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Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me page: www.elsev ier .com/ locate /co lsur fa

issimilar effects of solubilized p-toluidine on the shape of micelles ofifferently charged surfactantsulbir Singhb, Nilesh Dharaiyaa,∗, D. Gerrard Marangonib, Pratap Bahadura

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, IndiaDepartment of Chemistry, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada

i g h l i g h t s

Anionic, cationic, nonionic, zwitter-ionic and cationic–nonionic surfac-tants were used.Size/shape of differently chargedmicelles is altered by the presence ofp-toluidine.These changes can be tuned bychanging the solution pH.Different sites of p-toluidine inmicelles were ascertained by NOESY.

g r a p h i c a l a b s t r a c t

Micelle shapes of DTAB (cationic), SDS (anionic), DDMPS (zwitterionic), DMDAO (cationic–nonionic), andTriton X-100 (nonionic) surfactants with solubilized p-toluidine at different pH.

r t i c l e i n f o

rticle history:eceived 13 April 2013eceived in revised form 20 June 2013ccepted 23 June 2013vailable online 29 June 2013

eywords:-Toluidineicelle

a b s t r a c t

In the present study, pH controlled solubilization of p-toluidine (PTD) in micelles of surfactants havingdissimilar head group charge is examined. A connection is revealed between viscosity enhancement andlocus of solubilized PTD on the micelles of different surfactants viz. dodecyl trimethylammonium bromide(DTAB, cationic), sodium dodecyl sulfate (SDS, anionic) and dodecyl-N,N-dimethyl-3-amino-1-propanesulfonate (DDMPS, zwitterionic), dodecyl-dimethyl amine oxide (DMDAO, cationic–nonionic) and p-tert-octylphenoxy polyethylene (9.5) ether, Triton X-100 (nonionic) was examined by viscosity, dynamic lightscattering and 2D NOESY studies. PTD works as cationic hydrotrope in acidic pH and remains neutral inbasic pH. DTAB and DDMPS did not show any significant growth with solubilized p-toluidine, while pH

Hynamic light scatteringD NOESY

dependant micellar growth/transitions were observed for SDS, DMDAO and TX-100. Dissimilar effectswere observed for different surfactants; DMDAO micelles showed maximum growth at pH ∼5.0–6.0with almost no effect in more acidic pH, SDS micelles grew only in acidic conditions, while Triton X-100micelles grew more in basic media. The effect of temperature on DMDAO and Triton X-100 micelles in thepresence of PTD was also examined. The location of PTD in micelle was obtained from two-dimensional

ncem

nuclear Overhauser enha

. Introduction

Surfactant monomers aggregate and form colloidal entitiesalled micelles above their critical micelle concentration (CMC).

∗ Corresponding author. Tel.: +91 989 8771871; fax: +91 261 2256012.E-mail addresses: ksingh@stfx.ca (K. Singh), nilesh.dharaiya@yahoo.com,

ilesh.dharaiya01@gmail.com (N. Dharaiya), gmarango@stfx.ca (D.G. Marangoni),bahadur2002@yahoo.com (P. Bahadur).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.06.038

ent spectroscopy (2D-NOESY).© 2013 Elsevier B.V. All rights reserved.

Micellar characteristics and shape transitions depend on theirmolecular structure (nonpolar tail, polar head group and counter-ion), and also on conditions such as solvent medium, temperature,ionic strength and pH. Micellar growth is also directed by elec-trostatic and hydrophobic interaction between surfactant andadditive. Screening of electrostatic interaction decreases the sur-

face area of the head group causing an increase in packingparameter, p, and thus allows micellar growth. Ionic and nonionichydrotropes can alter micellar and phase behavior of surfactantsolutions. Several interesting aggregate morphologies e.g. rod-like,

5 hysicochem. Eng. Aspects 436 (2013) 521– 529

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22 K. Singh et al. / Colloids and Surfaces A: P

orm-like, and vesicles of anionic surfactant–cationic hydrotrope1–7] and cationic surfactant–anionic hydrotrope mixtures [8–14]ave been reported and reviewed [15,16]. Micellar transition inDS solutions in the presence of toluidine hydrochloride haseen studied by scattering [1,5], NMR [2], time-resolved fluores-ence anisotropy [3] and stopped-flow [4] measurements. Sometudies report that polar aromatic solutes like phenols [17–19],mines [20–22], carboxylic acids [23–25] efficiently induce micellarrowth. These types of investigations are essential because rod-likend worm-like micelles are applied as drag reducing agents, in oilelds [26–28] and many home and personal care products [29].

