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Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 67–75

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h om epage: www.elsev ier .com/ locate /co lsur fa

H controlled size/shape in CTAB micelles with solubilized polardditives: A viscometry, scattering and spectral evaluation

ijay Patela,∗, Nilesh Dharaiyaa, Debes Rayb, Vinod K. Aswalb, Pratap Bahadura

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, IndiaSolid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

i g h l i g h t s

CTAB micelles show structuralchanges with solubilized additives.p-Toluic acid, p-toluidine and p-cresol showed pH dependent micel-lar growth.The dissimilar effects of these addi-tives are explained from pH depend-ent changes.

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

pH-dependent growth of CTAB micelles in the presence of p-toluic acid.

r t i c l e i n f o

rticle history:eceived 3 January 2014eceived in revised form 5 April 2014ccepted 8 April 2014vailable online 16 April 2014

a b s t r a c t

In this manuscript we report pH induced micellar transition in aqueous solution of cetyltrimethylammo-nium bromide (CTAB) in the presence of three weakly polar aromatic additives viz. p-toluic acid, p-cresoland p-toluidine scrutinized by viscosity, nuclear magnetic resonance (NMR), dynamic light scattering(DLS) and small-angle neutron scattering (SANS) measurements. Interaction between these additives andCTAB micelles changes the size and shape of the micelles. Variation in pH alters the charge on the polar

eywords:TABicellar growth

olubilizationH

group and leads to protonation/deprotonation of acidic/basic group of the additives. Depending uponthe pH of solution additives interact with CTAB micelles and accordingly change the solution behav-ior/aggregation characteristics. Morphological changes of surfactant aggregates as a function of additiveconcentration and pH were monitored by SANS measurements. NMR studies reveal a pH dependentlocation of the additive molecules in the micelles.

. Introduction

Surfactant self-assembly in aqueous solutions is a coopera-

ive phenomenon and entropy driven process mainly governedy hydrophobic interaction. The size and shape of ionic micellesepend on hydrocarbon tail, polar head group and counter ion

∗ Corresponding author. Tel.: +91 9979509888.E-mail addresses: vijaypatel4686@gmail.com, vijayipatel444@yahoo.com

V. Patel), nilesh.dharaiya@yahoo.com (N. Dharaiya), debes@barc.gov.in (D. Ray),kaswal@barc.gov.in (V.K. Aswal), pbahadur2002@yahoo.com (P. Bahadur).

ttp://dx.doi.org/10.1016/j.colsurfa.2014.04.025927-7757/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

in the surfactant based formulation [1]. Micellar transition in anaqueous solution is a technologically important area. Sphericalmicelles form at concentration, above the critical micelle concen-tration (CMC) and may grow to rod-like or worm-like structures oreven transform into vesicles under different solvent conditions. Thespherical, ellipsoidal, rod or worm-like structures may also dependon the presence of additives. Highly viscous or viscoelastic sys-tems with entangled worm-like micelles are widely used in drag

reduction and oilfield industries [2,3].

Cetyltrimethylammonium bromide (CTAB) is the most exten-sively studied cationic surfactant. It forms spherical micelles havinga diameter ∼2–3 nm [4,5], CMC ∼1 mM [6] and a krafft point around

6 Physic

∼thdicbt[C1

s

rcagrhwpTmiahmveahT[ag[

l[mpttrtsp

e(aciooai

TC

8 V. Patel et al. / Colloids and Surfaces A:

25 ◦C [7] in water. These micelles undergo sphere-to-rod transi-ion above ∼250 mM [8,9]. The micellar transition in CTAB solutionsas been studied by viscosity [10,11], static light scattering [12],ynamic light scattering [4,5,13], small angle neutron scatter-

ng [14,15] and small angle X-ray scattering [16]. Microstructuralhanges in CTAB micelles have also been successfully monitoredy electron microscopy [9,10,17,18]. Also a variety of liquid crys-alline phases have been identified from polarizing microscope19]. The solubilization and the location of aromatic additives inTAB micelles have been determined by spectral techniques such asH NMR [20–23], 2D NMR [23,24], fluorescence [25] and UV visiblepectroscopy [26].

