DOI: 10.1002/cphc.201100866

The Chameleon-Like Nature of Zwitterionic Micelles:The Effect of Ionic Liquid Addition on the Properties ofAqueous Sulfobetaine MicellesVishal Govind Rao, Chiranjib Ghatak, Surajit Ghosh, Sarthak Mandal, and Nilmoni Sarkar*[a]

1. Introduction

Understanding the different interactions involved in the micel-lization process is very important for the application of micellarsystems in nanomaterial synthesis,[1–3] drug delivery,[4, 5] separa-tions,[6] pharmaceutical formulations, and other dispersanttechnologies.[6, 7] The interaction of ions with micelles of aque-ous ionic surfactants is generally described in terms of electro-static and ion-specific nonelectrostatic interactions.[8, 9] General-ly, electrolyte addition reduces the critical micellar concentra-tion (CMC) due to interaction of the added electrolyte with themicellar interface.[10–13] Nonionic and zwitterionic surfactantshave lower CMC values than similar ionic surfactants because,in the case of the former, ionic dipoles interact withoutcharged head group repulsion. Zwitterionic surfactants, suchas sulfo- and carboxybetaines, show chameleon-like behavior:they are formally neutral, but their micelles incorporate anionsowing to the higher charge density of the cationic ammoniuminner surface compared with that of the anionic sulfonate orcarboxylate surface.[14–22] The interactions between the ions ofthe added electrolytes and the sulfobetaine micelles werestudied by Okada et al. and other groups by using partitionand ion-pair models.[20, 23–32] In the partition model, the ionicchemical potential term is included in the Poisson–Boltzmannequation, whereas in the ion-pair model the inner and outerpotentials of the zwitterionic micelle are considered. Anionsrather than cations bind to zwitterionic sulfobetaine micellesand binding increases with soft anions (Pearson definition) fol-lowing the Hofmeister series.[24, 25, 33–37] Less hydrophilic anions(e.g. ClO4

�) bind much more strongly than halide ions, the

binding of which follows the sequence I�>Br�>Cl� ; OH� isonly weakly bound.[36–38] The softer, less hydrophilic anions canenter the head group region and interact with the quaternaryammonium ion, whereas the sulfonate ion remains stronglyhydrated, which makes it incapable of interacting stronglywith cations. The incorporation of inorganic anions into sulfo-betaine micelles was studied by capillary electrophoresis[35] andother physical methods. Anion addition makes the sulfobetainemicelle an anionoid, with an increasingly negative surfacecharge; further addition of salt results in interactions of theseanionoid micelles with cations of the electrolytes or H3O+ pres-ent in the system.[22, 23, 25, 33–36] One can perform both acid-cata-lyzed and anionic nucleophilic substitution reactions in thezwitterionic sulfobetaine micelles by the use of suitable elec-trolytes. The incorporation of anions in zwitterionic surfaces isrelevant for the understanding of the behavior of biologicalmembranes.[24, 25, 37, 39, 40]

The addition of anions changes the zwitterionic surfaces, af-fecting both the packing and surface charges, and shouldmodulate properties of these surfaces. Nowadays, favorablymodifying the physicochemical properties of aqueous surfac-tant solutions by the addition of environmentally benignroom-temperature ionic liquids (RTILs) has received enormous

We used fluorescence probing, z potentials, and dynamic lightscattering measurements to study the interactions betweenthe zwitterionic surfactant N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) and three ionic liquids (ILs), 1-ethyl-3-methylimidazolium ethylsulfate ([C2mim][C2SO4]), 1-ethyl-3-methylimidazolium n-butylsulfate ([C2mim][C4SO4]), and1-ethyl-3-methylimidazolium n-hexylsulfate ([C2mim][C6SO4]).The three ILs have the same cationic part and their anionicparts differ only in the length of the alkyl chain. The aim of ourwork is to offer a comparative study and establish the role ofthe alkyl chain length of the anion of ILs on 1) the incorpora-tion of these anions in the zwitterionic micelles of SB-16 (selec-tivity of anions) and 2) the physicochemical properties of aque-ous solutions of SB-16. Results show that, at lower concentra-

tions (i.e. �20 mm), the different ILs modify the properties ofthe aqueous SB-16 solution in similar manner. All of thembring about a decrease in the critical micelle concentration(CMC) and also in size, and increase the aggregation numberof the SB-16 micelles ; these effects are more dramatic when[C2mim][C6SO4] is used as the additive rather than [C2mim]-[C4SO4] and [C2mim][C2SO4]. It is proposed that, in case of[C2mim][C6SO4] , the presence of a hexyl chain on the hexylsul-fate ion allows the ion to align itself with the tail part of SB-16,whereas, in the case of [C2mim][C2SO4], the presence of ethylchain in the ethylsulfate ion is not sufficient to bring about asimilar alignment of the ethylsulfate anion with the tail part ofSB-16. This difference in the location of the anions of the ILs isresponsible for the different behavior of the ILs.

