Journal of Colloid and Interface Science 360 (2011) 672–680

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Journal of Colloid and Interface Science

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Comparative studies on brij reverse micelles prepared in benzene/surfactant/ethylammonium nitrate systems: Effect of head group sizeand polarity of the hydrocarbon chain

Sumit Ghosh ⇑Unilever R&D Bangalore, 64, Main Road, Whitefield, Bangalore 560 066, India

a r t i c l e i n f o

Article history:Received 9 March 2011Accepted 3 May 2011Available online 11 May 2011

Keywords:Ionic liquidReverse micellesNon-ionic surfactantsDynamic light scatteringVisible spectroscopy

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.05.006

Abbreviations: EAN, ethylammonium nitrate; DLS,methylene blue; MO, methyl orange.⇑ Fax: +91 80 2845 3086.

E-mail address: sumit07ghosh@hotmail.com

a b s t r a c t

Nonaqueous reverse micelles of brij surfactants are prepared in benzene and ethylammonium nitrate(EAN). The effect of polar head group bulk on reverse micellar size was studied with brij-52, brij-56and brij-58 whereas the effect of polarity of hydrocarbon chain was investigated taking brij-52 andbrij-93 with varying Ws (Ws = [EAN]/[surfactant]). Dynamic light scattering (DLS) has been employedto reveal the size and shape of the reverse micelles. Micropolarities of these reverse micelles were inves-tigated by visible spectroscopy using methylene blue (MB) and methyl orange (MO) as molecular opticalprobes. It has been revealed from the experimental results that with increase in polar head group sizereverse micellar size increases. Moreover, it is also observed that with increasing polarity of the hydro-carbon chain the average size of the reverse micelles decreases. It can be concluded that polar head groupsize and polarity of hydrocarbon chain play important roles in determining reverse micellar size of thebrij surfactants apart from the Ws ratio, nature of the solvent medium, and concentration of thesurfactants.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Surfactants can form reverse micelles in a non-polar solvent.The polar head groups of surfactants point inwards and hydrocar-bon chains point towards the nonpolar medium. However, distinc-tion between reverse micelles and reverse microemulsions is notso clear. The convention used by De and Maitra [1] is followedfor this study. They defined reverse micelles as noncontinuous,noninteracting spherical aggregates of surfactant monomers thatdelineate a polar phase from a non-polar phase. Whereas micro-emulsions can be defined as solutions containing a ternary orhigher order mixture that display a single phase. These microemul-sions can exhibit various structures like, discrete spherical drop-lets, interconnected bicontinous water channels, liquid crystalsetc. Reverse micellar systems have attracted researchers due totheir applications in different fields as they can be used as nanore-actors [1,2] and have been shown to stabilize species that are insol-uble in nonpolar phases. They enhance reactions by stabilizingreactants, especially radicals, in nonpolar media and by increasinglocal concentration of reactants prior to reaction. These systems

ll rights reserved.

dynamic light scattering; MB,

are also excellent models for biological membranes and compart-mentalization [3–7]. Reverse micelles are prepared from a surfac-tant and cosurfactant pair dissolved in a nonpolar phase. Oftenpolar solvent is solubilised inside the micelles which causes swellup to many times their size [1]. Researchers have been interestedfor several years in the study of interaction of dye molecules insidereverse micelles to elucidate the properties of these organized sys-tems [8,9]. Dye molecules can be located in a reverse micelle inthree different places: (a) external organic solvent, (b) micellarinterface formed by a surfactant monolayer, and (c) internal polarcore depending upon nature of dye and medium. Hence, by moni-toring spectral behavior of dye molecules, insight about nature andsize of reverse micelles can be obtained and this procedure hasbeen used extensively by many researchers especially for reversemicelles of non-ionic surfactants [8,9].

Protic ionic liquids are receiving much attention for preparingreverse micelles, as a class of neoteric solvents, because of theirspecial physicochemical properties, such as low volatility, wideelectrochemical window, nonflammability, high thermal stability,and wide liquid range [10–14]. They are characterized as fusedsalts having melting point below 100 �C, with negligible vaporpressure. Many of them are described as environmentally friendlytoo [15]. Their protic nature imparts some crucial features whichenables them to be of use in biological applications [16], organicsynthesis [17–19], chromatography [20], as protein conducting

S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680 673

electrolytes for polymer membrane fuel cells [21], self assemblymedia [22–28], as catalysts [29], and as propellant or explosives[23,30]. Ethylammonium nitrate (EAN) is widely studied protic io-nic liquid and it is known that EAN has many similarities to waterwhich includes high polarity, the ability to form a hydrogenbonded network and ability to promote amphiphilic self assembly[28].

Brijs are non-ionic surfactants with varying numbers of polarhead group containing polyoxyethylene groups and separatehydrophobic tail consisting of polymethylene chain. Their polyoxy-ethylene head group has the potential to be solvated by H-bondingwith ethylammonium cation, giving them a stronger amphiphiliccharacter. They have attracted researchers in recent times becauseof their potential uses as drug-release systems [31], biodegradationenhancer for remediation of diesel enhancers [32], micellar cata-lyzed systems [33], organized assembly biomimicking systems[34], and pH responsive nanoparticles [35]. It has been observedthat brijs are most useful for soluble cell free expression of mem-brane proteins [36]. However, reverse micelles of brij surfactantsare not well investigated in different media.

