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Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 252– 259

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

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

ight scattering and NMR studies of Triton X-100 micelles in theresence of short chain alcohols and ethoxylates

ilesh Dharaiyaa,∗, Prashant Bahadurb, Kulbir Singhc, D. Gerrard Marangonic,ratap Bahadura

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, IndiaDepartment of Chemical Engineering, Lamar University, Beaumont, TX 77710, USADepartment of Chemistry, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada

i g h l i g h t s

Solutions of TX-100 were examinedin the presence of alcohols and glycolethers.Dynamic light scattering showsincrease/decrease in micelle size bythese additives.Cloud point can be modulated bythese alcohols and ethoxylates.Location of additives in micelles fromNMR depends on their hydrophobic-ity.

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

Location of alcohols and their ethoxylates in Triton X-100 micelle.

r t i c l e i n f o

rticle history:eceived 13 March 2013eceived in revised form 17 May 2013ccepted 7 June 2013vailable online 14 June 2013

a b s t r a c t

Micellar characteristics of nonionic surfactant p-tert-octyl-phenoxy polyethylene (9.5) ether (Triton X-100) in aqueous media containing short-chain alcohols and their ethoxylates CnEm (n = 2, 4, 6 and m = 0, 1,2) were examined by dynamic light scattering (DLS) and nuclear magnetic resonance (NMR). The micellesize increased with the addition of C6Em and decreased when C2Em alcohols were added to Triton X-100 solution; the increase and decrease in the micellar size in the presence of varying amounts of C4Em

eywords:loud pointriton X-100icelleMR

alcohols depends on the number of polar ethoxylate groups. The results are supported by viscosity andcloud point data and explained on the basis of solvophobic interaction. The interaction and locationof additives in micelles is examined by 1H and NOESY NMR. The studies indicate that C2Em moleculesmostly remain in bulk water; C6Em molecules get solubilized toward the core of the aggregates, whileC4Em molecules are localized in the shell region of micelle according to their octanol/water partitioncoefficient values.

. Introduction

Short and medium alkyl chain alcohols [1] and their ethersith ethylene glycol [2], propylene glycol [3] and glycerol [4] are

∗ Corresponding author. Tel.: +91 9898771871; fax: +91 261 2256012.E-mail addresses: nilesh.dharaiya@yahoo.com, nilesh.dharaiya01@gmail.com

N. Dharaiya), pbahadur82@hotmail.com (P. Bahadur), ksingh@stfx.ca (K. Singh),marango@stfx.ca (D.G. Marangoni), pbahadur2002@yahoo.com (P. Bahadur).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.06.014

© 2013 Elsevier B.V. All rights reserved.

important solvents used in detergents, cosmetics, pharmaceuticals,coatings, paint and ink industries. These short-chain ethers aresometimes called “chameleonic solvents” or “solvo-surfactants”since they are nonionic amphiphiles, exhibiting typical solventproperties at the same time. Low vapour pressure and miscibilitywith water and many organic liquids make certain monoalkyl

ethers of ethylene glycol and diethylene glycol industrially usefulsolvents. Ethers with fairly long alkyl chain are amphiphilic andshow surface activity and aggregation behavior in water [5].Many glycol ethers are eco-friendly and exhibit low aquatic and

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N. Dharaiya et al. / Colloids and Surfaces A:

ammalian toxicity [6,7]. Alcohols and glycols can alter theicellar behavior of surfactants. Marangoni and coworkers [8,9]

eported critical micelle concentrations (CMC) of ionic surfactantsn the presence of alkoxyethanols and the location of these addi-ives in micelles by NMR. Chavda et al. [10–12] recently examinedhe effect of different alcohols, glycols and glycol ethers on micellarehavior of cationic surfactants. Marszall [13] investigated theffect of short-chain glycol ethers on the cloud point and CMC ofonionic surfactants. Mahajan et al. [14] investigated the effect oflycol oligomers on size, shape, and aggregation number of Tweenicelles. The motive of the present work is to find comparative

ffect of alcohols and their ethoxylates on Triton X-100 micelle.Triton X-100 (TX100) is a nonionic surfactant of alkyl phenol