[a] V. G. Rao, C. Ghatak, S. Ghosh, S. Mandal, Prof. N. SarkarDepartment of ChemistryIndian Institute of TechnologyKharagpur 721302, WB (India)E-mail : nilmoni@chem.iitkgp.ernet.in

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attention.[41–46] Due to its unusual properties, an IL may demon-strate a unique role in altering the properties of aqueous sur-factant solutions.[44] Increasing scientific interest in RTILs stemsfrom their environmentally friendly properties, the most impor-tant of which are low vapor pressures, wide liquid temperatureranges, and the ability to control some physical properties ofthe liquid by incorporation of appropriate functionalgroups.[47–51] The idea of forming micelles with desirably modi-fied physicochemical properties within aqueous IL solutions isexciting, useful, and economically convenient. Pandey et al.and others have shown the change in the behavior of aqueoussurfactant solutions with the addition of RTILs.[41–46]

Understanding the effects of RTIL addition on the propertiesof aqueous surfactant solutions is very important. Due to thedesigner nature of the ILs,[47–51] one can design different typesof ILs with different properties. Therefore, it is not possible togeneralize the effects of RTIL addition on the properties ofaqueous surfactant solutions. The variation in CMC of a surfac-tant solution with the addition of RTILs can be monotonic aswell as nonmonotonic. The CMC of sodium dodecyl sulfate(SDS), decreases with the addition of low concentrations of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]),whereas it increases with the addition of high concentrationsof [C4mim][BF4] .[41] Smirnova et al.[43] have shown that the CMCof SDS, decreases monotonically with the addition of long-chained ILs capable of forming micelles in their individualaqueous solutions. In many cases, addition of the IL does notalter the CMC value to a great extent, but the structure of themicelle is modified to a great extent.[42] Zheng et al. showedthat addition of the IL 1-butyl-3-methylimidazolium bromide([C4mim][Br]) affected aggregation of the triblock copolymerPluornic P104, and at a high concentration of the RTIL (>1 m)very large aggregates with diameter of around 500 nmformed.[52] Recently, Dey et al. showed that the addition of theRTIL1-pentyl-3-methylimidazolium bromide ([C5mim][Br]) to a5 wt % triblock copolymer (P123) micelle led to the formationof giant P123–RTIL clusters 40 nm in diameter in 0.9 m and3500 nm in diameter in 3 m RTIL.[53] These clusters are muchlarger than the P123 micelle (18 nm).

Recently, Behera and Pandey showed changes in importantphysicochemical properties of an aqueous solution of the zwit-terionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-pro-panesulfonate (SB-12) upon the addition of the hydrophilic IL[C4mim][BF4] .[45] They also carried out a comparative study onthe changes in physicochemical properties of aqueous SB-12upon addition of [C4mim][BF4] and 1-butyl-3-methylimidazoli-um hexafluorophosphate ([C4mim][PF6]).[46] Their work revealedthat the addition of [C4mim][PF6] modifies the physicochemicalproperties of aqueous SB-12 to a greater extent than that of[C4mim][BF4] .

Herein, we used N-hexadecyl-N,N-dimethylammonio-1-pro-panesulfonate (SB-16) as the zwitterionic surfactant. We havecarried out a comparative study of the changes in the physico-chemical properties of an aqueous solution of SB-16 upon theaddition of three different ILs, 1-ethyl-3-methylimidazoliumethylsulfate ([C2mim][C2SO4]), 1-ethyl-3-methylimidazolium n-butylsulfate ([C2mim][C4SO4]), and 1-ethyl-3-methylimidazolium

n-hexylsulfate ([C2mim][C6SO4]). All three ILs have the same cat-ionic part and the anionic parts only differ in the alkyl chainlength. Due to longer alkyl chain length, [C2mim][C6SO4] ismore hydrophobic than [C2mim][C2SO4] and [C2mim][C4SO4].Therefore, it is expected that the three ILs will modify thephysicochemical properties of the SB-16 solution, such as CMCand aggregation number, in different manners. We used z po-tential measurements to show the chameleon-like nature of anaqueous micellar solution of SB-16 and the differential incorpo-ration of ILs.