Cylindrical micelles obtained from ionic surfactants haveecently attracted considerable interest because of their unique vis-oelastic properties. It is generally believed that inorganic [9,27]nd organic salts/hydrotropes [22,28,29] can the change micellareometry. Inorganic counterions bind with the head group andeduce charge repulsion which results in micellar growth whileydrophobic organic ions exert strong electrostatic interactionith oppositely charged micelles and are often situated in thealisade layer which effectively changes the packing parameters.herefore, aromatic counterions are very effective in promotingicellar growth even at comparatively lower concentration than

norganic salts. Several studies on CTAB micelles in the presence ofnionic hydrotropes such as benzoate, tosylate and salicylate ionsave been reported [14,22,29]. Worm-like micelles and vesicles inixtures of CTAB with other surfactants [30–32] and characteristic

iscoelastic fluids with anionic polyelectrolytes [33,34] have beenxamined. Weakly polar aliphatic compounds like medium chainlcohols [5,35–37], amines [38,39] and carboxylic acids [40,41]ave shown a remarkable effect on micellar growth and transition.he aqueous solution behavior of CTAB in the presence of phenols17,20,21], aromatic amines [16,23], aromatic acids [18,22,42,43]nd benzyl alcohol [44] have also been examined. The micellareometry of CTAB with additives could be tuned by temperature45], light [46] and a pH [43,47].

Change in pH of aqueous micellar solutions alters the micel-ar behavior of pH responsive surfactants such as zwitterionic48,49], alkyldimethylamine oxide [50,51] and particularly poly-

eric surfactants [52,53] show interesting solution behavior. SomeH responsive polymeric surfactants have been studied for con-rolled release of drugs [54,55]. For non-pH responsive surfactants,heir aggregate morphology can be tuned by adding some pHesponsive compounds in their solution. There are few reports onhe micellar behavior of pH insensitive conventional surfactantuch as SDS [56], Triton-X 100 [56,58] and CTAB [43,47,59] in theresence of pH responsive polar aromatic compounds.

In this paper, we examined growth of CTAB micelles in the pres-nce of three weakly polar aromatic compounds viz. p-toluic acidPTA), p-cresol (PCL) and p-toluidine (PTD). These additives contain

pH responsive phenolic, amino and carboxylic acid group whichan be protonated/deprotonated up to a different extent by chang-ng the solution pH. Since a small change in pH can lead to formation

f micelles with different geometries, our aim is to study the effectf pH on the size/shape of CTAB micelles in the presence of thesedditives on the entire pH range. DLS and SANS techniques werentroduced to gain microscopic view on pH effect in the system. The

able 1haracteristics of additives added to CTAB solution.

Additive pKa Log Po/wa Solubility in water at 25 ◦C, g/100 ml

p-Toluic acid 4.3 2.27 ∼0.10p-Cresol 10.2 1.94 ∼2.15p-Toluidine 5.1 1.39 ∼0.72

a Data of Table 1 is taken from Ref. [60] (RSC website).

ochem. Eng. Aspects 455 (2014) 67–75

pH modulated micellar growth by such weakly polar solubilizatesoffers practical interest in surfactant based formulations.

2. Experimental

2.1. Materials

Cetyltrimethylammonium bromide (CTAB) from Sigma–Aldrichwas used as received. p-Toluidine (PTD), p-toluic acid (PTA) andp-cresol (PCL) were supplied by Fisher Scientific. Deionized waterfrom a Millipore Milli-Q system was used for viscosity and DLS mea-surements. 50 mM CTAB solution was used for all measurements.The pH of the solutions was adjusted by using HCl/NaOH. The aque-ous solubility, pKa, and octanol/water partition coefficients (Po/w)of the additives are shown in Table 1.

2.2. Methods

2.2.1. ViscosityCalibrated Cannon Ubbelohde viscometers were used to mea-

sure relative viscosities of the solutions in a temperature controlledwater bath. Viscometers were used having a size of 25, 150 and 300with viscometer constants of 0.001869, 0.03462 and 0.2530 cSt s−1,respectively [61]. The flow time of the solution was measuredin seconds and multiplied by the viscometer constant to get thekinematic viscosity in centi-stokes. This kinematic viscosity wasconverted into centi-poise by multiplying with the density of water(taken as ∼1 g cm−3). The relative viscosity of solution was obtainedfrom the ratio of viscosity (centi-poise) of solution and water (sol-vent) [62].

