Journal of Colloid and Interface Science 378 (2012) 118–124

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Journal of Colloid and Interface Science

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Phenol solubilization in aqueous Pluronic� solutions: Investigating the micellargrowth and interaction as a function of Pluronic� composition

R. Ganguly a,⇑, K. Kuperkar b, P. Parekh b, V.K. Aswal c, P. Bahadur b

a Chemistry Division, Bhabha Atomic Research Center, Mumbai, Indiab Department of Chemistry, Veer Narmad South Gujarat University, Surat, Indiac Solid State Physics Division, Bhabha Atomic Research Center, Mumbai, India

a r t i c l e i n f o

Article history:Received 6 February 2012Accepted 12 April 2012Available online 23 April 2012

Keywords:PluronicPhenolDLSSANSRheologyMicelle

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.04.034

⇑ Corresponding author. Fax: +91 22 25505150.E-mail addresses: rajibg@barc.gov.in, rajugang

ketankuperkar@gmail.com (K. Kuperkar), paresh78vkaswal@barc.gov.in (V.K. Aswal), pbahadur2002@ya

a b s t r a c t

Pluronics� are considered as potential materials for the removal of contaminants like phenol from pol-luted water sources because of their superior solubilizing capacity of aromatic compounds. Systematicstudies on the influence of solubilization of phenol on room temperature aggregation characteristics ofPluronics� in water are, however, conspicuous by their absence. In this manuscript, we thus reportDLS, SANS and rheological studies on the influence of phenol on the aggregation characteristics of fourPluronics� viz. F127, P123, P104 and P103. The aim of this study has been to understand the role playedby the composition of the Pluronics� in determining growth and interaction of the micelles induced bysolubilization of phenol. The study shows that in the case of F127 and P123, phenol solubilization leadsto a large increase in light scattering intensity due to an onset of attractive intermicellar interactions andconsequent formation of micellar clusters. P123 being smaller than F127 shows a subsequent timedependent micellar growth, leading to a sphere-to-rod shape transitions in micelles. The copolymersP103 and P104, which are smaller and less hydrophobic than P123, respectively, exhibit a large increasein solution viscosity in the presence of phenol owing to a rapid sphere-to-rod micellar growth. The obser-vation of such a fine interplay between the growth and interaction of the pluronic micelles in the pres-ence of a hydrophobic solvent is first of its kind and highlights the role of composition of pluronic indetermining the kinetics of the micellar restructuring process.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

The commercial poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) based triblock copolymers,called Pluronics�, are an important class of non-ionic surfactantsbecause of their rich structural polymorphism and numerousindustrial applications [1–13]. The uniqueness of these copolymersvis-à-vis the conventional ionic and non-ionic surfactants is thetemperature dependence in their self-assembly characteristics inaqueous media [10]. They exist as unstructured molecular solu-tions at low temperature, but start forming core–shell micellescomprising hydrophobic PPO blocks as core and hydrophilic PEOblocks as shell (corona) at their characteristic temperature calledthe critical micellar temperature (CMT) [11–13]. Above the CMT,an increase in the hydrophobic character of the copolymer mole-cules with temperature leads to an increase in their aggregationnumber and a simultaneous decrease in their degree of hydration

ll rights reserved.

@yahoo.co.in (R. Ganguly),84@gmail.com (P. Parekh),

hoo.com (P. Bahadur).

[11,14,15]. For some of these copolymers, such micellar restructur-ing process leads to a sphere-to-rod micellar shape transition whenthe size of the core becomes equal to the length of the stretchedPPO chain [16]. A few of these copolymers on the other hand showan onset of an intermicellar attractive interaction and the conse-quent formation of large micellar clusters with increase in temper-ature before their solutions phase separate at the cloud point (CP)[17–19].

Studies on the dynamics of aggregation characteristics of Plur-onics� in aqueous medium show that the above three mentionedprocesses, that is, the formation of micelles, their restructuringand finally their cluster formation, can be identified with threetypes of relaxation processes [19,20]. It has also been found thatthe micellar restructuring process in these copolymer solutionshas an activation barrier associated with it, which leads to a timedependent micellar growth in the case of copolymers with largemolecular weight and high hydrophobicity [19–25].

