated (i.e., today's energy needs are met in large part by crude oil). Petroleum products are crucial to the global economy today due to increasing energy demand approximately 1.5% per year [1] associated with population growth and improving life styles, limited proven oil reserves (i.e., shortage of current oil resources), declining oil production since 1995, difficulties in finding a new oil field, nonproductive primary and secondary recovery, significant oil remaining after secondary recovery (~45–55% of original oil in place, OOIP), and forecasts for tightening oil supply which driving the need to maximize the extraction of the original oil in place for every reservoir, and accelerating the development of enhanced oil recovery (EOR) technologies. EOR can be defined as any processes that increase oil recovery by reduction of the residual oil saturation (*S*or) after primary and secondary production. Indeed, EOR techniques refer to any process that involves the injection of a fluid not normally present in the reservoir (e.g., polymers, foams, and surfactants) where the injected fluids interact with a crude oil/brine/rock (COBR) system to create favorable conditions, which maximize oil recovery [2]. Tertiary or enhanced oil recovery techniques include chemical, thermal, and miscible flooding [3] for recovering up to 40% of the OOIP. Thermal EOR involves injection of steam or hot water to reduce heavy oils viscosity, thus improving its flow. Miscible methods employ hydrocarbon gases (natural gas and flue gas) nitrogen, supercritical CO<sup>2</sup> to displace oil from a depleted oil reservoir. Gas flooding improves oil recovery by dissolving in, swelling, and reducing the viscosity of oil. Chemical flooding was, up to 2000s, less common EOR method than thermal and gas flooding but now, huge projects are initiated or revisited. In chemical EOR methods, an agent that is not normally present in the reservoir is injected to enhance the oil displacement. The chemical flooding processes involve injection of three kinds of chemicals: alkaline, surfactant, and polymer (soluble and cross-linked polymers), in addition to other chemicals such as foaming agents, acids, and solvents [4] and/or combination of alkaline-surfactant-polymer (ASP) flooding, and surely the most important substance in these methods is polymer flooding [5]. In the polymer flooding method, water-soluble polymers aimed to reduce mobility of displacing fluid leading to a more efficient displacement of moderately viscous oils. Addition of surfactant to the polymer formulation may, under very specific circumstances, reduce oil-water interfacial tension (IFT) and hence remobilizing the trapped oil [6], changing the wettability of the surface, forming emulsions, so enhance the oil production. For all chemical flooding processes, inclusion of a viscosifier (usually a water-soluble polymer) is required to provide an efficient sweep of the expensive chemicals through the reservoir. To increase the oil recovery efficiency in oil-wet reservoirs (unswept regions), different techniques have to be pursued [7].

(1) Improving volumetric sweeping efficiency by adjusting the oil/water mobility ratio through polymer flooding agents, which increase displacing fluid viscosity in order to modify the viscous forces being applied to drive oil out of the pores [8], thus increasing the produced crude oil amount. A polymer solution has increased viscosity and decreased relative permeability so it is an attractive option to decrease the mobility ratio and increase the volumetric sweep efficiency of the injection [9].


## **2. Surfactants nature and applications in EOR**

**1. Introduction**

76 Application and Characterization of Surfactants

have to be pursued [7].

Crude oil is the most critical energy source in the world, especially for transportation, provision of heat and light as there has not been a sufficient energy source to replace crude oil has broadly integrated (i.e., today's energy needs are met in large part by crude oil). Petroleum products are crucial to the global economy today due to increasing energy demand approximately 1.5% per year [1] associated with population growth and improving life styles, limited proven oil reserves (i.e., shortage of current oil resources), declining oil production since 1995, difficulties in finding a new oil field, nonproductive primary and secondary recovery, significant oil remaining after secondary recovery (~45–55% of original oil in place, OOIP), and forecasts for tightening oil supply which driving the need to maximize the extraction of the original oil in place for every reservoir, and accelerating the development of enhanced oil recovery (EOR) technologies. EOR can be defined as any processes that increase oil recovery by reduction of the residual oil saturation (*S*or) after primary and secondary production. Indeed, EOR techniques refer to any process that involves the injection of a fluid not normally present in the reservoir (e.g., polymers, foams, and surfactants) where the injected fluids interact with a crude oil/brine/rock (COBR) system to create favorable conditions, which maximize oil recovery [2]. Tertiary or enhanced oil recovery techniques include chemical, thermal, and miscible flooding [3] for recovering up to 40% of the OOIP. Thermal EOR involves injection of steam or hot water to reduce heavy oils viscosity, thus improving its flow. Miscible methods

employ hydrocarbon gases (natural gas and flue gas) nitrogen, supercritical CO<sup>2</sup>