thoxylate category and possesses excellent surfactant perfor-ance in detergency, emulsifying and wetting characteristics over

fairly broad temperature range and is readily biodegradable15]. TX100 has an eight carbon branched hydrophobic chain, ∼–10 units ethylene oxide as the hydrophilic moiety, HLB ∼ 13.5nd CMC ∼ 0.26 mM [16,17]. Due to its ability to bind proteins,riton-X series surfactants have been used in extraction/separationf biomolecules such as amino acids, proteins, nucleic acids etc.18–20]. It is the most extensively investigated nonionic surfactantnd forms normal micelles in water [21] and reverse micelles inrganic liquid [22]. The solution behavior of nonionic ethoxylatedurfactants has been explained on the basis of Hofmeister seriesf inorganic ions [23–25], structure and hydrophobicity of organicdditives [26–39], pH responsivity of weakly acidic/basic solutesike anthranilic acid [40], salicylic acid [41] and p-cresol [42].

Mahajan et al. [30] studied the clouding behavior of TX100 in theresence of alcohols and glycol ethers while Gu and Galera-Gomez28] did a similar study in the presence of aliphatic alcohols, acids,sters and ketones. Additives alter the phase and micellar behaviorf surfactant on the basis of solubilization site in micelle. However,e find no systematic study on the effect of alcohols, cellosolves and

arbitols on micellar characteristics and location in TX100 micellesn the literature. This study was therefore undertaken to examinehe effect of mono ethyl-, mono n-butyl- and mono n-hexyl ethersf ethylene glycol and diethylene glycol (and the correspondinglcohols viz. ethyl, n-butyl, n-hexyl alcohol for comparison) on thehase and micellar behavior of TX100 micelles. DLS and NMR stud-

es were done to observe the micellar behavior of TX100 in theresence of these organic additives. 2D NOESY spectra were usedo determine solubilization site of additive in micelle. Their struc-ures and octanol/water partition coefficient values are shown inable 1.

. Experimental

.1. Materials

Polyoxyethylated isooctylphenol (Triton X-100) was procuredrom Sigma–Aldrich. Alcohols were supplied by Fisher scientific.thyl, butyl and hexyl cellosolves and carbitols were from DOW andigma Aldrich and used as received. Solutions for cloud point andLS measurements were prepared in Milli-Q water. D2O was used

or the sample preparation in 1H and NOESY NMR experiments.

.2. Methods

.2.1. Cloud pointThe cloud point (CP) was observed at a fixed concentration of

he surfactant (5 wt%) at different additives concentrations. Theolution was heated in thin glass tubes immersed in a well stirredeating bath. The sample temperature was raised gradually alongith constant stirring. The sudden change in the appearance of the

ochem. Eng. Aspects 436 (2013) 252– 259 253

solution from being clear to the first turbidity is considered as cloudpoint. The CP values were reproducible up to about ±0.5 ◦C.

2.2.2. Viscosity measurementsAn Ubbelohde viscometer, thermostated at 30.0 ± 0.1 ◦C, was

used for viscosity measurements. The viscometer was cleaned andair dried before every use. The time of flow for each viscosity mea-surement was recorded minimum for three times in order to checkthe reproducibility. It was found that the viscosity values werereproducible within ±1.0%.

2.2.3. Dynamic light scattering (DLS)DLS measurements were performed on a Zetasizer Nano-ZS

4800 (Malvern Instruments, UK). The light source was a He–Ne laseroperating at a wavelength of 633 nm at 90◦ scattering angle. Eachmeasurement for a particular micellar system was repeated at leastthree times. By using the Stokes–Einstein relationship, apparenthydrodynamic diameter (Dh) of the micelles was calculated.

2.2.4. Nuclear magnetic resonance (NMR)The 1H NMR and NOESY spectra were obtained on a Bruker

Avance-II 400 MHz spectrometer. D2O was used as a solvent forNMR experiments; solvent suppression eliminated the HDO peakdue to residual water. The 1H NMR chemical shifts were referencedto the deuterium lock signal. The mixing and the delay times for theNOESY experiments were estimated from the spin–lattice relax-ation time. The acquisition delays and mixing times were adjustedto ≈3 × T1 of ≈1 × T1, respectively.