The most remarkable ability of the reverse micelles is that theycan solubilize high amount of polar solvent present in the mediumwith increasing Ws. It is well established that size of reverse mi-celles depends predominantly on Ws values and on nature of thesolvent medium in which it is prepared. Earlier it was reported thatthe size of reverse micelles of non-ionic surfactants (triton-X-100)increase as Ws ratio increases [8,37]. Reverse microemulsion ofaerosol-OT was investigated extensively by earlier researchers[38,39] using different experimental techniques. However, notmuch work has been done to study the effect of head group andhydrocarbon chain on the size and structure of reverse micelles.The aim of this work is to investigate the effect of polar head groupsize and polarity of hydrocarbon chain on reverse micelles formedby brij surfactants in EAN and benzene which can be used as mic-roreactors for different reactions. In this work, the hydrodynamicdiameters of reverse micelles of brij-52, brij-56, brij-58 and brij-93, prepared in EAN and benzene have been reported for the firsttime. Effect of polar head group size and polarity of hydrocarbonchain on hydrodynamic diameters of brij-52, brij-56, brij-58 andbrij-93 reverse micelles have been thoroughly investigated usingdynamic light scattering and visible spectroscopy using methyleneblue (MB) and methyl orange (MO) as molecular optical probes.

2. Experimental

2.1. Materials and sample preparation

Brij-52, brij-56, brij-58 and brij-93 all were purchased fromSigma Aldrich (99% pure) and used as received. Chemical struc-tures of the surfactants are shown in Scheme 1a. Benzene and ethyl

(A)

(B)

(C)

(D)

Scheme 1a. Structures of (A) brij-52, (B) brij-56, (C) brij-58, and (D) brij-93.

alcohol were purchased from Merck, Germany, of HPLC grade andused without prior purification. Ethylamine (70% in water, Flukachemika) and nitric acid (70% in water, Merck) were also used asin the form purchased.

EAN was synthesized under anaerobic condition following stan-dard procedures recommended by earlier researchers [15]. Equi-molar concentration of nitric acid and ethylamine was taken toprepare EAN. Nitric acid was added slowly to ethylamine containedin a round-bottom flask over ice in stirring condition. Temperatureduring the reaction was maintained below 10 �C. Excess water wasremoved by drying under vacuum at >0.01 Torr. Prepared EAN wasdecolorized with activated charcoal and filtered through aluminumoxide prior to use. Melting point of the product was determined tobe 14 �C in agreement with previous reports. Water content wasdetermined by Karl Fischer titration technique and found to be0.5% v/v which is in well accordance with previous reports. Struc-ture of EAN is shown in Scheme 1b. MB and MO were procuredfrom Sigma Aldrich (99% pure) and used as in the form received.Stock solutions of dyes were prepared in ethyl alcohol with higherconcentration. The structures of the dyes are shown in Schemes 1c(i) and 1c (ii).

2.2. Dynamic light scattering study

Hydrodynamic diameter of brij reverse micelles in presence ofEAN and benzene were determined by DLS (‘Brookhaven Instru-ments Corporation’) equipped with BI-200SM research Goniome-ter, a BI9000AT auto-correlator and an Argon Ion Laser fromLEXEL (linearly polarized) operating at 488 nm, where surfactants,benzene and EAN have no absorption. Laser power was maintainedat 30mW throughout the experiment. All measurements weremade at scattering angle of 90� with a scan time of 2minutes cor-responding to scattering vector, q = 0.027 nm�1. BIC Dynamic lightscattering software was used for analyzing data. The data was fit-ted using cumulant algorithm [40]. Aliquots were withdrawnthrice from stock solutions and taken for DLS study. Measurementfor each set of aliquot was performed in triplicate and the mean va-lue was taken. The solutions were filtered through 0.22 lm Milli-pore Millex LCR filter prior to each measurement. Temperaturethroughout DLS measurement was kept constant at 25 ± 0.5 �C.However, at two higher temperatures (30 ± 0.5 �C and 35 ± 0.5 �C)measurements were performed to elucidate the interactions pres-ent between the surfactant molecules and EAN following the sameprotocol. Measurements on hydrodynamic diameter are accurate

Scheme 1b. Structure of ethylammonium nitrate (EAN).

Scheme 1c (i). Structure of methylene blue (MB).

Scheme 1c (ii). Structure of methyl orange (MO).

160

170

180

190

dia

met

er (

nm)

674 S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680

up to ±0.5 nm. Error bars are indicated in figures. The observationangle was changed between 15� and 150� (with 5� increment) inorder to check the accuracy of the diffusion coefficients measured.Relaxation time (s (lS)) for all measurement was calculated fromautocorrelation function and the mean value was taken for eachcase. Similar protocols were followed for calculating relaxation fre-quencies (C S�1).