oil from a depleted oil reservoir. Gas flooding improves oil recovery by dissolving in, swelling, and reducing the viscosity of oil. Chemical flooding was, up to 2000s, less common EOR method than thermal and gas flooding but now, huge projects are initiated or revisited. In chemical EOR methods, an agent that is not normally present in the reservoir is injected to enhance the oil displacement. The chemical flooding processes involve injection of three kinds of chemicals: alkaline, surfactant, and polymer (soluble and cross-linked polymers), in addition to other chemicals such as foaming agents, acids, and solvents [4] and/or combination of alkaline-surfactant-polymer (ASP) flooding, and surely the most important substance in these methods is polymer flooding [5]. In the polymer flooding method, water-soluble polymers aimed to reduce mobility of displacing fluid leading to a more efficient displacement of moderately viscous oils. Addition of surfactant to the polymer formulation may, under very specific circumstances, reduce oil-water interfacial tension (IFT) and hence remobilizing the trapped oil [6], changing the wettability of the surface, forming emulsions, so enhance the oil production. For all chemical flooding processes, inclusion of a viscosifier (usually a water-soluble polymer) is required to provide an efficient sweep of the expensive chemicals through the reservoir. To increase the oil recovery efficiency in oil-wet reservoirs (unswept regions), different techniques

(1) Improving volumetric sweeping efficiency by adjusting the oil/water mobility ratio through polymer flooding agents, which increase displacing fluid viscosity in order to modify the viscous forces being applied to drive oil out of the pores [8], thus increasing the produced crude oil amount. A polymer solution has increased viscosity and decreased

to displace

Surfactants are surface-active components, low to moderate molecular weight polar compound, consisting of an amphiphilic molecule, with a water soluble hydrophilic part called "head" (anionic, cationic, amphoteric, or nonionic) and a water insoluble hydrophobic part called "tail" [15] as shown in **Figure 1**. Surfactants used in EOR applications in order to [16]:


Depending upon the nature of the hydrophilic head group, the surfactants are classified, as shown in **Figure 2**, into the following:

(1) **Anionic**: the surface-active portion of the molecule (hydrophilic group) bears a negative charge such as carboxyl (RCOO<sup>−</sup> M<sup>+</sup> ), sulfonate (RSO3−M<sup>+</sup> ), sulfate (ROSO3−M<sup>+</sup> ), or phosphate (ROPO3−M<sup>+</sup> ) (e.g., RC6 H4 SO<sup>3</sup> − Na+ , alkyl benzene sulfonates).


Surfactant flooding in enhanced oil recovery processes is considered uneconomical and remains challenging, especially in a high-salinity, high-temperature environment due to the following drawbacks:


These previously reported shortages made the one think in an alternative when speak about EOR project [27].

**Figure 1.** Surfactant skeletal structure (http://conf.sej.org/pollution-environmental-health/, 2011).

#### Hydrophobic Polymers Flooding http://dx.doi.org/10.5772/intechopen.69645 79

**Figure 2.** Classification of surfactants depending upon hydrophilic group nature.

## **3. Polymeric surfactants (surfmers)**

(2) **Cationic**: the surface-active portion bears a positive charge such as the quaternary

(3) **Amphoteric (zwitterionic)**: where the molecule contains both a negative and a positive charge on the principal chain (surface-active portion) such as the sulfobetaines,

COO<sup>−</sup>

(4) **Nonionic**: where the surface-active portion (hydrophilic group) bears no apparent ionic charge (has no net charge) but gets its water solubility from highly polar groups such

Surfactant flooding in enhanced oil recovery processes is considered uneconomical and remains challenging, especially in a high-salinity, high-temperature environment due to the

(1) Loss of chemicals by adsorption in porous media [21], which dictate, the economics of an

(2) Surfactant aggregates exhibit relatively low tolerance to divalent ions, salinity, and high-

(3) As described by Austad and Taugbøl [23] and Berger et al. [24], high performance surfactants, greatly lower oil/water IFT, but do not favor capillary-driven imbibition during

These previously reported shortages made the one think in an alternative when speak about

, salt of a long chain amine).

, long chain amino acid).

), sugars, or similar groups (e.g., RCOOCH-

) (e.g., RNH3+Cl<sup>−</sup>

H2 CH<sup>2</sup>

OH, monoglyceride of long chain fatty acid).

C [22].