There are several reports on location of solubilizate in micelle30–35] and temperature [23,36], pH [19,37–40], electrolytes41,42] play significant role in solubilization. In particular, the

icellar transitions in the presence of some solubilizates can beonveniently modulated by altering the pH. For examples, phe-ol resides at micelle–water interface/head group region for bothnionic [43,44] and cationic micelles [17,35,37,41] while in basic pHue to deprotonation, phenol molecules penetrate in palisade layerf cationic micelles. The effectual growth in cationic gemini micelley p-toluidine diminishes at pH ∼3 [22]. In acidic pH, p-toluidineets protonated and thus can influence micellar structures whichake it better feature for application point of view.We earlier examined p-toluidine induced micellar transitions

n solutions of several cationic surfactants with varying nonpolarail/polar head group/counter ion [21]. Despite the available liter-ture on the effect of weakly polar aromatic solutes on size/shaperansformation in micelles, there is no systematic pH dependanttudy on the effect of p-toluidine on different (type) surfactants.he present work focuses on the comparison of change in micellartructural properties (size and shape) and macroscopic propertiesf ionic surfactants SDS, DTAB, DMDAO, DDMPS (anionic, cationic,ationic–nonionic, zwitterionic) each with 12C nonpolar tail in theresence of p-toluidine. These surfactants except DMDAO formpherical micelles independent of pH [45]. Additionally, micellarnd phase behavior of nonionic surfactant Triton X-100 in the pres-nce of PTD were systematically examined at different pH. Micellarrowth and viscosity enhancement observed from DLS and viscos-ty measurement are correlated to locus of solubilization of PTD inurfactant micelles evaluated form 2D NOESY.

. Experimental

.1. Materials

Dodecyltrimethylammonium bromide (DTAB), sodium dodecylulfate (SDS), dodecyl-dimethylamine oxide (DMDAO), dodecyl-,N-dimethyl-3-ammino-1-propane sulfonate (DDMPS), and p-

ert-octylphenoxy polyethylene (9.5) ether (Triton X-100) werebtained from Sigma–Aldrich and used as received. p-ToluidinePTD) was supplied from Fluka. D2O from Sigma, USA was used forD NOESY experiments. Millipore water was used for viscosity andLS measurements. All measurements were performed at 50 mMoncentration of surfactants. The pH of solutions was adjusted bysing HCl/NaOH.

.2. Methods

.2.1. ViscosityRelative viscosities were measured using an Ubbelohde sus-

ended level capillary viscometer. The viscometer was cleanedhoroughly and air dried each time before every measurement. Theiscometer was placed vertically in a thermostat at constant tem-erature ±0.1 ◦C. Calibrated stopwatch was used to measure the

Scheme 1. Molecular structure of p-toluidine (PTD) at different pH.

flow time of solutions which recorded minimum three times inorder to check the reproducibility.

2.2.2. Cloud pointCloud points were investigated by visual examination of surfac-

tant solution which was heated in thin glass tubes immersed in awell stirred heating bath. The sample temperature was increasedslowly under constant stirring by using a magnetic stirrer with aheater. The temperature is noted as cloud point at which surfac-tant solution being initial turbid from clear. The CP values werereproducible up to 0.5 ◦C.

2.2.3. Surface tensionSurface tension measurements were carried out using a Kruss

(Model K10T) tensiometer. Before each measurement the plate wascleaned with distilled water and flamed. All surfactant solutionswere prepared in millipore water. Detail procedure of experimentalprocedure can be found elsewhere [46].

