Conductometric and tensiometric studies on the mixed micellar systems of surface-active ionic liquid and cationic surfactants in aqueous medium

    Conductometric and tensiometric studies on the mixed micellar systems of surface-active ionic liquid and cationic surfactants in aqueous medium Anwar Ali, Ummer Farooq, Sahar Uzair, Rajan Patel PII: DOI: Reference:

S0167-7322(16)31371-X doi: 10.1016/j.molliq.2016.08.082 MOLLIQ 6243

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

30 May 2016 19 August 2016 20 August 2016

Please cite this article as: Anwar Ali, Ummer Farooq, Sahar Uzair, Rajan Patel, Conductometric and tensiometric studies on the mixed micellar systems of surface-active ionic liquid and cationic surfactants in aqueous medium, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2016.08.082

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ACCEPTED MANUSCRIPT Conductometric and tensiometric studies on the mixed micellar systems of

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Anwar Ali 1,*, Ummer Farooq 1, Sahar Uzair 1, Rajan Patel 2

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surface-active ionic liquid and cationic surfactants in aqueous medium

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1 Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India 2 Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia (Central University), New

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Delhi 110025, India

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*Corresponding author:

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Prof. Anwar Ali

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Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi- 110025, India Tel: + 91- 11-26981717 Extn. 3257, Fax: +91 – 11- 26981232 E- mail: [email protected]; anwar–[email protected]

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ACCEPTED MANUSCRIPT ABSTRACT Physicochemical properties of aqueous surfactant solutions can be suitably modified by the

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addition of room temperature surface-active ionic liquids (SAILs). As green solvents, these

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SAILs are used as the ideal additives for modifying the aqueous surfactant properties. Such

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mixed surfactant + SAIL systems in aqueous solutions find enormous applications in several technological fields. The changes in the micellar behavior of cationic surfactants cetylpyridinium

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chloride (CPC) and cetylpyridinium bromide (CPB) have been investigated in the presence of

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SAIL 1-decyl-3-methylimidazolium chloride [C10mim][Cl] employing conductometric and tensiometric methods. Micellar and interfacial parameters such as critical micelle concentration,

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cmc micellar mole fraction, X1, of component 1 (CPC/CPB), micellar interaction parameter, β,

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activity coefficients ƒ1 and ƒ2 of component 1 and component 2 (SAIL) in the mixed micelles,

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0 excess Gibbs free energy of micellization, Gex0 , standard Gibbs free energy, Gmic , enthalpy,

0 0 H mic , and entropy, S mic of micellization, surface excess concentration, Гmax, minimum surface 0 area per molecule, Amin, and standard Gibbs free energy of adsorption Gad at the interface were

evaluated. In addition, packing parameters of amphiphiles in the micelles, P, volume contribution of the hydrophobic chain, v , and its effective length, l C have also been evaluated for the pure and mixed systems. The interactions between CPC/CPB and SAIL in the mixtures are found to be non-ideal and synergestic, and that mixed micelles are richer in CPC/CPB monomers. Of the several interactive interactions, hydrophobic interaction seems to be dominant between the components of the mixed systems. Adsorption of SAIL molecules at air-solution interface is found to be richer in SAIL than CPC/CPB molecules, which is also supported by the

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ACCEPTED MANUSCRIPT 0 0 higher negative Gad values than Gmic values. The micelles/ mixed micelles formed have

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spherical geometry. Keywords: Conductivity; Surface tension; Cationic surfactants; surface active-ionic liquid;

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Mixed system; Interactions.

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ACCEPTED MANUSCRIPT 1. Introduction Room temperature ionic liquids (ILs) are a class of molten organic electrolytes that are

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liquids at or near room temperature, they are composed of bulky organic cations with organic or

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inorganic anions. ILs have negligible vapor pressure, high ionic conductivity, outstanding

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catalytic properties, are stable up to very high temperatures (300 0C or more), are not flammable, can easily be recycled for continued use, and are easy to handle [1,2]. These unusual and

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interesting properties make ILs highly desirable, and environmentally friendly green solvents.

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Because of their unique properties, ILs are highly preferred solvents over volatile and toxic organic solvents, and have been extensively used in chemical synthesis, electrochemistry,

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separation processes, and as electrolytes in batteries and capacitors [2,3]. Recently, imidazolium

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based surface-active ionic liquids (SAILs) such as 1-alkyl-3-methylimidazolium halides, due to their amphiphilic nature form aggregates (micelles) in aqueous solution, find wide range of

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applications in various fields, have emerged as novel solvents and have been a subject of growing interest among scientific and academic communities [4,5]. Interestingly, the structure of imidazolium ring resembles with many biologically important molecules such as amino acid, histidine, that have an imidazole side chain and play important role in the structure and binding functions of hemoglobin [6]. Conventional surfactants particularly cationic alkyl pyridinium halides and their derivaties are used as corrosion inhibitors for steel protection [7], mouth wash due to their antibacterial properties [8], quenchers in fluorescence spectroscopy [9], in separation techniques like ultrafiltration [10], and have recently attracted more attention with reference to their interaction with DNA and lipids [11]. For many practical applications micelles of the amphiphilic molecules play important role [12,13]. For example, micelles are able to solubilize the organic compounds which are poorly soluble in water by incorporating them in the miceller 4

ACCEPTED MANUSCRIPT phase, due to large surface area micelles are conveniently exploited to act as catalysts for many chemical reactions, can alter the reactions pathways, rates and equilibria [14,15], they are also

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used in the synthesis of nanomaterials [16], pharmaceutical formulations, and in drug delivery

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[17]. That is why surfactant micelles have been widely investigated to clarify their structure-

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function correlation as well as their applications in many colloidal domains [18,19]. Physicochemical properties of an aqueous surfactant solution depend on the nature of the

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surfactant, remain almost fixed at ambient conditions and at given concentration, and are

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difficult to modify [20]. But, for many practical applications, surfactants used are usually multicomponent rather than single surfactant in aqueous solutions [20,21]. Therefore,

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modifications/ alterations in the properties of an aqueous surfactant solution are often tailored to

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the needs of specific application. The usual way to modify the properties of a given aqueous surfactant solution is to use external additives/modifiers (e.g. polar organics, cosolvents,

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electrolytes, cosurfactants, etc) [1,2,13,20-22]. ILs are environmentally benign solvents, have ability to design and tune the physicochemical properties of aqueous surfactant solutions, and as additives have attracted wide attention of academic and industrial research communities [1-4, 22]. Extensive literature survey indicates that considerable interest has been shown by several researchers to investigate the micellar behavior of surfactants in imidazolium based ILs. Fletcher and Panday [1] have reported the aggregation of nonionic surfactants (Brij-35, Brij-700, Tween20, and Triton X-100), while no aggregation was observed for the cationic surfactant cetyltrimethylammonium bromide (CTAB) in ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulphonyl) imide [C2mim][Tf2N]. Pandey’s group has studied the interactions of ILs 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] and hexafluorophosphate [C4mim][PF6] with zwitterionic surfactant [20] and IL [C6mim][Br] with CTAB [22], using 5

ACCEPTED MANUSCRIPT solvatochromic probe method. Sharma et al. [2] have studied the interactions of IL [C14mim][Br] with

cationic

surfactants

tetradecyltrimethylammonium

bromide

(TTAB)

and

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dimethylditetradecylammonium bromide (DTDAB), and observed that the interactions for

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[C14mim][Br]+ TTAB mixtures were non-ideal and antagonistic while with the double chain surfactant (DTDAB) the interactions were synergistic. Recently, Zhang et al.[23] have reported

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the interaction between the long chain IL [C12mim][BF4] with nonionic surfactant Triton X-100

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in aqueous solution by using surface tension, electrical conductivity, 1H NMR and FF-TEM measurements; Inone and co-workers [24] studied the aggregation behavior of nonionic

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surfactants in ILs [C2mim][BF4] and [C6mim][BF4] by means of 1H NMR and dynamic lightscattering measuremens, and reported that the surfactants are highly solvophilic to the latter IL,

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while highly solvophobic to the former IL; Pal and Chaudhary [25] employed conductometric,

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fluorescence and 1H NMR techniques to study the interaction of ILs [C5mim][Br] and

of

cationic

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[C7mim][Br] with anionic surfactant SDS. Recently, Chabba et al. [26] reported the interactions ILs

[Cnmim][Cl]

(n=8,10,and12)

with

anionic

surfactant,

sodium

dodecylbenzenesulfonate (SDBS) in aqueous medium using tensiometry, steady state fluorescence, DLS, and small angle neutron scattering (SANS). These authors suggested strong synergistic interactions between cationic ILs and SDBS. Javadian and co-researchers [27] investigated the aggregation behavior of the cationic surfactant CTAB with ILs [C6mim][Br] and [C4mim][BF4] using tensiometry, conductometry, TEM, and cyclic voltametry (CV) methods, and demonstrated that ILs have significantly altered the physicochemical properties of aqueous CTAB.

