EffectofSurfactantsonAssociationCharacteristicsofDi-and ... · 2Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India 3UGC-DAE Consortium for Scientiﬁc

Hindawi Publishing CorporationInternational Journal of Polymer ScienceVolume 2011, Article ID 570149, 13 pagesdoi:10.1155/2011/570149

Research Article

Effect of Surfactants on Association Characteristics of Di- andTriblock Copolymers of Oxyethylene and Oxybutylene in AqueousSolutions: Dilute Solution Phase Diagrams, SANS, and ViscosityMeasurements at Different Temperatures

Sanjay H. Punjabi,1 Nandhibatla V. Sastry,1 Vinod K. Aswal,2 and Prem S. Goyal3

1 Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India2 Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India3 UGC-DAE Consortium for Scientific Research, Mumbai Center, Mumbai 400 085, India

Correspondence should be addressed to Nandhibatla V. Sastry, nvsastry [email protected]

Received 7 March 2011; Revised 29 June 2011; Accepted 7 July 2011

Academic Editor: Jan-Chan Huang

Copyright © 2011 Sanjay H. Punjabi et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The interactions in poly(oxyethylene) (E) – poly(oxybutylene) (B) of EB or EBE type block copolymers-sodium dodoecylsulfate (SDS) or dodecyltrimethylammonium bromide (DTAB) and/or t-octylphenoxy polyethoxyethanol, (TX-100) have beenmonitored as a function of surfactant concentration and temperature. The addition of ionic surfactants to copolymer micellarsolutions in general induced not only shape transition from spherical to prolate ellipsoids at 30◦C in the copolymer micelles butalso destabilize them and even suppress the micelle formation at high surfactant loading. DTAB destabilizes the copolymer micellesmore than SDS. TX-100, being nonionic, however, forms stable mixed micelles. The block copolymer-surfactant complexes arehydrophilic in nature and are characterized by high turbid and cloud points. Triblock copolymer micelles got easily destabilizedthan the diblock copolymer ones, indicating the importance of the interaction between the hydrophilic E chains and surfactants.The effects of destabilization of the copolymer micelles are more dominating than the micellar growth at elevated temperatures,which is otherwise predominant in case of copolymer micelles alone.

1. Introduction

Amphiphilic block copolymers of type EnBm or EnBmEn(E = oxyethylene and B = oxybutylene, and m and n indicatethe number of units) have several advantages over EPE(P = oxypropylene) copolymers. For example, B block ismore hydrophobic than P block and EB or EBE copolymersare fairly homogeneous in composition [1]. The surfactant-like properties including gelation behavior of several of labo-ratory synthesized or commercial EnBn and EnBmEn copol-ymers in aqueous media have been the subject of manyscientific investigations [2–21]. In colloidal engineering-based applications such as detergency, dispersion, stabiliza-tion, foaming, emulsification, lubrication, control release ofdrugs, the block copolymers and low molar mass surfac-

tants are often used in combination with each other. There-fore, the studies on copolymer-surfactant mixture solutionsare essential to find out best suitable conditions for achievingtheir optimum performance [22, 23]. A literature surveyreveals that much of the research work on amphiphilic co-polymer-surfactant mixed systems in water focused on EPEcopolymers-sodium dodecyl sulfate (SDS), sodium dodec-ylbenzene sulfonate (SDBS), cetyltrimethylammonium chlo-ride (CTAC) or bromide (CTAB)) or polyether-based non-ionic surfactants, and so forth [23]. Detailed investigationsdealing with the establishment of binding isotherms, mea-surement of critical micelle concentrations, determinationof size and aggregation number in mixed systems haveindicated that the surfactant molecules interact with thehydrophobic as well as hydrophilic parts of the copolymers

2 International Journal of Polymer Science

and induce interesting and dramatic changes in associationcharacteristics depending upon the molecular characteristicsof copolymers, namely, length of E or P blocks, P/E ratioand also on the type of hydrophobic block and concen-tration of the surfactants, and so forth. Whether the addedsurfactants would form simply mixed micelles, inducethe copolymer micellization or form copolymer-surfactantcomplexes, break the complexes and inhibit the copolymermicellar growth, were highly dependent on the surfactantconcentrations and also on the overall hydrophilicity orhydrophobicity of the copolymers. The mixed micelle for-mation in EPE copolymer-surfactants systems were alsoreported [24–28]. The interaction parameter was calculatedusing regular solution theory. It has been emphasized thatthe mixing behavior deviated from ideality and the systemsexhibited synergistic interactions.