2.2.4. Dynamic light scattering (DLS)The DLS experiments were done using Zetasizer Nano-ZS 4800

(Malvern Instruments, UK). The light source was He–Ne laser oper-ating at a wavelength of 633 nm at 90◦ scattering angle. Eachsample is filtered by 0.45 �m filter before use. The apparent hydro-dynamic diameter (Dh) of the micelles was calculated by using theStokes–Einstein relationship.

2.2.5. Nuclear magnetic resonance (NMR)2D-NOESY spectra were measured on a Bruker Avance-II

400 MHz spectrometer. D2O was used as the solvent for all NMRexperiments. In NOESY experiments, the mixing and the delaytimes were estimated from the spin–lattice relaxation time. Theacquisition delays and mixing times were adjusted to ≈3 × T1 of≈1 × T1, respectively. This corresponds to a mixing time of ≈3.5 s.Detailed procedures for experiments can be found elsewhere [21].

3. Results and discussion

p-Toluidine (PTD) is polar aromatic weak base having pKa ∼5.1.At lower pH it converts into protonated form (cationic hydrotrope)and remains unprotonated at higher pH (Scheme 1). This type ofremarkable pH dependant properties of PTD may bring changes inthe geometry of different micelle. Thus, we have taken differentlycharged surfactants to study the effect of PTD at varying pH con-ditions. First we examined micellization of pure surfactants andcompared CMC and other parameters from surface tension mea-surements.

The surface tension-log concentration plots for SDS (anionic),DTAB (cationic), TX-100 (nonionic), DMDAO (cationic–nonionic)and DDMPS (zwitterionic) measured at 30 ◦C are shown in Fig. 1.Typical surfactant behavior showing linear decrease in surface

K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529 523

Fig. 1. Surface tension vs. log concentration plots for different surfactants at 30 ◦C.

Table 1CMC and adsorption parameters for surfactants at air–water interface.

Surfactant CMC, mM �cmc, m Nm−1 Area/molecule, Å2

DTAB 14.8 36.5 52 [50] [47]SDS 8.2 33.5 51 [49] [48]DDMPS 3.5 40.5 62 [61] [49]

tGca

3

s(ich

F

DMDAO 1.5 32.2 50 [47] [50]TX-100 0.2 30.1 61 [64] [51]

ension close to CMC and constant values above CMC is seen. Theibbs adsorption equation was used to calculate area/molecule. Thealculated data for CMC, �cmc and area/molecule for each surfactantre reported in Table 1 along with the literature values.

.1. Effect of PTD concentration

Relative viscosities of 50 mM surfactant solutions were mea-ured in the presence of different PTD concentrations at 30 ◦CFig. 2). The solution viscosity showed an increase for the non-

onic TX-100 and DMDAO in the presence of PTD. However, nohange in viscosity is observed for SDS and DTAB solutions even atigh PTD concentration. DMDAO molecules remain uncharged in

ig. 2. Effect of PTD concentration on the relative viscosity of 50 mM surfactants.

Fig. 3. Effect of PTD concentration on cloud point (CP) and hydrodynamic diameter(Dh, nm) of 50 mM TX-100.

neutral/basic condition but positively charged at lower pH.Uncharged DMDAO micelles show less repulsion between headgroup as compared to ionic surfactants DTAB and SDS. PTD pen-etrates in DMDAO micelle and decreases effective area of the headgroup and increases effective area of the hydrocarbon chain whichlead to proper packing for micelle growth. In case of taken ionicsurfactants the positioning of PTD in micelle does not facilitate toefficiently diminish of electrostatic repulsion between head group,as a result no change in micellar geometry is seen even at higherPTD concentrations. In DDMPS solutions, PTD is solubilized up to∼40 mM and no significant increase in viscosity was noticed. Signif-icant micellar growth in hexadecyltrimethylammonium bromide(CTAB) micelles with PTD observed earlier [21] is due to its 16Clong nonpolar tail.