**Figure 1.** Surfactant skeletal structure (http://conf.sej.org/pollution-environmental-health/, 2011).

CH<sup>2</sup> O <sup>−</sup>

ammonium halides (R4

78 Application and Characterization of Surfactants

CH<sup>2</sup> CH<sup>2</sup> SO<sup>3</sup> −

as polyoxyethylene (POE or ROCH<sup>2</sup>

oil recovery or remediation process.

temperature condition ≥90<sup>o</sup>

water flooding [25, 26].

EOR project [27].

RN+

2

(CH3)<sup>2</sup>

CHOHCH<sup>2</sup>

following drawbacks:

N+ X−

(e.g., RN+

Polymeric surfactants (surface-active monomers) are one kind of functional surfactants, which not only have amphiphilic structure composed of hydrophobic tail and hydrophilic head group [28], but also contain polymerizable vinyl double bonds [29] in their molecular architecture, resulting in novel physicochemical properties distinct from conventional surfactants [30] as follows:


to copolymerize with acrylamide (AM) forming hydrophobically associating polyacrylamide (HAPAM), which has been widely used in enhanced oil recovery, drilling fluids, coats, and paintings [33].

Freedman et al. [34] reported the first synthesis of vinyl monomers [35], which also functioned as emulsifying agents [36, 31]. Typical polymerizable groups that have been exploited are vinyl, allyl, acrylate, methacrylate, styryl, and acrylamide [37]. The position of the polymerizable group either "H-type" where the polymerizable group located in the hydrophilic head group, or "T-type" where the polymerizable group located in the hydrophobic tail have a profound effect on the surfactant self-assembly and properties [38, 39].

## **4. Wettability of porous media**

Wettability defined as the preferential affinity of the solid matrix for either the aqueous phase or the oil phase "the tendency of one fluid (wetting fluid) to spread on or adhere to a solid surface in the presence of another immiscible fluid (nonwetting fluid)" [40]. Reservoir wettability is an important and elusive petrophysical parameter in all types of core analyses, which affect saturation and enhanced oil recovery processes [41]. There is a consensus in petroleum engineering that preferentially water-wet cores flood more efficiently than oil-wet cores; since, more oil is recovered from water-wet cores in the early flooding stages than from oil-wet cores [42]. This can be attained due to the strong wetting preference of the rock for water yields the most efficient oil displacement and due to imbibition phenomenon and other complex interactions occurring in the reservoir during production [43]. Modified polymers can affect markedly mineral wettability. The copolymer of acrylamide-methacrylamido propyl trimethyl-ammonium chloride can alter surface of oil-wet quartz with adsorbed dodecyl amine into water-wet one. Where the polymer masking the surfactant layer on the quartz particle accounts for the water wettability [44]. Consequently, in the present study, the authors try to prepare a copolymer and a nanocomposite modified by silica addition to alter wettability of sandstone rock from oil-wet to water-wet.

## **5. Principle and mechanism of polymer flooding for enhanced oil recovery**

The polymer flooding process involves injection of polymer "slug" followed by continued long-term water flooding to drive the polymer slug and the oil bank in front of it toward the production wells, as shown in **Figure 3**. By water injection, it seeks layers of high permeability and since the oil is highly viscous as compared with injected water so, water will finger through oil resulting in decreased sweeping efficiency [5].

Mobility ratio (M) is defined as mobility of the displacing phase divided by the mobility of the displaced phase. Based on the principle of mobility ratio, water-soluble polymers reduce water mobility by two mechanisms: (1) increase the viscosity of the water phase and (2) reduce

#### Hydrophobic Polymers Flooding http://dx.doi.org/10.5772/intechopen.69645 81

**Figure 3.** Polymer flooding mechanism.

to copolymerize with acrylamide (AM) forming hydrophobically associating polyacrylamide (HAPAM), which has been widely used in enhanced oil recovery, drilling fluids,

Freedman et al. [34] reported the first synthesis of vinyl monomers [35], which also functioned as emulsifying agents [36, 31]. Typical polymerizable groups that have been exploited are vinyl, allyl, acrylate, methacrylate, styryl, and acrylamide [37]. The position of the polymerizable group either "H-type" where the polymerizable group located in the hydrophilic head group, or "T-type" where the polymerizable group located in the hydrophobic tail have