2.2.2. Dynamic light scattering (DLS)A Zetasizer Nano-ZS 4800 (Malvern Instruments, UK) was used

for DLS experiments. The light source was an Ar-ion laser operatingat a wavelength of 633 nm at 90◦ scattering angle. Each sample wasfiltered by 0.45 �m filter before use. Stokes-Einstein relationshipwas used to calculate the apparent hydrodynamic diameter (Dh) ofthe micelles from cumulant analysis.

2.2.3. Nuclear magnetic resonance (NMR) spectroscopy1H NMR measurements were performed on Bruker, Avance II

(400 MHz). All measurements were performed at 30 ◦C. D2O wasused as solvent for sample preparation. The detailed procedure isreported in literature [23].

2.2.4. Small-angle neutron scattering (SANS)Small-angle neutron scattering experiments were performed at

the SANS diffractometer at Guide Tube Laboratory, Dhruva reac-tor, BARC, Mumbai, India [63]. In SANS, one measures the coherentdifferential scattering cross-section (d�/d�) per unit volume as afunction of wave vector transfer Q (=4�sin(�/2)/�, where � is thewavelength of the incident neutrons and � is the scattering angle).It provides information about the shape and size of the scatteringparticles in the length scale of 10–1000 A. The scattering data for amean incident wavelength of 5.2 A with ��/� = 15% were measuredat 30 ◦C in the Q-range of 0.017–0.35 A−1. The measured SANS datawere corrected for the background, the empty cell contributionsand the transmission and normalized to absolute cross-sectionalunit using standard protocols.

SANS analysis:The differential scattering cross-section per unit volume

(d˙/d˝) as measured for a system of monodisperse particles ina medium can be expressed as [64,65](

d˙

d˝

)(Q ) = nV2(�p − �s)

2P(Q )S(Q ) + B (1)

Physicochem. Eng. Aspects 455 (2014) 67–75 69

wraftt

sF

P

F

m

F

F

x

b

ttopdpoa

u

ws

wctt

u

wa

f

w(b

d

V. Patel et al. / Colloids and Surfaces A:

here n denotes the number density of particles, �p and �s are,espectively, the scattering length densities of particle and solventnd V is the volume of the particle. P(Q) is the intraparticle structureactor and S(Q) is the interparticle structure factor. B is a constanterm representing incoherent background, which is mainly due tohe hydrogen present in the sample.

Intraparticle structure factor P(Q) is decided by the shape andize of the particle and is the square of single-particle form factor(Q) as determined by

(Q ) = 〈|F(Q )|2〉 (2)

For a spherical particle of radius R, F(Q) is given by

(Q ) = 3

[sin(QR) − QR cos(QR)

(QR)3

](3)

For a prolate ellipsoidal particle with semi-major and semi-inor axes a and b, respectively,

(Q ) =∫ 1

0

F(Q, ) d (4)

where

(Q, ) = 3(sin x − x cos x)x3

(5)

with

= Q [a22 + b2(1 − 2)]1/2

(6)

where in the above equations refers to the cosine of the angleetween the directions of a and Q.

S(Q) describes the interaction between the particles present inhe system and it is the Fourier transform of the radial distribu-ion function g(r). g(r) gives the probability of finding the centerf another particle at a distance r from the center of a referencearticle. S(Q) is calculated using the mean spherical approximationeveloped by Hayter and Penfold [66]. In this approximation, thearticle (in this case, micelle) is treated as a rigid equivalent spheref diameter d = 2(ab2)1/3 interacting with another micelle through

screened coulomb potential u(r) given by the relation

(r) = u0d exp[−(r − d)

r

], r > d (7)

here u0 is the potential at r = d and the Debye–Huckel inversecreening length is evaluated by using the expression

=(

8�NAe2I

103εkBT

)1/2

(8)

here NA, e, I, ε, kB and T denote Avogadro number, electronicharge, ionic strength of the micellar solution, dielectric constant ofhe solvent, Boltzmann constant and absolute temperature, respec-ively.

The polydispersity in size distribution of particle is incorporatedsing the following integration [67]

d˙

d˝(Q ) =

∫d˙

d˝(Q, R)f (R)dR + B (9)

here f(R) is the particle size distribution and usually accounted by log-normal distribution as given by

(R) = 1√2�R�

exp

[− 1

2�2

(ln

R

Rmed

)2]

(10)

here Rmed is the median value and � is the standard deviation

polydispersity) of the distribution. The mean radius (Rm) is giveny Rm = Rmed exp(�2/2).