Remarkable growth of TX-100 micelle in the presence of PTD isconfirmed with increase in solution viscosity as well as enlargedmicelle size (Figs. 2 and 3). Interaction of PTD with TX-100 wasalso examined by cloud point, a characteristic feature of the non-ionic surfactant. With increase in PTD concentration, CP of TX-100decreased (Fig. 3). Such a phase behavior shown by nonionicsurfactant solutions in the presence of salting out ions or non-polar/weakly polar solubilizates often results from dehydration.Kandori et al. [52] explained that phenolic OH group makes H-bond with polyoxyethylene (POE) group of shell and replaces thewater molecules in case of nonionic surfactant. Similar H-bondinteraction between NH2 group of PTD and POE group of TX-100 ispossible for PTD/TX100 systems. There is also hydrophobic inter-action between phenyl ring of TX-100 and PTD. Thus addition ofPTD dehydrates POE chain of TX-100 and consequently decreasesthe CP and increases micelle size.

3.2. Effect of temperature

Temperature is very effectual to alter water structure aroundthe nonionic micelles by changing hydration. To probe deeper intoviscosity profiles of DMDAO/PTD and TX-100/PTD; viscosity mea-surements were done for solutions with/without PTD at differenttemperature. In the absence of PTD, the viscosity of DMDAO solu-tion is not altered and that of TX-100 solution increased with

increasing temperature (Fig. 4). The different trend in viscosity forDMDAO and TX100 can be explained on the basis of effect of tem-perature on aqueous solubility of various components. In case ofDMDAO + PTD system solubility of surfactant and PTD is increased

524 K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529

DMD

ca1piao

3

clsm2onbsbmi

PewcPfftitftfSritbio

p

region and at the water–micelle boundary. This explains the vis-cosity trend observed for DDMPS/PTD systems. PTD cannot bringviscosity change without being solubilized slightly deeper in the

Fig. 4. Effect of temperature on the relative viscosity of 50 mM (a)

ausing decrease in viscosity of solution. For TX-100 + PTD systems,lthough total solubility of PTD is increased but the solubility of TX-00 keeps on decreasing with overall increase in micelle size. Theolyoxyethylene chain of TX-100 progressively dehydrates with

ncrease in temperature and thus intermicellar interactions areltered which lead to micellar growth and transition [53]. The effectf temperature is more pronounced in the presence of PTD.

.3. Location of PTD

Additives can be solubilized anywhere in between interface toore region of micelle depending upon their polarity. The solubi-ization site of additives has significant effect on the shape andize of micelle. Thus the location of PTD in different surfactanticelle is determined using two-dimensional NOESY spectroscopy.

D NOESY is a noninvasive technique to examine the dissolutionf solubilizates in surfactant micelles [54]. In this experiment sig-ificant, positive NOESY cross peaks obtained indicate closenessetween pairs of coupled protons. From this technique interactionite of the solubilizate and surfactant molecule and location of solu-ilizates in these surfactant systems can be known. In Scheme 2, theolecular structure of different surfactants with proton numbering

s given.Fig. 5 shows 2D NOESY spectra of SDS in the presence of PTD.

henyl protons (P2, P3) of PTD have a strong cross peak with near-st methylene group proton to head group (S1) and comparativelyeak cross peaks with chain protons (S3) of SDS. The strong intense

ross peaks of CH3 group proton (P1) and phenyl protons (P2, P3) ofTD with terminal core proton S4 of SDS are produced by motionalreedom of hydrocarbon tail of SDS. The strong cross peak is alsoound between P1 and S3. All cross-peaks in the spectra indicatehat PTD molecules remain in micelle with phenyl moiety in towardnterior just below the head group, methyl group toward core dueo the hydrophobic interaction and amine group close to polar sur-ace of micelle. The strong cross peak between P1 and S1 showshat CH3 group of PTD stays close to polar head group which mani-ests that micelles become rich by PTD at higher PTD concentration.uratkar et al. [43] from 1H NMR concluded that phenol moleculeseside in SDS micelles in such a manner that the hydroxyl groups close to polar surface. Similarly, Jacobs et al. [44] also explainedhe location of phenols in SDS micelle at hydrophilic/hydrophobicoundary. Hawrylak et al. [54] have studied the location of benzene

n SDS micelles by 2D NOESY and concluded that the distributionf benzene molecules is throughout the SDS micellar interior.

Fig. 6 shows that there is a strong cross peak between phenylrotons (P2, P3) and proton (M4) of terminal methyl group of

AO (b) TX-100 with (solid circle) and without (hollow circle) PTD.