3. Results and discussion

3.1. Solubilization of additives

Surfactant micelles have an astounding ability to solubilize avariety of chemical compounds; depending upon their polarity,these compounds can be solubilized anywhere from the micellecore to its periphery. Additive solubilization has direct consequenceon the physical characteristics of the mixed surfactant micellesthus formed, and in particular, can have a profound effect on theshape and size of micelle. The presence of various additives inthe TX100 micelles can be easily ascertained from the changesin many physico-chemical properties upon addition of chemicaladditives. In the present case, we used 1H NMR chemical shiftvalues and/or peak broadening, proportional to the spin–spin relax-ation time of the protons of TX100 protons under observationto investigate the solubilization of short chain alcohols and theirethoxylate derivatives. The solubilization of aromatic hydrocarbons[43], nitrotoluenes [44] and phenols [42] in TX100 micelles hasbeen reported from the analysis of NMR chemical shift changes.Proton labeling for various protons of all constituents is presentedin Fig. 1. Fig. 2 gives an overlay of a series of TX100 1H NMR spectrain the presence and absence of additives at a surfactant concen-tration above the CMC of TX100. The observed changes in chemicalshift values for all the protons of TX100 in the presence of increasingadditive concentration is a strong indication that a significant quan-tity of the additive molecules are being solubilized into the micellesof TX100. Owing to hydrophilic characteristics of all additives in thepresent paper, we expect that the micropolarity of TX100 micellesgets disturbed after solubilization. However, it should be pointed

out that the chemical shift changes only provide evidence of a sig-nificant uptake of the additives in the TX-100 micelles; the exactlocation of the solubilizate and consequences of solubilization arenot clear from these experiments (see below).

254 N. Dharaiya et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 252– 259

Table 1Structures and octanol–water partition coefficient values (log Po/w) of additives.

Additives Structure Log Po/w

Ethyl alcohol (C2E0) −0.31

Ethyl cellosolve (C2E1) −0.32

Ethyl carbitol (C2E2) −0.54

n-Butyl alcohol (C4E0) 0.88

n-Butyl cellosolve (C4E1) 0.83

n-Butyl carbitol (C4E2) 0.56

n-Hexyl alcohol (C6E0) 2.02

n-Hexyl cellosolve (C6E1) 1.85

D

3

attttsa

n-Hexyl carbitol (C6E2)

ata of the table is taken from Refs. [11,39].

.2. Effects of solubilization

As mentioned earlier, the addition of organic additives can have profound effect on many physico-chemical micelle characteris-ics, including substantial changes in the physical characteristics ofhe micelles. In the present systems, changes in the size and shape ofhe micelles can perturb the temperature where the mixture starts

o phase separate; hence, cloud point measurements can provideignificant insights into the trend in the micellar growth with theddition of solubilizates.

Fig. 1. Proton labeling for T

1.70

3.2.1. Micellar growthA slight broadening of NMR peaks for some of the additives in the

present study can be taken as a vague indication of micellar growth.However, viscosity and DLS measurements can provide much bet-ter insight into the TX100 micelle growth upon the addition of shortchain alcohols or ethoxylates. Viscosity measurements of TX100solutions were carried out as a function of additive concentration

to look for evidence of changes in the micellar structure with theaddition of alcohols and ethoxylates. From Fig. 3, it is apparent thatthe adding of C6E0 and C6E1 to a 5.0 wt% TX100 solution signifi-

X100 and additives.

N. Dharaiya et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 252– 259 255

ns in D

caitoasatwts

F

Fig. 2. Chemical shift of 5% TX100 proto

antly increases the viscosity of the micelle containing solution; theddition of C4E0 to a 5.0 wt% TX-100 solution results in a marginalncrease in the solution’s viscosity, whereas the addition of C4E1o a 5.0 wt% TX100 solution has negligible impact on the viscosityf the system, even up to 8.0 wt% of added glycol. Since the rel-tive viscosity of surfactant solutions depends on the size and/orhape of micelles, it is clear that in the case of long chain alcoholsnd ethoxylates, there is a significant change in the morphology ofhe micelles as the added alcohol and surfactant co-assemble. We

ould expect that the high partitioning of the C6E0 and C6E1 addi-

ives deep in the interior of the micelles would induce significantwelling and, of course, a substantial increase in the aggregation

ig. 3. Effect of additives on relative viscosity (�rel) of TX100 (5%) solution at 30 ◦C.