Recently, Kelarakis et al. [29, 30] had monitored theinteractions of SDS with series of poly (oxyethylene)—poly (oxybutylene) diblock and poly(oxyethylene)—poly(oxybutylene)—poly (oxyethylene) triblock copolymers(E18B10, B20E610, B12E277B12, and E40B10E40) using DLS,electrical conductance, volumetric, ultrasonic velocity, andsmall angle X-ray scattering (SAXS) measurements. Unlikein EPE copolymer-surfactant systems, the addition of SDS toB18E10 (which is a predominantly hydrophobic copolymer)led to the formation of large vesicles with positive deviationsfrom the ideal mixing. It was also emphasized that thelength of the hydrophilic block of the copolymer plays animportant role not only in the interactions but also in thefinal size and shape of the particles formed in mixed systems.A clear picture and understanding on which way the addedsurfactants affects the self-association and phase behaviorof EB or EBE copolymers is yet to emerge. Therefore, ourpresent study reports the systematic measurements onthe dilute aqueous solution phase characteristics, smallangle neutron scattering (SANS), and dilute solutionviscosity for mixture solutions of a diblock E18B9 ortriblock E13B10E13 copolymers SDS, DTAB, a nonionicsurfactant, t-octylphenoxy polyethoxyethanol, and TX-100in water at different temperatures as a function of surfactantconcentration. The results are hoped to shed some lighton how the two opposing effects of micellar destabilization(by surfactants) and micellar growth (by temperature) getbalanced or predominate over each other in such systems.

2. Experimental Section

2.1. Materials. The block copolymers were obtained as giftsamples from The Dow Chemical Company, Freeport, Tex,USA and were used as received. The diblock copolymer,BM-45, has a structure of type, MeO-(EO)18-(BO)9-OHwhile the triblock copolymer, B-40, has a structure, HO-(EO)13-(BO)10-(EO)13-OH. EO and BO in the structuresrepresent oxyethylene and oxybutylene units, and MeO de-notes a methoxy group. The di- and triblock copolymershave been denoted as B-1 (E18B9) and B-2 (E13B10E13).SDS was of Fluka made with purity >98% on mass basis.DTAB and TX-100 were Aldrich products with purities of99% and 98%. These samples were used as received without

any purification. The critical micelle concentration (CMC)values as determined from surface tension isotherms at 30◦Cfor the surfactants are 0.23, 0.49, and 0.021 g·dl−1 for SDS,DTAB, and TX-100 respectively.

2.2. Methods. The solutions of copolymers were prepared bydissolving the known amount either in triple distilled wateror in a given surfactant solution and allowed to be swirledon a magnetic stirrer till homogenous, and thorough mixingis achieved. A stirring period of 60–90 minutes was found tobe sufficient. The solutions were always stored in stopperedglass vials. The solutions of the individual surfactants werehowever, prepared by simple dissolution of required amountswith a care to minimize the foam formation. The solutionswere allowed to stand for few hours till the overhead foam (ifany) is fully settled.

2.3. Phase Diagrams. The dilute aqueous solution phasecharacteristics for each of the solutions were monitored byvisual observation of typical changes in the appearance ofsolution through warming and cooling cycle. The averagevalue of temperature for the two sessions was finally notedto identify the turbid (Tp) and cloud points (Cp). Theuncertainty in these points is ±0.5◦C.

2.4. Small Angle Neutron Scattering. The SANS experimentswere carried out on B-1 or B-2 solutions and also oncopolymer-surfactant mixture solutions in D2O using anindigenously built SANS spectrometer at the DHRUVA Reac-tor, (Trombay, India). The D2O (with at least 99.5 atom%purity) was obtained from heavy water division of BARC,Mumbai, India. The use of D2O instead of H2O for preparingthe solutions provides a very good contrast between theassociates of solute and the solvent in SANS experiment. Thesolutions were held in 0.5 cm path length UV-grade quartzsample holders with tight and fitting teflon stoppers sealedwith parafilm. The sample to detector distance was 1.8 m forall runs. The spectrometer makes use of a BeO-filtered beamand has a resolution (ΔQ/Q) of about 30% at Q = 0.05 Å−1.The angular distribution of the scattered neutron is recordedusing one-dimensional position—sensitive detector. Theaccessible wave transfer Q (= 4π sin0.5 θ / λ, where λ is thewave length of the incident neutrons and θ is the scatteringangle) range of this instrument is between 0.02 and 0.3 Å−1.The mean neutron wavelength was λ = 5.2 Å.