Wettability defined as the preferential affinity of the solid matrix for either the aqueous phase or the oil phase "the tendency of one fluid (wetting fluid) to spread on or adhere to a solid surface in the presence of another immiscible fluid (nonwetting fluid)" [40]. Reservoir wettability is an important and elusive petrophysical parameter in all types of core analyses, which affect saturation and enhanced oil recovery processes [41]. There is a consensus in petroleum engineering that preferentially water-wet cores flood more efficiently than oil-wet cores; since, more oil is recovered from water-wet cores in the early flooding stages than from oil-wet cores [42]. This can be attained due to the strong wetting preference of the rock for water yields the most efficient oil displacement and due to imbibition phenomenon and other complex interactions occurring in the reservoir during production [43]. Modified polymers can affect markedly mineral wettability. The copolymer of acrylamide-methacrylamido propyl trimethyl-ammonium chloride can alter surface of oil-wet quartz with adsorbed dodecyl amine into water-wet one. Where the polymer masking the surfactant layer on the quartz particle accounts for the water wettability [44]. Consequently, in the present study, the authors try to prepare a copolymer and a nanocomposite modified by silica addition to alter wettabil-

**5. Principle and mechanism of polymer flooding for enhanced oil** 

The polymer flooding process involves injection of polymer "slug" followed by continued long-term water flooding to drive the polymer slug and the oil bank in front of it toward the production wells, as shown in **Figure 3**. By water injection, it seeks layers of high permeability and since the oil is highly viscous as compared with injected water so, water will finger through

Mobility ratio (M) is defined as mobility of the displacing phase divided by the mobility of the displaced phase. Based on the principle of mobility ratio, water-soluble polymers reduce water mobility by two mechanisms: (1) increase the viscosity of the water phase and (2) reduce

a profound effect on the surfactant self-assembly and properties [38, 39].

coats, and paintings [33].

80 Application and Characterization of Surfactants

**4. Wettability of porous media**

ity of sandstone rock from oil-wet to water-wet.

oil resulting in decreased sweeping efficiency [5].

**recovery**

the relative permeability of water to the porous rock by adsorption/retention of the polymer in the rock pore throats [45], and thereby creating a more efficient and uniform front to displace unswept oil from the reservoir (i.e., the mobility ratio (M) is inversely proportional to the water viscosity. With a reduced mobility ratio, the sweep efficiency is increased and, as a consequence, oil recovery is enhanced [46].

## **6. Hydrophobically associating polyacrylamide polymers (HAPAMs)**

Hydrophobically associating polyacrylamide are prepared conveniently by micellar copolymerization of hydrophilic and hydrophobic monomers [47], or through grafting or incorporating hydrophobic chain cross-linking segments onto main chain of partially hydrolyzed polyacrylamide (HPAM). A lot of small molecule surfactants need to be added in order to enable the hydrophobic monomer to be solubilized into micelles, and the addition of small molecule surfactants brings many negative influences [30]. During HAPAM polymerization, hydrophilic surfmers dissolve in an aqueous phase resulting in homogeneous phase copolymerization of hydrophilic surfmers and acrylamide [48], which avoid drawbacks of surfactant addition. Moreover, above the critical micellar concentration (CMC) of surfmer, a microblock copolymerization mechanism carried out which means that a surfmer will be inserted into the backbone structure of acrylamide main chain, which gives rise to enhanced hydrophobic properties [49], stronger thickening property of HAPAM [50], and improved salinity resistance of HAPAM. As surfmer copolymerized with monomer and inserted in its main chain so surfmer separation from the polymer chain is prohibited [51]. These enhanced stability properties of polymers [52] have been reported for mechanical stability [53], electrolyte stability of the latex [54], a decrease of surfactant migration [51], and control of surface charge density [55]. Since surfactants are simply adsorbed onto the surface of particles in conventional emulsion polymerization, consequently increase emulsion stability by permanently fixing of the head groups. It has been long desired to obtain nanosized latexes containing higher polymer contents at lower surfactant concentrations [56]. Moreover, it was realized many years ago that polymerization of surfactants can lead to well-defined polymeric surfactants and potentially to polymerized micelles [57]. Recently, monomers composed of hydrophobic tail groups and hydrophilic head groups as well as a polymerizable group have been investigated [58]. Introduction of ionic groups contained in surfmer into polymer chains will improve the water solubility accompanying with perturbation of the hydrophobic association resulting in lowering the thickening effect [59]. The presence of phenyl group in the surfmer structure is well known to induce stronger van der Waals interactions than typical aliphatic groups due to their planar and polarizable structure, so the incorporation of one or more aromatic group(s) can stabilize hydrophobic associations involving the alkyl chain. Furthermore, the benzene rings can act as spacers, increasing the rigidity of polymer chains [60]. Consequently, incorporation of phenyl rings into the polyacrylamide (PAM) backbone through surfmer will improve its flooding characteristics in EOR applications. In addition, introduction of cationic groups into the PAM structure increases water solubility [61, 62] and decreases the water phase permeability (*K*w) as it flows through porous media, which improve oil recovery in oil-displacing applications [63]. Hydrophobic polymers have attracted much attention on both academic and industrial laboratories for polymer flooding in enhanced oil recovery [64, 65] owing to their unique characteristics [66] which can be summarized as follow;