2O with and without additives at 30 ◦C.

number of the micelle and its hydrodynamic radius. From the vis-cosity analysis, we can infer that C4E0 functions in much the samemanner as the more hydrophobic C6E0; however, due to the shorterchain and the reduced hydrophobicity, C4E0 does not significantlypenetrate in the micelles and would have a much less pronouncedeffect on the micellar aggregation number.

Dynamic light scattering (DLS) is a powerful technique forstudying hydrodynamic radii of meso-scale and nano-scale col-loidal particles. In principle, DLS measures the time-dependentfluctuations in the intensity of scattered light from a colloidalsolution. In the present case, we have studied the effect of con-centration of short chain polar compounds on the micelle size(hydrodynamic diameter) of TX100 micelles. Though Triton X-100is slightly polydispersed (EO chain ∼9.5), its micelle in water havelow polydispersity comparable to that with homogeneous CnEm

surfactants. The hydrodynamic diameter of TX100 is ∼10–12 nm[21,42,45] and its shape has been observed as oblate [46]. In Fig. 4,it can be seen that the addition of 0.5% C6E0 results in a notice-able increase in the hydrodynamic diameter (∼20 nm) of TX100micelles. To produce an approximate similar size increase, 1.0%of C6E1 is required. C6E0 being more hydrophobic can penetratemore easily deep inside the micelle and induce micellar growth ata much lower concentration than either C6E1 or C6E2. The additionof C2E0 and C2E2 results in a minor decrease in micellar size. For theC4 series, butanol (C4E0) shows significant micelle growth whilea slight increase in the TX100 micellar size is observed with theaddition of C4E1 and a size decrease is observed with added C4E2.

In Fig. 4, C4E0 addition only slightly increases the micellar sizeup to 5.0 wt% of additive; a marked increase in aggregate size(∼30 nm) was observed at nearly 8.0 wt% of additive indicating thatat low concentrations, C4E0 behaves as a cosolvent (or remainsat micelle–water interface) and above 5%, its molecules pene-

trate into the micelle and induce micellar growth. Hence, C4E0switches its additive behavior against its own concentration gra-dient. Since C4E2 does not penetrate deep inside the micelle, due

256 N. Dharaiya et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 252– 259

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bitol. We observed that short hydrocarbon chain (C2) compoundsincrease the CP, whereas long chain (C6) compounds decrease the

ig. 4. Effect of additives on hydrodynamic diameter (Dh, nm) of TX100 (5%) micellest 30 ◦C.

ts low Po/w, it gradually decreases the micelle size. For a 5.0 wt% and0.0 wt% of added C4E2, the micelle size is 8.1 and 6.4 nm, respec-ively, indicating that C4E2 favors demicellization at higher alcoholoncentrations. The presence of C4E2 decreases the hydrophobicnteractions, which is a main driving force for micellization and thushe micelle–monomer equilibrium shifts toward the monomerictate and micellization become less spontaneous. C4E1 at higheroncentrations slightly increases the micelle size and gives an inter-ediate effect when compared to C4E0 and C4E2. Ethyl alcohol

C2E0) and its ethers with ethylene glycol and diethylene glycolehave as cosolvents and only very slightly affect the physico-hemical properties of the micelles and as a result small decreasen micelle size.

For nonionic surfactants, the temperature effect is very pro-ounced. Polyoxyethylene chains of nonionic surfactants can beasily hydrated through H-bonds with water molecules. Fig. 5hows the effect of temperature for TX100 micelles in the presencef additives. As it can be seen, addition of any additives changes theater structure around the oxyethylene units and consequently theicelle size. The C2 series compounds decrease micelle size with

n increase in temperature. C4E1 and C4E2 do not alter micelleize but C4E0 increases the micelle size with temperature. C6E1nd C6E0 are more hydrophobic and with only an addition of 0.5%,icelle diameter increases with temperature. If the compound isore hydrophobic, the decrease in the hydrophobic hydration and

ydration in shell region should have a lot more pronounced tem-erature dependence; this results in a greater sensitivity of theicelle size as a function of temperature.

