Imidazolium-Based Ionic Liquids as Modulators of PhysicochemicalProperties and Nanostructures of CTAB in Aqueous Solution: TheEffect of Alkyl Chain Length, Hydrogen Bonding Capacity, and AnionTypeS. Javadian,* V. Ruhi, A. Asadzadeh Shahir, A. Heydari, and J. Akbari

Department of Chemistry, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Iran

*S Supporting Information

ABSTRACT: The aggregation behavior of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), in aqueoussolution of various imidazolium-based ionic liquids (ILs) was investigated using tensiometry, conductometry, transmissionelectron microscopy (TEM), and cyclic voltammetry (CV) methods. The data obtained from tensiometry and conductometrymeasurments were used to specify the critical micelle concentrations (CMCs) and other physicochemical parameters. The CMCof the surfactant was found to significantly decrease in the presence of ILs. The surface activity parameters of CTAB wereobserved to be a function of structural properties of ILs. Furthermore, intermicellar interaction parameter (kd) was measuredusing cyclic voltametry. The intermicellar interaction parameter was also found to decrease slightly with addition of IL. However,in the presence of HMImBr or HMImCl, the intermicellar interaction parameter is not changed significantly, as a result of phasetransition from spherical to cylindrical micelles, which was also confirmed by transmission electron microscopy (TEM)micrographs. The size of CTAB nanoparticles grew from 5 nm to 20−30 nm in the presence of ILs.

1. INTRODUCTION

The self-aggregation capability of surfactants greatly dependsupon their structure, concentration, solubilizing media, and themethod through which self-assembly takes place.1 From thestandpoint of chemical kinetics, phytochemistry, analyticalchemistry, chromatography, and nanorelated technologies, themicelles of ionic surfactants are regarded as a suitableenvironment for various chemical reactions. Therefore, thestudies on properties of surfactant micelles and the methods ofcontrolling their structures are of great importance. Themodulation of surfactants’ CMCs and micellization behaviorcan be achieved by varying the nature of surfactants such astheir net charge, polar head groups, and/or counterions.Another method is the application of mixed surfactant systems.On this basis, the mixed micelles of room-temperature ionicliquids (RTILs) and surfactants have been recently paid greatattention. It is now well-known that the CMC and even thestructure of micelles changes in the presence of ionic liquids(ILs). Thus, the investigation of IL/surfactant mixed systemsgets more important since the self-assembly behavior of asurfactant can be optimized with ILs in order to be used inparticular systems.1−4

ILs are compounds that consist of a cationic part and ananionic part (such as inorganic salts), and their melting pointsare below 100 °C. Their properties are dependent upon thegroups attached to the cationic portion. This broadens theirapplication in different chemical industries.4−7 In fact, theimportant characteristic of these composites is their designcapability for application in different situations. Moreover, theunique characteristics of ILs, especially their negligible vaporpressure, makes them a good alternative solvent in greenchemistry.8 ILs are widely used as solvents for synthesis and

catalysis of inorganic and organic compounds as well as inchromatography and extraction, and also as electrolytes inbatteries or solar cells.9 ILs have also been reported to act assurfactants and/or liquid crystals.10

Most of the ILs influence surfactant self-assembly systems ina way quite different from inorganic salts, because of theirmolecular structure, which enables them to form hydrogenbonding along with electrostatic and hydrophobic interactions.In addition, the effect of hydrocarbon chain length of ILs onsurfactant micellization depends directly upon the concen-tration of ILs.1,11 In high concentrations of ILs, the addition ofIL increases the CMC but the CMC value starts to decrease asits hydrocarbon chain gets longer. Beyaz et al. have studied theCMC of sodium dodecyl sulfate (SDS) (7 mM in water) in thepresence of 30 mM imidazolium-based RTILs with varying sidechains as additives. Their study showed that the CMC of SDSdecreases from 170 mM to 1.9 mM as the side chain length ofILs goes from C1 to C8.