Experimental Section

Materials and Sample Preparation

SB-16, pyrene, and cetylpyridinium chloride (CPC) were purchasedfrom Sigma–Aldrich and used as received. Coumarin-153 (C-153)and Rhodamine 6G (R6G; laser grade, Exciton) were used as re-ceived. [C2mim][C2SO4], [C2mim][C4SO4], and [C2mim][C6SO4] wereobtained from Bioniqs (>99 % purity) and were also used as re-ceived. Doubly distilled deionized water (Milli-Q water) was usedfor sample preparation. Stock solutions of pyrene and CPC wereprepared in ethanol and water, respectively. The final concentrationof probe molecule (pyrene) in all measurements was kept at 5 �10�7

m. Aqueous SB-16 solutions of the probe were prepared bytaking appropriate aliquots of the probe from the stock solutionand then evaporating the ethanol by using a stream of nitrogengas. The desired concentration of aqueous SB-16 was added toachieve the required final probe concentration. Calculatedamounts of the ILs were added directly to the aqueous SB-16 solu-tions. All experiments were performed at 298 K. The structures of[C2mim][C2SO4], [C2mim][C4SO4], [C2mim][C6SO4], pyrene, R6G, CPC,and zwitterionic surfactant SB-16 are shown in Scheme 1.

Instrumentation

The fluorescence spectra were recorded by using a Hitachi (modelno. F-7000) spectrofluorimeter. All samples were excited at 337 nm.For size and z potential measurements, we used a Malvern NanoZS instrument employing a 4 mW He–Ne laser operating at awavelength of 633 nm. For anisotropy measurements, we used aPerkinElmer LS-55 luminescence spectrometer equipped with afilter polarizer and a thermostated cell holder. For the time-re-solved anisotropy measurements, we used a time-correlatedsingle-photon counting (TCSPC) instrument. For viscosity measure-ments at 298 K, we used a Brookfield DV-II + Pro viscometer. Thetemperature was controlled by using a circulating bath (ThermoNeslab, RTE 7).

2. Results and Discussions

2.1. Determination of CMC Values by Using Pyrene as aFluorescence Probe

The unique response of pyrene fluorescence to solvent polaritywas utilized to determine the CMC of SB-16 in the presenceand absence of ILs. The ratio of fluorescence intensities of thefirst and third vibronic bands, II/IIII, decreases with decreasingsolvent polarity. With the formation of micelles, the nonpolarpyrene molecules partition into the “hydrophobic” micellarphase, resulting in an abrupt decrease in II/IIII. This abrupt de-

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crease in II/IIII forms the basis of CMC determination for mi-celle-forming surfactant solutions. Following the procedure re-ported by Baker et al.[54] we plotted pyrene II/IIII versuslog [SB-16] in the presence of different amounts of ILs(Figure 1). CMC values were determined by normalizing thetotal sigmoidal response in II/IIII to 1.0 and then estimating theconcentration at which the amplitude decreased to 0.01. TheCMC values of aqueous SB-16 in the presence of differentamounts of ILs are reported in Table 1.

There are few reports in the literature on the effects of vari-ous external additives on the properties of aqueous zwitterion-ic surfactants[55–58] that can be used to explain the observed re-sults. It was shown that the addition of a common electrolyte,NaCl, resulted in a decrease in CMC of aqueous zwitterionicSB-12.[57] The trend in CMC variation for aqueous solutions ofSB-16 upon the addition of [C2mim][C2SO4] , [C2mim][C4SO4],and [C2mim][C6SO4] is same as that observed for the CMC var-iation of aqueous cationic cetyltrimethylammonium bromide(CTAB) upon the addition of [C6mim][Br][59] and aqueous anion-ic SDS,[41] as well as the effect of aqueous zwitterionic SB-12[45]

upon the addition of [C4mim][BF4] (a decrease at lower [IL] isfollowed by an increase as the [IL] is increased). Therefore, wecan conclude that, at lower concentrations, the three ILs act aselectrolytes. The decrease in CMC at low concentrations of ILsfollows the order of [C2mim][C6SO4]> [C2mim][C4SO4]>[C2mim][C2SO4] . This observed trend can be explained by thefact that, in the case of [C2mim][C6SO4] , the presence of the