.2.2. Cloud pointPolyoxyethylene based nonionic surfactants and some neu-

ral water soluble polymers undergo phase separation and showlouding behavior, though its mechanism has yet not been clearlynderstood [37,47,48]. The threshold temperature where cloudingccurs for nonionic surfactants is called the cloud point. The cloud

Fig. 5. Effect of temperature on hydrodynamic diameter (Dh, nm) of TX100 (5%)micelles in the presence of additives.

point of a nonionic surfactant depends on the balance betweenthe hydrophilic and hydrophobic interactions and is very sensitiveto the presence of additives [27,28]. Stability of mixed surfactantsolutions with respect to temperature, need to be known prior totheir uses in many applications, especially for high temperatureapplication areas. Fig. 6 shows the CP values for 5.0 wt% TX100 solu-tions in the presence of different additives like alcohols, cellosolvesand carbitols having different chain lengths. A comparison is madebetween the same hydrocarbon chain of alcohol, cellosolve and car-

Fig. 6. Effect of additives on cloud point of 5% TX100.

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N. Dharaiya et al. / Colloids and Surfaces A:

P while medium chain compounds exhibit an intermediate effectn the cloud point.

C2E0, C2E1 and C2E2 all increase the CP and the effect is in therder C2E0 < C2E1 < C2E2 and is in excellent agreement with theirctanol–water partition coefficients (Po/w). Compounds having amaller Po/w have small number of molecule penetrating the micellend those that do are only able to exert an affect near the palisadeayers of the aggregates. Those with high Po/w penetrate deepernside the micelles [11]. Therefore, C2Em (m = 0–2) molecules do notet incorporated into the micelles to any great extent and behaves co-solvents and almost exclusively exist in the bulk water phase.

uch chemicals act as water structure breakers and induce H-bondsetween water and the polyoxyethylene shell of the TX100 micelleausing a decrease in the cohesive energy density, opposing mice-

ig. 7. NOESY spectra of (a) 5% TX100 + 5% C4E1 and (b) 5% TX100 + 5% C4E2 in D2Ot 30 ◦C.

ochem. Eng. Aspects 436 (2013) 252– 259 257

llization. The effect of Po/w becomes clearer when we examine adecrease in the CP for C4E0 and an increase in the CP for C4E2. Ifwe examine the trends in the CP values of the TX100 aggregates forthe C6 series, one can easily see that the CP value decreases witha decrease in number of ethoxy units. It is also evident from Fig. 6that an increase and decrease in CP value has an approximate cutoff Po/w and this value is very close to 0.83.

3.3. Location of alcohols and their ethoxylates in TX100 micelleusing 2D-NOESY

The 2D-NOESY offers information about the intra- and inter-molecular interactions between the protons of solute andsurfactant molecules in micellar system. In all cases, when the pro-

tons have closeness <5 A, then significant information from intraand inter cross-peaks intensities is obtained [49]. The observedpositive cross peaks in the NOESY experiments signify that the

Fig. 8. NOESY spectra of (a) 5% TX100 + 2% C4E0 and (b) 5% TX100 + 8% C4E0 in D2Oat 30 ◦C.

258 N. Dharaiya et al. / Colloids and Surfaces A: Physic

FD

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ath[l2ar

of TX100 aqueous solution. This study is considered to be of high

ig. 9. NOESY spectra of (a) 5% TX100 + 0.5% C6E2 and (b) 5% TX100 + 0.5% C6E0 in2O at 30 ◦C.

otions of the protons on both the solubilizates and surfactantsre in the extreme narrowing range. The NOESY contour plotsor each solubilizate–surfactant systems, given in Figs. 7–9, cane interpreted in terms of the locus of solubilization of the addi-ives inside the TX100 micelles. All NOESY spectra are discussedccording to proton labeling for TX100 and additives as shown inig. 1.