The aggregation characteristics of Pluronics� in water are mod-ified quite significantly in the presence of different additives thathave strong influence on their solubilization characteristics[26–38]. Substances that increase the hydrophobic character ofthese copolymer molecules favor micelle formation and growth

R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124 119

by reducing the critical micellar concentration (CMC), CMT and thesphere-to-rod transition temperature [26–38]. Prominent mem-bers in this category are the hydrophobic solvents that have beenfound to have strong affinity toward the micelles when solubilizedin aqueous Pluronic� solutions [31–38]. Phenol and its variantsbeing sparingly soluble in water also get solubilized in thesemicellar systems. This makes Pluronics� potential materials forthe removal of these aromatic compounds from industrial wastewaters [37–40]. A recent study shows that solubilization of phenolin aqueous Pluronic� P65 (EO19PO29EO19, mol. wt. 3400 g)solutions leads to micellar dehydration and growth [37] at roomtemperature. The influence of phenol on the aggregation character-istics of Pluronics� of other available compositions has, however,not been studied so far. Such studies could be important in deter-mining the applicability of Pluronics� as phenol extracting agentssince the self-assembly characteristics of these copolymers areknown to be modulated quite remarkably as a function of theircomposition, in particular their hydrophilic–lipophilic balance(EO/PO ratio) and molecular weight [1,2]. In view of this, we havestudied the effect of phenol on the aggregation characteristics ofPluronics� of four different compositions with widely differentEO/PO ratios and molecular weights, viz. F127, P123, P103 andP104, in aqueous media. The studies showed an interesting inter-play of micellar growth and interaction characteristics that havebeen explained based on copolymer composition dependent kinet-ics of the micellar restructuring process under the influence of thedehydrating effect of phenol on the copolymer micelles.

2. Experimental

2.1. Materials and sample preparation

The triblock copolymers Pluronics� P123, P103, P104 and F127were procured from BASF Corp. Parsippany, NJ, USA, and were usedas received. Phenol (analytical grade) was purchased from E.Merck. The copolymer solutions were prepared by weighing re-quired amounts of water and copolymer and keeping them inrefrigerator overnight in tightly closed glass stoppered vials.

2.2. Methods

2.2.1. ViscometryThe absolute viscosities of the solutions were measured in a

temperature controlled water bath using size 50, size 150 and size300 calibrated cannon Ubbelohde viscometers with 0.004065,0.03925 and 0.2775 centi-stokes/s viscometer constants, respec-tively [41]. The measured flow time of the solutions in secondswas multiplied with the viscometer constant to get the kinematicviscosity of the solutions in centi-stokes. The spread in the flowtime of each solution was found to remain within ±5 s. These kine-matic viscosities were then multiplied by the density of water toobtain the viscosity of the solutions in centi-poise. The relative vis-cosities of the solutions were calculated by dividing the obtainedviscosity values with the viscosity of water.

2.2.2. Dynamic light scattering (DLS)DLS measurements of the solutions were performed using a

Malvern 4800 Autosizer employing 7132 digital correlator. Thelight source was He–Ne laser operated at 633 nm with a maximumoutput power of 15 mW. The average decay rates (C) of the electricfield autocorrelation function and subsequently the diffusion coef-ficient (D = C/q2, q = scattering vector) of the micelles were ob-tained by analyzing, g1(s) vs. time data using a modifiedcumulants method or a bi-exponential equation [42]. The modifiedcumulants method overcomes the limitations of cumulants analy-

sis to fit the correlation function data of samples with large poly-dispersity at long correlation time [43]. The apparent equivalenthydrodynamic radii (RH) of the micelles were calculated usingStokes–Einstein equation [RH = kT/(6pgD), g = viscosity of the sol-vent]. Measurements were made at five different angles rangingfrom 50� to 130�. To determine whether the scatterers are diffusivein nature, the average decay rate (C) for the samples was plottedagainst q2 (q being the magnitude of the scattering vector givenby [4pn�sin(h/2)]/k, where n is the refractive index of the solvent,k is the wavelength of laser light, and h is the scattering angle).CONTIN [44] analyses of the correlation function data were carriedout based on Laplace transform:

g1ðtÞ ¼Z a

�asAðsÞe�t=sdðln sÞ ð1Þ

2.2.3. Small angle neutron scattering (SANS)SANS measurements were carried out on the samples prepared

in D2O at the SANS facility at DHRUVA reactor, Trombay (India).The mean incident wavelength was 5.2 Å with Dk/k = 15%. Thescattering was measured in the scattering vector (q) range of0.017–0.3 Å�1. The measured SANS data were corrected for thebackground, the empty cell contributions and the transmissionand were placed on an absolute scale using standard protocols.Correction due to the instrumental smearing was taken into ac-count throughout the data analysis [45].