Instrument was calibrated before and during the course ofexperiments using several size standards. Measurements of97.5 ± 0.8 nm polystyrene spheres in water (Duke scientific)yielded a diameter of 97.8 ± 0.5 nm, and 38 nm poly(vinyltoluene)spheres in water (Coulter) yielded a diameter of 38.4 ± 0.4 nm. Sur-factant solutions in benzene were prepared by volumetric dilutionfrom stock solutions. To introduce EAN calibrated micro syringewas used. The Ws ratio was varied between 0–2.0 for all surfactantsused. It was not possible to prepare solutions with higher values ofWs due to turbidity problem. Lowest value of Ws (Ws = 0) corre-sponded to the system where EAN was not added. Concentrationsof surfactants in all cases were fixed at 0.05 M, much above theircritical micellisation concentration. However, at room temperatureEAN was insoluble in benzene as well as in benzene/brij solutionsat brij concentrations lower than its critical micellisation concen-tration. (Critical micellisation concentrations of brij surfactantsused here are as follows in water: brij-52: 0.0067 mM, brij-56:0.002 mM, brij-58: 0.007 mM, and brij-93: 24.845 mM [41].)

130

140

150

0 0.5 1 1.5 2 2.5

A

B C

Ws=[EAN]/[surfactant]

Hyd

rody

nam

ic

A

B C

Fig. 1a. Variation of hydrodynamic diameter of (A) brij-52, (B) brij-56, and (C) brij-58 with respect to Ws at 25 ± 0.5 �C.

136

138

140

142

144

146

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150

152

154

156

0 0.5 1 1.5 2 2.5

Ws=[EAN]/[surfactant]

Hyd

rody

nam

ic d

iam

eter

(nm

)

A

B

A

B

Fig. 1b. Variation of hydrodynamic diameter of (A) brij-93, and (B) brij-52 withrespect to Ws at 25 ± 0.5 �C.

2.3. Visible spectroscopy study

Visible light absorption measurements were performed on aPerkin Elmer Lambda-35 spectrophotometer with a thermostattedcell holder. Measurements were done within the scan range of400–700 nm (visible region) at room temperature (25 ± 0.5 �C) ina quartz cell of 1 cm path length. Concentration of MB and MOwere kept constant throughout all experiments at 1.0 � 10�5 M.Appropriate amount of ethanolic solution of MB and MO weretransferred in volumetric flask to prepare 1.0 � 10�5 M concentra-tion of it in reverse micellar medium. Ethanol was evaporated bybubbling dry air/N2 and benzene/surfactant solution was thenadded to residue in both cases. Concentrations of the dyes usedhere follow Bouguer–Lambert–Beer law. Wavelength of maximumabsorbance was measured by taking midpoint between two posi-tions of the spectrum where absorbance was equal to 1.0 � Amax,

here Amax denoted maximum absorbance. Measurements of kmax

are accurate up to 0.1 nm. Error bars are provided in all cases.The pH of the solutions containing reverse micelles and dyes wasmeasured by a control dynamics pH meter from Mettrel (ModelNo.-1213). The pH meter was calibrated with appropriate pH solu-tions before each measurement. Each measurement was repeatedthrice and the mean value was taken for each measurement. ThepH of the solutions was found to be in the range of �5.2–5.6 forall cases as EAN is slightly acidic. It is well known from literaturethat MB changes it color from blue to pink at pH � 11.2–13.2[42], and MO changes it color in the pH range of �3.1–4.4 [43].Therefore, there was no effect of pH on the visible absorbance spec-tra of the dye molecules.

3. Results and discussions

3.1. Dynamic light scattering study

DLS is a useful technique to study the formation and propertiesof these organized self-assembled systems and used extensively byearlier researchers [8]. Hydrodynamic diameters (nm) of brij-52,brij-56 and brij-58 are plotted (Fig. 1a) together with respect toWs to study the effect of variation of polar head groups and brij-52 as well as brij-93 (Fig. 1b) to compare the effect of the polarityof hydrocarbon chain on the size of reverse micelles. Correlationfunction for each measurement is presented in Fig. S1 (Supplemen-tary material). Excellent fit with cumulant algorithm for all the sys-tems confirm accuracy level of each measurement. Earlier reportselicit how hydrodynamic diameter of reverse micelles of non-ionicsurfactants increases as Ws ratio increases [8,37]. But in this case, itis observed from DLS data that, the size of reverse micelles de-creases almost linearly as Ws increases. Now the question ariseswhy hydrodynamic diameter of brij surfactants are decreasingwith increasing Ws ratio? This can be envisaged considering twoeffects. First, the lowering of size with increasing Ws ratio can pri-marily be resulted by poor encapsulation of EAN over the reversemicelles. Size of the reverse micelles decreases significantly forbrij-58 compared to brij-56 and brij-52 due to less encapsulationby EAN. It is already known that with increasing polar head groupbulk, ability to form H-bonded network between the head group of