In the present chapter, the authors try to overcome the shortage in chemical EOR candidates through synthesis of a novel surfmers (H-type) by the reaction of a 1-vinyl imidazole as a polymeric moiety containing double bond and 4-dodecyl benzene sulfonic acid surfactant, then hydrophobically associating polyacrylamide (HAPAM) prepared by free radical emulsion polymerization of acrylamide (AM) monomer, divinyl sulfone as a hydrophobic cross-linked moiety and surfmers, to chemically anchor a surfmer and hydrophobic cross-linker moiety onto the hydrophilic backbone of acrylamide chain. After that a hydrophobically associating polyacrylamides-SiO<sup>2</sup> (HAPAM-SiO<sup>2</sup> ) nanocomposite was prepared through copolymerization of acrylamide monomer with silica nanoparticles through one-shot synthesis. The rheological properties of copolymer solutions were investigated with respect to the polymer concentration, shear rate, shear stress, temperature, and salinity. Moreover, evaluation of behavioral characteristics and performance of these copolymers solution on wettability alteration, mobility ratio reduction, interfacial tension (IFT) reduction, and recovered oil amount under harsh reservoir are also reported [78, 79].

## **7. Experimental design and procedure**

head groups. It has been long desired to obtain nanosized latexes containing higher polymer contents at lower surfactant concentrations [56]. Moreover, it was realized many years ago that polymerization of surfactants can lead to well-defined polymeric surfactants and potentially to polymerized micelles [57]. Recently, monomers composed of hydrophobic tail groups and hydrophilic head groups as well as a polymerizable group have been investigated [58]. Introduction of ionic groups contained in surfmer into polymer chains will improve the water solubility accompanying with perturbation of the hydrophobic association resulting in lowering the thickening effect [59]. The presence of phenyl group in the surfmer structure is well known to induce stronger van der Waals interactions than typical aliphatic groups due to their planar and polarizable structure, so the incorporation of one or more aromatic group(s) can stabilize hydrophobic associations involving the alkyl chain. Furthermore, the benzene rings can act as spacers, increasing the rigidity of polymer chains [60]. Consequently, incorporation of phenyl rings into the polyacrylamide (PAM) backbone through surfmer will improve its flooding characteristics in EOR applications. In addition, introduction of cationic groups into the PAM structure increases water solubility [61, 62] and decreases the water phase permeability (*K*w) as it flows through porous media, which improve oil recovery in oil-displacing applications [63]. Hydrophobic polymers have attracted much attention on both academic and industrial laboratories for polymer flooding in enhanced oil recovery [64, 65] owing to

their unique characteristics [66] which can be summarized as follow;

precursors [70].

82 Application and Characterization of Surfactants

ates forming aggregates or micelles.

viscosity upon shear decreasing [72].

and tertiary oil recovery [77].

(1) In aqueous solutions, above a critical association concentration (C<sup>∗</sup>

bic groups develop intermolecular hydrophobic associations in nanodomains, leading to building up of a 3D-transient network structure [67] in high ionic strength medium, so providing excellent viscosity building capacity [68, 69], remarkable rheological properties, and better stability with respect to salts than the unmodified HPAM

(2) Reduce interfacial tension at the solid/liquid interface, since hydrophobic moiety associ-

(3) Shows an unusual adsorption isotherm [71] so can be considered as a wettability modifier. (4) Does not undergo mechanical degradation under high shear stress such as those encountered in pumps and near the well bore area, since the physical links between chains are disrupted before any irreversible degradation occurs, also they reform and retain their

(5) High resistance to physicochemical conditions (temperature, pH, and ion content) prevailing around the wells, so considered a prospective EOR candidate as thickeners or rheology modifiers in high-temperature, high-pressure reservoirs [73–75], reservoir stimulation [76],

In the present chapter, the authors try to overcome the shortage in chemical EOR candidates through synthesis of a novel surfmers (H-type) by the reaction of a 1-vinyl imidazole as a polymeric moiety containing double bond and 4-dodecyl benzene sulfonic acid surfactant, then hydrophobically associating polyacrylamide (HAPAM) prepared by free radical emulsion polymerization of acrylamide (AM) monomer, divinyl sulfone as a hydrophobic cross-linked

), their hydropho-

## **7.1. PROTOCOL 1: synthesis of polymeric surfactant (surfmer)**

The addition reaction was carried out in a 250 ml three-necked Erlenmeyer flask equipped with a reflux condenser, mechanical stirrer, and nitrogen inlet/outlet. Note that 0.106 mol of 1-vinylimidazole was added dropwise to 0.106 mol of 4-dodecyl benzene sulfonic acid in ethyl acetate (150 ml) in an ice bath under a N2 atmosphere. The reaction mixture maintained at 0°C for 2 h and then stirred for 12 h at 45°C. The white product was precipitated and recrystallized in 50 ml ethyl acetate upon cooling [78]. The yield was about 73%. The proposed structure is shown in **Scheme 1**.