The measured scattering intensities of neutrons werecorrected for the background, empty cell scattering, andsample transmission. The intensities then were normalizedto absolute cross-section units. Thus, plots of dΣ/dΩ versusQ were obtained. The uncertainty in the measured scatteringintensities is estimated to be 10%. The experimental pointsare fitted using a nonlinear least squares method.

2.5. Viscosity. The flow times of individual copolymeraqueous solutions as well as mixtures of B-1 and B-2surfactants were obtained by using Ubbelohde suspendedlevel viscometers. Two viscometers were used to obtainflow times in the range of 130–360 s, thus avoiding anykinetic corrections. Three consecutive flow times agreeing

International Journal of Polymer Science 3

Phase seperation

Clear

Turbid

Cloudy

Fully dense clouds

1 2 3 4 5

50

60

70

80

90

100

CTX-100 (g·dL−1)

T(◦

C)

(a)

Clear

Turbid

Cloudy

Fully dense clouds

Phase seperation

1 2 3 4 5

CTX-100 (g·dL−1)

50

60

70

80

90

100

T(◦

C)

(b)

Figure 1: Dilute aqueous solution phase diagram for block copolymers-TX-100 mixtures: (a) B-1–TX-100 and (b) B-2–TX-100.

within ±0.02 s were recorded, and the mean flow time wasconsidered. Shear corrections were not taken into consid-eration, because obtained intrinsic viscosities were alwaysless than 3 dl g−1. The flow volume was greater than 5 mL,making drainage corrections unimportant. The viscometersduring the measurements were suspended in thermostaticwater baths maintained at constant temperature accurate to±0.01◦C.

3. Results and Discussion

3.1. Dilute Aqueous Solution Phase Diagrams. The featuresfor aqueous solutions of B-1 and B-2 copolymers have beendiscussed previously [31, 32]. The clear copolymer solutions(at room temperature) turned turbid before dense cloudinesssets in at higher temperatures. Therefore, typically, eachsolution possess a turbidity point (Tp) and a cloud point(Cp). Diblock copolymer, has lower Tp and Cp values ascompared to triblock copolymer and the temperature rangeover which the turbidity persisted in the solutions has alsobeen found to be more for B-2 than B-1 copolymer. Thesetrends are directly attributed to the fact that the triblockE13B10E13 has longer hydrophilic E blocks than the diblockE18B9. Therefore, the turbidity set in the solutions is due tothe increased immiscibility of hydrophobic oxybutylene moi-ety at elevated temperatures. Similarly, our previous detailedSANS studies[31] on aqueous solutions of these copolymersrevealed that they form spherical micelles consisting of anhydrophobic inner core surrounded by outer hydrophiliccorona. The size and association number in the micelles sys-tematically increased with the increase in temperature, and,in fact, the spherical micelles of B-2 copolymer underwenta shape transformation to highly asymmetrical ellipsoids attemperatures close to Tp. It was noticed that the appearanceof turbidity in solutions coincided and was related to thegrowth of the micelles into large structures.

The dilute aqueous solution phase diagrams for a fixed2 wt% (w/v) aqueous solutions of B-1 or B-2 in presenceof varying amounts of each of the surfactants; that is, SDS,DTAB, or TX-100 was constructed. The common featuresnoted from the diagrams are (i) initial clear solutions turnturbid to cloudy, (ii) further increase in temperature resultedinto formation of fully dense clouds, and (iii) result in phaseseparation at high temperatures. A typical phase diagramfor block copolymers-TX-100 mixtures is shown in Figure 1.The phase diagrams for other mixtures are given in FigureS1 of Supplementary Material, which is available online atdoi: 10.1155/2011/570149. The summary of the turbid andcloud pints as extracted from these diagrams is collected inTable 1. A perusal of the data from Table 1 reveals that (i)the copolymers-SDS, -DTAB mixture solutions have higherCp values as compared to the copolymer solutions alone inwater, (ii) the successive addition of surfactants as additivesincreased the Cp values and the same appeared to be greaterthan 100◦C at very high SDS and DTAB loading (iii) theaddition of nonionic TX-100 to the copolymer solutionson the contrary did not produce such large effects and thecloud points are well below 100◦C, and (iv) the copolymer-TX-100 mixture solutions got phase separated at elevatedtemperatures.