The data have been analyzed by comparing the scattering fromifferent models to the experimental data. Throughout the data

Fig. 1. Relative viscosity of 50 mM CTAB solution in the presence of (о) PTA, (�)PCL, and (�) PTD at 30 ◦C.

analysis, corrections were made for instrumental smearing, wherethe calculated scattering profiles smeared by the appropriate reso-lution function to compare with the measured data [68]. The fittedparameters in the analysis were optimized using nonlinear least-square fitting program to the model scattering [69].

3. Results and discussion

3.1. Effect of polarity of additives on CTAB micelles

Aromatic compounds having different polar groups, pKa values,water solubility and Po/w may give distinct effects on the micellarcharacteristics of CTAB (Table 1). Such pH responsive micellar sys-tem could realize interesting findings. The morphological changein an aqueous solution of surfactant can easily be monitored bya change in viscosity of the solution. In view of this, viscositymeasurements for 50 mM CTAB solution at different additive con-centrations were carried out and these results are shown in Fig. 1.We have determined the effect of wide range of concentration ofPCL and PTD on CTAB solution while lower solubility of PTA confinesto identify its higher concentration effects on micelle transition.Polarity of additives is deciding factor for their interaction withCTAB micelles. All the three additives show an increase in viscosity,but more polar PCL induces a marked increase in viscosity reflectingmore prominent micelle growth than PTA and PTD. The progressiveaddition of PCL leads to considerable increase in the solution vis-cosity giving maximum presenting micellar growth. The decreasein viscosity after maximum has been ascribed to the morpholog-ical changes in micelles [43]. In case of PTD, viscosity increasesmoderately till its maximum concentration. PTA has limited sol-ubility in CTAB micelles and is not much effective in promoting themicelle growth. The change in morphology of surfactant aggregates

depends on the location of solubilizates and their extent of interac-tion with CTAB micelles. If the solubilizate molecules are situatedin the palisade layer or between the head groups that diminished

70 V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 67–75

Table 2Micellar parameter for 50 mM CTAB in the presence of additives at 30 ◦C.

[Additive] Semi-major axis, Å (a) Semi-minor axis, Å (b) Axial ratio (a/b) Aggregation number (Nagg)

0 33.7 24.0 1.4 14525 mM PTA 83.9 21.0 3.9 27725 mM PCL 89.0 21.9

50 mM PCL 125.9 21.6

75 mM PCL 98.2 23.5

Fig. 2. Effect of additive concentration on SANS pattern of 50 mM CTAB at 30 ◦C. Thesa

em

iSiptsepcmn(cigflwdmttToPtrmto

olid lines represent the fitted data. Data have been shifted in vertical direction by constant factor for clarity.

lectrostatic repulsion and increase in packing parameter of theicelle facilitate micellar growth.Microstructural changes in micellar systems can easily be mon-

tored by SANS. To shed light on additives induced micellar growthANS measurements were performed for PCL and PTA correspond-ng to viscosity and DLS measurements. Hirata et al. [16] haveroved the existence of cylindrical rod-like micelles in CTAB solu-ion in the presence of PTD using small angle X-ray scattering. Fig. 2hows SANS plots for 50 mM CTAB solution in the absence and pres-nce of PTA and PCL at varying concentration. A typical correlationeak is observed in SANS spectra of CTAB solution presenting aharged system [70,71]. Data analysis shows that in 50 mM, CTABicelles have semi-major axis, semi-minor axis and aggregation

umber (Nagg) 33.7 A, 24 A and 145, respectively. The axial ratio1.4) suggests that micelle shape is prolate ellipsoidal [15]. A shift inorrelation peak toward low Q-region observed for CTAB solutionsn the presence of 25 mM PTA as well as PCL reveal the micellarrowth. Here, ellipsoidal micelles are elongated which is evidentrom increased axial ratio and Nagg. However, due to inaccessibleower Q cut-off, SANS can see only elongated ellipsoidal micelles,

hich can be seen as small rod-like in cryo-TEM [17]. Viscosityata reveal that PTA at higher concentration does not induce largericellar growth as compared to PCL. With increase in concentra-