#### **7.2. PROTOCOL 2: synthesis of HAPAM copolymer**

An aqueous solution of acrylamide in distilled water was gently bubbled with nitrogen gas for 30 min. The emulsion polymerization was carried out as previously reported in our literature [78] where designated reactants as listed in **Table 1** were added in a jacketed autoclave under an inert nitrogen environment for 12 h at 60°C. After reaction completion, viscous polymer gel was precipitated by acetone, redissolved in water, and reprecipitated in acetone then subjected to Soxhlet extraction with methanol for 24 h until a white solid obtained. The proposed structure is shown in **Scheme 2**.

**Scheme 1.** Structure of surfmer.


*Notes*: × Surfmer concentration is (3–45) times its CMC value. A; monomer (acrylamide) concentration, mol L−1. B; surfmer concentration, mol L–1. C; initiator (KPS) concentration, mol L−1. D; cross-linker (DVS) concentration, mol L−1. E; temperature, °C. F; pH-value. G; reaction time, h. H; deionized water, g.

**Table 1.** Reactants concentration and reaction conditions in the case of HAPAM.

#### **7.3. PROTOCOL 3: preparation of HAPAM-SiO2 nanocomposite**

After determination of optimum polymerization conditions and optimum reactants concentration, (3-aminopropyl) triethoxy silane was added in different molar ratios, as shown in **Table 2**. The polymerization procedure was carried out typically as shown previously in **PROTOCOL 2** with regarding addition of 3-aminopropyl triethoxy silane and KPS-initiator at the same time individually at reaction temperature of 60°C. The proposed structure is shown in **Scheme 3**.

**Scheme 2.** Structure of HAPAM copolymer.


**Table 2.** Reactants concentration and reaction conditions in the case of HAPAM-SiO<sup>2</sup> .

Further discussions of chemical synthesis and spectroscopic characterization of surfmer, HAPAM copolymer, and nanocomposite by means of FTIR, <sup>1</sup> H-NMR, <sup>13</sup>C-NMR, transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) as well as optimum polymerization conditions are reported in our previous literature [9, 78, 80]. Moreover, critical micelle concentration, surface excess concentration, and surface area of prepared surfmer and original surfactant indicate higher surface activity of prepared surfmer, which increases latex stability [78].

**Scheme 3.** Structure of HAPAM-SiO<sup>2</sup> nanocomposite.

**Scheme 2.** Structure of HAPAM copolymer.

**7.3. PROTOCOL 3: preparation of HAPAM-SiO2**

 8.44 × 10−1 1.69 × 10−2 3.03 × 10−3 5.68 × 10−3 50 1.69 3.38 × 10−2 6.07 × 10−3 1.14 × 10−2 60 3.38 6.75 × 10−2 1.21 × 10−2 2.27 × 10−2 65 6.75 1.35 × 10−1 2.43 × 10−2 4.54 × 10−2 70

84 Application and Characterization of Surfactants

temperature, °C. F; pH-value. G; reaction time, h. H; deionized water, g.

**Table 1.** Reactants concentration and reaction conditions in the case of HAPAM.

 **nanocomposite**

After determination of optimum polymerization conditions and optimum reactants concentration, (3-aminopropyl) triethoxy silane was added in different molar ratios, as shown in **Table 2**. The polymerization procedure was carried out typically as shown previously in **PROTOCOL 2** with regarding addition of 3-aminopropyl triethoxy silane and KPS-initiator at the same time indi-

*Notes*: × Surfmer concentration is (3–45) times its CMC value. A; monomer (acrylamide) concentration, mol L−1. B; surfmer concentration, mol L–1. C; initiator (KPS) concentration, mol L−1. D; cross-linker (DVS) concentration, mol L−1. E;

vidually at reaction temperature of 60°C. The proposed structure is shown in **Scheme 3**.

**Run # A** ×**B C D E F G H** 1 4.22 × 10−1 8.44 × 10−3 1.52 × 10−3 2.84 × 10−3 40 5.4 12 260 Rheological and solution properties were evaluated under simulated reservoir conditions as a function of polymer concentration and reservoir salinity, temperature, and shear rate. The results show good salt and temperature resistance, interfacial tension reduction and enhanced viscosity characteristics. The capability of polymer and nanocomposite to increase oil recovery was assessed through a linear packed sandstone model, as previously reported [78, 80]. Core flood tests were carried out under simulated reservoir conditions where a sand cleaning procedure, packing, flooding experiments, and recovered oil amount were discussed elsewhere [78, 80].