S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680 675

surfactants and polar solvent molecules increases. In case of brij-58, the extent of H-bonding will be more compared to brij-56and brij-52, as it has the highest number of polar polyoxyethylenehead groups. Therefore, in case of brij-58, with increasing Ws, EANtries to move towards the inner core of the reverse micelles byforming favorable H-bonded network with the polar head group.Slight deviation from linearity in case of brij-58 can be explainedby considering deviation in spherical micellar structure and drop-let–droplet attractive interaction present in the medium, the de-tails of which are discussed later [8]. Secondly, lowering of thevalues of hydrodynamic diameter is a characteristic feature for for-mation of reverse micelles in highly immiscible nonaqueous sol-vents [44]. For these systems, the polar solvent predominantlystays in reverse micellar core instead of locating near the reversemicellar interface. Here, EAN and benzene are immiscible. So, withincreasing EAN concentration, more and more EAN molecules aresolubilised in reverse micellar core instead of staying near themicellar interface. Increasing concentration of EAN leads to morefavorable H-bond interaction with polar head groups of the surfac-tants. So, favorable and strong H-bonding interaction with EANhelps to hold the polar head groups much near to the micellar core.This fact is manifested by the lowering of hydrodynamic diameterof the reverse micelles [44]. To confirm the role of H-bond forma-tion, hydrodynamic diameter of the reverse micelles were mea-sured in two different higher temperatures, 30 ± 0.5 �C and35 ± 0.5 �C. These results are presented in Figs. S2a and S2c (Sup-plementary material). It is well known from literature, that withincreasing temperature the capability of forming H-bond decreases[45]. Therefore, at higher temperature, formation of strong andfavorable H-bond becomes difficult between the polar head groupsof brij-58 and EAN. This leads to the lower extent of decrease insize (180.8–161.6 nm at 30 ± 0.5 �C and 180.5–164.2 nm at35 ± 0.5 �C) at higher temperatures compared to the decrease(180.8–145.4 nm) observed at 25 ± 0.5 �C for brij-58 with increas-ing Ws. In case of brij-56 and brij-52, there is not much effect ofincreasing temperature on the size, confirming lower extent ofH-bond formation between the polar head groups of these systemsand EAN as discussed earlier.

Blokhuis and Sager reported that transformation from oneshape (spherical) to another shape (cylindrical), in case of reversemicelles; depend on the ratio of polar solvent to non polar solventas well as on surfactant concentration [46]. Therefore, the size ofresultant reverse micelles depends on both, concentration of thepolar solvent solubilised in micelles and total surfactant concentra-tion in the medium. It is observed that size of reverse micelles pre-pared in this medium is greater than reverse micelles preparedusing water as polar solvent. The reason behind this can be ex-plained by considering the fact that the molar volume of EAN ishigher than compared to that of pure water [8,47].

The shape of reverse micelles is an interesting topic for investi-gation. According to Bernheim-Groswasser and coworkers, forma-tion of cylindrical or lamellar reverse micelles can lead to higherhydrodynamic diameter [48]. Gao and coworkers reported ellipsoi-dal shape for their reverse microemulsion system of triton-X-100in benzene and 1-butyl-3-methylimidazolium tetarfluoroborate([bmim]BF4) for Ws ratio varying from 0.71 to 1.55 and hydrody-namic diameter of 150.51 nm [49]. But, Falcone and coworkers[8] reported that reverse micelles of triton-X-100 in benzene and1-butyl-3-methylimidazolium tetarfluoroborate ([bmim]BF4) werespherical in shape based on low polydispersity index obtainedfrom DLS measurement and linear trends observed in the hydrody-namic diameter vs. Ws ratio profiles. They reported smaller hydro-dynamic diameter compared to the values reported by Gao andcoworkers. According to them, the shape of reverse micelle primar-ily depends upon the initial surfactant concentration and droplet–

droplet interaction present in the medium. Higher surfactant con-centration makes the droplet–droplet interaction easier and conse-quently changes the shape and size of these reverse micellarsystems [8]. Initial concentrations of the surfactants used in thiswork were above their corresponding cmc values in benzene andEAN, similar to the protocol followed by Falcone and coworkers [8].

Therefore, very low polydispersity index (Fig. S3, Supplemen-tary material) in DLS measurement as well as linear trend observedover the whole range of Ws (very small deviation observed only incase of brij-58 and the reason behind this is explained earlier) ex-clude possibility for presence of any branched or elongated cylin-drical network in all the systems studied [8]. Formation of nonstructured or structured microemulsion is also nullified. So, forall these systems studied here, the shape of the reverse micellescan be considered as discrete spherical or near spherical droplets.

Diameter of the reverse micelles also depends on effective pack-ing parameter of surfactant, ‘v/alc’, where ‘v’ and ‘lc’ are volume andlength of the hydrocarbon chain, respectively, and ‘a’ is the effec-tive surfactant head group area. Size increases when packingparameter values are smaller and vice versa [8]. As mentioned ear-lier that brij-58 has larger head group area compared to brij-56 andbrij-52, so packing parameter value is smaller for brij-58 than brij-56 and brij-52. Therefore, average size of the reverse micelles de-creases in the following order: brij-58 > brij-56 > brij-52(Scheme 2). Moreover, with increasing Ws, EAN tries to move in-side the reverse micellar interface, as it is completely insoluble inbenzene. But due to difficult encapsulation over reverse micellesof brij surfactants which consist of hydrophobic polymethylenechains, it repels out from interface and moves toward the reversemicellar core. This results continuous decrease in effective ‘a’ val-ues, packing parameter value increases and finally leads to de-crease in size for all these systems [8,44].