In view of these observed sharp contrasting cloudingbehavior between the copolymers-ionic surfactant mixtureson one hand and copolymers-nonionic surfactant mixtureson the other, we thought it interesting to undertake detailedSANS measurements in mixture solutions under solutionconditions of either varying surfactant concentrations at agiven temperature and or either varying temperatures (fromroom temperature to high temperatures close to the Tp)for a fixed surfactant concentration so that we hope to getan insight in to the changes in the geometrical features ofcopolymer micelles under two competitive effects of micelledestabilization and micellar growth.


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Figure 2: SANS distribution for copolymer or surfactant solution in D2O at different temperatures (in ◦C) (a) 2% (w/v) B-1: (�) 30◦C, (�)50◦C; (b) 2% (w/v) B-1: (�) (30◦C), (�) 50◦C, (©) 70◦C; (c) (©) 2.4% SDS at 30◦C, (�) 1% DTAB 30◦C, (•) 3% and (�) 6% TX-100 at30◦C. Curves represent the model fitted values.

3.2. Small Angle Neutron Scattering (SANS). SANS mea-surements were carried out first on the solutions of theindividual copolymers or surfactants in D2O at differenttemperatures and the spectra are shown in Figure 2. Theconcentrations chosen are far excess than the CMC ofrespective pure components. The analysis of the spectra tookinto the consideration of the differential scattering cross-section per unit volume as a function of scattering vector Qand is expressed as [33–37]

dΣdΩ

= nmV2m(ρm − ρs

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

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dΣdΩ

= nmV2m(ρm − ρs

)2P(Q) + B, (2)

where nm denotes the number density of micelles, Vm isthe micellar volume, and ρm and ρs are scattering lengthdensities of the micelles and solvent, respectively. P(Q) isthe single particle (intraparticle) structure factor, and S(Q)is the interparticle structure factor. B is a constant term thatrepresents the incoherent scattering from the background,which is mainly due to hydrogen in the sample. The P(Q)for spherical micellar core can be written as

P(Q) =[

3

(QRc)3 (sin(QRc)−QRc cos(QRc))

]2, (3)

where Rc is the hydrophobic core radius which is attributedto the size of the micellar core. The interparticle structure


factor S(Q) depends on the spatial distribution of micellesand is given by [38]

S(Q) = 1 + 4 πn∫ (

g(r)− 1) sin (Qr)Qr

r2dr, (4)

where g(r) is the radial distribution function describing thearrangement of the micelles.Percus and Yevick approxima-tion [39] for describing a direct correlation between twoscattering objects, defines the analytical form of structurefactor as [40–42]

S(Q) = 11 + 24φG

(2QRhs,φ

)/(2QRhs)

, (5)

where Rhs, the hard sphere micellar radius, consisting of bothpoly (oxybutylene) and poly (oxyethylene) and is the physicalsize of the micelle. φ is the hard sphere volume fraction of themicelles in the solution. G is a function of x = 2 QRhs and φ.

The φ value can mathematically be made equivalent to

(Cm4πR3hsNA

)/3N1000, (6)

where Cm is the concentration of copolymer molecules inassociated (micellized) form and has an unit in terms ofmol dm−3.

The micelles of copolymers or surfactants in waterconsist of hydrophobic core surrounded by hydrophiliccorona. The scattering length densities of polyoxybutylene(0.212 × 1010 cm−2), SDS (−0.392 × 1010 cm−2), DTAB(0.392 × 1010 cm−2), TX-100 (0.388 × 1010 cm−2), andD2O (6.38 × 1010 cm−2) were used in the calculations.There is a very good contrast between the hydrophobic coreand the solvent. However, because of large amount of D2O(water of hydration) present in the outer polyoxyethylenecorona, the scattering contrast between the hydrated coronaand the solvent shall expected to be poor. In view of above,we assume thatP(Q) depends only on the hydrophobic coreradius. Thus, in fitting experimental neutron scatteringdata from solutions to (1), three unknown parameters, coreradius, Rc, hard sphere interaction radius, Rhs and volumefraction of the micelles, φ have been considered as fittingparameters in the analysis. The association number, N, thencan be obtained from the knowledge of the core size