## **8. Rock wettability evaluation**

#### **8.1. Quantitative assessment**

Wettability was evaluated by measuring the contact angle between oil droplet and rock surface at temperature = 90°C and salinity = 40,000 ppm) and polymer solution concentration of 2 g L−1. The contact angles measured through a static sessile drop method for a spherical core plate for a period of 2 days as reported in **Figure 4**. After aging with crude oil for a day at elevated temperature, the plate was found to be oil-wet. The plate is then immersed in polymer/nanocomposite-brine solution under reservoir conditions, where oil droplet hanged on the plate lower surface and photographed for 48 h. Images are analyzed mathematically to calculate the contact angle. It is observed that advancing contact angle decreases with time and stabilizes at a value of about 74 and 68° in the case of HAPAM and HAPAM-SiO<sup>2</sup> , respectively [78]. Wettability alteration by can be explained on basis of (1) by polymer/nanocomposite adsorption on the rock surface, physicochemical properties altered, where thin wetting water film becomes unstable at the interface [81] and ruptured so, creating a continuous oil path for oil displacement which in turn increases oil recovery, (2) positively charged nitrogen bases adsorb on negatively charged sandstone rock surface at pH = 6, so wettability

**Figure 4.** Contact angle photograph after 48 h. (A) HAPAM copolymer and (B) HAPAM-SiO<sup>2</sup> nanocomposite.

changed from oil-wet to water-wet, (3) flood detergency improved by SiO<sup>2</sup> -nanoparticles addition leading to higher recoveries of residual oil, since associating polymer chain with nanoparticles enables a nanofluid to act as wetting agents, demulsifiers, and surface tension reducers at the very smallest of contact angles, which greatly enhances the removal of "foreign" materials such as oil, paraffin, and polymer residues, leaving the substrate water-wet. This is confirmed by reducing contact angle to nearly 74 and 68° in the case of HAPAM and HAPAM-SiO<sup>2</sup> , respectively [78].

#### **8.2. Qualitative assessment**

**Figure 5** shows qualitative evaluation of wettability through a two-phase separation test, it is shown that grinded sandstone grains are oil-wet as it is dispersed in the oil phase in the case of oil and brine solution, as shown in **Figure 5A**. While the sandstone grains sink into the aqueous phase of polymer solution in the case of polymer/nanocomposite, as shown in **Figure 5B** and **C**. This means that sandstone grains become water-wet. So, we can conclude that the polymer/nanocomposite able to alter wettability of the rock from oil-wet to water-wet so, improve recovered oil amount [78].

**Figure 5.** Two-phase separation test.

**Figure 4.** Contact angle photograph after 48 h. (A) HAPAM copolymer and (B) HAPAM-SiO<sup>2</sup>

Rheological and solution properties were evaluated under simulated reservoir conditions as a function of polymer concentration and reservoir salinity, temperature, and shear rate. The results show good salt and temperature resistance, interfacial tension reduction and enhanced viscosity characteristics. The capability of polymer and nanocomposite to increase oil recovery was assessed through a linear packed sandstone model, as previously reported [78, 80]. Core flood tests were carried out under simulated reservoir conditions where a sand cleaning procedure, packing, flooding experiments, and recovered oil amount were dis-

Wettability was evaluated by measuring the contact angle between oil droplet and rock surface at temperature = 90°C and salinity = 40,000 ppm) and polymer solution concentration of 2 g L−1. The contact angles measured through a static sessile drop method for a spherical core plate for a period of 2 days as reported in **Figure 4**. After aging with crude oil for a day at elevated temperature, the plate was found to be oil-wet. The plate is then immersed in polymer/nanocomposite-brine solution under reservoir conditions, where oil droplet hanged on the plate lower surface and photographed for 48 h. Images are analyzed mathematically to calculate the contact angle. It is observed that advancing contact angle decreases with time and stabilizes at a value of about 74 and 68° in the case of HAPAM and HAPAM-SiO<sup>2</sup>

respectively [78]. Wettability alteration by can be explained on basis of (1) by polymer/nanocomposite adsorption on the rock surface, physicochemical properties altered, where thin wetting water film becomes unstable at the interface [81] and ruptured so, creating a continuous oil path for oil displacement which in turn increases oil recovery, (2) positively charged nitrogen bases adsorb on negatively charged sandstone rock surface at pH = 6, so wettability

cussed elsewhere [78, 80].