2.2.4. RheologyRheological measurements were conducted using an Anton Parr

Physica MCR101 rheometer in a double gap concentric cylindergeometry (DG 26.7) with a Peltier temperature control.

3. Results and discussion

3.1. Studies on F127 and P123 solutions

Phenol is known to improve the aggregation characteristics ofthe Pluronic� P65 micelles in aqueous medium by reducing theCMC, CMT and CP of P65 solutions [37]. In the present case, we willfirst discuss the influence of phenol on the aggregation character-istics of P123 (EO20PO70EO20, mol. wt. � 5800) and F127 (EO100-

PO70EO100, mol. wt. � 12,600) by dynamic light scatteringstudies. These two copolymers have same hydrophobic PPO blocksize, but their self-assembly characteristics are widely differentbecause of the large difference in the size of their hydrophilicPEO block. The F127 is having bigger size of the EO block (EO/POratio 2.86), and its CMC, CMT and CP in the aqueous medium aresignificantly higher than those of P123 (EO/PO ratio 0.57). The evo-lutions of the light scattering intensity and the aggregation charac-teristics as a function of phenol concentrations for the aqueoussolutions of these two copolymers obtained from DLS studies arerepresented in Figs. 1 and 2. The corresponding correlation func-tion plots are shown in Figure A of the supplementary material.These figures show that for both the copolymers, the solubilizationof phenol leads to a large increase in the scattering intensity (Fig. 1)and an associated change in the shape of the correlation functionplot (supplementary material). This could arise either due to alarge micellar growth or due to the formation of micellar clustersin the presence of an attractive intermicellar interaction. CONTIN[44] analyses of these correlation function are represented inFig. 2. They reflect the presence of bigger structural aggregatesalong with smaller spherical micelles in the presence of 60 mMand 130 mM phenol for P123 and F127 solutions, respectively.These two concentrations are the maximum phenol concentrationsstudied beyond which the copolymer solutions become too cloudy

Fig. 1. Scattering intensity vs. phenol concentration plots recorded at 130�scattering angle and at 30 �C for 5% PI23 and 5% F127 solutions.

Fig. 2. Relaxation time distribution plots obtained from CONTIN analysis of thecorrelation function data of (a) 5% P123 solution and (b) 5% F127 solution.

Fig. 3. (a) The hydrodynamic radius and (b) the relative viscosity of 5% P123solution vs. time plots at 30 �C in the presence of 45 mM phenol concentration.

120 R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124

to carry out DLS measurements as they approach their cloudpoints. The results in Fig. 2 suggest that the observed increase inthe scattering intensity upon the addition of phenol results dueto the presence of bigger micellar aggregates or clusters, with thesize of the micelles remaining practically constant. Quite interest-ingly, the viscosities of the copolymer solutions remain unchanged(not shown) in the above mentioned concentration range of phe-nol. This also rules out the possibility that a large growth of themicelles leading to a sphere-to-rod shape transition is responsiblefor the observed change in the correlation function plot. Thisbehavior is very much different from the observed sphere-to-rodmicellar growth and associated viscosity enhancement in the P65solutions [37]. This difference could result due to slower restruc-turing process of the P123 (mol. wt. � 5800 g) and F127 (mol.wt. � 12,600 g) micelles due to their significantly higher molecularweights as compared to that of P65 (mol. wt. � 3400 g) [19–24].Since the addition of phenol leads to a decrease in the CMC andthe CMT of the Pluronic� solutions [37], the observed formationof bigger clusters cannot be of the type that are reported to formup to 10–12 �C above their CMT [13]. The observation of such clus-ter formation in aqueous Pluronic� L64 solutions [17,18] and inother non-ionic micellar systems [46,47] has been explained basedon the presence of attractive intermicellar interaction. Phenol isexpected to reside in the palisade layer [37] of the Pluronic� mi-celles, and the dehydration of the micellar corona caused by itcould result in the onset of intermicellar attractive interaction.