hexyl chain on the hexylsulfate ion allows it to align with thetail part of SB-16, whereas in case of [C2mim][C2SO4] the ethylchain in the ethylsulfate ion is apparently unable to align itwith the tail part of SB-16. Hence, this difference in the loca-tion of the anions of ILs is responsible for the different behav-ior of the ILs. The increase in CMC at higher concentrations ofILs can be attributed to the fact that [C2mim][C2SO4], [C2mim]-[C4SO4] , and [C2mim][C6SO4] act as cosolvents at higher con-centrations, and low solvophobicity between the IL and hydro-phobic tail of the surfactant molecule leads to a high CMC.This reversal in the role of ILs at higher concentrations is sup-ported by earlier reports.[41, 45, 59]

2.2. Determination of Micellar Aggregation Number (Nagg)by the Fluorescence Quenching Method

The aggregation numbers (Nagg) of aqueous SB-16 in the pres-ence of different amounts of [C2mim][C2SO4], [C2mim][C4SO4],and [C2mim][C6SO4] were obtained by observing the fluores-cence quenching behavior of pyrene by CPC. Equation (1) wasused to determine the micellar aggregation number:[60–64]

lnIO

IQ

� �¼< n >¼ Qmicelle

micelle½ � ¼CPC½ �micelle

micelle½ �

¼ CPC½ �micelle

Nagg

SB� 16½ � � CMC

� � ð1Þ

in which I0 and IQ are fluorescence intensities of pyrene in theabsence and presence of quencher CPC, respectively; <n> isthe mean occupancy number of the quencher molecule (CPC);Qmicelle (i.e. [CPC]micelle), [micelle] , and [SB-16] are the concentra-tions of quencher CPC within the micellar phase, micelles, andSB-16 surfactant, respectively. The aggregation number is de-termined from the slope of a plot of ln IO

IQ

� �versus [CPC]micelle

for all of the systems. Plots of ln IO

IQ

� �versus [CPC]micelle for

pyrene quenching by CPC within a 28 mm aqueous solution ofSB-16 in the presence of different amounts of [C2mim][C2SO4] ,[C2mim][C4SO4] , and [C2mim][C6SO4] are shown in Figure 2. Theaggregation numbers of aqueous SB-16 in the presence of dif-ferent amounts of ILs is reported in Table 2.

As far as micellar aggregation number (Nagg) is concerned, itwas shown that increasing [NaCl] increased the Nagg of SB-12[65]

and n-alkyl betaines with C12, C16, and C18.[58] An increase in Nagg

of aqueous SB-12 was also observed when the concentrationof an anionic cosurfactant, sodium dodecyl sulfonate, in-creased from 0.2 to 0.7 molar fraction.[65] It is also noteworthythat Nagg increases with the addition of 2 wt % [C4mim][BF4] toaqueous zwitterionic SB-12.[45] In the same way, we observed adramatic increase in Nagg at lower concentrations of ILs, whichfurther confirmed that, at lower concentration, the three ILsacted as electrolytes. The aggregation number of the SB-16 mi-celle was 81. Interestingly, with the addition of 20 mm [C2mim]-[C6SO4] the aggregation number of the SB-16 micellar aggre-gate increases to 113, whereas with the addition of 20 mm

[C2mim][C2SO4] it increases to only 86. This observation furtherconfirms very strong interactions of [C2mim][C6SO4] with qua-

Scheme 1. Structures of the ILs [C2mim][C2SO4], [C2mim][C4SO4], and[C2mim][C6SO4] , as well as those of the fluorescence probes pyrene and R6G,the quencher CPC, and the zwitterionic surfactant SB-16.