3 In low concentrations of ILs, theaddition of IL decreases the CMC value but the amount of thisreduction in CMC value gets smaller as the hydrocarbon chainof IL gets longer.12 Generally, IL acts as a cosolvent in its highconcentrations and increases the CMC value, while it acts as anelectrolyte in lower concentrations and decreases the CMCvalue by electrostatic stabilization of micelle surfaces.13−15 Theeffect of the anbions of ILs on the micellization of surfactants ispartially similar to the effect of inorganic salts. However,depending upon the cation structure, the anion of ILs can have

Received: July 10, 2013Revised: September 10, 2013Accepted: September 13, 2013Published: September 13, 2013

Article

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different effects. For example, in most ILs, anions have thecapability of forming dipole−dipole interactions or hydrogenbonding with the cationic part of IL, which may also affect thesolvophobicity of surfactants.16 Given the hydrogen bondingsites in the surfactant structure, the type of the effect of ILs canbe different.13,14,17 On this basis, the behavior of RTIL/surfactant mixed systems is so complicated that it stillencourages colloid science researchers to conduct furtherstudies on unknown aspects of such systems in order todevelop and/or improve the processes that are prevalent inindustry and/or science.The present work aims to study the aggregation behavior of

cationic surfactant, cetyl trimethylammonium bromide(CTAB), in 0.1 and 0.2% (w/w) aqueous solutions of theILs, 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), 1-hexyl-3-methylimidazolium chloride ([HMIm]Cl), 1-butyl-3-methylimidazolium bromide ([BMIm]Br), 1-hexyl-3-methyl-imidazolium bromide ([HMIm]Br), 1-butyl-2,3-dimethylimida-zolium chloride ([BM2Im]Cl), 1-butyl-3-methylimidazoliumtetrafluoroborate ([BMIm]BF4), and N-butyl imidazoliumchloride [N-BIm]Cl (see Figure 1) using tensiometry,

conductometry, transmission electron microscopy (TEM),and cyclic voltammetry techniques. The variations of CMCand interfacial properties were interpreted by taking account ofthe role of ILs as an electrolyte and cosurfactant, as well as theirability to form hydrogen bonding, which makes them differentfrom electrolyte salts. The size of CTAB nanoparticles was alsofound to be modified in the presence of ILs.

2. EXPERIMENTAL SECTION2.1. Materials. Hexadecyltrimethylammonium bromide

(CTAB) was purchased from Merck. The ILs1-butyl-3-methylimidazolium chloride ([BMIm]Cl), 1-hexyl-3-methyli-midazolium chloride ([HMIm]Cl), 1-butyl-3-methylimidazo-lium bromide ([BMIm]Br), 1-hexyl-3-methylimidazolium bro-mide ([HMIm]Br), 1-butyl-2,3-dimethylimidazolium chloride

([BM2Im]Cl), 1-butyl-3-methylimidazolium tetrafluoroborate([BMIm]BF4), and N-butylimidazolium chloride ([N-BIm]-Cl)were synthesized using the method described in theliterature. (The synthesis and detailed characterization data ofthe ionic liquids prepared are summarized in the SupportingInformation.)18

All manipulations were performed in a nitrogen atmosphereunder standard conditions. All of the solvents used wereobtained commercially. The ILs were prepared using thepreviously reported method without any modification. Thepurity of the ILs were examined by NMR spectroscopy.Doubly distilled deionized water was used for sample

preparation and dilution. The stock solutions of ILs with theconcentrations of 0.1% and 0.2% (w/w) were prepared and alsoused for preparation of concentrated CTAB solution. Alltitrations were carried out using Hamilton syringes.

2.2. Methods. 2.2.1. Specific Conductivity Measurements.Conductometric titrations were done using a Jenway Model4510 conductometer at 298 K. After any injection of CTABsolution, the solution was stirred and allowed to equilibrate for10 min, and then three successive measurements of specificconductivity were performed. The uncertainty of the measure-ments was ±0.01 μS cm−1.

2.2.2. Surface Tension Measurements. Surface tensionmeasurements were made with a Kruss Model K12tensiometer, employing the Du-Nouy ring method underatmospheric pressure at 298 K. First, the glass container wasrinsed sequentially until the surface tension of distilled waterwas measured to be 70−72 mN m−1. The platinum ring wasthoroughly cleaned by acetone and distilled water, and flameduntil glowing temperature before each measurement. Hereagain, the solution was stirred and allowed to equilibrate for 10min after any injection of concentrated CTAB solution. In allcases, more than three successive measurements were carriedout and the standard deviation did not exceed 0.05 mN m−1.The uncertainty of the measurements was ±0.1 mN m−1.