It has been shown in the literature that the slow restructuringprocess of the P123 micelles results in a time dependent micellar

growth with either an increase in temperature or in the presenceof salts that have water structure making properties [21–25]. Thishas been attributed to slow micellar restructuring processes ofP123 micelles because of the large molecular weight (� 5800 g)and hydrophobicity (EO/PO = 0.57) of P123 that gives rise to a largemicellar aggregation number [19–22]. In view of this, we have car-ried out time dependent DLS studies on 5% P123 solutions in thepresence of 45 mM phenol. The analyses of the correlation functiondata as a function of time were carried out based on bi-exponentialequation because of the presence of scattering species with twowidely different size ranges. The bi-exponential equation is definedas:

gð1ÞðtÞ ¼ Af � expð�t=sfÞ þ As � exp½�ðt=ssÞ� ð2Þ

where Af and As are the amplitudes for the fast and slow relaxationmodes corresponding to the relaxation time sf and ss, respectively.The relaxation time for the faster mode is associated with the diffu-sion of the micelles, and the relaxation time for the slower mode isassociated with the diffusion of the bigger micellar aggregates.

The results of these analyses (shown in Fig. 3) suggest that theP123 micelles undergo monotonous growth with time at 45 mMphenol concentration, which leads to a slow but steady increasein the viscosity of the copolymer solution on standing. Similar timedependent micellar growth and viscosity enhancement uponmicellar dehydration were earlier observed [21–24] in aqueousP123 solutions due to the slow restructuring of the P123 micelles[19,20]. In the present case, the observed micellar growth isaccompanied by a decrease in the value As, that is, the fraction ofclusters present in the micellar solutions. The increase in the vis-cosity associated with the micellar growth suggests that the micel-lar growth observed in this system by our DLS studies isaccompanied by a simultaneous sphere-to-rod shape transition.The observation of such time dependent sphere-to-rod growth ofP123 micelles could be attributed to time dependent micellarrestructuring and growth following the dehydration of the micellarcorona in the presence of phenol. No such time dependent micellargrowth, however, is observed in F127 solutions. This can be attrib-uted to even slower micellar restructuring of F127 micelles be-cause of the very high molecular weight of the F127 molecules(� 12,600 g) as compared to that of P123 (� 5800 g) [19].

Additional insight into the observed time dependent behaviorof the P123-phenol-water system can be obtained from the SANSstudies that are shown in Fig. 4a. The figure shows that thepresence of phenol in 5% P123 solution leads to a significant in-crease in the scattering intensity, though the viscosity of thosesolutions remains practically constant. This could be attributed to

Fig. 4. (a) SANS patterns of 5% aqueous P123 solutions recorded at 30 �C. (b) Thecorresponding pair distance distribution function [p(r)] plots. Fig. 5. (a) Correlation function diagram of 5% P103 solution recorded at 130� angle

and at 30 �C. The solid lines represent tit to the data. (b) the relaxation rate vs. q2

plot of the fast mode, that is, for the micelles.

Fig. 6. Relaxation time distribution plots obtained from CONTIN analysis of thecorrelation function data of 5% P103 and 5% P104 solutions in the absence andpresence of phenol.

R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124 121

the formation of micellar cluster as has been suggested by the DLSstudies. The shape of the plot for the phenol containing solutionundergoes a change with time, accompanied by an increase inthe viscosity (not shown), which is expected in the light of whatis observed from our DLS studies. In the presence of simultaneousmicellar cluster formation and growth in these solutions, the anal-ysis of these data could be a difficult proposition. However, to con-firm whether the micelles do undergo shape change with time ornot, we calculated the corresponding pair distance distributionfunctions [p(r)] using the program GENOM made by Svergunet al. [48,49]. As shown in the inset of Fig. 4b, the asymmetry ofthe p(r) plot increases significantly with time, which is a clear sig-nature of an increase in anisotropy of the micelles with progress intime. SANS studies on similar time dependent micellar shape tran-sitions in P123 solutions caused either by continuous heating attemperatures close to the cloud point or in the presence of waterstructure making additives like Na3PO4 and NaF have also been re-ported earlier [21,22]. The observed results thus support the con-clusions made from the viscosity and DLS studies that P123micelles undergo time dependent sphere-to-rod micellar shapetransition under the influence of phenol. The observed time depen-dent micellar growth is, however, different from the rapid growthobserved in the P65 solution by Mata et al. [37] from the SANSstudies. As discussed earlier, the difference in the molecular weightof P123 and P65 is probably responsible for such variation in themicellar growth behavior.