Comparison between the hydrodynamic diameters (Fig. 1b) ofbrij-52 and brij-93 shows, as Ws increases, size of brij-93 reversemicelles remains almost constant, whereas in case of brij-52, sizedecreases significantly. Brij-93 has higher hydrocarbon chainlength than brij-52. But, surprisingly it shows lower size thanbrij-52 corresponding to Ws ratio 0.5 and 1.0. This observationclearly indicates that diameter of reverse micelles depend not onlyon size of surfactant head group but also on the polarity of hydro-carbon chain. In case of brij-93, presence of the double bond inoleyl group (hydrocarbon chain) helps polar EAN to encapsulateits reverse micelles more effectively because of the favorable elec-trostatic interaction between electron rich double bond and elec-tron deficient ethylammonium cation of EAN. In this case, EANcan locate near the reverse micellar interface instead of migratingto the core. This fact is also prominent from hydrodynamic diame-ters of brij-52 and brij-93 corresponding to Ws = 1.5 and Ws = 2.0.The size of brij-52 reverse micelles is smaller compared to the sizeof brij-93 reverse micelles for these two cases. With increasing EANconcentration, for brij-93 reverse micelles; more EAN moleculestry to move near the micellar interface. Therefore, effective ‘a’ val-ues for brij-93 reverse micelles in these two cases increase, whichresults higher hydrodynamic diameter. So, with increasing Ws va-lue, brij-93 reverse micelles show almost constant size althoughthe average size is smaller compared to the reverse micelles ofbrij-52 (Scheme 2). The size of brij-93 and brij-52 reverse micelleswere measured and compared at two higher temperatures to con-firm the role of electrostatic interaction for brij-93 reverse mi-celles. It is known that at elevated temperature (Fig. S2b andS2d, Supplementary material) electrostatic interaction is favored[45], so more EAN molecules can stay near the interfacial area lead-ing further increase in size for the brij-93 system.

Recent work of Jean-Luc Lemyre and coworkers confirmed thathydrodynamic diameter of reverse micelles is in direct proportion

Average size of reverse micelles decreases as polar head group bulk decreases

Brij-58 Brij-56 Brij-52

Brij-52 Brij-93Average size of reverse micelles decreases on increasing polarity of hydrocarbon tail

Scheme 2. Average size of the reverse micelles decreases with decreasing polar head group bulk and with increasing the polarity of hydrocarbon chain.

676 S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680

with aggregation number [50]. According to them, the relationshipbetween hydrodynamic diameter and aggregation number for re-verse micelles can be expressed using the following equation,

d2¼ 3

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3N=4p ½EAN�

½surf :�VEAN þ V surf :

� �sð1Þ

where N is aggregation number, d is hydrodynamic diameter, and[EAN], [Surf.], VEAN and Vsurf. are EAN and surfactant concentrationsand molecular volumes, respectively. Values for group molar vol-umes given by Funasaki and coworkers were taken to calculatethe molecular volumes of the surfactants [51] following the meth-ods used by Preu and coworkers [52]. Density of EAN was takenfrom the work of Kanzaki and coworkers [53]. Molar volume ofEAN was taken as 97.99 cm3/mol [15]. This equation is based on amodel assuming the micelles as a compact sphere and neglectspresence of any polar solvent molecules between the surfactanthydrophobic tails which would be the ideal case if two perfectlyimmiscible solvents are used. The detailed protocol for this calcula-tion can be found elsewhere [50]. Therefore, as hydrodynamicdiameter decreases with increasing Ws, aggregation number alsodecreases, which is evident from Table T1 (Supplementary mate-rial). This decrease in aggregation number of reverse micellarassembly is again evident from the plots of diffusion coefficient asa function of Ws, as the concentration of surfactant solution addedin all cases are same (Figs. S4a and S4b, Supplementary material).Moreover, it is also observed from table T1 (Supplementary mate-rial) that for a fixed Ws ratio, aggregation number decreases withincreasing head group bulk due to steric hindrance among the headgroups. The calculated aggregation numbers are higher than previ-ously reported, which provides the reason for obtaining higherhydrodynamic diameter in all these cases [54]. Diffusion coefficientis inversely proportional to hydrodynamic diameter of reverse mi-celles. Diffusion coefficient values increase as Ws ratio increases

for all cases supporting above analysis. However, for brij-93 system,diffusion coefficient values remain almost constant indicating itsconstant size over the range of Ws. It can be shown that diffusioncoefficient is related with the volume fraction of dispersed phase(U) by the following equation,

D ¼ D0ð1þ aUÞ ð2Þ

where ‘a’ is dynamic virial coefficient and D0 is the diffusion coeffi-cient at infinite dilution. Further details of deriving this equationcan be found elsewhere [55]. Volume fraction of the dispersedphase can be calculated using following equation [44],

U ¼ VTEAN þ VScV surf:

V totalð3Þ

where VTEAN is total volume of EAN present, VS is the volume of sur-

factant stock solution added in benzene, c is the concentration ofthe stock solution prepared, Vsurf. is the molecular volume of thesurfactants, and Vtotal is the total volume of the solutions preparedin each case. In this calculation it is assumed that all surfactantsused here are completely dry and pure. Dynamic virial coefficientis equal to 1 [55] if reverse micelles behave as perfect hard spheresand is negative if attractive interactions among them are strongerthan repulsive interactions [55]. Calculated values of ‘a’ (Fig. S5,Supplementary material) reveal presence of attractive interactionamong spherical droplets for all brij reverse micelles as the valuesare all negative. These values are higher for brij-58 and brij-93 indi-cating the presence of greater attraction force between the reversemicelles of these two systems. Unimodal distributions (Figs. S6(a–d), Supplementary material) are observed in all cases confirmingpresence of spherical aggregates.