2.2.3. Cyclic Voltammetry Measurements. Cyclic voltam-metry (CV) was performed using a SAMA Model 500electrochemical analyzer. A three-electrode system consistedof a platinum disk electrode (0.0314 cm2) as the workingelectrode (the interface at which the redox reaction of interestoccurs), a saturated Ag/AgCl reference electrode with a saltbridge containing 3 M aqueous solution of KCl, and a counterelectrode (a Pt wire through which all the current required tomaintain the redox reaction at the working electrode flows) wasemployed. The working electrode was polished with a slurry ofalumina powder and then washed carefully with distilled waterbefore each measurement. Ferrocene was used as theelectroactive probe (the required concentration of ferrocenewas determined based on previous reports19) and NaCl (0.1mol dm−3) was used as the supporting electrolyte. Because ofthe insolubility of ferrocene in water, ferrocene was firstdissolved in ethanol before the small volume of the resultingethanol solution was added to the aqueous surfactant solution.The small volume of the ethanol solution, compared to the finalsolution volume, meant that the effects of the ethanol werenegligible. It is worth mentioning that the concentration offerrocene used does not significantly affect the micellization ofthe surfactants and their mixtures. Electrodes were plunged insolutions for 10 min and then voltammograms were recordedfor each solution, with a scan rate of 100 mV s−1 and the fifthcycles were saved.

Figure 1. Molecular structures of (a) different imidazolium-based ILsand (b) cetyl trimethylammonium bromide (CTAB).

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2.2.4. Transmission Electron Microscopy (TEM) Measure-ments. The solutions of certain concentrations were preparedand delivered to the operator. Transmission electron micro-graphs were recorded on an electron microscope (Zeiss, ModelEM-10C) operating at 100 kV. Samples were placed on carbon-coated grids and kept waiting for the solvent to evaporate, andthen the micrographs were acquired.

3. RESULTS AND DISCUSSION3.1. The Effect of ILs on CMC and Micellization

Behavior of CTAB. The CMC values for pure CTAB solutionand its mixtures with 0.1 and 0.2 wt % of various ILs wereobtained from conductivity (Figure 2) and surface tension

(Figure 3) measurements. The degree of micelle ionization (α)was also calculated from the slopes of conductometric plotsusing Frahm’s method;20 these values are listed in Table 1. It isobvious that the CMCs obtained by two techniques are in goodaccordance.The effect of ILs in aggregation behavior of surfactants can

be discussed from the standpoints of concentration, anion type,and hydrocarbon chain length of ILs. It has been previouslyreported that low concentration of ILs in mixed IL/surfactantsystems has a decreasing effect on surfactant CMC.1,11 Our

findings confirm this once more, since the addition of all ILsdecreases the CMC of CTAB (see Table 1). ILs interact withsurfactant micelles in two ways: (a) their hydrocarbon chaininteracts with CTAB chains through hydrophobic forces toform mixed micelles, and (b) they also act as an electrolyte andfacilitate CTAB micellization through electrostatic stabilizationof micellar surface.11 Acting together, these interactionsproduce a cosurfactant effect which appears as the CMCreduction. On this basis, as the concentration of ILs doubles(from 0.1% to 0.2%), the electrostatic repulsion of surfactanthead groups will even be more shielded by counterions andCMC values show further reduction. This decreasing effect ofILs on CMC is similar to the electrolyte effect of inorganic salts(NaCl) but is slightly different from it, since the contribution ofthe hydrophobic effect of their cationic portion results in afurther CMC reduction.This electrostatic effect shows itself once again when

discussing the effect of IL anion type on surfactantmicellization. The bigger the anion is, the more difficult itssolvation and the easier its binding to the micelle surface. Thus,the CMC value is even smaller in the presence of ILs withbigger hydrophilic anions, compared to small ones. The ionicradii ranking of anions in this study is given as

> >− − −BF Br Cl4

and, hence, the CMC of CTAB varies in a reverse sequence inthe presence of ILs with the same chain length but differentcounterions as follows:

< <

−

− −

BMImBF (0.36 mmol dm )

BMImBr (0.37 mmol dm ) BMImCl (0.42 mmol dm )4

3

3 3

Figure 2. Conductometric plots of (●) pure CTAB, (◇) CTAB in0.1% w/w BMImCl, (▲) CTAB in 0.1% w/w BM2ImCl, (×) CTABin 0.1% w/w N-BImCl, (■) CTAB in 0.1% w/w BMImBr, (□) CTABin 0.1% w/w BMImBF4, (+) CTAB in 0.1% w/w HMImCl, and (○)CTAB in 0.1% w/w HMImBr at 298.15 K.

Figure 3. Tensiometric plots of (●) pure CTAB, (○) CTAB in 0.1%w/w BMImCl, (■) CTAB in 0.1% w/w BMImBr, and (□) CTAB in0.1% w/w BMImBF4 at 298 K.