3.2. Studies on P103 and P104 solutions

To understand whether the slow micellar restructuring andgrowth in the aqueous P123 system does indeed have a root inthe copolymer’s high molecular weight and hydrophobicity, wehave studied the effect of phenol on the micellar aggregation char-acteristics of two other copolymers P103 and P104. P103(EO17PO60EO17) has same PEO weight fraction (EO/PO ratio � 0.57)but has a smaller molecular weight (mol. wt. � 4950 g) as com-pared to that of P123 (mol. wt. � 5800 g). P104 (EO27PO61EO27),on the other hand, has similar molecular weight (mol.wt. � 5900 g), but with higher PEO weight fraction (EO/POratio � 0.89) than that of P123. The evolutions of the micellaraggregation characteristics as a function of phenol concentrationfor these two copolymers are shown in Figs. 5 and 6.

Fig. 5a shows change in the correlation function plot of P103micelles in 5% P103 solution upon the addition of phenol. A shift

in the correlation function plot to higher time scale and a concom-itant change in the nature of plot from single exponential to bi-exponential is suggestive of occurrence of growth of P103 micellarunder the influence of phenol. The analysis of the correlation func-tion data was carried out based on modified cumulant method atlow phenol concentration and stretched bi-exponential equationwhen viscosity becomes high, as reported earlier in concentratedrod-like Pluronic� micellar system showing large viscosity[50–52]. The stretched bi-exponential equation is represented as:

gð1ÞðtÞ ¼ Af expð�t=sf Þ þ As exp½�ðt=ssÞb� ð3Þ

where Af and As are the amplitudes for the fast and slow relaxationmodes corresponding to the relaxation time sf and ss, respectively[50–52]. The relaxation time for the fast mode is associated withthe diffusion of the rod-like micelles and that of the slow modecan be ascribed to coupling between concentration fluctuationand stress relaxation in the entangled rod-like micelles. The expo-nent b (0 < b 6 1) is inversely proportional to the width of the dis-tribution of the relaxation times of the slow mode [52].

Fig. 7. (a) The hydrodynamic size of the micelles measured at 130� scattering angleand at 30 �C. (b) The relative viscosity of the solutions measured at 30 �C.

Fig. 8. The cloud point of the 5% P104 solution as a function of phenolconcentration.

122 R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124

The results of this analysis are shown in Figs. 6 and 7a, where itcan be observed that the size of the micelles indeed increases withincrease in phenol concentration till the copolymer solution under-goes phase separation. Apart from the micellar growth, the CONTINanalysis (Fig. 6) of the correlation function data (Figure B of thesupplementary material) also shows the signature of the presenceof a slower and second relaxation mode associated with the pres-ence of entangled rod-like micelles. The linearity of the relaxationrate plot of the fast mode (Fig. 5b) is suggestive of diffusive natureof the rod-like micelles. The relaxation rate of the slower mode(not shown here), however, do not show an expected linear behav-ior with q2/b [50–52]. This could result due to the interference inthe relaxation process of the entangled rod-like micelles from themicellar clusters that are likely to be present in the system nearcloud point [17–19]. Fig. 7b shows that the observed growth ofP103 micelles is associated with a simultaneous increase in viscos-ity of the copolymer solution of about three orders of magnitude,which suggests that the growth of the micelles is associated withtheir shape transition from spherical to rod like. These results thusshow that unlike in the case of P123, the restructuring of P103 mi-celles occurs quite rapidly and appears instantaneous in the timescale of viscosity and DLS measurements. As discussed in the pre-vious sections, this could be attributed to smaller molecular weightof P103 as compared to that of P123. P123 and P103 micelles showsimilar difference in their growth rate in the presence of waterstructure making salts like Na3PO4 and NaF too [22].

As shown in Figs. 6 and 7, similar behavior is also exhibited by5% P104 solutions, in which a rapid micellar growth, and a simul-taneous and large viscosity enhancement is observed upon theaddition of phenol. The observed fast micellar restructuring andgrowth in this system could be attributed to smaller micellaraggregation number arising due to lesser hydrophobicity of P104molecule as compared to that of P123. The observed behaviors inthe case of P103 and P104 are thus very similar to that reportedfor P65, which also show simultaneous micellar growth and viscos-ity enhancement in aqueous solutions [37]. We will now comparethis observation with that of F127, which is both significantly morehydrophilic and at the same time significantly bigger than P104.The absence of any micellar growth in case of F127 micelles sug-gest that very high molecular weight of F127 prevails over itslow hydrophobicity in determining the micellar restructuringprocess.