Diffusion coefficients, (D) can be calculated from followingrelationship,

S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680 677

Cq2 ¼ D when q! 0 ð4Þ

where C is relaxation frequency (C = s�1 S�1) and q (nm�1) is thewave vector defined as the following equation,

q ¼ 4pnk

sinh2

� �ð5Þ

where k is the wavelength of the incident laser beam (488 nm), h isthe scattering angle, and n is the refractive index of the medium.The hydrodynamic diameter (d) can be calculated by usingStokes–Einstein relationship:

d2¼ kBT

6pgCq2 ¼ kBT

6pgDð6Þ

where kB is the Boltzmann constant, T is the temperature, and g isviscosity of the medium. So from these equations, it can be deduced,that relaxation time (s (lS)) is proportional to hydrodynamic diam-eter of the reverse micelles. Relaxation time (s (lS)) was calculatedfrom autocorrelation function and plotted in Fig. 2 as a function ofWs for all systems. Further background of the calculation details canbe found elsewhere [56].

It is observed that relaxation time (s (lS)) decreases withincreasing Ws ratio. Linear plots can be explained by consideringformation of spherical aggregates in the medium. Slight deviationsfrom linearity are observed for brij-93 and brij-58. These devia-tions are observed due to the presence of greater attraction forceamong the reverse micelles of brij-58 and brij-93, as calculated val-ues of ‘a’ are higher for brij-58 and brij-93 supporting previousanalysis. Figs. S7(a–d) (Supplementary material) represent theplots of normalized relaxation time distribution (G(s)) as a functionof relaxation time (s (lS)). Figs. S8(a–d) and Figs. S9(a–d) (Supple-mentary material) represent the normalized distribution of diffu-sion coefficients (G(D)) with diffusion coefficients (D cm2 S�1) andnormalized distribution of relaxation frequencies (G(C)) withrelaxation frequencies (C S�1), respectively. Unimodal distribu-tions are observed for all cases further confirming formation ofspherical aggregates. Fig. S10 (Supplementary material) representcorrelation of relaxation frequencies (C S�1) with Ws ratio for allbrij surfactants studied here confirming previous analysis.

3.2. Visible spectroscopy study with MB

Spectral study with an optical probe can provide much informa-tion on polarity and size of reverse micellar core by showing per-turbation in its visible absorbance spectrum. So, many

490

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510

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530

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0 0.5 1 1.5 2 2.5

A

B

C

D

Ws=[EAN]/[surfactant]

Rel

axat

ion

time

[µS]

A

B

C

D

Fig. 2. Variation of relaxation time, s (lS) calculated from autocorrelation functionsof light scattering measurement with respect to Ws ratio for (A) brij-52, (B) brij-56,(C) brij-58, and (D) brij-93 reverse micelles prepared in benzene and EAN at25 ± 0.5 �C.

researchers have studied properties of reverse micellar core usingdye molecules [57,58]. MB is a well known cationic dye in this con-text and it can form aggregates in polar medium. Among all othercationic dyes it has a unique property of showing monomer–dimerequilibrium. Monomer–dimer equilibrium of MB is sensitive tonature of surfactant present, size and concentration of the surfac-tant micelles; polarity of medium and on micellar equilibrium ofsurfactant [59]. Shift in monomer–dimer equilibrium of MB canbe studied spectroscopically. Peak at 663 nm (A1) in absorptionspectra of MB is due to monomers and peak at 610 nm (A2) repre-sents MB dimers. Absorbance spectrum of MB in presence of EAN ispresented in Fig. 3 (graph A) which shows MB monomers are fa-vored than dimers in this medium.

It clearly indicates absence of any type of interaction, (likehydrogen bonding) between EAN and MB. It is well reported thatdyes can form charge transfer complexes with surfactants[60,61], but here no evidence whatsoever is observed for existenceof charge transfer complex between MB and surfactant molecules.MB is practically insoluble in benzene. So, it will remain purely inEAN medium. Figs. S11(a–d) (Supplementary material) representplots of kmax for A1 peak of MB as a function of log[surfactant con-centration]. Surfactant concentrations were varied from 0.01–0.05 M in benzene and EAN. Decrease in kmax is observed at�0.03 M indicating respective cmcs of surfactants near that con-centration range in the medium [62]. So, at 0.05 M surfactant con-centration, formation of reverse micelles can be ensured for all thesystems [8,9]. These plots also reveal that with decreasing surfac-tant concentration kmax of A1 peak of MB shifts bathochromicallywhich confirm reverse micelles formation for all systems [62]. Con-trol experiments with different concentrations of all brij surfac-tants (in Millipore Milli-Q water, 18.2 MX/m) from premicellarregion to postmicellar region were performed; however resultsare not presented here. These experiments reveal absence of anyeffect of head group size and polarity of the hydrocarbon chainon the formation of brij micelles in water, as visible absorbancespectra of MB remain unperturbed in all the cases. Reason behindthis can be attributed to the fact that with water, polar polyoxyeth-ylene head group part of non-ionic brij molecules can form uni-form H-bonded network, which is altered in experimentalmedium; as both of the solvents have different properties com-pared to water [63].

It is already known that hydrophobic interaction prevails be-tween MB and reverse micelles of non-ionic surfactants. Due tohydrophobic interaction dye molecules can be effectively stackedin reverse micelles which can be explored for elucidation of sizeof reverse micellar core [62]. As it is mentioned earlier that mono-mer–dimer equilibrium of MB is dependent on size of surfactantmicelles, so this property of MB can be used to investigate reverse

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 450 500 550 600 650 700

Abs

orba

nce

Wavelength (nm)

B

A

B

A

Fig. 3. Visible absorbance spectra of 1 � 10�5 (M) (A) methylene blue (MB) and (B)methyl orange (MO) in ethylammonium nitrate (EAN) at 25 ± 0.5 �C.