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ternary ammonium ions of SB-16 micellar aggregates and dif-ferences in the location of the anions of the added ILs in theSB-16 micellar pseudophase, which in turns leads to differentaggregation numbers. This influence of binding strength ofadded electrolyte on the micellar properties is supported byLei et al.[66] The aggregation number decreases at higher con-centrations of ILs. With the addition of 800 mm [C2mim][C6SO4]and [C2mim][C2SO4] the aggregation number of the SB-16 mi-cellar aggregates decreases to 5 and 62, respectively. This de-crease in aggregation number at higher concentrations of ILscan be attributed to the fact that [C2mim][C2SO4] , [C2mim]-[C4SO4] , and [C2mim][C6SO4] act as cosolvents at higher con-

centrations and low solvophobicity between the IL and the hy-drophobic tail of surfactant molecules lead to lower aggrega-tion numbers. The huge decrease in Nagg in the case of[C2mim][C6SO4] addition can be explained by the presence ofthe hexyl chain on the hexylsulfate ion. This makes it more hy-drophobic in nature than the other two ILs, [C2mim][C2SO4]and [C2mim][C4SO4] . Consequently, solvophobicity betweenthis IL and the hydrophobic tail of the surfactant molecule islowest, which is responsible for unfavorable micellization and adramatic lowering of Nagg. The decrease in aggregationnumber and increase in CMC with the addition of different or-ganic solvents has been reported by many groups.[67–70] This re-versal of the role of ILs at higher concentrations is also sup-ported by other earlier reports.[41, 45, 59]

2.3. Determination of Average Aggregate Size by DynamicLight Scattering

We used dynamic light scattering (DLS) to obtain the averagesize of the aggregates of aqueous SB-16 in the presence of dif-ferent amounts of [C2mim][C2SO4], [C2mim][C4SO4], and[C2mim][C6SO4] . Figure 3 shows the variation in scattering in-tensity with the diameter of the micellar aggregates. The sizesof the micellar aggregates in the presence of different

Figure 1. Pyrene (5 � 10�7m) II/IIII versus log [SB-16] in the presence of different amounts of ILs.

Table 1. CMC values of aqueous SB-16 in the presence of differentamounts of ILs.

Concentration CMC [mm] from pyrene fluorescenceof additive [mm] [C2mim][C2SO4] [C2mim][C4SO4] [C2mim][C6SO4]

0 0.096�0.005 0.096�0.005 0.096�0.00520 0.083�0.005 0.064�0.004 0.054�0.004100 0.108�0.007 0.115�0.008 0.212�0.006400 0.639�0.006 1.179�0.009 2.689�0.008800 2.233�0.010 5.742�0.014 15.156�0.0261400 7.434�1.543 16.289�2.724 54.242�5.423

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amounts of ILs are reported in Table 2 and plotted in Figure 4.The size of the SB-16 micelle was 7.4 nm. With the addition of20 mm [C2mim][C6SO4] the size of the SB-16 micellar aggregatedecreased to 4.0 nm, whereas with the addition of 20 mm

[C2mim][C2SO4] it decreased to 5.8 nm. The trend in sizechange for the aqueous SB-16 solution upon addition of[C2mim][C2SO4] , [C2mim][C4SO4], and [C2mim][C6SO4] was sameas that observed for the size change of aqueous zwitterionicSB-12[45] upon addition of [C4mim][BF4] . The decrease in size atlower concentrations of ILs followed the order of [C2mim]-[C6SO4]> [C2mim][C4SO4]> [C2mim][C2SO4]. As in the case ofCMC variation, this order can be explained on the basis of

varying structural traits(length of the alkyl chain)of the three ILs, which isinstrumental in aligningthe anion of the ILs withthe tail part of SB-16. Incase of [C2mim][C6SO4] ,the hexyl chain on thehexylsulfate ion enables itto align with the tail partof SB-16, whereas in caseof [C2mim][C2SO4], the

ethyl chain in the ethylsulfate ion is apparently unable to dothe same.

2.4. Determination of z Potential

The z potential indicates the degree of repulsion between ad-jacent similarly charged aggregates in the solution. The abso-lute value of z potential is the potential difference betweenthe dispersion medium and the stationary layer of fluid at-tached to the dispersed micellar aggregates. The magnitude ofthe z potential gives an indication of the potential stability ofthe colloidal system. We used a Malvern Nano ZS instrument

Figure 2. Plots of ln IO

IQ

� �versus [CPC]micelle for pyrene (5 � 10�7

m) quenching by CPC in a 28 mm aqueous solution of SB-16 in the presence of differentamounts of ILs.

Table 2. Aggregation number (Nagg) and average aggregate diameter (�5 %) of a 28 mm aqueous solution of SB-16 inthe presence of different amounts of ILs.