To highlight the dehydrating effect of phenol on the pluronicsolutions, we have attempted to study the phase separation char-acteristics of P103 and P104 solutions as a function of phenol con-centrations. Our recent study showed that aqueous P123 solutionsdo not show sharp cloud points owing to a slow and time depen-

dent micellar rearrangement processes [21]. In the present casethough we observed rapid micellar growth in both P103 andP104 solutions, no sharp cloud point and no micellar growth areobserved in the P103 solution in the absence of phenol. PureP103 solutions thus do not exhibit complete phase separationand any viscosity enhancement with increase in temperature uponheating up to 100 �C, which can be attributed to slow micellarrestructuring process in them. In Fig. 8, we have thus shown thecloud point of only 5% P104 solutions as a function phenol concen-tration. The observed progressive decrease in the cloud point downto room temperature with increase in phenol concentration sug-gests that like in the case of P65 [37], the micellar dehydration in-deed is the reason for the observed sphere-to-rod growth andcluster formation of the pluronic micelles.

We will now discuss the above mentioned results from the per-spective of the suitability of these pluronics as phenol extractingagent. The extraction process of phenol involves its solubilizationin the aqueous copolymer solution and its subsequent removaleither by cooling the copolymer solution below CMC or by heatingthem above the cloud point (cloud point extraction) [37–40]. Ourstudies show that pluronics with higher EO/PO ratio show highersolubilization capacity of phenol mainly because phenol preferablyresides in the micellar corona region [37]. F127 having highestEO/PO ratio of 2.86 thus exhibits maximum solubilization capacity.For the removal of phenol, our results pertain more to cloud pointextraction [37] as the observed micellar growth and onset of inter-micellar interaction occurs near the cloud point of the copolymersolutions. In that respect too, F127 will be the preferred pluronicas the observed viscosity enhancement of the aqueous solutionsof other three copolymers on approaching their cloud points canhinder the process of separation of phenol from the aqueoussolutions.

3.3. Rheological studies

To understand more about the observed phenol-induced roomtemperature growth of the pluronic micelles, we have carried outrheological studies on the 5% P123 and 5% P104 solutions with55 mM and 110 mM phenol, respectively. The aim has been tocompare the rheological aspect of the restricted growth observedin P123 solutions vis-à-vis the rapid one in P104 solution. In thecase of P123 solution, the measurements were taken after twoweeks to allow the micelles to undergo sphere-to-rod growth.

In Fig. 9, we show the evolution of the zero shear viscosity ofP123 and P104 solutions as a function of temperature up totheir phase separation temperature. As shown in the figure, theviscosities of the P104 solution increase by about three orders of

Fig. 9. Variation of zero shear viscosity of 5% P104 and 5% P123 solutions as afunction of temperature.

Fig. 11. Plots of the storage modulus (G0) and the loss modulus (G00) of 5% P123solution with 55 mM phenol as a function of frequency of the oscillatory shear.

R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124 123

magnitude as the micelles undergo rapid sphere-to-rod growth onapproaching its cloud point. The observed decrease in the viscosityabove 30 �C after a sharp increase can be attributed to branching ofthe micelles, which provide additional mechanism for the stressrelaxation as the branch points can slide along the length of the mi-celles [53].

The situation is very different in the case P123 solution as nomicellar restructuring and growth is expected in the time scaleof the measurements because of the restricted nature of the micel-lar growth in it. Since in this solution the micelles have alreadygrown significantly after two weeks time, its zero shear viscosityis enhanced compared to that of the P104 solution at low temper-ature. It, however, remains practically constant with increase intemperature till it starts falling as the process of phase separationbegins to occur. These two plots thus clearly represent the differ-ence in the nature of the micellar growth in the P123 and P104solutions.

An important feature of the rheology of the worm like micellarsystems is the deviation from the Newtonian behavior in the formof shear dependence of viscosity in the semi-dilute regime [25]. Tosee whether such behavior is also associated with the micellargrowth in the present system, we studied the shear dependenceof viscosity of both P104 and P123 solutions. As shown in Figs.10 and 11, the observed sphere-to-rod growth in these solutionsis associated with shear thinning of the viscosity. Such shear thin-ning arises due to the alignment of the wormlike micelles along thedirection of the shear induced flow and consequent breaking oftheir entangled network structure.