Table 1. Critical Micelle Concentration (CMC) and αValues Obtained by Tensiometry and ConductometryTechniques for CTAB Solutions in the Absence andPresence of Various ILs and NaCl

CMC (mmol dm−3)

% (w/w) of IL/salt via tensiometry via conductometry α

Pure CTAB0 0.91 0.92 0.30

BMImCl0.1 0.42 0.41 0.270.2 0.35 0.32 0.24

HMImCl0.1 0.57 0.54 0.230.2 0.38 0.38 0.15

BMImBr0.1 0.37 0.33 0.250.2 0.12 0.12 0.17

HMImBr0.1 0.53 0.49 0.210.2 0.21 0.21 0.13

BMImBF40.1 0.36 0.32 0.21

N-BImCl0.1 0.39 0.38 0.24

BM2ImCl0.1 0.44 0.41 0.32

NaCl0.1 0.51 0.330.2 0.43 0.34

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Nonetheless, it seems that there are other factors rather thanelectrostatic and hydrophobic effects that influence themicellization behavior of surfactants in the presence of ILs,since the effect of the IL chain length on CMC is somehowunusual. According to some previous studies, the reduction insurfactant CMC becomes smaller as the IL chain lengthincreases. This attitude is unlike our expectation of furtherCMC decrease as a result of stronger hydrophobic interactionsof longer chains (as is in surfactants mixed systems21) and hasbeen attributed to the hydrogen bonding ability of ILs.12 Thereare numerous reports on hydrogen bonding ability of C(2)−H,C(4)−H, and C(5)−H hydrogen atoms of imidazolium ringwith their counterions as well as with water molecules.22−24 Aschematic model of such hydrogen bondings is shown in Figure4. These interactions, along with electrostatic attraction

between the CTAB headgroup and anions, result in thetrapping of oppositely charged anions at the micelle’s polarshell and are expected to produce a cosurfactant effect whichdecreases the surfactant CMC, but it is observed (see Table 1)that, as the alkyl chain of ILs changes from butyl to hexyl, theCMC of CTAB increases. Thus, it seems the hydrogen bondingis not the main cause of the mentioned CMC increase in thepolar shell.In addition, N-BImCl and BM2ImCl were chosen as the

compounds with the maximum and minimum hydrogenbonding sites, respectively. Their molecular structures areshown in Figure 5. The hydrogen at the C(2) position formsthe strongest hydrogen bonding.23 In BM2ImCl, this position isoccupied by a methyl group. Based on our expectation, if thehydrogen bonding were responsible for the observed CMCincrease, this increase would be more noticeable for N-BImCland trivial for BM2ImCl compared to BMImCl, but this is notthe case. In other words, CMCs are almost the same (CMC inthe presence of BM2ImCl is even greater). Therefore, it islogical to say the polar shell hydrogen bondings do not affectthe CMC in this manner.

The severity of the decrease in CMC means that thesolubility of CTAB molecules in water rises under the influenceof a cosolvent factor of longer IL chains. Dissolving in water, ILmolecules disturb the water structure through hydrogenbondings. A disturbed solvent structure hosts CTAB moleculesbetter and delays their micellization. The steric effect becomesgreater when the IL’s alkyl chain is longer,resulting in a furtherdisturbance of the water structure. The severity of thedisturbance also increases with concentration in such a waythe water structure is completely disturbed and surfactantmolecules dissolve easier at high IL concentrations, thus theirCMC rises considerably.3 In fact, the steric effect of alkyl chainalong with the ability of imidazolium ring to disturb waterstructure through hydrogen bonding acts as a cosolvent effectand may be the main cause for the observed CMC increasewith IL’s alkyl chain elongation.The overall effect of ILs on the surfactant CMC seems to be

a combination of the mentioned factors, including electrolyteeffect, hydrophobic effect, steric effect, and hydrogen bondingability in both the solution and micellar phases. Depending onthe physical conditions (concentration, chain length, etc.), onefactor may dominate the others, but all of the factors act onsurfactant micellization simultaneously.

3.2. Degree of Micelle Ionization (α). When micelles areformed in solution, counterions are adsorbed on their surfacesand the concentration of free charged particles decreases,resulting in a reduction in the specific conductivity of thesolution. In conductometric plots, this phenomenon appears asa decrease in slope after the CMC point. The ratio of post-CMC slope to pre-CMC slope is defined as micelle ionizationdegree (α), which is an index of counterion bonding to themicelle surface; that is, a small α value means that counterionsare more bonded to the micelle surface. As seen from Table 1,α increases in the presence of inorganic salt (NaCl). Suchbehavior has been already reported by Jakubowska, whoreported that the CMC is usually decreased in the presence ofan electrolyte but the value of α may decrease or increase,depending on surfactant structure.25 Unlike NaCl, ILs reducethe value of α. As discussed above, the hydrogen bondingbetween their anions and imidazolium rings, along with theelectrostatic attraction between negative and positive charges,lead to an accumulation of counterions on the micelle’s polarshell and, subsequently, to a reduction in α value. As theconcentration of ILs rises, more IL molecules, together withbound counterions, enter the polar shell and a further reductionin α value is observed. Because of the limited hydrogen bondingcapacity and short alkyl chain of BM2ImCl molecule, itsbehavior is quite similar to that of an inorganic salt; that is, itraises the α value like NaCl. Here again, anion size affects the