86 Application and Characterization of Surfactants

**8.1. Quantitative assessment**

**8. Rock wettability evaluation**

nanocomposite.

,

## **9. Conclusion**

Improved oil recovery by polymer flooding involves injection of a mobility control agent (e.g., polyacrylamide and its hydrophobically associated derivatives) in order to displace the mobilized oil to the producing well, and improve seeping efficiency. In this chapter, the authors reported about synthesis of hydrophobically associating polyacrylamide (HAPAM) prepared by free radical emulsion polymerization and its modified nanocomposite derivative. Chemical structure of the prepared latexes was proven through different techniques such as FTIR, 1 H-NMR, 13C-NMR, scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction, while particle size and particle size distribution were characterized by dynamic light scattering (DLS) and thermal properties characterized by thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) as reported in our previous literature [78, 80]. Rheological properties were assessed in accordance with salinity and temperature tolerance, polymer concentration, and shear rates. Core flooding carried out via a linear pressurized packed model [9, 78–80]. Based on the experimental results, the following conclusions can be drawn:


In addition to the aforementioned aspects, and to the best of our knowledge, no polymers had previously reported to alter sandstone rock wettability, consequently the novel copolymer and nanocomposite considered as a promising candidates for EOR applications as a wettability-modifying agent in high-temperature and high-mineralization oil fields as compared to currently applied commercial polyacrylamides. On an industrial scale, we hope that a novel polymer applied as an EOR candidate to solve some of energy shortages as recovered oil amount reach to 26% from original oil in place (OOIP).

## **Author details**

**9. Conclusion**

88 Application and Characterization of Surfactants

(1) HAPAM-SiO<sup>2</sup>

erization reactions.

polymer flooding projects.

mer and HAPAM-SiO<sup>2</sup>

the following conclusions can be drawn:

(2) The prepared HAPAM copolymer and HAPAM-SiO<sup>2</sup>

FTIR, 1

Improved oil recovery by polymer flooding involves injection of a mobility control agent (e.g., polyacrylamide and its hydrophobically associated derivatives) in order to displace the mobilized oil to the producing well, and improve seeping efficiency. In this chapter, the authors reported about synthesis of hydrophobically associating polyacrylamide (HAPAM) prepared by free radical emulsion polymerization and its modified nanocomposite derivative. Chemical structure of the prepared latexes was proven through different techniques such as

H-NMR, 13C-NMR, scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction, while particle size and particle size distribution were characterized by dynamic light scattering (DLS) and thermal properties characterized by thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) as reported in our previous literature [78, 80]. Rheological properties were assessed in accordance with salinity and temperature tolerance, polymer concentration, and shear rates. Core flooding carried out via a linear pressurized packed model [9, 78–80]. Based on the experimental results,

shot synthesis via Aza-Michael addition reaction, so we can overcome shortages arising from agglomeration and coagulation of modified silica particles during emulsion polym-

property of retaining the viscosity and strong non-Newtonian behaviors (i.e., exhibit shear thinning behavior); so they can be considered as a promised EOR candidates for

(3) They respond to *in situ* reservoir stimuli (temperature, ionic strength, pH, and shear stress) also, show good thermal, rheological, and salt resistant properties even under res-

(4) They effectively reduce interfacial tension to ultralow values, so increase mobilization of

(5) Wettability assessment by a static sessile drop method indicates that the HAPAM copoly-

In addition to the aforementioned aspects, and to the best of our knowledge, no polymers had previously reported to alter sandstone rock wettability, consequently the novel copolymer and nanocomposite considered as a promising candidates for EOR applications as a wettability-modifying agent in high-temperature and high-mineralization oil fields as compared to currently applied commercial polyacrylamides. On an industrial scale, we hope that a novel polymer applied as an EOR candidate to solve some of energy shortages as recovered

which in turn will increase a recovery factor as there is a consensus in petroleum engineer-

nanocomposite can alter rock wettability from oil-wet to water-wet,

residual crude oil, which resemble the behavior of interfacial tension agents.

ervoir conditions, and consequently improve sweeping efficiency.

ing that water-wet reservoirs recover more oil than oil-wet ones.

oil amount reach to 26% from original oil in place (OOIP).

nanocomposite prepared by introducing silica nanoparticles through one-

nanocomposite had the perfect

Abdelaziz N. El-hoshoudy\*, Saad M. Desouky, Mohamed H. Betiha and Ahmed M. Alsabagh

\*Address all correspondence to: azizchemist@yahoo.com

Egyptian Petroleum Research Institute, Cairo, Egypt

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