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ACCEPTED MANUSCRIPT Inspite of the fact that a lot of work has been reported on the effect of imidazolium based ILs on the micellization and surface behavior of cationic surfactants, to the best of our

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knowledge reports on the effect of SAIL 1-decyl-3-methylimidazolium chloride [C10mim][Cl] in

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altering the physicochemical properties of cationic surfactants, cetylpyridinium chloride (CPC) and cetylpyridinium bromide (CPB) have not been reported in the literature. These

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considerations led us to investigate the effect of SAIL [C10mim][Cl] on the physicochemical

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properties of CPC and CPB in aqueous solutions. In addition to the experimental study, it would be equally important to predict the physicochemical properties of the mixed systems CPC/CPB

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+[C10mim][Cl] by using theoretical models. Clint [28] proposed pseudo-phase separation model, according to which, above the cmc, the micelles are considered as macroscopic separate pseudo-

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phase and are in equilibrium with the monomers in the solution. Clint’s ideal mixing model successfully predicts the properties of the mixed systems composed of surface-active

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amphiphilic molecules having similar structures. However, when applied to the mixed systems of dissimilar structures, it failed, as the model neglects interaction among the entities in the micellar state [2,29,30]. The model developed by Rubingh [31] and Rosen et al. [32] using regular solution theory in conjunction with the pseudo-phase separation model, is the most widely used theory for the study of synergism and antagonism of non-ideal mixed micelles or adsorbed films [29]. Although, in the recent past, there have been a lot of studies on the binary mixed systems composed of conventional surfactants, no theoretical study using Clint’s and Rubingh models has been reported in the literature on the mixed systems composed of SAIL [C10mim][Cl] and surfactants CPC/CPB in aqueous solutions. However, Mahajan’s group has successfully demonstrated the importance of theoretical study on cationic SAIL [C14mim][Br] with cationic

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ACCEPTED MANUSCRIPT surfactants TTAB/ BTDAB [2] and cationic SAILs [Cnmim][Cl] (n= 8, 10, and 12) with anionic surfactant SDS [26] in aqueous solutions employing the above models.

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In this paper, the experimental results have been coupled with the theoretical results in order to

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understand the complete mechanism by which the cationic SAIL [C10mim][Cl] modifies the bulk

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and interfacial properties of the cationic surfactants CPC and CPB in aqueous solutions. Various micellar and interfacial properties such as cmc, α, Gm0 , H m0 , S m0 , Гmax, Amin , P ,  , etc. have

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been estimated. The experimental techniques employed are conductivity and surface tension. The

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principal merit of the electrical conductivity measurement compared to many other physicochemical techniques is that it is much more precise and accurate in recording the bulk

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properties of the ionic solutions [4,15,33]. On the other hand, surface tension measurement is a

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valuable tool, it reveals informations about the forces operating on the air-solution interface and,

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hence, provides informations about the structure of the interface [2,3]. The study will help in analyzing and comparing the experimental and theoretical results to explain the synergism and antagonism of the mixed systems together with the energetics of the process, and the factors such as the length and type of the hydrophobic chains, polar head groups, nature of the counterions, steric factors among their non-polar and polar groups that affect the mutual interactions between the SAIL and surfactant molecules on the air-solution interface and in the bulk solution in the mixed systems. Thus, understanding of the mechanism of such interactions will enhance our knowledge about the interfacial and bulk properties of the present binary mixed systems, and their subsequent practical applications.

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ACCEPTED MANUSCRIPT 2. Experimental

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2.1. Materials and methods The cationic surfactants, cetylpyridinium chloride (CPC) (Batch No. T-8361484, 99 %, Sisco

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Research Laboratories, India) and cetylpyridinium bromide (CPB) (Batch No. T-8321102, 98 %,

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Sisco Research Laboratories, India) were recrystallized from ethanol (CAS No. 64-17-5

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,99.9 %, S.D.Fine Chem. Ltd., India) and were dried in a vaccum desiccators. SAIL, 1-decyl-3methylimidazolium chloride [C10mim][Cl] (CAS No. 171058-18-7, ≥97 %, Aldrich, USA) was

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dried at 70 0C for 2-3 days to remove water content immediately prior to its use. Karl Fischer analysis of [C10mim][Cl] using a karl Fischer titrator (Model No. MA-101-13, SPECTRALAB,

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India) indicates water content less than 600 ppm. Stock solutions of SAIL and CPC/CPB were

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prepared about ten times higher than the cmcs of pure components by dissolving requisite

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amounts of SAIL and surfactants in double distilled deionized water having specific conductivity less than 1.5×10-6 S cm-1 and pH ~ 6.99 at 298.15K. The solutions were prepared with utmost care and were stored in ambered glass viols in inert atmosphere to avoid moisture absorption and contamination. All the glasswares were dried in oven so that traces of moisture, if any, are removed. The different mole fractions of the mixtures of CPC/CPB+ [C10mim][Cl] were obtained by mixing appropriate volumes of the stock solutions using Biosystem micropipette, accuracy up to ± 1×10-3 mL. The weighings were done on precisa XB-220 A (Swiss-make) electronic balance with a precision of ± 0.1mg. The temperature of the samples was maintained by an electronically controlled thermostated water bath (Julabo, Model MD, Germany) within ± 0.02 K.

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ACCEPTED MANUSCRIPT 2.2. Conductivity measurements The conductivities of the samples were measured using a digital conductivity meter, PICO+

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(Labindia Instruments, Pvt. Ltd.) operating at 50 Hz. Prior to use conductivity meter was

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calibrated at atmospheric pressure and at experimental temperatures by measuring the

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conductivities of 0.01 and 0.10 N solutions of KCl (E. Merck, Purity > 99%). The cell constant (1.007± 0.001 cm-1) of the conductivity cell was measured according to the method of Fuoss et

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al. [34]. The conductivity cell with two square–shaped platinum electrodes sealed with glass was

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dipped in a corning glass tube containing the sample solution. The glass tube containing sample solution and conductivity cell was properly sealed and immersed in thermostated water bath. The

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conductivities of the sample solutions were recorded after 30 minutes to ensure that the solution

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attained thermal equilibrium. In each case, a minimum three conductivity readings were taken and mean values were used in all the calculations. Using standard techniques, the combined

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expanded uncertainty at the 95% confidence level (coverage factor k=2), in the measurement of conductivity is estimated to be 2%. 2.3. Surface tension measurements Surface tension measurements of aqueous solutions of [C10mim][Cl], CPC, CPB, and their mixtures were made with a high precision Delta- Pi Langmuir microtensiometer ( Kibron, Helsinki, Finland) by Du- Nouy- Padday method, Padday et al. [35], at 298.15, 308.15, and 318.15 K and atmospheric pressure. The temperature of the measurement cell was controlled by a Grant GD120 water thermostat with temperature stability ± 0.02 0C. The tensiometer uses a small diameter (0.51 mm) special alloy wire for the measurement which was cleaned by red hot burning from butane gas through blazer. It has a dynamic range of 0 to 300 mN.m -1 and a

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ACCEPTED MANUSCRIPT resolution of 10 µN.m-1. It has automatic monitoring and display of temperature in the course of the experiment. The surface tension of pure or mixed samples was recorded after thorough

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mixing and thermal equilibration. The calibration of the tensiometer was carried out by

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measuring the surface tension of pure water and methanol.

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In all the cases, at least three successive measurements were carried out until the values are reproducible. The accuracy of the surface tension measurement was ascertained by measuring 

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values of pure water and methanol, and were found to be 72.0 and 22.3 mN.m-1, respectively at

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298.15 K, which compare well with the literature values [36]. The combined expanded uncertainty at the 95% confidence level (k=2) in the measurement of surface tension is estimated

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3. Results and discussion

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to be 1%.

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3.1. Critical micelle concentration from conductance and surface tension measurements The representive plots of specific conductance,  , and surface tension,  with concentration of pure [C10mim][Cl], CPC, and CPB have been shown in Supplementary Figs. S1 and S2, those of CPC/CPB + SAIL mixtures at different mole fractions of CPC/CPB and temperatures in aqueous solutions are displayed in Figs. 1-4. The plots clearly show an abrupt change in  and  over a narrow concentration range, termed as the critical micelle concentration (cmc). Below cmc, the amphiphilic molecules are unassociated, behave as 1:1 electrolyte and obey DebyeHuckel- Onsager equation [14,33], while above the cmc the surfactant molecules aggregate and form micelles [12,21]. The sharp decrease in the conductivities of pure SAIL, CPC, CPB and their mixtures above the cmc is due to the sharp increase in mass of aggregates with net decrease in the surface charge density of the material as a result of the formation of the micelles. On the 11

ACCEPTED MANUSCRIPT other hand, the value of  decreases with increase in the concentration of SAIL, CPC, and CPB and their mixtures up to a certain concentration after which the surface tension remains almost

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constant. The break point in  vs. concentration plot is taken as the cmc of the pure and mixed

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surfactants. The decrease in  for the pure and mixed aqueous solutions in the pre-micellar

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concentration range is due to the fact that the hydrophobic groups of SAIL and surfactant molecules avoid contact with the polar water molecules, adsorb at the air-solution interface till

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the interface is completely occupied. However, with further increase in the concentration, the

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molecules form aggregates (micelles) in the solution, and the surface tension becomes almost constant beyond the cmc [18]. By utilizing these typical properties, namely, adsorption at the airsolution interface and formation of micelles, the surfactant molecules lower the free energy of

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the solution [12,13,18]. It should be noted that the observed limiting values of  of the studied surfactants and SAIL lie in the range from 40 to 28 mN. m-1, which are in the excellent

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agreement with the reported range 40 to 30 mN. m-1 of limiting  values, depending on the nature of the surfactant [18]. The cmc values obtained for the pure components SAIL, CPC, CPB and their mixtures in aqueous solution using conductance and surface tension measurements are summarized in Tables 1 and 2. It is clear from the tables that the values measured by surface tension slightly differ with those determined by the conductance method. Because of the fact that different methods sense different steps of micellization process, it is well documented that the cmc is a method depend property and so are the other micellar properties [11,37]. The experimental values of the cmc determined by conductivity method are in good agreement with the corresponding values reported in the literature [5,6,38-41], as shown in Table 3.