ion of PCL up to 50 mM, the correlation peak at low-Q arises due tohe increased interaction among the elongated ellipsoidal micelles.he axial ratio is increased up to ∼5.8 which suggests the presencef larger aggregates and Nagg jumps to 439. With further increase inCL concentration up to 75 mM, increased interaction drives themo form unilamellar vesicles, showing a slope of (−2) in the low-Q

egion of SANS data [72]. As a consequence the number density oficelles decreases and correlation peak vanishes and at the same

ime decrease in axial ratio and Nagg is also observed. The formationf unilamellar vesicles (at 75 mM PCL) is responsible for decrease in

4.0 3195.8 4394.1 405

viscosity after peak [37]. These observations clearly manifest thatprogressive addition of PCL to 50 mM CTAB solution leads to the for-mation of unilamellar vesicles. Micellar parameters obtained fromSANS data analysis are listed in Table 2.

1H NMR experiments were performed to get better insite onPTA solubilization in CTAB micelle (Fig. 3). The 1H NMR spectrumof CTAB shows broadening of all protons of CTAB in the presenceof PTA which implies some micellar growth. The broadening in 1HNMR spectra of the head group protons (f) and alkyl chain protons(a, b and c) reveal the micellar transition.

Earlier, the solubilization of PTD in different cationic surfactantshaving dissimilar chain length, counter ion and head group havebeen reported [23]. In the present study, we have compared themicellar behavior of PTD with PCL and PTA. In the presence of PTD,an increase in the viscosity of the surfactant solution is significantlyless as compared with PCL. This is due to the relatively lower Po/wand polarity of the NH2 group which results in less hydrophobicand electrostatic interactions between PTD and cationic micelle ascompared with PCL. Smaller amount of PCL is required to increasethe viscosity than of the other studied solubilizates due to thehigher polarity of the OH group which provides strong electro-static interactions with quaternary head group and changes thepacking parameter of CTAB micelles. Mata et al. [15] have shownthe location of phenol at the micelle–water interface (between headgroup) in CTAB micelles. Hence, PCL molecules may penetrate morein CTAB micelles as compared to phenol and thus lead to significantchanges in packing parameter of the micelles. Agarwal et al. [17]determined the microstructure of CTAB micelles in the presence ofphenol and its derivatives. It was observed that PCL was more effec-tive to induce micellar geometry than phenol. Consequently, uponan increase in PCL concentration, spherical CTAB micelles trans-form into rod-like and worm-like structures with very few vesicularstructures. All these results indicate that the polarity, concentrationand location of molecules play vital role in modulating the micellargeometry of CTAB surfactant.

3.2. Effect of pH

The presence of pH responsive groups such as OH, NH2 andCOOH, in the studied aromatic compounds show pH sensitivity.

The extent of ionization of these solubilizates depends on pH.In order to study the effect of pH on microstructures of aggre-

gates, first we observed the variation in the viscosity of the solutionas a function of pH. Fig. 4 shows the relative viscosity of 50 mM CTABin presence of 25 mM PTA at varying pH at 30 ◦C. The pH of the solu-tion having 25 mM PTA solubilized in 50 mM CTAB is ∼2.6. Viscosityand micelle size is low at pH ∼2.6 but significantly increases onits either side which present micellar growth further confirmed bySANS (discussed in the later part of the manuscript). Aromatic acidshave tendency to deprotonate when the pH is above their pKa value.PTA has a pKa value 4.3 [73]. The solution viscosity is increasedabove pH 4.3 but slightly decreases below it. If the solution pH

increases above pH 2.6 then the equilibrium of PTA molecules shiftstoward anionically charged PTA. As negatively charged species PTAexhibits electrostatic interaction with the head groups of CTAB andtends to neutralize the surface charge of micelles. There is also

V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 67–75 71

absen

atPsih

Fswat

Fig. 3. 1H NMR spectra of 50 mM CTAB in the

hydrophobic interaction between the phenyl ring of PTA andhe nonpolar chain of the surfactant. This association of ionizedTA with the polar head group promotes micellar growth. If the

olution pH < pKa, equilibrium of PTA molecules shifts toward lessonized species having low aqueous solubility and solubilization byydrophobic effect takes place.

ig. 4. Effect of pH on relative viscosity and hydrodynamic size of 50 mM CTABolution in the presence of 25 mM PTA at 30 ◦C. In DLS measurements 10 mM NaBras added. *Highlighted red colored portion in figure shows the solution without

ny pH adjustment. (For interpretation of the references to color in this figure legend,he reader is referred to the web version of the article.)

ce and presence of 25 mM PTA and at 30 ◦C.