Concentrationof

Nagg Diameter [nm]

additive [mm] [C2mim][C2SO4] [C2mim][C4SO4] [C2mim][C6SO4] [C2mim][C2SO4] [C2mim][C4SO4] [C2mim][C6SO4]

0 81�8 81�8 81�8 7.4 7.4 7.420 86�9 97�8 113�9 5.8 4.8 4.0100 75�7 62�7 46�9 6.6 6.3 5.4400 67�9 50�9 22�6 7.5 7.2 4.8800 62�8 41�6 5�3 8.4 8.2 4.41400 46�8 27�9 – 8.6 8.5 –

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to determine the z potential of SB-16 solutions in the presenceof different amounts of ILs, [C2mim][C2SO4] , [C2mim][C4SO4],and [C2mim][C6SO4] . Since z potential drastically changes witha slight variation in pH, we used sodium tetraborate buffer(pH 9.0) for sample preparation. Figure 5 shows the variationof z potential of SB-16 solutions with the concentration of ILs.The negative value of the z potential became more negativewith the addition of ILs. The negative value of z potential in-

creases more rapidly with the addition of [C2mim][C6SO4] thanthose of [C2mim][C2SO4] and [C2mim][C4SO4]. The z potentialsof SB-16 solutions in the presence of different amounts of ILsare reported in Table 3.

The z potential of the SB-16 micelle in sodium tetraboratebuffer (pH 9.0) was �24.0 mV. Interestingly with the additionof 20 mm [C2mim][C6SO4] the z potential of the SB-16 micellaraggregate decreased to �69.0 mV, whereas with the addition

Figure 3. DLS results for the variation in size of a 28 mm aqueous solution of SB-16 in the presence of different amounts of ILs.

Figure 4. Variation in size of a 28 mm aqueous solution of SB-16 in the pres-ence of different amounts of ILs.

Figure 5. The variation in z potential of 28 mm SB-16 (in sodium tetraboratebuffer, pH 9.0) in the presence of different amounts of ILs.

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of 20 mm [C2mim][C2SO4] it decreased to �31.2 mV. The rapidincrease in negative z potential with the addition of [C2mim]-[C6SO4] clearly indicates very strong interactions of [C2mim]-[C6SO4] with quaternary ammonium ions of SB-16 micellar ag-gregates, which lead to more anionoid SB-16 micellar aggre-gates. In keeping with the earlier line of reasoning, one can as-cribe this strong interaction once again to the presence of thelong alkyl chain in the hexylsulfate ion, which helps in the in-corporation of [C2mim][C6SO4] in the micellar system. At higherconcentrations of ILs the negative values of the z potential de-crease to less negative values (Table 3), which clearly indicatesthat initial anion incorporation of the ILs leads to the formationof the SB-16 micelle anionoid and further addition of ILs leadsto incorporation of cations of the ILs.

2.5. Determination of the Dipolarity of the MicellarPseudophase from the Fluorescence Probe Behavior ofPyrene

We used the pyrene II/IIII ratio to determine the changes in thedipolarity of the micellar pseudophase within 28 mm aqueoussolutions of SB-16 in the presence of different amounts of ILs.Figure 6, which represents the pyrene II/IIII ratios for post-micel-lar aqueous SB-16 in the presence of different amounts of ILs,clearly indicates that the pyrene II/IIII ratio appears to be inde-pendent of the presence of [C2mim][C2SO4]. This indicates thatthe cybotactic region of pyrene remains unchanged by the ad-dition of [C2mim][C2SO4]. In the case of [C2mim][C6SO4] addi-

tion, the pyrene II/IIII ratio increases almost linearly at higherconcentrations. It is well established that, on average, pyrenepositions itself in the palisade layer of the micellar pseudo-phase,[71–73] so an increase in the pyrene II/IIII ratio clearly indi-cates that the cybotactic region is modified by the addition of[C2mim][C6SO4] , which further confirms penetration of the SB-16 micelle by [C2mim][C6SO4]. It is noteworthy to mention thatat higher [C2mim][C6SO4] concentrations a rapid reduction inNagg (with no significant change in micellar size) may allow in-creased water penetration into the micellar pseudophase,which in turn can also lead to increased dipolarity.[45] The mi-crofluidity result (discussed in Section 2.6) also supports thisproposition. We also measured the pyrene II/IIII ratio in varioussolvents and compared these values with those obtained forpost-micellar aqueous SB-16 in the presence of differentamounts of ILs. For the latter, the pyrene II/IIII ratios vary from1.064 to 1.095, which are almost equal to that in 1-propanol(1.08). Therefore, we can say that the polarity of the palisadelayer in the SB-16 micelle, where pyrene is solubilized, is moreor less similar to 1-propanol.