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ACCEPTED MANUSCRIPT It is well established that the cmc decreases with increase in the hydrophobicity or in other words with increase in the number of carbon atoms in the hydrophobic tail, and a general

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rule for the ionic surfactants or SAIL is that the cmc is halved by the addition of one methylene

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group (-CH2) to the straight-chain hydrophobic group [12]. For example, the observed cmc (58.80 mM) for [C10mim][Cl] in this work and the literature values 13.47, 3.68, and 0.86 mM

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for [Cnmim][Cl] (n= 12,14 and 16) [42], respectively, at 298.15K, truly endorse the above rule

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and strongly confirm the dependency of cmc on the number of carbon atoms in the hydrophobic group of the surface active ILs and also for surfactants CPC and CPB [12]. The decrease in cmc

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with increase in the number of carbon atoms in the hydrophobic group is attributed to the increased hydrophobic–hydrophobic interactions, resulting in early micelle formation, hence,

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decreased cmc values. Although, hydrophobic interaction between hydrophobic groups of monomers is the major cause of micelle formation, the nature of head-group also plays

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significant role on the process of micellization. As the head- groups of CPC and CPB are more hydrophobic than the imidazolium ring of [C10mim][Cl] [42,43], micellization of the first two surfactants will be more favored than SAIL. This will result in lowering the cmc values for CPC and CPB than SAIL, this is evident from the cmc values reported in Tables1and 2. Further, the basis for the rule that the cmc of ionic surfactants is halved for each increase in the hydrophobic chain by one carbon atom is the empiral equation proposed by Klevens [44] and confirmed by others [12,45]. According to this equation, there is logarithmic decrease in the cmc as the number of carbon atoms, n, in the hydrophobic chain of a homologous series increases, as follow: log cmc = A- B n,

(1)

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ACCEPTED MANUSCRIPT where A is a constant for a particular ionic head at a given temperature and B is an other constant equal to 0.30 at 35 0C for the ionic types of surfactants [12]. The micellization process, in

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general, depends on two factors: (a) hydrophobic-hydrophobic interaction between the

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hydrophobic tails of the component molecules and (b) electrostatic interaction between the charged head groups of the unlike molecules. The combined effect of these two interactions

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decides the ease with which the micelles are formed in the mixed surfactant. In the present study,

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the head groups of the cationic surfactants CPC and CPB as well as of SAIL in the mixed systems CPC/CPB +SAIL are positively charged, electrostatic repulsion between them will try to

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destabilize the micelle and micelle formation is opposed. It is, therefore, the hydrophobic interaction which dominates over the electrostatic repulsion between [C10mim][Cl] and

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CPC/CPB molecules and favors micelle formation. From the Tables 1 and 2 it is clear that there is a sharp decrease in the cmcs with increase in the concentration of CPC and CPB in the mixed

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CPC/CPB + SAIL systems, and that the cmcs of the mixtures lie in between the cmcs of the pure components at each studied temperature. This is attributed to the increased hydrophobic interaction between the hydrophobic tails of SAIL and CPC/CPB molecules as the amount of CPC/CPB increases in the mixtures. Also, as the concentration of CPC/CPB increases that of SAIL decreases which, in turn, helps in reducing the electrostatic repulsion between the cationic head groups of CPC/CPB and SAIL, this further facilitates the formation of micelles and, hence, decreases the cmc of the mixed system. A comparison between CPC+SAIL and CPB+SAIL mixtures indicates that the cmc values of the latter mixture are lower than the former one. This can be explained by considering the effect of the counterions on the cmcs of the mixtures CPC/CPB + [C10mim][Cl]. In general, for a given hydrophobic group and cationic head group, the cmc is found to decrease with increase in the radius of the counterion [12,27,39], because

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ACCEPTED MANUSCRIPT bigger the counterion, more difficult is its hydration and stronger will be its binding to the oppositely charged ionic head groups of the micelle. This will decrease mutual repulsion

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between the ionic head groups, resulting in enhanced hydrophobic interaction between the nonpolar groups of the mixed surfactants, a decrease in the cmc is expected. Accordingly, in the

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present study, because the ionic radius of the counterion Br– is larger than Cl– ion, it is weakly

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hydrated and, hence, binds to the surface of the micelle more strongly than Cl – ion. The observed

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smaller values of cmc of CPB + [C10mim][Cl] than those of CPC + [C10mim][Cl] mixed surfactant (Tables 1 and 2) clearly support the above view. An other convenient way to examine

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the dependency of cmc on the nature of counterion and head group of the surfactant is to employ Collins model [46]. According to this model, ions are either kosmotropes or chaotropes, the

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former have high charge density hence strongly hydrated whereas the latter have lower charge density and are weakly hydrated, and interaction between similar ions is stronger than between

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unlike ions. Pyridinium head groups of CPC and CPB molecules behave as chaotrope due to their large surface area and low charge density [47]. In the present study, counterion Cl– is more kosmotropic than Br–, therefore, weakly binds with the chaotropic pyridinium head group, resulting in the higher cmc values of CPC than those of CPB. The effect of temperature on the cmc of surfactants in aqueous medium is interesting, in the low temperature range it first decreases with increase in temperature to a minimum value around 298.15 K for ionic surfactants [12,48] and around 323.15 K for nonionic surfactants [12,49] due to decrease in the hydration of the ionic head group which enhances hydrophobicity of the surfactant and hence lowers the cmc. Whereas, in the higher temperature range above the mentioned temperatures, breakdown of the structured water surrounding the hydrophobic groups occurs, which results in the decreased hydrophobic interactions, leading to an increase in the cmc [12,21,48]. The increase in cmcs of

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ACCEPTED MANUSCRIPT SAIL, CPC, CPB, and their mixtures (Tables 1 and 2) in the investigated temperature range

3.2. Mixed micellar composition and interaction parameters

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clearly suggests the dominance of the second effect over the first one.

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The mixed solutions of surface active IL and surfactants may be ideal or non-ideal depending

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on the nature of the component molecules. The cmc of mixed CPC/CPB + SAIL systems can be

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evaluated from the cmc1, and cmc2 of the component 1 (CPC/CPB) and component 2 (SAIL) respectively, using the following equations [30,31,50]:

(2)

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n i 1  cmc i 1 f i cmci

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where  i is the total mole fraction of the component i in the solution and f i is the activity

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coefficient in the mixed micelle. In the case of ideal mixing (with no net interaction), f i = 1, Eq. (2) takes the following form, known as Clint equation [28]:

 (1  1 ) 1  1  cmc * cmc1 cmc2

(3)

where cmc* is the ideal state mixed cmc. It is clear (Tables 1 and 2) that the experimental values of cmc at different mole fractions of CPC/CPB +SAIL mixtures are lower and show negative deviations (Figs. 5 and 6) from the ideal cmc* values predicted by Clint model based on ideal mixing, indicating the non-ideal behavior of the mixed systems. This indicates that micelles are formed at lower concentration than predicted from the ideal mixing, which, in turn, suggests that interactions between CPC/CPB and SAIL are favorable for the formation of mixed micelles. Furthermore, applying the Motomura′s theory [51], which is based upon the excess

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ACCEPTED MANUSCRIPT thermodynamic quantities, the ideal value of the mole fraction of CPC/CPB in the mixed micelle

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1cmc2 1cmc2  (1  1 )cmc1

(4)

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 ideal 

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(Xideal) was evaluated using the following equation:

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A clear understanding and quantitative interpretation of the interactions between CPC/CPB

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and SAIL in the mixed micelles showing non-ideal behavior can be made by using regular solution theory based upon the pseudo-phase separation model [31,32,52]. Hence, the following

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equations can be employed to calculate the micellar mole fraction, X1, of component 1 in the

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SAIL in the micelles:

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mixed micelles and micellar interaction parameter, β, between the monomers of CPC/CPB and

(5)

(6)

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ACCEPTED MANUSCRIPT The computed values of cmc*, Xideal, X1, and β are listed in Tables 1 and 2. These tables reveal that the values of both Xideal, and X1 increase as the mole fraction of CPC or CPB increases at

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each studied temperature. The increase in X1 with an increase in αCPC/CPB suggests that mixed

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micelle formation is favored and that more molecules of CPC/CPB contribute to the mixed

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micelles. The values of Xideal are greater than X1, indicating that the mixed micelles are rich in CPC or CPB monomers in comparison to the ideal mixed state. The nature and extent of

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interaction between the components in the mixed micelles can be conveniently studied using the interaction parameter, β. Generally, a negative value of β indicates synergistic interaction, i.e.,

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upon mixing, there is stronger attraction between the components than before mixing whereas a positive β value is attributed to antagonistic interaction, i.e., weak interaction, its value close to

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zero indicates no interaction hence an ideal behavior on mixing [12,52]. The observed values of β (Tables 1 and 2) for both the mixed systems CPC+SAIL and CPB+SAIL are negative at each

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studied temperature, become more negative as the mole fraction αCPC/CPB in the mixtures increases. This indicates increased attractive interactions between CPC/CPB and SAIL molecules, suggesting synergism in the mixed system. This is expected, the hydrophobic interaction between the longer hydrophobic tails of CPC/CPB and SAIL becomes increasingly stronger as the mole fraction of CPC/CPB in the mixed systems increases. The above fact is also supported by negative deviation of the cmc from cmc* (Figs. 5 and 6). The negative values of β and │ β│> │ln (cmc1/cmc2) │, clearly suggest the synergism in the mixed systems [12,53]. As mentioned above, though the major contribution to negative β values is due to the strong hydrophobic interactions between the CPC/CPB and SAIL molecules, attractive interactions involving imidazolium cation of SAIL and π-electrons of pyridinium rings of CPC/CPB, ringstacking through π-π interactions between π-electrons of aromatic rings of component 18

ACCEPTED MANUSCRIPT molecules, and H-π interactions between the most acidic proton at C-2 of the imidazolium ring and π-electrons of the pyridinium rings of CPC and CPB also play significant role in making β

PT

negative for the studied mixed systems [26].