N = 4π(RC)3

3nVBO, (7)

where n is the number of oxybutylene units in the hydropho-bic block, Rc is the radius of the anhydrous core and VBOis the volume of one oxybutylene unit (116.5% 10−24 cm3).From the estimated values of N, the number density ofmicelles, ζ that is the number of copolymer micelles per unitvolume is calculated by following relation:

ζ

cm−3= C ×NA × 10

−3

N, (8)

where C = concentration in mol dm−3, NA = Avogadro’snumber.

B-1 or B-2 Spherical Micelles in Water. Various micellarparameters such as core radius, Rc, and hard sphere radius,Rhs, volume fraction, φ, and intermicellar distance, ζ , asobtained from the above treatment of SANS data at 30◦C arelisted in Table 2. SANS intensities for the diblock copolymersolutions were greater than that of the triblock copolymersolutions, and accordingly, the association number and num-ber density of the diblock copolymer were higher than the fortriblock copolymer micelles. The observed smaller micellardimensions for a triblock copolymer can be explained by thefact that core part of its micelle has two block junctions at thecore/fringe interface as compared to only one such constraintin the micelle of a diblock copolymer. This effectively restrictsthe freedom of hydrophobic chain within the core of themicelle.

The analysis of the SANS curves at high temperaturescould only be done by assuming ellipsoidal P(Q) factor,while the S(Q) factor at high temperatures has been setequal to unity as correlation peaks were absent. The variousparameters of copolymer micelles at different temperaturesare also listed in Table 2. It can be seen that the increase intemperature induced the shape transformation in copolymermicelles. The increase in temperature (50◦C to 70◦C for B-2) resulted in a drastic increase of the major axis “b”. Thedistortion of the spherical shape at elevated temperatureswas also followed by an increase in association number, andthese changes are mainly attributable to the expulsion ofwater molecules from the core as well as the hydrated coronaof the micelles. Under these conditions, more copolymermolecules can be accommodated in a micelle and hence Nof the micelles increases.

Surfactant Micelles in Water. The SANS distributions andtheir detailed analysis for anionic sodium dodecyl sulfate,cationic tetradecyltrimethylammonium bromide DTAB, andnonionic TX-100 in water have been reported elsewhere [43].The surfactant micelles were typically prolate ellipsoidal inshape with their hydrocarbon tails occupying the interiorhydrophobic core surrounded by the head groups. Themicellar parameters for these three surfactants, as extractedfrom the SANS analysis are summarized in Table 3.

Copolymer-Surfactant Mixtures. SANS measurements on thesolutions of 2% B-1 + SDS (0.001, 0.05 and 1%), + DTAB(0.05, 0.1 and 1%), and + TX-100 (6%) mixtures at 30◦C aswell as at elevated temperatures were and the correspondingSANS curves are shown in parts (a)–(c) and (d)–(f) of FigureS2 of Supplementary Material. In the analysis of mixturespectra, the scattering length, SL and volume of hydrophobicparts, V for the mixture solutions as calculated from therelations

SLmix = SLS −(CCopCCop

− CS)

SLCop,

Vmix = VS −(CCopCCop

− CS)VCop

(9)

were fed into the analytical program. The subscripts Copand S indicate copolymer and surfactants, respectively.


Table 2: Micellar parameters for 2% (w/v) copolymer solutions in D2O at different temperatures.

T/◦C Rc Å Rhs Å φ N T/◦C Rc Å Rhs Å φ N

B-1 B-2

Hard Sphere Hard Sphere

30 43 98 0.028 318 30 35 70 0.024 154

a Å b Å a/b N a Å b Å a/b N

Prolate ellipsoidal Prolate ellipsoidal

50 136 37 3.7 50 50 28 1.8 136

70 117 29 4.0 356

Table 3: Micellar parameters for prolate ellipsoids of surfactant solutions in D2O at 30◦C.