DMDAO indicating that hydrophobic chain of DMDAO bendstoward PTD molecule. There is a weak intense cross peak betweenM3 and P3 protons. There is a strong intense cross peak betweenthe CH3 protons (P1) of PTD and M4 and M3 protons of DMDAO.All these points indicate that phenyl group of PTD may be solubi-lized deeper into micelle and CH3 group of PTD will have strongtendency to remain in interior part of micelle. Such a location ofPTD induces micellar growth in DMDAO solution.

Fig. 7 shows weak intensity peaks between phenyl protons (P2,P3) of PTD and D5, D8 protons of outer head group region. Here D7proton of DDMPS and P1 proton of PTD merge together so the spec-tra cannot give the actual interaction of these protons with otherprotons. The absence of cross peak between the phenyl protons ofPTD and alkyl chain protons (D2, D3 and D4) of DDMPS indicatesthat no penetration of phenyl moiety in palisade layer of micelle.From the spectra we conclude that PTD remains in head group

Scheme 2. Schematic presentation of structure and proton numbering of surfac-tants and PTD.

K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529 525

Fig. 5. 2D NOESY spectra of 50 mM SDS in the presence of 50 mM PTD.

Fig. 6. 2D NOESY spectra of 50 mM DMDAO in the presence of 50 mM PTD.

Fig. 7. 2D NOESY spectra of 50 mM DDMPS in the presence of 40 mM PTD.

526 K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529

sisq

PtpcirTtccrscmCTtisawp

tttcg

3

tdc

The effect of pH is studied at different concentration of PTD in50 mM SDS, high concentration of PTD [30 mM] leads to greaterincrease in viscosity under acidic pH conditions. An aqueous

Fig. 8. 2D NOESY spectra of 50 mM TX-100 in the presence of 50 mM PTD.

urfactant micelles; simply because there will not be enough pack-ng parameter change. In contrast to this, we have seen deeperolubilization of PTD in DMDAO and TX100 micelles and conse-uent micellar growth and viscosity enhancement.

2D NOESY spectra of 50 mM TX-100 in the presence of 50 mMTD is presented in Fig. 8. Many cross peaks are visible betweenhe protons of different section of TX100, along with some crosseaks between protons of PTD and TX-100. TX-100 has an isooctylhain, benzene ring and polyoxyethylene group. In micellar statesooctyl chains make core and hydrophilic polyoxyethylene chainseside in the periphery of the micellar core. Around the surface ofX-100 micellar core, polyoxyethylene (POE) chains are coiled andwisted thus form wide layer in contact with water. Due to thisonformation there is strong intermolecular interaction of POEhains with protons of micellar core (isooctyl chain) and phenyling protons confirmed by NOSEY spectra of TX-100 [55]. Fig. 8hows the cross peak between methyl proton (P1) of PTD withore protons T1 and T2. The hydrophobic chain is free to move inicelle and bends toward PTD due to the hydrophobic force. A

ross peak is observed between P1 proton of PTD and shell proton4 of TX-100. Phenyl rings of both PTD and TX-100 are mergedogether and therefore cross peaks are not considered for locationnformation. We may assume that due to possible interaction, PTDolubilized deep in TX-100 micelle at core–shell interface as wells in shell region causes micellar transition. In previous studies,e investigated p-cresol is located in core–shell interface andalisade layer of TX-100 micelle [19].

We examined the location of PTD in CTAB micelle in earlier workhat indicates PTD in the palisade layer with its CH3 group towardhe micellar core. We have not given the NOESY spectra of DTAB inhe presence of PTD. Because it is similar to CTAB except has smallerhain length, the location of PTD may be same without any micellarrowth in the concentration studied.