RI

The activity coefficients ƒ1 of CPC/CPB and ƒ2 of SAIL in the mixed micelles were

SC

calculated with the help of the following equations based on the regular solution theory



2



(7)

(8)

D

f 2  exp 1



MA



f1  exp  (1  1 )2

NU

developed by Rubingh [52]:

TE

The values of ƒ1 and ƒ2 are included in Tables 1 and 2. Both ƒ1 and ƒ2 are less than unity for all

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the mole fractions, αCPC/CPB, for both the mixed systems CPC/CPB+SAIL at each studied temperature. This again confirms the non-ideal behavior of the mixed systems, and attractive interaction between CPC/CPB and SAIL are stronger than between individual components. Now, it can be concluded that negative deviations of the experimental cmc from ideal value cmc*, large negative values of β, and the values ƒ1 and ƒ2 less than unity clearly suggest the synergestic behavior of the studied systems. Synergestic behavior, i.e., non-ideality of cationic SAIL [C14mim][Br] + cationic surfactant [DTDAB][2] and cationic +cationic mixtures (CTAB+CPC [54] and CTAB+CBZC [55]) composed of conventional surfactants for which (cmc < cmc*, β < 0, and 0 < (ƒ1 and ƒ2 ) < 1) have also been reported by others.

19

ACCEPTED MANUSCRIPT The values of ƒ1, ƒ2, and X1 are utilized to evaluate the excess Gibbs free energy of micellization, Gex , for the mixed systems using the following equation[2,56]: (9)

RI

PT

Gex  RT X 1 ln f1  (1  X 1 ) ln f 2 

SC

As mentioned earlier, for ideal mixing, ƒi = 1 (i= 1 and 2), Gmic becomes zero, i.e., there is no net interaction between the component molecules. However, for non-ideal systems, as in the

NU

present case, ∆Gex ≠ 0. It′s values (Tables 1 and 2) for both the mixed systems are negative at

MA

each temperature over the entire mole fraction range, αCPC/CPB, become increasingly more negative as the concentration of CPC/CPB in the mixed systems increases. This further confirms

D

that the mixed micelles are thermodynamically more stable than the micelles of the individual

TE

components. Moreover, the stability of the mixed micelles increases with the increased concentrations of CPC/CPB in the mixtures, again reinforcing the view based upon the

AC CE P

interaction parameters.

3.3. Thermodynamics of micellization 0 0 Thermodynamic parameters such as standard Gibbs free energy, Gmic , enthalpy, H mic , and

0 entropy, S mic of micellization provide an understading of the mechanism of the process of

micellization and the factors that affect this mechanism. Because of the fact that for the determination of thermodynamic properties of micellization and to have a clear understanding of the mechanism of almost all transport and equilibrium measurements suitable models are frequently employed [57]. We have used here the pseudo-phase separation model [21,49] for the calculation of the said thermodynamic properties for the micellization of CPC/CPB + [C10mim][Cl] mixed systems in aqueous solutions. This model considers micelle formation as a 20

ACCEPTED MANUSCRIPT phase separation phenomenon, micellar phase is in its standard state and constitutes new phase, and the concentration of free amphiphilic monomers is assumed to be equal to the cmc.

PT

0 Accordingly, Gmic for the ionic surfactants and their mixed systems is calculated by using the

RI

equation [12,21,39,50]:

(10)

SC

Gm0  (2  g ) RT ln X cmc

NU

where Xcmc is the cmc in mole fraction unit since it is more accurate to use mole fraction in place of cmc (mol. dm-3) in the evaluation of the energetics of micellization process [39], R and T are

MA

the gas constant and temperature and g is the counterion dissociation, obtained from the ratio of the slopes of the two intersecting straight lines in the post- micellar region (S2) to that in the pre-

TE

D

micellar region (S1) of the conductivity vs molarity plots [12,21]. The quantitative estimation of g was carried out by Buckingham et al. [58]. Earlier, the suitability of the method in estimating g

AC CE P

was verified by Kale and co-workers [59] and also by Bandhopadhyay and Moulik [60], who have obtained the values of g using ion-selective membrane electrodes and reported that there was good agreement with those estimated conductometrically. Like cmc, g is also found to be experimental technique dependent [21] and both cmc and g are affected by the size of the anion involved. For example, as stated above, Br– ion being larger in size than Cl– ion, is poorly hydrated and thus binds to the micellar surface stronger than Cl– ion, this results in lower g (0.36, 0.46 and 0.48) values for Br– ion than for Cl– ion (0.44, 0.47, and 0.50) at the studied temperatures. Applying the same reasoning Javadian et al. [27] have shown that for CTAB + [C4mim][Cl] / [Br] / [BF4] mixed systems, with common cationic part [C4mim]+, the value of g decreases in the sequence: Cl– (0.27) > Br– (0.25) > BF 4 (0.21) ; BF 4 being the largest anion shows strongest binding to the micellar surface and hence lowest g value than Cl– and Br– ions.

21

ACCEPTED MANUSCRIPT The standard enthalpy of micellization, ∆H0mic, is obtained by using the Gibbs- Helmholtz

(11)

SC

  RT 2 2  g  .  ln X cmc T P  ln X cmc g T P 

RI

PT

equation:

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The values of  ln X cmc T P  and g T P  were obtained by fitting the ln X cmc and g values

MA

to the following polynomial equations, as described elsewhere [39]:

(12)

D

ln X cmc  a  bT  cT 2

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g  a I  bIT  cIT 2

TE

and

(13)

where a, b, c, a I , b I , and c I are the repective polynomial constants and have been determined by least-squares regression analysis. As the term

g

T P  is very small over the investigated

temperature range compared to the first term on the right hand side of Eq. (11), it can be neglected. Thus the entropy of micellization, ∆S0mic can be obtained from the known values of 0 0 and H mic using the equation: Gmic

S m0 

1 0 0   H mic  Gmic T

(14)

22

ACCEPTED MANUSCRIPT 0 0 0 The values of Gmic , H mic and S mic , along with the values of g, for the pure and mixed

systems CPC + SAIL and CPB + SAIL are listed in Tables 4 and 5, respectively. It should be

PT

noted that g values are higher for pure CPC than those for CPB at each investigated temperature.

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This is ascribed to the smaller size of Cl– ion than Br– , the former ion being more hydrated than the latter one, gets weakly adsorbed on the oppositely charged micellar surface. As a result,

SC

ionization of Cl– ion is relatively more favored as compared to that of Br – ion, yielding higher g

NU

values for CPC than for CPB (Tables 4 and 5). Furthermore, it is interesting to see that the degree of counterion dissociation, g, tends to increase as the concentration of SAIL increases in

MA

the mixed systems. Increase in the concentration of ionic liquid increases the dielectric constant of the medium [27] which enhances the counterion dissociation from the mixed micelles, hence,

TE

D

0 increased g values. In general, for ionic surfactants, the values of Gmic are reported to lie in the

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0 range from -23 to -45 kJ.mol–1 at 298.15K [12,48]. The observed Gmic values for the pure

components CPC, CPB, SAIL and their mixed systems lie in the same range at the corresponding 0 temperature. The overall uncertainty in Gmic was estimated to be less than 2.0%. It would be 0 useful to compare the values of Gmic of pure components with those available in the literature.

0 Table 4 shows that Gmic (-42.0 kJ. mol–1) of CPC duplicates well with its value -42.0 kJ. mol–1 0 [39] at 298.15 K; that for [C10mim][Cl], the observed Gmic (-24.0 and -22.0 kJ. mol–1) are close

to the reported values (-25.16 and -23.38 kJ. mol–1) [6] at 298.15 and 308.15 K, respectively. The 0 standard free energy of micellization, Gmic , is negative for the pure components as well as for

the mixed systems (Table 4 and 5) over the entire composition and temperature range considered, indicating that micelle / mixed micelle formation is thermodynamically spontaneous. 0 It is further emphasized that a marked decrease in Gmic , like cmc, of the mixed systems is

23

ACCEPTED MANUSCRIPT observed with the increase in the concentration of CPC and CPB in the mixtures. This is attributed mainly to the increased hydrophobicity of the mixed systems as the concentration of

PT

the components CPC and CPB, having long hydrophobic tails (n=16) and less polar head-groups

RI

than [C10mim][Cl] (n=10) [42], in the mixtures increases, resulting in the pronounced decrease in

SC

0 0 values. It is well established that Gmic decreases with an increase in the number of carbon Gmic

atoms (n) in the hydrophobic tails of the surface active ionic liquids [Cnmim][Cl] (n=10,12,14,

NU

and 16) [42] and also in the case of conventional surfactants [12,21]. Tables 4 and 5 show that

MA

0 0 slightly increases with increase in the temperature. Small changes in Gmic with Gmic

temperature are typical for surfactants in aqueous solutions and are attributed to the entropy-

D

0 enthalpy compensation effect [12,21,49]. It is worth to mention that calculation of Gmic

TE

requires the knowledge of the cmc and g. In the present study, conductivity method has been

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employed to obtain the cmc and g values. This method is simple and does not take into account the conductivity of the micelle (a macro-ion), which leads to slightly higher g values than calculated by Evans equation which takes care of the micellar conductivity [21,42]. 0 It is observed that the values of H mic (Tables 4 and 5) are large negative for pure CPC,

CPB, and mixed systems CPC/CPB + SAIL, and positive for pure SAIL, at the studied temperatures. This indicates that the micellization of the pure as well as mixed systems, expect for the pure SAIL, is exothermic, whereas in the case of SAIL it is endothermic. It is interesting to note that though the micellization of pure SAIL is endothermic, its mixtures with CPC and CPB, exhibit exothermicity over the whole composition range, at the studied temperatures. 0 Positive H mic values for [C10mim][Cl] have also been reported by others [6], which may be due

to the reduction in the degree of hydration of the hydrophobic part of SAIL during its transfer

24

ACCEPTED MANUSCRIPT 0 from bulk to the interior of the micelle on micellization. On the other hand, negative H mic

values for the mixed systems CPC/CPB+ SAIL, suggest the dominance of interactive

PT

interactions ( hydrophobic – hydrophobic, П- П, and H- П) over the repulsive interaction

RI

between positively charged head groups of the components in the mixed systems, as it is

SC

0 recognized that hydrophobic and electrostatic interactions contribute to H mic [12,21,43].