A very interesting observation was found below pH ∼2.0 wherethe viscosity drastically increases presenting the micellar growthwhich is supported by increase in Dh of the micelles. Commonly,PTA molecules are solubilized in surfactant solution with a fewPTA molecules stay in the bulk water due to an aqueous solubilityand remaining solubilized in micelle. Below pH ∼2, PTA moleculesbecome less soluble in water and get penetrated into the micelleswhich results in micelle growth. Size distribution curves for 50 mMCTAB in the presence of 25 mM PTA at 30 ◦C is shown in supportingmaterial (S1). It provides better idea for change in hydrodynamicsize of micelles as a function of pH. The micelle size of CTAB is2–3 nm and in the presence of 25 mM PTA (and 10 mM NaBr)increases up to about ∼10 nm and a further considerable growthoccurs at lower and higher pH. The Dh of micelles ∼25 nm (pH ∼ 1.2)and ∼19 nm in alkaline medium (pH ∼ 9.0) indicates elongated rod-like micelles. It clearly indicates that the increase in viscosity ismore pronounced in acidic medium which suggests that hydropho-bic interaction is superior over electrostatic interaction. It can beeasily understood that ionized form of PTA molecules induce themicelle growth but it is some unlike finding that less ionized formalso promote micelle growth. For more information on the pHeffect, the 1H NMR spectra for CTAB + PTA system at varying pHwere recorded (Fig. 5). On evaluating this system for pH ∼ 9.0 and1.5 with pH ∼ 2.6, merging of the head group protons e and f aswell as chain protons a, b and c can be seen. This observation con-cludes that the CTAB–PTA system shows a considerable micellargrowth in higher acidic and basic pH conditions. Kumar et al. [22]explained that benzoic acid gets soluibilized in the palisade layer ofthe CTAB micelle. Here, considerable broadening/shifting of headgroup as well as chain protons of CTAB indicates that PTA is solu-bilized in slight deep in palisade layer of micelle (at pH ∼ 2.6). Inbasic pH ∼ 9.0, PTA attains a negative charge and remains near thecationic head group and the phenyl ring stays in the palisade layer.At pH ∼ 1.5, the spectrum shows more upfield shift of the headgroup and core protons as compared to pH ∼ 9.0. It shows more

penetration of PTA molecules in micelle at pH ∼ 1.5. Furthermore,the spectrum pattern is same for pH ∼ 1.5 and 9.0 indicating thatthe cationic charge of the head group is neutralized or shielded byaromatic �-electrons [59]. Furthermore, the methyl protons and

72 V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 67–75

Fig. 5. Chemical shift of 50 mM CTAB in presen

Fa

pPTaT

otimaiiM

Costvia

CTAB micelle in the presence of 25 mM PCL (at 5.8 pH) whichmay further grow in alkaline pH ∼ 10. From pH ∼ 10 to 12 viscositystarts decreasing drastically but hydrodynamic size consequentlyincreases and slightly decreases at higher pH (Fig. 7). This indicates

Table 3Micellar parameter for 50 mM CTAB + 25 mM additives at varying pH at 30 ◦C.

System Semi-major axis, Å

(a)

Semi-minor axis, Å

(b)

Axial ratio(a/b)

Aggregatio n number

(Nagg)PTA pH ~1.5 93.1 21.4 4.3 319pH ~2.6* 83.9 21.0 3.9 277pH ~10 92.6 20.8 4.4 300

PCLpH ~1.5 96.7 22.1 4.3 353

ig. 6. Effect of pH on SANS pattern of 50 mM CTAB in the presence of 25 mM PTAt 30 ◦C. The solid lines represent the fitted data.

henyl protons of PTA have a higher upfield shift indicating thatTA molecules are situated in the nonpolar portion of the micelle.his indicates that at lower pH, mostly PTA molecules are unionizednd well solubilized in the micelle and lead to micellar transition.he 1H NMR experiments show fine correlation with viscometry.