Figure 4. Schematic model of hydrogen bonding of imidazolium ringhydrogens with anions and water molecules on the micelle surface.

Figure 5. Molecular structures of the BM2Im+ and N-BIm+ cations.

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value of α. BF4− is the largest anion and binds to the micelle

surface stronger than smaller anions (Br− and Cl−), resulting inthe maximum reduction of α (30→ 21). Replacement of thebutyl chain by the hexyl chain in ILs decreases the value of α,because the hydrophobic interactions between IL and CTABmolecules becomes stronger and the mole fraction of ILs inmixed micelles and, subsequently, the charge density andnumber of hydrogen bonding positions on micelle surface areincreased. Thus, more anions are adsorbed on the micellesurface, which, in turn, leads to the further reduction of α.3.3. Interfacial Properties. The surface activity of CTAB

molecules in the presence and absence of ILs, such as surfaceexcess concentration (Γmax), minimum surface area perheadgroup (Amin), surface tension at CMC point (γCMC), andsurfactant efficiency (pC20, which is the negative logarithm ofthe surfactant concentration needed to reduce the solutionsurface tension by 20 mN m−1) and effectiveness (πCMC = γ0 −γCMC) were specified using tensiometry data and aresummarized in Table 2.A quantitative measure of the surfactant adsorption at the

air/solution interface is usually obtained by the surface excessconcentration (Γmax), which can be determined using the Gibbsadsorption equation for dilute solutions:21

γΓ = − ⎡⎣⎢

⎤⎦⎥nRT C

12.303

ddlog

T Pmax

, (1)

Here, R is the gas constant, T the temperature, γ the surfacetension, C the surfactant concentration, and n the number ofspecies formed in solution during monomer dissociation. Theminimum area per surfactant headgroup (Amin) at the CMCpoint was also calculated using the following equation:21

=Γ

AN

(nm /molecule)10

min2

18

A max (2)

where NA is Avogadro’s number. To calculate Γmax,tensiometric plots were fitted to a polynomial equation (y =ax2 + bx + c) and then, the slope of the tangent at CMC pointwas considered as (dγ/dlog C).26 Thomas et al. have shown

that, if adsorption reaches a plateau before the CMC, then theanalysis is straightforward and can be reasonably accurate.However, both direct measurements at higher concentrationsand less direct measurements using added electrolyte indicatethat adsorption of anionic surfactants has generally not reacheda packing limit at the CMC, i.e., it is necessary to fit apolynomial to the data below the CMC and, because this isabruptly terminated at the CMC, there are no effectiveobjective criteria for the choice of that polynomial.27 Also,Menger et al. has argued that the interface is, in fact, notsaturated in region B.28,29 Nonlinear curvefitting to linearregion B slightly improves the calculated molecular areas.Recently, Pan have successfully modeled CTAB30 and TTAB(myristyltrimethylammonium bromide) adsorption at the air/water interface without the Gibbs adsorption isotherm.31

Inorganic salts have been shown to enhance the efficiencyand effectiveness of ionic surfactants. The addition ofelectrolyte to surfactant solution decreases the repulsionbetween surfactant headgroups at surface monolayer andfacilitates the further adsorption of surfactants at the air/solution interface. Therefore, the value of Γmax is also increased(the value of Amin decreases) in the presence of electrolytes.32

As shown in Table 2, ILs, unlike inorganic salts, slightlydecrease CTAB adsorption at the air/solution interface. Thisdemonstrates that their attitude toward surfactants is somethingmore than only an electrolytic effect. This is because of theirhydrocarbon chain, which increases their surface activity (seesection 3.4). IL molecules are themselves partially adsorbed atthe air/solution interface, which is confirmed by the lowersurface tension of IL solutions, compared to that of pure water(γ0 values). As a result, a part of the solution surface is occupiedby IL molecules and fewer CTAB molecules can enter thesurface monolayer. This phenomenon appears as a reduction inΓmax value and is more noticeable in the presence of HMImCland HMImBr, because of their longer chains and greatertendency toward the solution surface. Furthermore, an increasein the concentration of ILs also increases their mole fraction inthe surface monolayer (see section 3.5), resulting in the furtherreduction of Γmax. The Γmax order for ILs with different anions