0 Negative H mic , however, is mainly due to the transfer of hydrophobic tails of the surfactant/

NU

SAIL monomers from aqueous to the interior of the micelle accompanied by the release of

MA

solvated water molecules [12,21]. Additionally, at this point, it is important to mention that the 0 values of H mic (conductometrically) obtained from the temperature depence of the cmc and g

D

markedly differ from those determined directly by calorimetric method [39,43]. For example, the

TE

0 values of H mic (conductivity) for CPC in aqueous solution, calculated by Eq. (10), are -9.00 and

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0 -10.2 kJ. mol–1 ( this work) while H mic (calorimetry) are -4.5 and -17.9 kJ.mol-1 [39] at 298.15

and 318.15 K, respectively; Galgano and Seoud [43] reported that for the most cases, the ratio 0 0 (conductivity)/ H mic (calorimetry) is 2.5 ± 0.5; for example for the surface – active ionic H mic 0 liquid, [C16mim][Cl] the values of H mic (conductivity) are -16.53, -24.38, and -32.99 kJ. mol-1 0 and those of H mic (calorimetry) are -2.75, -10.08, and -16.50 kJ. mol-1 at 298.15, 308.15, and

0 318.15 K, respectively. Such discrepancy in H mic obtained by the two methods has also been 0 reported elsewhere [61,62]. The difference between the data on H mic obtained by the two

methods is traced to the fact that several factors such as the changing aggregation number, 0 shapes, and binding of the counterions to the micelles influence H mic , are not properly 0 considered in the determination of H mic (conductivity), whereas, in the direct determination of

25

ACCEPTED MANUSCRIPT 0 0 (calorimetry) all the above factors are considered [61,62]. The values of S mic (Tables 4 H mic

and 5) are positive for both the pure components and mixed systems, suggesting that the process

PT

0 of micellization is favored by entropy gain. Further, an increase in temperature causes S mic to

RI

decrease probably due to the decrease in the amount of structured water by the hydrophobic tails

SC

and bound by the polar head-groups of the components in the non-micellar species, and subsequent transfer of the hydrophobic tails from aqueous medium to the more ordered interior

NU

0 of the micelle on micellization [12]. The observed S mic (0.104 kJ.K-1.mol-1) for CPC at 308.14

MA

K is in close agreement with its value 0.110 kJ.K-1.mol-1 at 303.15 K [11]. 0 It is important to analyze the enthalpic and entropic contributions to Gmic for the studied

TE

D

0 systems. It is evident from the Tables 4 and 5 that the values of (- TS mic ) are found to be higher

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0 than the values of H mic for both pure as well as mixed systems, this suggests that micellization

is primarily entropy dominated, associated with the transfer of hydrophobic tails of surfactant molecules from the aqueous environment to the interior of the micelles [12,21]. Thus, during micellization breaking up of the structured water molecules around the hydrophobic tails of the surfactants takes place when it is transferred from the aqueous to the interior of the micelle wherein the hydrophobic tails enjoy increased freedom due to the non-polar nature of the interior of the micelle compared to the polar aqueous environment [12,21].

26

ACCEPTED MANUSCRIPT 3.4. Interfacial properties Interfacial properties such as surface excess concentration, Гmax, minimum surface area per

PT

molecule at the interface, Amin , and the standard Gibbs free energy of adsorption at the interface,

RI

0 of pure surface-active CPC, CPB, SAIL [C10mim][Cl], and their mixed systems Gad

SC

CPC/CPB+[C10mim][Cl] were calculated using surface tension data and are displayed in Table 6.

NU

Surface- active molecules (amphiphiles) adsorb at the air- solution interface and decrease the surface tension of the solution. A quantitative measure of the maximum adsorption of a

MA

surfactant at air-solution interface at cmc is obtained by surface excess concentration, Гmax, using

D

the Gibbs adsorption equation in the form[12]:

TE

(15)

AC CE P

where n is the number of species formed in solution as a result of dissociation of surfactant / SAIL molecules and C is the surfactant (amphiphilic) concentration,

was obtained from

the slope of  vs log C plot at constant T and P. The value of n is taken as 2 for CPC and CPB and 3 for mixed systems [11,62]. The maximum error in the estimation of the slope was less than 2.5%. The minimum area occupied by each amphiphilic molecule at the saturated air- solution interface at the cmc was calculated by using the following equation [11,62]:

Amin 

1018 N A max

(16)

27

ACCEPTED MANUSCRIPT where N A is the Avogadro′s number and the factor 1018 is used to convert the area from m2 to nm2. The units of Гmax and Amin are in mol. m-2 and nm2. molecule-1, respectively. The values of

PT

Гmax and Amin are listed in Table 6. It is clear from Table 6 that Гmax value of pure SAIL is greater

RI

than those of the conventional surfactants CPC and CPB. This suggests that SAIL molecules are

SC

themselves appreciably adsorbed at the air-solution interface compared to those of CPC and CPB, and this fact is substantiated with the highest surface tension,  , reduction due to SAIL

NU

compared to those due to CPC and CPB in aqueous solution. Also, for the CPC/CPB +SAIL

MA

mixtures (Table 6) Гmax increases with the increase in concentration of SAIL, indicating that the air-solution interface is richer in SAIL molecules compared to the CPC or CPB molecules. The

D

magnitude of Гmax is considered as the combined effect of (i) the attractive interaction between

TE

the hydrophobic tails of CPC/CPB and SAIL in the monolayer formed at the air-solution interface of the mixed CPC/CPB + SAIL molecules (ii) steric hindrance introduced by the bulky

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hydrophobic tails of CPC and CPB molecules. Thus, increasingly large Гmax values with increase in the concentration of SAIL suggest weak hydrophobic interaction between component molecules present in the interface than similar interaction in the bulk solution: In addition, due to longer hydrophobic chains (16 carbon atoms) in CPC and CPB, their molecules tend to bend or get coiled with a consequent increase in the cross- sectional area of the molecules at the airsolution interface [2,12]. As a result, the adsorption of CPC and CPB molecules is hindered, making Гmax increasingly smaller as the concentration of CPC/CPB increases in the mixed systems. Similar behavior, i.e., increase in Гmax with increase in the concentration of ionic liquids have also been reported for the mixed systems tetraethylammonium tetrafluoroborate + TX-100 [4] and [C10mim][Cl] + SDBS [26] in aqueous solution. As expected, an opposite trend in Amin with respect to Гmax for both the mixed systems (Table 6) suggests that higher the adsorption at 28

ACCEPTED MANUSCRIPT the air-solution interface, smaller the effective area of the molecules at the surface. Lowest value of Amin for SAIL in the mixed systems is attributed to the fact that air-solution interface is closely

PT

packed with the SAIL molecules than CPC and CPB molecules. Furthermore, as stated above,

RI

the bending/ coiling of the longer hydrophobic tails of CPC and CPB causes steric hindrance,

SC

hence, loose packing in the interface, resulting in the increase in the effective area Amin with increase in their concentration in the mixed systems (Table 6). The results observed in the

NU

present study are in accordance with the view that ionic liquids, like inorganic electrolytes, cause

MA

decrease in the effective area of the surfactants in their systems [2, 27]. For example, the decrease in Amin with an increase in the concentration of the ionic liquids [C14mim][Cl] and

D

[C8mim][Cl] in their respective mixtures with the cationic TTAB/ DTDAB [2] and anionic

TE

SDBS [26] surfactants in aqueous solution have also been reported.

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0 The standard Gibbs free energy of adsorption, Gad , at the air-solution interface was

evaluated using the relation [2,11,12]:

(17)

where  cmc (  0   cmc ) is the surface pressure, a measure of the effectiveness of a surfaceactive molecule in reducing the surface tension;  0 and  cmc are the surface tension of the solvent 0 and of the mixed system at cmc, respectively. All the values of Gad are negative (Table 6) for

both the mixtures studied, indicating that the adsorption of the amphiphilic molecules at the air0 0 solution interface is spontaneous. Moreover, a comparison between Gmic and Gad shows that

the latter quantity is more negative than the former one, therebye, suggesting that adsorption process at the interface is more favorable than the formation of the micelles in the bulk solution. 29

ACCEPTED MANUSCRIPT The standard state for the adsorbed amphiphile is defined as a hypothetical monolayer at its

PT

minimum surface area per molecule when  cmc is assumed to be zero [2], the term

П

of

Eq. (16) represents the amount of work required in transferring the amphiphilic molecules from П

is very

SC

RI

monolayer at zero surface pressure to the micelle. Since, in the present study,

0 small in comparison with Gmic , indicating that negligible amount of work is required in

NU

0 transferring the amphiphilic molecule from monolayer to the micelle. Similar results on Gad

MA

have also been reported for CPC+TX-100 [11] and [C14mim][Cl] + TTAB / DTDAB [2] mixed systems.