2.6. Determination of Microfluidity (or Microviscosity) fromR6G Fluorescence and Time-Resolved AnisotropyMeasurements of C-153

Fluorescence anisotropy, which implies the extent of the polar-ization of the emitted radiation from a probe, depends on theoverall geometry of the rotating fluorescence unit as well asthe microfluidity of its cybotactic region.[74] Generally, thelower the microfluidity of the immediate environment for agiven rotor, the higher the resulting anisotropy. Accordingly,fluorescence anisotropy is a powerful tool to investigate thecybotactic region microfluidity surrounding fluorescent solutesin a variety of complex media, such as micelles, microemul-sions, polymers, and sol–gel-derived glasses.[74] The steady-state fluorescence anisotropy (r) is given by Equation (2):[55]

r ¼Ik � I?

Ik þ 2I?ð2Þ

in which Ik and I? are the parallel and perpendicular polarizedcomponents of the radiation emitted from a fluorophore fol-lowing photoexcitation with vertically polarized light. Figure 7shows the variation of steady-state anisotropy < r > of R6G(2.5 mm) in a 28 mm aqueous solution of SB-16 with increasingamounts of [C2mim][C2SO4] , [C2mim][C4SO4], and [C2mim]-[C6SO4] . In the case of [C2mim][C2SO4], the anisotropy remainsalmost unchanged, whereas it decreases rapidly in the case of[C2mim][C4SO4] and [C2mim][C6SO4]. This is the most counterin-tuitive result because with the addition of ILs the bulk viscosityof the SB-16 solutions increases (Table 4). Therefore, it is ex-pected that the addition of ILs to aqueous solutions of SB-16will reduce the microfluidity, but we have observed the re-verse. Microfluidity probes are solubilized within the more vis-cous SB-16 micellar pseudophase in the absence of ILs. In thecase of [C2mim][C2SO4] addition, the steady-state anisotropy re-mains almost unchanged, whereas it decreases rapidly in the

Table 3. The z potential (�5 %) of 28 mm SB-16 (in sodium tetraboratebuffer, pH 9.0) in the presence of different amounts of ILs.

Concentration z Potential [mV]of additive [mm] [C2mim][C2SO4] [C2mim][C4SO4] [C2mim][C6SO4]

0 �24.0 �24.0 �24.020 �31.2 �45.0 �69.0100 �24.3 �44.2 �61.6400 �21.0 �42.7 �53.2800 �20.3 �38.0 �44.01400 �20.0 �36.2 –

Figure 6. Pyrene (5 � 10�7m) II/IIII ratios in a 28 mm aqueous solution of SB-16

in the presence of different amounts of ILs.

ChemPhysChem 2012, 13, 1893 – 1901 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 1899

Chameleon-Like Nature of Zwitterionic Micelles

case of [C2mim][C4SO4] and [C2mim][C6SO4] at higher concen-trations. Once again we can conclude that, at higher [C2mim]-[C6SO4] concentrations, a rapid reduction in Nagg (with no sig-nificant change in micellar size) may allow increased waterpenetration into the micellar pseudophase, which in turn re-duces the microviscosity (or increases the microfluidity) aroundthe probe molecule (R6G). Furthermore, we determined themicroviscosity (hmic) inside the micellar pseudophase throughthe well-known Stokes–Einstein–Debye relationship [Eq. 3)]:

trh i ¼hmicV

kBTð3Þ

in which V is the volume of the fluorophore, kB is the Boltz-mann constant, htri is the average rotational relaxation time,and T is the absolute temperature. We used C-153 as a fluoro-phore, the volume of which was determined by following themethod reported by Choudhury et al.[75] From the steady-stateanisotropy of R6G, it is clear that the maximum change in mi-croviscosity of the SB-16 micellar pseudophase is observed inthe case of [C2mim][C6SO4] addition. Therefore, we measuredthe time-resolved anisotropy of C-153 in a 28 mm aqueous so-lution of SB-16 and in the same solution containing 800 mm

[C2mim][C6SO4] . The average rotational relaxation time of C-153 at 298 K was 1.12 ns for the former and it decreased to0.40 ns for the latter. The microviscosity obtained in a 28 mm

aqueous solution of SB-16 was quite large, about 18.8 cP, rela-tive to the bulk viscosity (0.92 cP). On the other hand, the mi-

croviscosity obtained for a 28 mm aqueous solution of SB-16containing 800 mm [C2mim][C6SO4] was about 6.7 cP, that is,only slightly larger than the bulk viscosity (2.18 cP). Therefore,we can say that, due to differential incorporation of ILs and arapid reduction in Nagg (with no significant change in micellarsize) in the case of [C2mim][C6SO4] addition, there is increasedwater penetration into the micellar pseudophase, which inturn reduces the microviscosity.