Fig. 6 shows SANS plots for 50 mM CTAB solution in the presencef 25 mM PTA at two different pH values at 30 ◦C. It clearly depictshat by changing the pH on either side of 2.6, scattering intensity isncreased to higher extent in acidic medium compared to alkaline

edium suggesting pronounced micellar growth at lower pH. It islso in accordance our viscosity and DLS data. Here, micellar growths perceived in acidic as well as alkaline medium as evident fromncrease in axial ratio and Nagg which is the aim of the present study.

icellar parameters obtained from SANS are recorded in Table 3.Further, the pH effect depends on the PTA concentration in the

TAB solution. At lower PTA concentration (10 mM), the viscosityf the solution remained unaltered over the entire pH rangetudied because the amount of PTA is not sufficient to cause a

ransition of CTAB micelles (figure not shown). For 25 mM PTA, theiscosity increased significantly up to pH ∼ 6 and a further increasen pH do not show a change. This is because most PTA moleculesre converted to anionic salt at pH ∼ 6 and gives their whole effect

ce of 25 mM PTA at different pH at 30 ◦C.

for micellar growth. Mata et al. [15] determined the size of CTABmicelles in the presence of phenol and found prolate ellipsoidalmicelles by using DLS and SANS. Sreejith et al. [37] studied themicellar geometry of CTAB in the presence of octanol/KBr by rhe-ology, DLS and cryo-TEM and found rod-like, worm-like micellesand vesicular structures with varying octanol concentration.

PCL shows a strong interaction with CTAB micelles, a changein pH leads to an interesting effect on the aggregation character-istics of this system. Fig. 7 shows the relative viscosity of 50 mMCTAB in presence of 25 mM PCL at varying pH at 30 ◦C. PCL hassome solubility in water but at lower pH its dissociation decreasesand penetration increases in CTAB micelle which makes tight pack-ing. A drastic increase in a viscosity indicates the formation oflarge aggregates such as rod-like micelles and illustrates similarbehavior of PCL and PTA at pH < 2. pKa of PCL is ∼10.2 [57] andviscosity peak maximum is found at pH ∼ 10 where ionized andunionized form of PCL may be in somewhat equal proportion. AspH moves from neutral to alkaline, ionization of PCL increases andit electrostatically interacts with CTAB micelles and leads to anincrease in viscosity at pH ∼ 10. Agarwal et al. [17] described theformation of rod-like micelles in CTAB solution in the presence of17 mM PCL. The SANS results also showed elongated ellipsoidal

pH ~5.8* 89.0 21.9 4.0 319pH ~10 96.5 22.2 4.3 356pH ~12 97.4 20.8 4.7 309

*Text highlighted in green color shows the solution without any pH adjustment.

V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 455 (2014) 67–75 73

Fig. 7. Effect of pH on relative viscosity and hydrodynamic size of 50 mM CTABsolution in the presence of 25 mM PCL at 30 ◦C. In DLS measurements 10 mM NaBrwat

twId[trr[lvvetna

mFp∼ibmapmipsp

pvt

more unionized PTD molecules destabilizes the formation of largeCTAB micelles. At pH > 10, the viscosity of this system increasesdrastically. At higher pH, the dissociation of PTD is decreased andit becomes a more hydrophobic molecule which has tendency to

as added. *Highlighted red colored portion in figure shows the solution withoutny pH adjustment. (For interpretation of the references to color in this figure legend,he reader is referred to the web version of the article.)

hat above pH ∼ 10, elongated micelles may form a branched net-ork or make entanglements and lead to a decrease in viscosity.

n literature, there are many examples for a peak in viscosity andifferent phenomenon for decreases in viscosities after this peak15,37,43]. In this regard, Verma et al. [43] studied the pH effect onhe morphology of CTAB micelle with solubilized anthranilic acid byheology, DLS and SANS and illustrated that branched micelles areesponsible for the decrease in viscosity at high pH. Sreejith et al.37] discussed that addition of octanol in the CTAB/KBr system firsteads to a spherical to worm-like transition which increases theiscosity and further addition of octanol leads to the formation ofesicle micelles which results in a drop in viscosity. Mata et al. [15]xamined that the addition of small amounts of phenol increaseshe size of the CTAB aggregates while high concentrations of phe-ol destabilize the formation of micelles and decrease the viscositynd micellar size.