D

Additionally, the mode of packing of amphiphiles in the micelles and micellar geometry

TE

in aqueous media find practical applications in determining the various properties of the

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amphiphilic solution, such as its viscosity, capacity to solubilize water- insoluble materials, and its cloud point [12], and can be investigated with the help of the packing parameter, P, which is obtained using the following relation [63]:

P

v Al C

(18)

30

ACCEPTED MANUSCRIPT where v is the volume occupied by the hydrophobic chain considered to be fluid and incompressible [11], A is the cross-sectional area of the hydrophilic head group at the micellar

PT

interface, and l C is the maximum effective length of the hydrophobic chain in the core. The

RI

volume contribution of the hydrophobic chain, v and its effective length l C for a saturated

SC

hydrocarbon chain having n number of carbon atoms were evaluated using the following equations suggested by Tanford [64]:

NU

v  (0.0274+0.0269 n) nm3

MA

l c = (0.154+ 0.1265 n) nm

(19) (20)

R

TE

v Al C

(21)

AC CE P

A  (3v / A) nm

D

The values of A and radius R of the micelle were calculated from the expressions [11,63,65]:

(22)

For mixed micelles, v was calculated by using the relation [11]:

v   X iRVi

(23)

i

31

ACCEPTED MANUSCRIPT where i refers to the ith species and X iR refers to the micellar mole fraction calculated from Rubingh′s equation [31] and the component with longer chain length was taken into consideration, l C (CPC/CPB) in the

PT

present case, as l C (CPC/CPB) > l C (SAIL) for the calculation of l C [11] of the mixtures

RI

CPC/CPB+SAIL. The values of l C , v , A, P, and R for the pure and mixed systems thus obtained

SC

are summarized in Table 7. It shows that both v and A increase with increase in the

NU

concentration of CPC/CPB in the mixtures. The trends in v and A are in consistent with each other and also reinforce the increasing values of Amin with the concentration of CPC/CPB in the

MA

mixed systems. The magnitude of the packing parameter, P, can be used to predict the type and shape of the micelle formed in aqueous media. For example, for spherical micelles 0 < P < 0.33;

D

for cylindrical micelles 0.33 < P < 0.50; for lamellar micelles 0.5 < P < 1; and for reversed

TE

micelles in nonpolar media P > 1 [12]. Thus, the observed values of P clearly indicate that the

AC CE P

micelles/ mixed micelles formed in this study are spherical in nature. This observation is truly endorsed by the fact that for spherical micelles in aqueous media the radius of the micelle, R, should not exceed the effective length, l C , of the hydrophobic chain, which is actually the observed result for the micelles/ mixed micelles of the studied systems (Table 7). Furthermore, the observed P value for CPC (0.16) is in close agreement with the literature value, and that using P value, formation of the spherical micelles for the pure CPC and its mixtures with nonionic surfactant TX-100 has been reported by others [11]. Also, the formation of spherical micelles (i.e., P < 0.33) has been reported for the binary mixtures of ionic liquid [C14mim][Br] with cationic surfactants TTAB/ DTDAB in aqueous medium [2].

32

ACCEPTED MANUSCRIPT 4. Conclusions The mixed systems composed of CPC/CPB + green solvent [C10mim][Cl] exhibit synergestic

PT

behavior. The observed values of the cmc < cmc*, β < 0, and (ƒ1 and ƒ2) < 1 clearly indicate the

RI

non-ideal behavior of the mixtures, and also suggest that stronger interactions exist between

SC

CPC/CPB and [C10mim][Cl] molecules in the mixed micelles than in the pure components. The counterion Br–, being less kosmotropic than Cl– strongly binds with the micellar surface of CPB

NU

+ [C10mim][Cl] than Cl– binds with CPC + [C10mim][Cl] mixed micelles, resulting in the lower

MA

cmc values for the former mixture than the latter one. Importantly, X1 < Xideal at each αCPC/CPB shows that the mixed micelles are richer in CPC or CPB monomers in comparison to the ideal

D

mixed state. In addition to the predominantly hydrophobic interactions between CPC/CPB and

TE

[C10mim][Cl] molecules, attractive interactions between imidazolium cation of [C10mim][Cl] and

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π- electrons of pyridinium rings of CPC/CPB, ring stacking via π- π interactions between πelectrons of aromatic rings of component molecules and H- π interactions involving C-2 proton of imidazolium ring and π- electrons of aromatic rings of CPC/CPB molecules also significantly 0 contribute towards the synergism of the investigated mixtures. Negative Gmic values both for

the pure components as well as for the mixtures indicate that micelle formation is 0 thermodynamically spontaneous and exothermic, as H mic is also negative. Moreover, higher 0 0 values of (  TS mic ) than H mic are due to the fact that the process of micellization of the studied

systems is primarily entropy dominated one. Higher adsorption of [C10mim][Cl] at the airsolution interface compared to CPC and CPB in the pure state as well as in the mixtures is clearly evident from the larger values of Гmax , and maximum reduction in  , by [C10mim][Cl] than by CPC and CPB molecules. Thus, the air- solution interface is richer in [C10mim][Cl]

33

ACCEPTED MANUSCRIPT molecules than CPC or CPB molecules, which may be due to the steric hindrance by the bulky hydrophobic tails of CPC/CPB molecules. The above view is further supported by the lowest

PT

Amin, values for [C10mim][Cl] than CPC and CPB in the mixed systems. Higher negative values

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0 0 of Gad than those of Gmic indicate that the adsorption at air-solution interface is more

SC

favorable than the formation of micelles in the bulk solution. Furthermore, the values of P are less than 0.33, suggesting that the micelles/ mixed micelles formed are spherical in nature. The

NU

calculated radius, R, of the micelle/ mixed micelle does not exceed the length of the hydrophobic

MA

tail of the surfactants which further substantiates the above view.

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Acknowledgement Ummer Farooq is thankful to UGC for providing the scholarship in the form of BSR (Basic Scientific Research).

34

ACCEPTED MANUSCRIPT References

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[1] K.A. Fletcher, S. Pandey, Langmuir 20 (2004) 33-36.

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[2] R. Sharma, S. Mahajan, R.K. Mahajan, Colloids Surf. A 427 (2013) 62-75.

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[3] D. Bhatt, K. Maheria, J. Parikh, J. Chem. Thermodyn. 74 (2014) 184-192.

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[4] A. Ali, M. Ali, N.A. Malik, S. Uzair, A.B. Khan, J. Chem. Eng. Data 59 (2014) 17551765.

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[5] J. Luczak, C. Jungnickel, M. Markiewicz, J. Hupka, J. Phys. Chem. B 117 (2013) 5653-

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5658.

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[6] A.B. Khan, M. Ali, N. Dohare, P. Singh, R. Patel, J. Mol. Liq. 198 (2014) 341-346.

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Solvents: Physical Properties and Methods of Purification, 4th ed. John Wiley & Sons,

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New York, USA (1986).

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[37] R.C. Bazito, O.A. Elseond, Langmuir 18 (2002) 4362-4366.

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Interface Sci. 342 (2010) 83-92.

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[48] S.K. Mehta, K.K. Bhasin, R. Chauhan, and S. Dham, Colloid Surf. A 255 (2005) 153-157.

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[49] L.J. Chen, S.Y. Lin, C.C. Huang, and E.M. Chen, Colloid Surf. A 135 (1998) 175-181; L.J.

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Chen, S.Y. Lin, and C.C. Huang, J. Phys. Chem. B 102 (1998) 4350-4356. [50] S. Ghosh, A.D. Burman, G.C. De, and A.R. Das, J. Phys. Chem. B 115 (2011) 11098-

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[51] K. Motomura, M. Yamanaka, and M. Aratono, Colloid Polym. Sci. 262 (1984) 948-955.

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[52] D.N. Rubingh, in: K. Mittal (Edo), Solution Chemistry of Surfactants, Plenum Press, New

[53] L. Fang, L. Gan-Zuoi, and C. Jian-Bo, Colloids Surf. A 145 (1998) 167-174 [54] M.E. Haque, A.R. Das, A.K. Rakshit, and S.P. Moulik, Langmuir 12 (1996) 4084-4089. [55] A.A. Dar, G.M. Rather, S. Ghosh, and A.R. Das, J. Colloidal Interface Sci., 322 (2008) 572-581. [56] N. Azum, M.A. Rub, A.M. Asiri, A.A.P. Khan, A. Khan, S.B. Khan, M.M. Rahman, and A.O. Al-Youbi, J. Solution Chem. 42 (2013) 1532-1544. [57] D. F. Evans and B.W. Niriham, J. Phys. Chem. 87 (1983) 5025-5032.

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ACCEPTED MANUSCRIPT [58] S.A. Buckingham, C.J. Garvey, and G.G. Warr, J. Phys. Chem. 97 (1993) 10236-10244.

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[59] K.M. Kale, E.L. Cussler, and D.F. Evans, J. Phys. Chem. 84 (1980) 593-598.