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

0 0.5 1 1.5 2 2.5

A

B

A1/

A2

Ws=[EAN]/[surfactant]

A

B

Fig. 4b. Plots of A1/A2 for methylene blue with variation of Ws for (A) brij-93, and(B) brij-52 in ethylammonium nitrate (EAN) and benzene at 25 ± 0.5 �C.

678 S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680

micellar size. Visible absorbance spectra of MB with respect to Ws

ratios varying from 0 to 2.0 in presence of different brij surfactantsare presented in Figs. S12a (brij-52), S12b (brij-56), S12c (brij-58),and S12d (brij-93) (Supplementary material); respectively. Thesefigures reveal that with increasing Ws, monomers of MB becomespredominant over dimers for all brijs studied here, as intensity ofA1 peak increases significantly in comparison with A2 peak. Thiscan be explained by considering that with increase in Ws ratio, sizeof micellar core decreases. So, possibility of incorporating dimers ofMB becomes difficult for all these cases. Ratio of intensities of A1and A2 peaks for MB represents ratio of concentration of monomerstructures to that of dimeric forms. Plots of Abs.(A1/A2) vs. Ws

varying from 0 to 2.0 for brij-52, brij-56 and brij-58 are presentedin Fig. 4a to study the effect of polar head group and that for brij-52and brij-93 in Fig. 4b to investigate the effect of hydrocarbon chainpolarity, on monomer–dimer equilibrium of MB. It is observed thatin case of brij-58 monomeric form of MB predominates exclu-sively; whereas for brij-56 it is not so predominant and for brij-52, monomeric and dimeric forms almost remain in equal ratiowith increasing Ws. This fact confirms the previous analysis.

However, Fig. 4b shows an interesting result because, here brij-52 and brij-93; both surfactants show almost comparable affinityfor incorporating monomers and dimers of MB, although brij-93shows almost constant size over the range of Ws as obtained fromDLS measurements. In case of brij-52, it is obvious because of de-crease in size of reverse micelles as Ws increases; amount of MBmonomers is slightly favored over dimers. From DLS study it is evi-dent that the size of brij-52 reverse micelles decreases not muchwith increasing Ws. So, brij-52 system shows almost equal affinityfor both monomers and dimers of MB. As mentioned earlier in DLSstudy, presence of polar double bond in hydrocarbon chain of brij-93 enhances electrostatic interaction. This electrostatic interactioncan change properties of sequestrated ionic liquids [8]. Due tofavorable electrostatic interaction, degree of ion pair formationfor EAN changes which leads to the change in consequent seques-tration of EAN over brij-93 reverse micellar phase. Polarity ofmicellar interface increases compared to the micellar core. Incor-poration of dimeric structure becomes possible for brij-93 and itshows almost equal affinity for both monomeric and dimeric formsof MB like brij-52.

Variation of kmax of A1 and A2 peaks of MB with respect to Ws

for brij surfactant is shown in Figs. S13(a–d) (Supplementary mate-rial). With increase in head group size, EAN tries to move towardsmicellar core, as can be concluded from the DLS study. It is alreadyknown that, free EAN molecules are more polar compared to EANmolecules that are bound to micellar head group by H-bond

1

1.1

1.2

1.3

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1.7

1.8

1.9

2

0 0.5 1 1.5 2 2.5

A

B

C

A1/

A2

Ws=[EAN]/[surfactant]

A

B

C

Fig. 4a. Plots of A1/A2 for methylene blue with variation of Ws for (A) brij-52, (B)brij-56, and (C) brij-58 in ethylammonium nitrate (EAN) and benzene at 25 ± 0.5 �C.

interaction. Bathochromic shift for MB is expected if polarity of sol-vent molecules increase [9]. Figs. S13b and S13d (Supplementarymaterial) represent variation of kmax for brij-52 and brij-93 forA1 and A2 peaks, respectively whereas Figs. S13a and S13c (Sup-plementary material) represent the same for brij-52, brij-56, andbrij-58 for A1 and A2 peaks, respectively. For all the systems stud-ied here, it can be observed that kmax for both A1 and A2 peaks ofMB, shifted bathochromically with increasing Ws value. This can beattributed to the fact, that with incorporation of EAN, polarity ofreverse micellar systems increases. But the extent of shift is differ-ent in each case showing the difference in polarity of microenvi-ronments for these systems. Negligible bathochromic shifts(1 nm for A1 peak, 1 nm for A2 peak with increase in Ws ratio)are observed for both the A1 and A2 peaks in case of brij-93 reversemicelles. Due to favorable electrostatic interaction between doublebond in hydrocarbon chain of brij-93 and with ethylammoniumcation, polarity of micellar core decreases. Negligible bathochromicshift in kmax observed in this case is comparable to the shift ob-served for brij-58. In brij-58 reverse micelles, EAN can form strongH-bonded network with polar polyoxyethylene head group. So,amount of free EAN molecules in micellar core is low in this case.However, the shift is more for brij-56 (3 nm for A1 peak, 2 nm forA2 peak with increase in Ws) and brij-52 (4 nm for A1 peak, 2 nmfor A2 peak with increase in Ws) compared to brij-58 (1 nm for A1peak, 1 nm for A2 peak with increase in Ws) confirming highermicropolarity for these two systems. Hypsochromic shift in kmax

was observed for both A1 and A2 peaks of MB, as the surfactantswere changed from brij-52 to brij-56 and then brij-58. Absorbancemaxima for A1 and A2 peaks of MB are shifted hypsochromicallyfor brij-93, compared to brij-52. These results confirm presenceof strong H-bonded network in case of brij-58 compared to brij-56 and brij-52 and support results from DLS experiments.Figs. S14a and S14b (Supplementary material) represent variationof kmax of A1 and A2 peaks of MB with respect to Ws for all brijsurfactants.