Table 2. Tensiometric Parameters of CTAB in the Absence and Presence of Various ILs at 298 K

% (w/w) of ILs Γmax × 106 (mol/m2) Amin (nm2/molecule) γ0 (mN/m) γCMC (mN/m) πCMC (mN/m) pC20

Pure CTAB0 1.79 0.93 69.11 34.9 34.24 0.8

BMImCl0.1 1.52 1.09 68.74 34.12 34.58 1.20.2 1.18 1.41 66.49 33.52 32.97 1.5

BMImBr0.1 1.62 1.03 68.61 33.61 34.99 1.60.2 1.26 1.32 65.80 33.14 32.66 1.8

HMImCl0.1 0.90 1.85 67.38 34.38 32.92 1.40.2 0.76 2.18 64.51 34.12 31.04 1.9

HMImBr0.1 0.93 1.79 66.92 33.83 33.09 1.80.2 0.87 1.91 63.71 33.29 30.42 2.3

BM2ImCl0.1 1.56 1.07 68.53 34.23 34.20 1.3

BmimBF40.1 1.71 0.97 68.55 32.26 36.34 1.9

N-BimCl0.1 1.49 1.11 67.83 34.47 34.36 1.0

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but same cations (BMImBF4 > BMImBr > BMImCl) is thesame as their anion size order (BF4

− > Br− > Cl−), showing thecontribution of electrolyte effect as well. The more-effectivereduction of electrostatic repulsion between CTAB headgroupsby larger anions enhances CTAB adsorption at the air/solutioninterface and slightly increases the Γmax, but it is not so muchthat it can completely compensate the effect of surfaceoccupation by IL molecules.The effectiveness of surfactant adsorption (πCMC) is related

to the interfacial area occupied by it, which, in turn, isdependent upon the structural groupings and orientation ofsurfactant molecules at the interface.21 On this basis, thegeneral reduction in effectiveness of CTAB (with someexceptions) can be attributed to the increase of Amin andstructural properties of the CTAB/IL mixed monolayer.Nonetheless, all ILs enhance the efficiency of CTAB adsorption(greater pC20). The adsorption of ILs at the air/solutioninterface provides a more hydrophobic environment for CTABmolecules, raising their tendency toward surface adsorption.For N-BImCl, BMImCl, and BM2ImCl surfactants, pC20 variesin the reverse order of the hydrogen bonding capacity andhydrophilicity of the ILs. We suppose that the water structure ismore disturbed by its hydrogen bondings in the presence of N-BImCl. Therefore, the solubility of CTAB molecules increasesand their tendency toward solution surface decreases contrarily.This phenomenon is observed as a reduction in CTABadsorption efficiency (pC20).These tensiometric parameters suggest a considerable

interaction between surfactant and IL molecules. Suchinteractions may lead to the modification of surfactants’interfacial properties and aggregation behavior, which is ofindustrial importance, through some processes such as mixedmicellization in surfactant/IL systems. The next section(section 3.4) discusses such a possibility.3.4. Surfactant−IL Interactions in the Aggregated

Phase and in Monolayers at the Liquid/SolutionInterface. Imidazolium-based ILs with alkyl chains longerthan four carbon atoms are generally known to behave asamphiphilic compounds with surface activity.33 Their alkylchain has a key role in their interfacial properties. The longerthe carbon chain is, the higher their surface activity will be.34

On this basis, HMImCl and HMImBr were chosen toinvestigate their intermolecular interactions with CTAB, bothin micellar and monolayer phases. The tensiometric plots ofthese two ILs are shown in Figure 6, and parameters of thesetwo ILs are given in Table 3. Their higher CMCs, compared tothose of conventional surfactants, are due to their shorter alkyl

chains. The bigger size of the Br− anion, in comparison with theCl− anion, reduces the CMC of HMImBr to some extentthrough the electrostatic stabilization of micelles. It is alsoobvious that the surface excess concentration is higher for ILsthan for surfactants, as a result of their smaller molecules,delocalized positive charge, and better shielding of electrostaticrepulsions through hydrogen bonding to counterions. Inaddition, their adsorption efficiency is observed to be quitelower than that of the surfactants, because of their greaterhydrophilicity.Considering the surface activity of ILs, CTAB molecules are

expected to effectively interact with them and form mixedmicelles and monolayer. The nature and strength of suchinteractions can be determined using the regular solutionmodel developed for surfactant mixtures by Rubingh et al.35−37