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[60] A. Bandhopadhyay and S.P. Moulik, Colloid Polym. Sci. 266 (1988) 455-461.

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[61] G.C. Kresheck, J. Phys. Chem. B 102 (1998) 6596-6600.

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[62] S.P. Moulik and D. Mitra, J. Colloid Interface Sci. 337 (2009) 569-578. [63] S. Sharma and S. Chauhan, Colloid Surf. A 453 (2014) 78-85.

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[64] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes,

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Wiley- Interscience, New York, 1973.

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[65] D. Das and K. Ismail, J. Colloid Interface Sci. 327 (2008) 198-203.

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140

CPC= 0.6

120 120

CPC= 0.8

60

298.15 K 308.15 K 318.15 K

40

20

80

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40

20

0

0

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180

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CPC = 0.4

60

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(c)

80

298.15 K 308.15 K 318.15 K

40 20 0

0.0

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-1

 (mS cm )

120 100

1.5

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CPC = 0.2

140

2.5

120

 (mS cm )

140

2.5

2.0

[Concentration] (mM)

(d)

-1

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298.15 K 308.15 K 318.15 K

1.5

1.0

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1.0

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(b)

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(a)

-1

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80

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100

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100

100

80

298.15 K 308.15 K 318.15 K

60

40

3.0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

[Concentration] (mM)

[Concentration] (mM)

Fig. 1. Plots of specific conductivity,  vs. [concentration] of mixed system, CPC + [C10mim][Cl] (a) αCPC = 0.8, (b) αCPC = 0.6, (c) αCPC = 0.4 and (d) αCPC = 0.2 at different temperatures.

41

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90

120 110

80

CPB = 0.8

90 80

-1

 (mS cm )

50

(a)

40 30

50

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20 10

0.0 0.2 0.4 0.6

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160 140

CPB = 0.4

120

80

(c)

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298.15 K 308.15 K 318.15 K

20 0

0.5

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2.5

0.8 1.0 1.2

1.4 1.6

1.8 2.0 2.2

[Concentration] (mM)

180 160

CPB= 0.2

140 120

(d)

100

298.15 K 308.15 K 318.15 K

80 60

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40

TE

-1

D

100

-1

0.4

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1.2

 (mS cm )

0.2

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[ Concentration] (mM)

0.0

(b)

30

10

0.0

60

40

298.15 K 308.15 K 318.15 K

20

70

RI

-1

 (mS cm )

60

 (mS cm )

CPB = 0.6

100

PT

70

40

1.0

3.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

[Concentration] (mM)

[Concentration] (mM)

Fig. 2. Plots of specific conductivity,  vs. [concentration] of mixed system, CPB + [C10mim][Cl] (a) αCPB = 0.8, (b) αCPB = 0.6, (c) αCPB = 0.4 and (d) αCPB = 0.2 at different temperatures.

42

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75

65 70

60

-1

)

60

mN m

(a)

45

55 50

40

35 -3.6

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-3.0

-2.8

-2.6

35 -4.0

-2.4

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log C (M)

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log C (M)

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40

(b)

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)

-1 50

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mN m

CPC= 0.6

65

70

65

65

45

40 -4.0

-3.8

-3.6

-3.4

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CPC = 0.2

-1

)

55

mN m

TE

(c)

AC CE P

)

-1 mN m

55

50

D

CPC= 0.4

60

(d)

50 45 40 35 30 -3.6

-2.4

-3.4

-3.2

-3.0

-2.8

-2.6

-2.4

log C (M)

log C (M)

Fig. 3. Plots of surface tension,  vs. log [concentration] of mixed system, CPC+[C10mim][Cl] (a) αCPC = 0.8 (b) αCPC = 0.6, (c) αCPC = 0.4 and (d) αCPC = 0.2 at 298.15 K.

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65 60

CPB = 0.8

50

-1

(a)

50

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55

mN m )

mN m

-1

)

55

45

45 40 35

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CPB = 0.6

(b)

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60

30

35

25 -4.0

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65

(c)

50

TE

mN m

-1

)

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CPB = 0.4

55

45

30 -8.5

-8.0

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40 35

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log C (M)

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65

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70

60

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log C (M)

60

CPB = 0.2

55

)

-8.5

-1

-9.0

mN m

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50

(d)

45

40

35

30 -8.0

-6.0

-7.5

-7.0

-6.5

-6.0

-5.5

log C (M)

log C (M)

Fig. 4. Plots of surface tension,  vs. log [concentration] of mixed system, CPB+[C10mim][Cl] (a) αCPB = 0.8 (b) αCPB = 0.6 , (c) αCPB = 0.4 and (d) αCPB = 0.2 at 298.15 K.

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5.5

4.5

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(a)

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cmc cmc*

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cmc cmc*

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cmc cmc*

2.0 1.5 1.0

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CPC

Fig. 5. Vairations of cmc (■) and cmc* (●) with αCPC at different temperatures (a) 298.15 (b) 308.15 and (c) 318.15 K.

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cmc (mM)

cmc (mM)

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Fig. 6. Vairations of cmc (■) and cmc* (●) with αCPB at different temperatures (a) 298.15 (b) 308.15 and (c) 318.15 K.

46

ACCEPTED MANUSCRIPT

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Supplementary material

80 100

70

-1

 (mS cm )

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 (mS cm )

50 40

298.15 K 308.15 K 318.15 K

30 20

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80

(a)

(b)

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40

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298.15 K 308.15 K 318.15 K

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0.6

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[CPC] (mM)

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[CPB] (mM)

TE

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(c)

320

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 (mS cm )

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300 280 260 240 220

298.15 K 308.15 K 318.15 K

200

20

30

40

50

60

70

80

90

[C10mim][Cl] (mM)

Fig. S1. Plots of specific conductivity,  vs. [CPB] (a), [CPC] (b) and [C10mim][Cl] (c) in aqueous solution at different temperatures.

47

ACCEPTED MANUSCRIPT

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75 70

65

(a)

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(b)

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-1 mN m

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50

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50 45

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35 -10.0

-9.5

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44

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log C (M)

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log C (M)

mN m )

-1

mN m )

55

PT

60 60

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

log C (M)

Fig. S2. Plots of surface tension,  vs. log [CPB] (a), log [CPC] (b) and log [C10mim][Cl] (c) at 298.15 K.

48

-6.5

ACCEPTED MANUSCRIPT

a cmc (mM)

X1

S.T.

Avg

f1

f2

∆Gex (kJ. mol-1)

AC CE P

308.15

4.50 2.38 1.61 1.21

CPC+[C10mim][Cl] 0.00 60.0 47.7 0.20 3.30 3.00 0.40 1.84 1.89 0.60 1.47 1.27 0.80 1.20 1.20 1.00 1.08 1.04

0.709 0.815 0.832 0.941

D

49.4 2.20 1.68 1.17 1.11 0.99

0.937 0.975 0.989 0.996

-4.367 -3.520 -4.359 -5.571

0.691 0.886 0.884 0.981

0.111 0.096 0.049 0.007

-0.819 -0.823 -1.002 -1.270

TE

40.0 2.00 1.60 1.14 1.08 1.02

MA

NU

T (K) 298.15 CPC+[C10mim][Cl] 0.00 58.80 0.20 2.40 0.40 1.77 0.60 1.20 0.80 1.14 1.00 0.96

β

Xideal

RI

Cond.

cmc*

(mM)

SC

αaCPC

PT

Table 1 Values of (cmc), average (cmc), ideal (cmc*), micellar composition (X1, Xideal), interaction parameter (  ), activity coefficients ( f 1 , f 2 ) and excess free energy of mixing ( Gex )for mixtures CPC + [C10mim][Cl] as a function of mole fraction (α) of CPC and temperature at atmospheric pressure.

53.85 3.15 1.86 1.37 1.20 1.06

5.00 2.67 1.81 1.36

0.723 0.931 0.791 0.973 0.860 0.987 0.904 0.995

-3.60 -3.80 -3.57 -4.82

0.758 0.847 0.932 0.957

0.152 0.0927 0.0713 0.0197

-0.78 -0.79 -0.85 -1.10

61.7 4.00 2.76 1.60 1.48 1.15

5.60 2.90 1.97 1.49

0.738 0.780 0.851 0.90

-3.40 -3.42 -3.92 -3.93

0.790 0.847 0.916 0.977

0.154 0.124 0.058 0.155

-1.1 -1.0 -1.2 -1.3

318.15 CPC+[C10mim][Cl] 0.00 70.0 53.4 0.20 3.50 4.50 0.40 2.40 3.12 0.60 1.56 1.65 0.80 1.47 1.50 1.00 1.20 1.10

0.935 0.974 0.988 0.995

a

Uncertainties in  CPC  ± 0.01 and cmc = ±0.1 mM. 49

ACCEPTED MANUSCRIPT

a cmc (mM)

Avg

CPB+[C10mim][Cl] 0.00 60.0 0.20 3.20 -0.34 0.40 1.71 -0.48 0.60 1.12 -0.74 0.80 0.92 -1.40 1.00 0.86

f1

f2

∆Gex (kJ. mol-1)

49.4 2.60

3.29

1.30

1.35

1.69

1.12

1.11

0.84

0.84

0.954

-1.6

0.939

0.305

-

0.86

0.982

-3.0

0.942

0.104

-

1.14

0.91

0.992

-3.4

0.966

0.061

-

0.86

0.97

0.998

-5.6

0.995

0.005

-

TE

0.85

D

40.0 2.40

AC CE P

308.15

β

NU

T (K) 298.15 CPB+[C10mim][Cl] 0.00 58.8 0.20 2.80 0.35 0.40 1.40 0.68 0.60 1.10 0.76 0.80 0.85 1.42 1.00 0.69

Xideal

RI

S.T.