3.3. Visible spectroscopy study with MO

Absorbance of MO is dependent on the polarity of the microen-vironment. Solvatochromism of MO has been used by earlierresearchers to investigate microenvironment of micellar as wellas reverse micellar systems [64]. Moreover, MO is insoluble in ben-zene due to its ionic character. So, MO can be considered mainlysolubilised in polar cores of reverse micelles which consist of poly-ethylene oxide head groups of brij surfactants [9]. It has beenshown that electronic transition energy of MO molecules, as envis-aged in the wavelength of their absorption maxima, is an

S. Ghosh / Journal of Colloid and Interface Science 360 (2011) 672–680 679

indication of the ‘polarity’ of their microenvironments. Bathochro-mic shift in absorbance maxima is observed if polarity is increased[9,64]. In presence of EAN, probe MO molecules can move to EANpool sequestrated in reverse micellar core or it can stay near micel-lar interface. If EAN molecules prefer to bind with the head groups,it is expected that kmax of MO will not change bathochromically [9].It is mentioned earlier, that polar solvent molecules, which arebound to polar head groups of surfactants exhibit restricted mobil-ity. Therefore, these molecules show much lower polarity com-pared to free polar solvent molecules in the micellar core. Here,it is observed that with increasing Ws ratio, the kmax of MO for allbrij surfactants shifted bathochromically. Visible absorbance spec-trum of MO in pure EAN is presented in Fig. 3 (graph B). Absor-bance maximum of MO is observed at 464 nm. Figs. S15(a–d)(Supplementary material) produce evidence that with increasingWs form 0 to 2.0, kmax of MO is also increased almost linearly from464 nm to 470 nm (6 nm) for brij-52. With increase in head groupsize, EAN can form uniform H-bonded network with polar headgroup; as it is evident from the previous analysis. Therefore, kmax

of MO for brij-56 (464–467 nm; increases 3 nm) and brij-58(464–465 nm; increases 1 nm) increases little. So, amount of freeEAN molecules which are responsible for enhanced polarity in re-verse micellar core decreases in the following order: brij-52 > brij-56 > brij-58. However, for brij-93, absorbance maximum of MOshifted bathochromically to almost 1 nm, which is comparable tothe shift observed in case of brij-58; showing decrease in the polar-ity of the microenvironment of reverse micellar core as in this caseEAN molecules can move into reverse micellar interface due tofavorable electrostatic interaction with double bond present inhydrocarbon chain. Fig. S16 (Supplementary material) representsshift of absorbance maxima for all surfactants as a function of Ws

ratio ascertaining previous analysis.

4. Conclusions

In this work, DLS and visible spectroscopic measurements areused to monitor formation of reverse micelles of brij surfactantswith different head groups and hydrocarbon chains prepared inbenzene and EAN. With increasing polar head group size, EAN triesto move out from reverse micellar interface and preferentiallymoves to reverse micellar core as it can form uniform H-bondednetwork with the polar head groups. This results in continuous de-crease in reverse micellar size with increasing Ws. So, average sizeof the reverse micelles decrease in the following order: brij-58 > brij-56 > brij-52. Similarly, from the study between brij-52and brij-93, it can be concluded that the polarity of hydrocarbonchain also plays an important role in determining the size of re-verse micelles. Presence of double bond in hydrocarbon chain ofbrij-93 enhances encapsulating ability of EAN over reverse micellesof brij-93 through electrostatic interaction. EAN molecules can staynear micellar interface and brij-93 reverse micelles shows almostconstant size over increasing Ws ratio. DLS experiments at highertemperatures confirm these observations. Absorbance spectra ofMO and MB confirm the fact that with increase in size of polar headgroup, ability to form H-bonded network with EAN molecules in-creases which results decrease of polarity in micellar core. In sum-mary, the effect of polarity of hydrocarbon chain as well as size ofpolar head group variation on hydrodynamic diameters of brijreverse micelles which can be used as potential microreactors fordifferent organic reactions is studied in the experimental temper-ature. Bending moment calculation using different theoreticalmodels may reveal more information regarding these systems[65]. This kind of study can be extended for other series of non-ionic surfactants like tweens and spans; having different polarhead group types and with different ionic liquids having different

characteristics; which can explore new features about these self-assembled systems.

Acknowledgments

Author gratefully acknowledges the reviewer and editor fortheir useful comments and suggestions as well as his colleagueDr. Prasun Bandyopadhyay for his critical inputs in to the manu-script. Dr. R. Srinivasa Gopalan helped in preparing EAN and pro-curing chemicals. Mr. Arindam Roy helped in measuring DLS.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcis.2011.05.006.

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