Conc.,g·dl−1 b Å a Å b/a N

Conc.,g·dl−1 b Å a Å b/a N

SDS DTAB

2.4 22 13 1.7 73 1 22 11 2.0 52

TX-100

3 12 38 3.2 190

6 12 39 3.3 198

By this way, we could calculate the association number ofsurfactants, NS, in mixture solutions. The scattering lengthscorresponding to the hydrophobic part of the individualcopolymers or surfactants are B-1 (2.223 × 10−12),B-2 (2.474 × 10−12), SDS (−1.374 × 10−12), DTAB(1.772 × 10−12), and TX-100 (1.453 × 10−12) cm,respectively. Similarly, the volume (in Å3) of hydrophobicpart of the respective copolymers and surfactants are B-1(1047), B-2 (1162), SDS (350.2), DTAB (452.2), and TX-100(374.6). The association number of the copolymer NCop wasthen calculated by the relation

NCop =(CCop/CCop − CS

)NS. (10)

B-1-SDS Mixtures. Our main aim is to monitor the effectof added SDS on the relative stability of B-1 micellarassociates, and therefore, the SANS data were analyzedby a similar procedure used for B-1 solutions in D2O.The various parameters as extracted from the analysis andformulations as described above are listed in Table 4. Aperusal of the data shows that the addition of SDS unimersor micelles systematically decreased the Rc and Rhs andalso the association number of copolymer micelles. Themixture containing 1% SDS is highly interesting, becauseits solution remained clear even up to temperatures close to100◦C. Since the mixed state is expected to have contributionfrom the micelles of both the components, it is quitelikely that the contribution of the copolymer micelles tothe aqueous solution phase characteristics would be bareminimum. In fact, our analysis revealed that there are2 B-1 copolymer molecules per about 93 SDS moleculesin this mixture, indicating that the micellization of B-1is completely suppressed at high SDS concentrations. Thedecrease of size and suppression of B-1 micellization inmixture solutions need to be accounted. SDS molecules

both in unassociated and associated form strongly interactwith the hydrophobic and hydrophilic parts of B-1 unimericmolecules to form charged complexes. It is possible thatseveral SDS unimers or micelles are associated with a singleB-1 molecule, as we noticed ∼1 : 45 ratio in a copolymer-SDS complex. The SDS-B-1 complex formation depletes thecopolymer unimers and the extent of depletion increaseswith the increase in SDS concentration. This leads to theshift in the equilibrium, unimer ↔ micelles progressivelyto the left, resulting into a decreased micellar dimensions.At high surfactant concentrations, the copolymer moleculesare saturated with the negatively charged SDS molecules.The resultant repulsions among the negative head groups ofcopolymer-bound SDS molecules or micelles suppress thecopolymer micelle formation all together. A direct evidencefor the existence of such repulsions among copolymer-SDS complexes comes from the fact that the Q valuecorresponding to the maximum peak (Qmax) systematicallyshifted to the right of the scale with the increase in theSDS concentration indicating that the inter particle distanceincreases at high SDS concentrations. Kelarakis et al. [29,30] have studied the same copolymer-SDS system by DLSand confocal microscope measurements at 25◦C and notedthat the addition of SDS (in the mole fraction range from0.25 to 0.8) induced the formation of particles much largerthan the micelles of copolymer and the super structuresgot decomposed at high SDS loading (xSDS > 0.8). Thepresent work covers a mole fraction of SDS, xSDS (0.0028to 0.735), and the analysis of SANS data did not showthe presence of any large structures, and on the contrary,SDS addition resulted in the destabilization of B-1 micellesusually observed with EPE copolymers–SDS systems[23].As indicated by the authors, the spherical micelles in 1%(w/v) solution of E18B10 in fact transformed to worm likemicelles in the temperature range of 50◦C to 60◦C due to


probable dehydration effects. Dehydration of corona part ofthe copolymer micelles can also be caused by the salt addition[44, 45].