X1

SC

Cond.

cmc*

(mM)

MA

αaCPC

PT

Table 2 Values of (cmc), average (cmc), ideal (cmc*), micellar composition (X1, Xideal), interaction parameter (  ), activity coefficients ( f 1 , f 2 ) and excess free energy of mixing ( Gex ) for mixtures CPB +[C10mim][Cl] as a function of mole fraction (α) of CPB and temperature at atmospheric pressure.

0.71

0.70

47.7 3.10

53.85 3.15

3.66

0.85

0.951

-1.5

0.970

0.330

1.61

1.66

1.88

0.90

0.981

-2.07

0.980

0.180

1.08

1.10

1.27

0.91

0.991

-3.2

0.960

0.070

0.87

0.89

0.96

0.96

0.997

-4.7

0.992

0.010

0.84

0.85

318.15 CPB+[C10mim][Cl] 0.00 70.00 53.40

61.70

50

3.60

3.65

4.20

0.850

0.951

-1.2

0.970

0.410

1.92

1.80

1.86

2.18

0.880

0.991

-2.4

0.960

0.142

1.24

1.30

1.27

1.47

0.883

0.991

-3.1

0.950

0.080

1.03

1.10

1.06

1.10

0.921

0.996

0.975

0.037

1.10

1.14

1.12

SC

D

MA

NU

Uncertainties in  CPB  ± 0.01, cmc = 0.1 mM.

TE

a

RI

3.70

AC CE P

0.20 -0.33 0.40 -0.48 0.60 -0.73 0.80 -1.26 1.00

PT

ACCEPTED MANUSCRIPT

51

-3.9

ACCEPTED MANUSCRIPT

cmc (mM)

RI

Compound

PT

Table 3 Comparison of (cmc) values for [C10mim][Cl], CPC, and CPB in aqueous solutions at different temperatures and atmospheric pressure.

Literature values

T (K)

NU

SC

This work

58.80

CPC

208.15, 308.15 and 318.15

0.96, 1.08, 1.20

0.96; 1.04; 1.15 [39]

CPB

208.15 and 308.15

0.69, 0.86

0.64 [40]; 0.82 [41]

AC CE P

TE

D

MA

[C10mim][Cl] 208.15

52

53.8 [5]; 59.9 [38]; 55.1 [6]

ACCEPTED MANUSCRIPT Table 4 Thermodynamic parameters for pure CPC, [C10mim][Cl] and their mixtures CPC+[C10mim][Cl] at different temperatures and atmospheric pressure. ∆S0mic (kJ. K-1. mol-1)

0.44 0.47 0.50

Pure [C10mim][Cl] 298.15 308.15 318.15

-24.0 -22.0 -21.0

0.57 0.60 0.65

-9.00 -10.0 -10.2

SC

-43.4 -42.0 -42.2

NU

Pure CPC 298.15 308.15 318.15

RI

)

0.114 0.104 0.100

T∆S0mic (kJ. mol-

33.98 32.04 31.81

0.097 0.088 0.083

28.92 27.11 26.40

-21.82 -24.36 -25.68

0.034 0.030 0.028

10.13 9.240 8.90

0.643 0.680 0.70

-18.05 -18.75 -19.07

0.056 0.052 0.047

16.69 16.02 14.95

CPC (0.6)+ [C10mim][Cl] 298.15 -37.73 308.15 -37.61 318.15 -37.91

0.560 0.607 0.640

-18.83 -18.69 -18.84

0.063 0.057 0.056

18.78 17.56 17.81

CPC (0.8)+ [C10mim][Cl] 298.15 -39.33 308.15 -40.80 318.15 -39.51

0.529 0.530 0.560

-16.29 -17.57 -17.61

0.077 0.075 0.069

22.95 23.11 21.95

CPC (0.4)+ [C10mim][Cl] 298.15 -34.82 308.15 -34.88 318.15 -34.01

D

0.716 0.658 0.630

TE

CPC (0.2)+ [C10mim][Cl] 298.15 -31.98 308.15 -33.78 318.15 -34.46

MA

5.00 5.23 5.42

AC CE P

1

∆H0mic (kJ. mol-1)

g

PT

∆G0mic (kJ. mol-1)

T (K)

53

ACCEPTED MANUSCRIPT Table 5 Thermodynamic parameters for pure CPB, [C10mim][Cl], and their mixtures CPB+[C10mim][Cl] at different temperatures and atmospheric pressure.

-16.92

-44.18

0.464

-16.97

0.0877

-43.60

0.483

0.0839

-24.0

0.57

-22.0

0.60

-21.0

0.65

CPB (0.4)+ [C10mim][Cl] 298.15 -36.38 15.80 308.15 -36.19 14.48 318.15 -35.60 13.99 CPB (0.6)+[C10mim][Cl] 298.15 -37.01 23.55 308.15 -36.65 22.80

-17.31

0.0965

5.00

0.097

5.23

0.088

5.42

0.083

0.829

-20.77

0.026

0.87

-21.41

0.022

0.90

-22.21

0.018

0.62

-20.69

0.053

0.64

-21.47

0.047

0.67

-21.68

0.044

0.61

-13.35

0.079

0.63

-13.75

0.074

AC CE P

CPB (0.2)+[C10mim][Cl] 298.15 -28.63 7.75 308.15 -28.25 6.77 318.15 -27.97 5.72

SC

0.364

NU

RI

PT

∆S0m (kJ. K-1. mol-1)

-45.71

MA

Pure [C10mim][Cl] 298.15 28.92 308.15 27.11 318.15 26.40

∆H0m (kJ. mol-1)

g

D

Pure CPB 298.15 28.77 308.15 27.02 318.15 26.69

∆G0m (kJ. mol-1)

TE

T (K)

54

T∆S0m (kJ. mol-1)

ACCEPTED MANUSCRIPT -14.09

0.071

0.53

-8.6

0.104

0.62

-8.7

0.66

-9.0

0.098

AC CE P

TE

D

MA

NU

SC

CPB (0.8)+[C10mim][Cl] 298.15 -40.39 31.60 308.15 -38.91 30.19 318.15 -38.61 29.58

0.66

PT

-36.44

RI

318.15 22.58

55

0.093

ACCEPTED MANUSCRIPT

αCPC Гmax

Amin ∆G 0ads (106 mol m-2) (nm2 molecule-1) (kJ. mol-1)

0.0 0.2 0.4 0.6 0.8 1.0

MA

D

-48.55 -53.08 -54.59 -58.61 -62.15 -63.36

TE

0.90 1.00 1.02 1.20 1.22 1.13

AC CE P

1.84 1.65 1.54 1.37 1.35 1.30

NU

SC

mol-1)

0.0 0.2 0.4 0.6 0.8 1.0

Гmax Amin ∆G 0ads (106 mol m-2) (nm2 molecule-1) (kJ.

RI

αCPB

PT

Table 6 Surface excess (Гmax ), minimum area per molecule (Amin), and free energy of adsorption (∆G 0ads ) for pure CPC, CPB, [C10mim][Cl] and mixtures for CPC/CPB+ [C10mim][Cl] as a function of mole fraction αCPC/CPB.

56

1.84 0.72 0.70 1.78 0.64 1.24

0.90 2.30 2.40 0.92 2.50 1.33

-48.55 -81.00 -90.38 -60.91 -102.57 -70.22

ACCEPTED MANUSCRIPT

PT

Table 7 Values of effective hydrophobic chain length ( l c ), volume of hydrophobic chain (v) , surface area of polar head group ( A) , packing parameter (P), and radius (R) for pure CPC, CPB, [C10mim][Cl] and mixtures CPC/CPB+ [C10mim][Cl]. lc (nm)

v (nm3)

A (nm2)

P

0.0 0.2 0.4 0.6 0.8 1.0

1.40 2.17 2.17 2.17 2.17 2.17

0.296 (0.296) 0.410 (0.432) 0.427 (0.434) 0.429 (0.440) 0.447 (0.450) 0.457 (0.457)

0.90 (0.90) 1.00 (2.30) 1.02 (2.40) 1.20 (2.45) 1.22 (2.50) 1.26 (1.33)

0.230 (0.230) 0.210 (0.090) 0.200 (0.087) 0.175 (0.086) 0.170 (0.080) 0.160 (0.150)

MA

NU

SC

RI

αCPC/CPB

AC CE P

TE

D

Values in brackets are for pure CPB and CPB+[C10mim][Cl] mixtures.

57

R (nm)

0.98 (0.98) 2.15 (0.95) 2.10 (0.90) 1.80 (0.88) 1.70 (0.87) 1.10 (0.66)

ACCEPTED MANUSCRIPT GRAPHICAL ABSTRACT

5.5

4.5

PT

5.0

At 308.15 K

RI

3.5

cmc cmc*

3.0

SC

cmc (mM)

4.0

2.5

1.5 1.0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

MA

CPC

NU

2.0

4.0

D

3.5

At 308.15 K

TE

2.5

2.0

1.5

1.0

0.5

cmc cmc*

AC CE P

cmc (mM)

3.0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

CPB

58

ACCEPTED MANUSCRIPT HIGHLIGHTS • Mixed micellization of cationic surfactants and surface-active ionic liquid (SAIL) has been

PT

investigated. • Synergestic behavior has been observed for the mixed systems.

AC CE P

TE

D

MA

NU

SC

RI

• Theoretical models also confirm interaction in mixed micelles.

59