To quantify our anticipation from viscosity data, SANS measure-ents were performed on CTAB solutions in the presence of PCL.

ig. 8 shows SANS plots for 50 mM CTAB + 25 mM PCL at varyingH at 30 ◦C. The pH of 50 mM CTAB with 25 mM PCL solubilized is5.8. It is obvious that elongated ellipsoidal micelles are present

n the system as evident for axial ratio ∼4. At pH ∼ 1.5, hydropho-ic interaction drives PCL molecules to interact more with CTABicelles and consequently ellipsoidal micelles are more extended

s evident from increased axial ratio. At pH ∼ 10 which is close toKa of PCL, electrostatic interaction drags PCL molecules in CTABicelles which results in micellar growth giving peak in viscos-

ty and an increase in Nagg. At pH ∼ 12, viscosity decreases aftereak which may be due to the formation of entangled/branchedtructures [43]. Micellar parameters obtained from SANS fits areresented in Table 3.

Fig. 9 shows the relative viscosity and Dh of 50 mM CTAB in theresence of 100 mM PTD at varying pH. For 100 mM PTD, a highiscosity is observed at pH ∼ 5.0 and pH > 10 and Dh of this sys-em follows the same trend as viscosity which implies formation

Fig. 8. Effect of pH on SANS pattern of 50 mM CTAB in the presence of 25 mM PCLat 30 ◦C. The solid lines represent the fitted data. Data have been shifted in verticaldirection by a constant factor for clarity.

of rod-like micelles. At pH ∼ 3, most PTD molecules are protonatedhaving cationic charge that leads to a repulsive interaction withthe cationic head groups of CTAB. Thus the protonated PTD can-not penetrate in the micelle and remains in the bulk phase. ThepKa of PTD is ∼5.1 [56] and the viscosity increased drastically frompH ∼ 3 to 5 because some PTD molecules are converted into unpro-tonated/neutral form and penetrate in the micelle. Form pH ∼ 5to 7 the viscosity of the solution is consequently decreased whichshows the formation of small micelles. It indicates that formation of

Fig. 9. Effect of pH on relative viscosity and hydrodynamic size of 50 mM CTABsolution in the presence of 100 mM PTD at 30 ◦C. In DLS measurements 10 mM NaBrwas added. *Highlighted red colored portion in figure shows the solution withoutany pH adjustment. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of the article.)

7 Physic

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enetrate in the micelle and cationic-� interaction leads to theormation of rod-like/elongated micelles. Earlier, the pH effect onTD solubilization was examined for dodecyltrimethyl ammoniumromide [56] and gemini [74] surfactants, but CTAB shows morehange in micellar solution.

. Conclusions

The solubilization of three weakly polar aromatic compoundsiz. PTA, PTD and PCL in CTAB micelles was examined to understandheir effect on micellar transition which depends on their elec-rostatic and hydrophobic interaction with the surfactant. Highlyolar PCL strongly interacted with CTAB micelles compared tohe other two additives showing pronounced micellar transitions.ynamic light scattering and viscosity data manifest the presencef large aggregates at varying concentration of additives and solu-ion pH. SANS study revealed that elongated ellipsoidal micellesnd unilamellar vesicles are formed as a consequence of progres-ive solubilization of PTA and PCL, respectively. The pH dependentrotonation/deprotonation of the pH responsive group of thesedditives and their interaction with surfactant’s polar head groupeads to the formation of extended ellipsoidal micelles. New insightn this study is that polar aromatic compounds when less ion-zed (more hydrophobic) get solubilized more in the micelles andationic-�/electrostatic interaction induces micellar growth. NMRtudies indicate the pH dependent location of PTA in micelle. Thistudy provides fundamental knowledge of pH modulated growthf micelles by weakly polar solubilizates useful in developing sur-actant based formulations and supportive for researchers in theeld of colloid and surface science.

cknowledgement

P. Bahadur and V. Patel thank UGC New Delhi for thenancial assistance (Project No: 37-527/(2009) SR). N. Dharaiyahanks to UGC for Rajiv Gandhi National Fellowship [No. F.16-860(SC)/2010(SA-III)].

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.colsurfa.2014.4.025.

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