.4. Effect of pH

As mentioned earlier electrical nature of PTD is very sensitiveo pH of solution, thus surfactant solution containing PTD can haveirect consequences from pH change. As shown in Scheme 1, PTDonverts into cationic hydrotrope in acidic pH and remains nonionic

Fig. 9. Effect of pH on the relative viscosity of surfactant solutions at different PTDconcentration.

at neutral/basic pH. It interacts electrostatically or hydrophobicallywith the surfactants according to their head group charge. The loca-tion of PTD in micelle is changed with pH and leads to interestingviscosity behavior of surfactant solution. Fig. 9 represents viscos-ity change for various surfactants with and without PTD addition;complementing to viscosity data micellar sizes for all these systemsare also presented in Fig. 10. Except DDMPS and DTAB all other sur-factant solution with solubilized PTD showed interesting viscositybehavior upon pH variation although there is no change in viscosityfor pure surfactant solutions in studied pH range.

Fig. 10. Effect of pH on the apparent hydrodynamic diameter of SDS, TX-100 andDMDAO micelles in the presence of PTD ([PTD] for SDS, TX-100 and DMDAO are 30,80 and 100 mM, respectively).

K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 521– 529 527

d TX-

spalabmatgpptt∼rt

rtoihDoisnCtfco∼tamcbAt

Scheme 3. Micelle shapes of DTAB, SDS, DDMPS, DMDAO an

olution of 50 mM SDS + 30 mM PTD shows low viscosity atH ∼8 which slightly increases at pH ∼5.0 and drastically increasest pH ∼3. The apparent hydrodynamic size of micelles is simi-arly increased. PTD leads to greater increase in viscosity undercidic pH conditions, this is expected from the ion-pair formationetween SDS and protonated PTD. Just below isoelectric point someolecules of PTD get protonated and start to form co-micelles as

result micelles shape changes from spherical to elongates dueo packing parameter constrains. At pH ∼3.0, most PTD moleculeset protonated and effect of PTD gets intensified. In neutral/basicH, PTD solubilizes slightly deep in interface portion but at lowerH the hydrophobic counterions of PTD electrostatically binds withhe head group of SDS with its phenyl ring in palisade layer. Thushe possible micellar transition occurs of SDS from spherical at pH8 to rodlike at pH ∼3 which is supported by viscosity and DLS

esults. Hassan et al. [1,5] investigated the rodlike micelle of SDS inhe presence of p-toluidine hydrochloride.

In contrast to SDS, TX-100 and DMDAO solutions showedeverse viscosity trend. Viscosity enhancement produced by addi-ion of PTD to TX-100 and DMDAO solutions almost completely diesut at pH ∼3. The viscosity of DMDAO solution is increased signif-cantly in the presence of 100 mM PTD (at pH ∼8) which becomeigher at pH ∼5–6 and lower at pH ∼3 (Fig. 9). Charge and shape ofMDAO micelle depends on the degree of protonation of its aminexide head group which alters electrostatic and hydrogen-bondingnteractions. DMDAO behaves as cationic surfactant and smallpherical micelles form in acidic pH while it forms small sphericalonionic micelles in alkaline pH. Maeda et al. [56] explained thatMC of DMDAO becomes lower at pH region 4–6 due to the forma-ion of a micellized dimer (DMDAO)2H+. This dimeric/double tailedorm of cationic and nonionic monomers having H-bond, decreasesurvature and forms larger micelles favored at high concentrationr in the presence of salt [45,57]. Higher viscosity in region of pH5–6 shows greater interaction of PTD with the dimeric/double

ailed structure of DMDAO molecules than their cationic (pH ∼3)nd nonionic (pH ∼8) micelle form. At pH ∼3, most PTD and DMDAOolecules get protonated and strong repulsion occurs between

ationic micelle of DMDAO and protonated PTD. Protonated PTDehaves as a cationic hydrotrope and is expelled out of micelles.ccording to discussed relation between micelle size and geome-

ry [45,56,57], we assume that micelle size of DMDAO in presence

100 surfactants with solubilized p-toluidine at different pH.

of 100 mM PTD indicates ellipsoidal micelles at pH ∼8, rod likemicelles in 5–6 pH region and spherical micelles at pH ∼3.