Based on this model, the interaction parameter (β) in micellarphase, which is a measure of interaction strength, can becalculated using the following equations:38

αα− − −

=X C X C

X C X C( ) ln( / )

(1 ) ln[(1 ) /(1 ) ]11

M 21 12

M1 1

M

1M 2

1 12M

1M

2M

(3)

βα

=−

C X CX

ln( / )(1 )

M 1 12M

1 1M

1M 2

(4)

where X1M is the mole fraction of surfactant in mixed micelles,

α1 the total mole fraction of surfactant in solution, C1M the

CMC of the pure surfactant, C2M the CMC of the pure IL, and

C12M the CMC of their mixture. Similarly, the interaction

parameter for mixed monolayer (βσ) is calculated using thefollowing equations:39−41

αα− − −

=Z C Z C

Z C Z Cln( / )

(1 ) ln[(1 ) /(1 ) ]11

21 12 1 1

0

12

1 12 1 20

(5)

βα

=−

σ C Z CZ

ln( / )(1 )

1 12 1 10

12

(6)

Here again, Z1 is the mole fraction of surfactant in the mixedmonolayer, α1 is the total mole fraction of surfactant at theconcentration required to produce a certain value of γ,andC1

0,C20, and C12 are the molar concentrations of surfactant,

IL, and their mixture, respectively. In our experiments, α1, C1,C2 and C12 are the values at γ = 40.0 mN m−1.The calculated data are listed in Table 4. A negative βM value

means that there is either greater attraction or smaller repulsionbetween two different molecules than the molecules of thesame type. This is an expected result, since the hydrophobicinteraction between hexyl chain of the ILs and the longer chainof CTAB is greater than that of between two IL chains. On theother hand, ILs increase the ionic strength of the solution,thereby reducing the electrostatic repulsion between adjacentmolecules.34 It is obvious that the electrostatic stabilizationeffect of the greater Br− anion makes the interaction evenstronger (more negative β value),which shows the major role ofelectrostatic stabilization in negativity of β. Increasing the ILconcentration, their mole fraction in monolayer and mixedmicelles is observed to rise, resulting in a more negative β value(stronger intermolecular interactions may be as a result offurther anion trapping through hydrogen bonding along withelectrostatic attractions), smaller ionization degree (α), lowerCMC and effectiveness, and higher Amin and efficiency ofCTAB molecules, as mentioned previously.Figure 6. Tensiometric plot of (●) HMImBr and (○) HMImCl ILs.

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3.5. Micelle Size, Morphology, and IntermicellarInteractions. Mixed micellization definitely affects thestructural properties of micelles. The CV technique wasemployed to investigate the modifying effect of ILs on CTABmicelles. To trace the micelles, ferrocene was chosen as theelectroactive probe. Ferrocene simply penetrates into themicelles without perturbing them and its oxidation (eq 7)current can be used to acquire the diffusion coefficient ofmicelles.42,43

⇔ ++ −FC FC e (7)

The peak current (ipa) of the CV voltammogram is related tothe diffusion coefficient through the Randles−Sevcik equa-tion:44

ν= ⎜ ⎟⎛⎝

⎞⎠i FACn

nF DRT

0.4463pa

1/2

(8)

where n is the number of electrons involved in redox reaction,A the area of the electrode, F the Faraday constant, R the gasconstant, T the absolute temperature, D the diffusioncoefficient of the electroactive probe, C the probe concen-tration in the solution, and v the scan rate. Ferrocene moleculesenter the micelles because of their hydrophobicity. Thus, D ineq 8 corresponds to the micelle diffusion coefficient (Dm).At concentrations slightly above the CMC, aggregates

mutually interact which leads the calculated Dm values to belower than the real values at the CMC point. Therefore, linearinteraction theory was used for the calculation of interparticlesinteraction parameters:45,46

= − −D D k C[1 ( CMC)]m m0

d s (9)

where kd is the interparticle interaction parameter, D0m the self-

diffusion coefficient in the absence of particle interaction, andCs the surfactant concentration. The plots of Dm versus (Cs −CMC) in various CTAB/IL/aqueous solutions are shown inFigure 7. The intercept and slope of these plots give the valuesof kd and Dm