The viscoelastic behavior of the entangled micelles in semi-dilute regime is analogous to that of living polymeric networks

Fig. 10. Plots of the storage modulus (G0) and the loss modulus (G00) of 5% P104solution with 110 mM phenol as a function of frequency of the oscillatory shear.

that often obey the simple Maxwell model. The dynamics of suchsystem has been studied in detail by Cates and coworkers[54,55], which involves two time scales, namely the reptation time(srep) and the breaking time (sb) of the micelles. The reptation timecorresponds to the curvilinear diffusion of a chain along its owncontour in a tube that is formed by the entangled neighboringchains. Both the reptation time and breaking time are functionsof the length of the rod-like micelles. An increase in length of themicelles results in an increase in reptation time and a simulta-neous decrease in breaking time [56]. The ratio of these two relax-ation times (f = sb/srep) controls the relaxation behavior of thewormlike micelles [55]. For very small values of f, a single relaxa-tion time dominates and Maxwellian behavior is observed. For lar-ger values of f on the other hand, existence of a spectrum ofrelaxation times leads to deviations from Maxwellian behavior,which is similar to that observed in the polymer solutions withwide molecular weight distributions [56].

Rheological studies on the wormlike micellar solutions ofpluronic P84, P103, P105 and P123 show that they belong to thesecond category because of their slow micellar breaking processes[57–60]. To understand the nature of the viscoelastic behaviorassociated with the phenol-induced sphere-to-rod growth of theP104 and P123 solutions, we have studied their variation of storagemodulus (G0) and loss modulus (G00) as a function of frequency (x)(Figs. 10 and 11). As shown in these figures, in both the cases, thecopolymer solutions show typical viscoelastic behavior with signif-icant contribution from the G0 (x) and G00 (x). As expected, the nat-ure of their variation does not show any compliance with theMaxwell behavior in the low frequency region because of relativelysluggish micellar restructuring processes in these copolymer solu-tions. In the case P104 solution (Fig. 10), at 23 �C, the storage mod-ulus exceeds the loss modulus at an intermediate oscillatoryfrequency called crossover frequency (xR). As the micelles growwith increase in temperature from 23 �C to 30 �C, the crossover fre-quency goes below the measured frequency range of 0.1. This andthe enhanced value of G0 at 30 �C suggest that the copolymer solu-tion becomes more solid like as its viscosity increases with in-crease in micellar growth as a function of temperature. In thecase of P123 on the other hand, due to limited growth, the copoly-mer solution shows less solid like behavior and the storage modu-lus never exceeds the loss modulus as a function of oscillatoryshear frequency (Fig. 11).

4. Conclusion

In conclusion, in view of the potential applicability of Pluronics�

as phenol extracting agents [38–40], we have studied the influenceof phenol on the aggregation characteristics of four Pluronics� withwidely different EO/PO ratios and molecular weights, viz. P123,

124 R. Ganguly et al. / Journal of Colloid and Interface Science 378 (2012) 118–124

P103, P104 and F127. The aim was to understand the role of hydro-phobicity and molecular weight of these copolymers in determin-ing the restructuring and growth of micelles due to thedehydrating effects of phenol on the micellar corona [37]. Theresults show that although phenol has dehydrating effect on themicelles of all these copolymers, the manifestation of it variesquite significantly with their compositions. In the case of P123and F127, the solubilization of phenol leads to the formation ofmicellar clusters and a simultaneous increase in the light scatter-ing intensity due to the onset of intermicellar attractive interac-tions, without any detectable increase in viscosity of thecopolymer solutions. Quite interestingly, however, while P123 mi-celles undergo a slow and time dependent sphere-to-rod growthand a consequent increase in the viscosity of the copolymer solu-tions, F127 micelles do not show any such effect. This has beenattributed to faster restructuring of the P123 micelles because ofthe significantly smaller molecular size of P123 as compared toF127 [19,20]. The other two copolymers, P103 and P104, with low-er molecular weights and hydrophobicity than those of P123,respectively, show even faster micellar restructuring and corre-spondingly a rapid sphere-to-rod micellar growth [19–21]. Theobservation of such a fine interplay between the growth and inter-action of the pluronic micelles induced by a hydrophobic solvent isthe first of its kind in aqueous pluronic solutions. The results couldalso have an important bearing in determining the suitability ofpluronics as phenol extracting agents.

Acknowledgment

Author sincerely acknowledges Board of Research in NuclearSciences (BRNS), BARC, INDIA, for their financial support underthe scheme (No: 2010/37C/31/BRNS).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.04.034.

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