In the presence of PTD, TX-100 micelles grow in basic pH. At pH∼8.0, higher viscosity and micelle size indicates growth of micelleof TX-100 with PTD. Above isoelectric point and at high pH, PTDexist as electrically neutral molecule (hydrophobic) and get solu-bilize in TX-100 micelle causing micellar growth and consequentlyviscosity enhancement. As pH decreases, PTD gradually gets pro-tonated and expels out from micelle showing decrease in viscosityand micelle size. Viscosity of TX-100 + PTD system becomes lowerat pH ∼3 and the micelle size ∼10 nm which is same as for TX-100 micelle in water. These results indicate that at pH ∼3, most ofPTD molecules get protonated and are pushed out of micelle andremain in bulk phase and micelle–water interface. Goyal et al. [58]from SANS measurements on TX-100 solutions over a wide rangeof concentration and temperature reported oblate ellipsoids micel-lar geometry. Therefore, we can assume here that, TX-100 micellesare more ellipsoidal or rod-like at pH ∼8 and less ellipsoidal at pH∼3 in the presence of PTD (80 mM). DTAB/DDMPS solutions do notshow considerable change in viscosity with PTD at wide range ofpH. Electric nature of DDMPS micelles is unaffected by pH changeand furthermore, pH alteration have minimum effect on the solu-bilization of PTD in DDMPS micellar solution. Viscosity profile forthis particular surfactant show negligible penetration of PTD intoDDMPS micelles, this point has been clarified in 2D NOESY section.From these observations, the micellar geometry of surfactant in thepresence of PTD can be described as in Scheme 3.

We have also determined clouding behavior of TX-100 in thepresence of PTD at different pH (Fig. 11). There is no noticeablechange in cloud point (CP) of TX-100 with varying pH. As shownin Fig. 3, CP of TX-100 decreases with addition of 80 mM PTD from64 to 32 ◦C at pH ∼8. CP of this system is increases toward lowerpH (72 ◦C at pH ∼2.0). This is because PTD is uncharged in basicmedium and reduces CP while in acidic medium behaves like acationic hydrophobic salt which drastically increases the CP of TX-100. Verma et al. [38] have studied the effect of pH on TX-100 inthe presence of anthranilic acid and found minimum in CP at pH

3.2. Because anthranilic acid is adsorbed in TX-100 micelle as acharged hydrophobic ions as either positive (lower pH) or nega-tive (higher pH) charge diminishes the net attractive interactionsbetween micelles as examined by DLS and SANS. For this system

528 K. Singh et al. / Colloids and Surfaces A: Physico

F

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ig. 11. Cloud point of 50 mM TX-100 in the presence of PTD as a function of pH.

icelle size remains approximately same with change in pH, whilen our TX-100/PTD system micelle size was higher at basic pH andower at acidic pH.

. Conclusion

This work sheds light on change in physicochemical propertiesf differently charged surfactants upon solubilization of aromaticmine, p-toluidine (PTD) under different pH conditions. The 2DOESY spectra reveal that PTD get solubilized in palisade layer ofMDAO and core-shell interface of TX-100 micelle leading to micel-

ar growth and viscosity enhancement. Contrary to this, PTD residet micelle-water interface in near head group region of DDMPSendering negligible viscosity enhancement against PTD concen-ration. Furthermore, locus of solubilization for PTD was controlledhrough surfactant solution pH. Solution viscosity and dynamicight scattering results show that size/shape of micelles from theseurfactants varies differently with pH changes in the presence ofTD. In acidic pH, protonated PTD strongly interacts with anionicSDS) while in alkaline pH, PTD interacts more with nonionic TX-00. Micellar growth was observed in DMDAO at intermediateH. It means pronounced micellar growth of ionic surfactant isbtained in the presence of oppositely charged ionic hydrotropeshile the growth of nonionic surfactants can be attained with non-

onic molecules. Thus pH can be used to control the interactionsetween the micelles and solubilizates for well-intended system.esearch findings are of considerable importance for fine tuning oficelles.

cknowledgment

N. Dharaiya gratefully acknowledges UGC, New Delhi for pro-iding financial assistance in the form of Rajiv Gandhi Nationalellowship [No. F.14-2(SC)/2010(SA-III)].

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[2] P.A. Hassan, S.R. Raghavan, E.W. Kaler, Microstructural changes in SDS micellesinduced by hydrotropic salt, Langmuir 18 (2002) 2543–2548.

[

[

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