0 .The kd and Dagg

0 values are summarized in Table 5. It can beseen that the addition of ILs decreases the value of intermicellarinteraction parameter (kd), since the surface charge of micellesare almost neutralized by them and the electrostatic interactionbetween micelles is lowered. The decrease in kd with theaddition of IL indicated that the linear spherical expansion ofmicelles becomes predominant. The CTAB micellar structures

remained spherical and no dramatic structural changes tookplace in the solutions. As shown in Figure 8, TEM imagesconfirmed the results of the CV studies. Furthermore, it can beseen that the size of CTAB micelles grow from 5 nm (ref 47) toroughly 20−30 nm in the presence of BMImBr. With furtheradditions of IL, the kd values slightly increased, because ofmicellar growth. A higher electrolyte concentration results inbetter electrostatic stabilization of micelles surface and therebyfurther micellar growth. The role of electrostatic stabilization iseven more obvious when the effect of anion size is taken intoaccount. It is seen that the kd follows a reverse size order:

< <BMImBF BMImBr BMImCl4

Nevertheless, the behavior of HMImCl and HMImBr isdifferent. As shown in Table 5, increasing the HMImBr orHMImCl concentration in the aqueous solution up to 0.2%resulted in no significant change in kd for the CTAB systems.Despite the decrease in micellar surface charge density with theaddition of IL, no significant variation in kd values was

Table 3. CMC Values and Tensiometric Parameters Obtained for the Aqueous Solutions of HMImCl and HMImBr ILs

CMC (mmol dm−3) Γmax × 106 (mol/m2) Amin (nm2/molecule) γ0 (mN/m) γCMC (mN/m) πCMC (mN/m) pC20

HMImCl358 2.99 0.56 70.17 33.38 36.79 −2.1

HMImBr312 3.14 0.53 70.17 33.15 37.02 −1.8

Table 4. Calculated Interaction Parameters for CTAB−HMImCl and CTAB−HMImBr Mixed Systems

% (w/w) of IL βm βδ X1m f1

m Z f1δ

HMImCl0.1 −4.57 −8.73 0.78 0.80 0.62 0.2910.2 −4.97 −10.79 0.69 0.63 0.52 0.083

HMImBr0.1 −5.13 −15.60 0.77 0.74 0.49 0.0160.2 −6.60 −13.56 0.63 0.41 0.47 0.023

Figure 7. (a) Plots of Dm vs CTAB concentration (Cs − CMC) foraqueous solutions of (■) pure CTAB, (▲) CTAB + 0.1% w/wBMImCl, (□) CTAB + 0.1% w/w BMImBr, ( × ) CTAB + 0.1% w/wHMImCl, (○) CTAB + 0.1% w/w HMImBr, and (●) CTAB + 0.1%w/w BMImBF4, in the presence of ferrocene and 0.1 M NaCl. (b)Plots of Dm vs CTAB concentration (Cs − CMC) for aqueoussolutions of (▲) CTAB + 0.2% w/w BMImCl, (□) CTAB + 0.2% w/w BMImBr, (×) CTAB + 0.2% w/w HMImCl, and (○) CTAB + 0.2%w/w HMImBr, in the presence of ferrocene and 0.1 M NaCl.

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observed. This unexpected behavior in kd values can be mainlyattributed to the spherical-to-cylindrical morphology transition.Representative TEM photographs of CTAB mixed micelles at0.2% HMImBr confirmed the above morphology results (seeFigure 8). The average length of the cylindrical structures inFigure 8 is ∼20−30 nm. The morphology modification is ofgreat importance for some applications such as drug deliveryand nanoenzyme synthesis, where the larger nanoparticles areneeded.

4. CONCLUSIONThe structural properties of ionic liquids (ILs) have a greateffect on their attitude toward surfactants. Specification of theireffect on the surface and micellization behavior of thesurfactant, and relating it to their structural properties, isquite complicated and should be done by taking into accounttheir electrolyte effect, hydrophobic interactions, and hydrogenbonding capacity. Although one of these factors may dominatethe others, depending on the physical and structural conditions,all of them simultaneously act on the surfactants to differentextents. On this basis, this study showed that the short-chainILs mainly act on cetyl trimethylammonium bromide (CTAB)as electrolytes, while for long-chain ILs, the hydrophobic effect

of the alkyl chain is also very important. The ability ofimidazolium-based ILs to form hydrogen bondings with anionsand water molecules also affects the surface activity of thesurfactant, as either a cosolvent or a cosurfactant factor. Resultsrevealed the considerable contribution of anion type to theelectrolytic effect of ILs. ILs were also found to generallymodify the CTAB micelles, both in size and morphology, whichis of great importance to their applications in industrialprocesses.

■ ASSOCIATED CONTENT*S Supporting InformationThis material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail addresses: javadians@yahoo.com, javadian_s@modares.ac.ir.NotesThe authors declare no competing financial interest.

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