3. Conclusion

The ILs [C2mim][C2SO4], [C2mim][C4SO4] , and [C2mim][C6SO4]can be used to modify the properties of conventional micellesin a controlled fashion. Additionally, micellar or micelle-like ag-gregates exist even in the presence of high concentrations of[C2mim][C2SO4] and [C2mim][C4SO4] (the concentrations ofwhich may be as high as 1400 mm) in a 28 mm aqueous solu-tion of SB-16. At lower concentrations of ILs, ethylsulfate, bu-tylsulfate, and hexylsulfate anions of [C2mim][C2SO4], [C2mim]-[C4SO4] , and [C2mim][C6SO4], respectively, penetrate into theSB-16 micelle and interact with the quaternary ammonium ionof the SB-16 micellar aggregate and favor the micellizationprocess. A more dramatic decrease in CMC, size, and increasein Nagg number of the SB-16 micelle was observed when[C2mim][C6SO4] was the additive compared with the effectsbrought about when using [C2mim][C4SO4] or [C2mim][C2SO4]as the additives. The hexyl chain on the hexylsulfate ion wasresponsible for the more pronounced penetration of the hexyl-sulfate anion of [C2mim][C6SO4] because it aided in aligningthe anion with the tail part of SB-16. But in case of [C2mim]-[C2SO4] , the ethyl chain in the ethylsulfate ion was apparentlyunsuccessful at performing a similar function and the ethylsul-fate anion was unable to align itself with the tail part of SB-16.It was this difference in the location of the anions of the ILsthat was responsible for the differences in behavior. Neverthe-less, a clear change was observed in the physicochemical be-havior of aqueous solutions of SB-16 at higher concentrationsof [C2mim][C2SO4] , [C2mim][C4SO4], and [C2mim][C6SO4]. The ILs[C2mim][C2SO4] , [C2mim][C4SO4], and [C2mim][C6SO4] acted ascosolvents at higher concentrations and this led to the ob-served increase in CMC and decrease in Nagg. Thus, low solvo-phobicity between the IL and the hydrophobic tail of surfac-tant molecule led to a high CMC and low Nagg. Our investiga-tion has outlined the unique role of different ILs and their con-centration dependence for modifying the properties of aque-ous zwitterionic surfactant solutions. We have established ahybrid, environmentally benign system made up of IL-modifiedaqueous surfactants. This may increase the potential applica-tions of these new, environmentally friendly solvents (RTILs).

Acknowledgements

N.S. is thankful to the Department of Science and Technology(DST), Council of Scientific and Industrial Research (CSIR), andBoard of Research in Nuclear Sciences (BRNS), Government ofIndia, for generous research grants. V.G.R. , C.G. , S.G. , and S.M.

Figure 7. Steady-state anisotropy, < r >, of R6G (2.5 mm) in a 28 mm aque-ous solution of SB-16 in the presence of different amounts of ILs.

Table 4. Viscosity (�5 %) of a 28 mm aqueous solution of SB-16 in thepresence of different amounts of ILs.

Concentration Viscosity [cP]of additive [mm] [C2mim][C2SO4] [C2mim][C4SO4] [C2mim][C6SO4]

0 0.92 0.92 0.9220 0.95 0.97 1.02100 1.00 1.05 1.07400 1.04 1.22 1.40800 1.36 1.65 2.181400 2.36 3.12 4.68

1900 www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 1893 – 1901

N. Sarkar et al.

are thankful to CSIR for research fellowships. We are thankful toUdita Brahmachari for correcting the English of this manuscript.

Keywords: anions · fluorescence · ionic liquids · micelles ·zwitterions

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Received: October 28, 2011

Revised: January 11, 2012

Published online on February 17, 2012

ChemPhysChem 2012, 13, 1893 – 1901 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 1901

Chameleon-Like Nature of Zwitterionic Micelles