Inside the micelle, the TX100 molecules are arranged in such way so that the hydrophobic micellar core is surrounded by ahick shell of hydrophilic polyoxyethylene groups that form a largeydrated, shell-type head group region of the surfactant aggregates50]. Various additives spontaneously get solubilized at differentocations in the micelles depending upon their hydrophobicity. The

D NOESY technique can be used to probe the average locationnd orientation of solubilized additives. Fig. 7a shows two sets ofeal peaks for C4E1/TX100 systems. These sets are (a) phenyl ring

ochem. Eng. Aspects 436 (2013) 252– 259

protons T7 and T8 of Triton X-100 with alkyl chain protons b1, b2and b3 of C4E1 and (b) T1 proton (chain proton) of TX100 with thealkyl chain protons of C4E1. The cross peaks between T1 proton ofTX100 with alkyl chain protons of C4E1 indicate that TX100 chainshave significant motional freedom inside the micelles. Similar crosspeaks with lesser intensities are observed for C4E2/TX100 systems(Fig. 7b). The existence of these cross peaks is an indication of theclose proximity of TX100 and C4E1/C4E2 in the respective mixedmicelles and collectively, all these cross peaks indicate that on anaverage C4E1 and C4E2 molecules reside at the periphery of the coreof the TX100 micelle with their ethoxylate groups maintaining aclose proximity with the shell region.

As discussed above, C4E0 switches its behavior as a function ofits concentration in solution. We attempted to probe its solubi-lization under low and high concentration conditions using the 2DNOESY technique, and the corresponding NOESY spectra are shownin Fig. 8a and b. Cross peaks between the alkyl chain protons a1, a2and a3 of C4E0 and the phenyl ring protons of TX-100; and a1 pro-ton of C4E0 with T1 core proton indicates that the average locationof C4E0 (e.g., 2% C4E0 system) is in the vicinity of the core regionof the aggregates. As the concentration of C4E0 is increased to ahigher value (e.g., 8% C4E0 system), the intensity of crosspeaks ofthe alkyl group protons of C4E0 with TX100 alkyl chain protons T1,T2 and T3 increases while cross peak intensity between all otherprotons markedly decreases (Fig. 8b), qualitatively suggesting thatat elevated concentrations, C4E0 prefers to be buried deep inside theTX100 micelle and consequently leads to the previously observedmicellar growth.

Fig. 9a shows interactions of alkyl chain proton e4 of C6E2 withthe core protons and phenyl ring protons of TX100. The terminalmethyl protons e1 of C6E2 exhibit a strong cross peak with thecore proton T1. Various other cross peaks are also found betweenthe alkyl chain protons e3 of C6E2 with the core protons T2 andT3 of TX100. Collectively, all these cross peaks indicate that themost preferred location of solubilization of C6E2 is in the core withits ethoxylate groups oriented toward the shell region. For C6E0(Fig. 9b), the cross peak patterns are nearly identical with thoseof its ethoxylated counterpart peaks except for the fact that nocross peak intensities are observed between the alcohol protonsd1 and d3 and the aromatic and POE chain (shell) protons T4 andT5 of TX100, indicating again that C6E0 appears to be preferen-tially solubilized deep in core. This explains the extreme viscosityenhancement of TX100 solution by C6E0.

4. Conclusion

The partitioning of different short chain alcohols, cellosolves andcarbitols and consequent effects on micellar properties of TX100is examined by cloud point, viscosity, dynamic light scattering(DLS), 1H and 2D NMR. It is revealed that long chain (C6E0, C6E1and C6E2) alcohol and its glycol derivatives are capable to formmixed micelle and consequently decrease CP and increase micellesize. Shortest chain additives prefer to act like co-solvents. Micel-lar growth in the presence of intermediate chain length compound(C4E0, C4E1, and C4E2) is very interesting and depends on the size oftheir polar group; most interestingly, C4E0 can act as a co-solventor can get solubilized into the TX100 micelles depending uponits concentration. Locus of solubilization of various additives hasbeen found to be a key element in altering the physical character-istics of TX100 micelles and thus the physicochemical properties

significance for formulation chemist and the research groups seek-ing fine tuning of surfactant micelles template for nanomaterialfabrication.

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N. Dharaiya et al. / Colloids and Surfaces A:

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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