B-1-DTAB. An examination of the micellar parametersnamely semiminor axis, a, semimajor axis, b, fractionalcharge, α, and association numbers of NB-1 and NDTABas listed in Table 4 shows that spherical micelles of B-1copolymer undergo a shape transition from hard sphereto ellipsoids upon its interaction with DTAB unimers. Thesuccessive increase of DTAB mole fraction (from 0.1149 to0.7219) decreased drastically the number of B-1 moleculesin mixed micelles and in fact completely suppressed the B-1micellization at high mole fraction. The Qmax value is initiallyshifted to a lower value followed by a gradual increase withthe increase in DTAB mole fractions. This observation is incontrast to the trends noted in B-1-SDS mixtures. Therefore,the structure and nature of the associates in B-1-SDS and B-1-DTAB mixtures need to be different. The (CH3)3N

+-headgroup of DTAB molecules is more hydrophobic and bulky ascompared to the −SO4− of SDS. Therefore, the arrangementof DTAB molecules or micelles within the B-1 molecular orassociate structures would be such that the methyl groupsof the head group are drawn towards the hydrophobic Bunits. Such twisting configuration not only facilitates moreDTAB molecules but also induce the shape transition fromhard spherical to open ellipsoidal structures as a result ofsteric as well as repulsive interactions. The repulsions amongthe charged complexes of B-1-DTAB would push away theindividual mixed micelle, and thus causing a shift in the Qmaxto lower values. At higher DTAB concentrations, however,the repulsions are so large and suppress the B-1 micelleformation, and the mixed micelles simply resemble DTABpure micelles.

B-1-TX-100. The SANS distribution trace for single mixtureof B-1-TX-100 (part c of Figure S2) is quite similar to thecurve for TX-100 solutions in D2O, and data was found tobe best reproduced with a form factor F(Q) of ellipsoidalcore. The contribution from S(Q) was considered to be unity(since no correlation peak was observed). The mixed micelleswith a predominant contribution from TX-100 moleculeswere formed at the expense of spherical copolymer micelles.

B-2-Surfactants Mixtures. In order to study how the copoly-mer architecture affects the associate structures in the mixedstate, we selected three mixtures with a fixed 2% (w/v)concentration of E13B10E13 triblock copolymer and maxi-mum concentration of individual surfactants. The analysisof SANS data revealed that the shape of the associatesis spherical (in B-2-SDS) and prolate ellipsoidal (in B-2-DTAB and -TX-100). The perusal of the micellar parameters,as listed in Table 4, further revealed that the addition ofthe surfactants in high concentrations, irrespective of theirnature, not only induce more dissociation of B-2 micellesas compared to B-1 micelles but also largely suppressed B-2 micelle formation. The associates in the mixed state arepredominantly dominated by surfactant micelles. B-1 and B-2 have almost identical number of B units but differ both in

the molecular architecture and number of E units. B-2 haslonger E blocks at both ends, while B-1 has one terminalshorter E block. Therefore, it can inferred that the E block ofthe copolymers plays an important role in their interactionswith the surfactants. The more the number of E units orlonger its length, more will be the binding of the surfactantsand hence more would be the destabilization or suppressionof copolymer micelle formation.

Temperature Dependence. The SANS distributions at differ-ent temperatures could be fitted by considering the P(Q)and S(Q) or only P(Q), or F(Q) based on ellipsoidal densecore depending upon whether the curves are characterizedby correlation peaks or not. The micellar parameters atdifferent temperatures, as extracted from the analysis, aregiven in Table 4. A close scrutiny of the data reveals that theaddition of surfactants facilitated the SANS measurementsat high temperatures, as solutions in the mixed state wereclear. As usual, the increase in the temperature inducedthe transformation of shape of the micelles (of both thecopolymers) (see Table 2) from spherical to prolate ellipsoidswith many fold increase in the association number. Theaddition of surfactants (in low concentration) decreasedthe association number of the mixed micelles indicatingthe predominance of contribution from copolymer-ionicsurfactant interaction over the usual temperature effects. Athigh loading of ionic surfactants, the copolymer micelles aredissolved to their unimeric or multimeric forms. The case ofcopolymers-TX-100 systems presented a different scenario.Irrespective of whether the TX-100 is in its unimeric ormicellar form, its addition at elevated temperatures resultedin the gradual decrease in the contribution of copolymermolecules in the mixed micelles. Even at very high TX-100 loading, considerable number of copolymer moleculesformed the part of the mixed micelles. Since both thecopolymers as well as TX-100 surfactants are nonionic innature, the copolymer-TX-100 complexes are more stablebecause of the absence of any repulsions, and this may bethe reason for the formation of stable mixed micelles in thissystem.

3.3. Viscosity Measurements. The utility of dilute solutionviscosity measurements on the copolymer aqueous solutionsis interesting, as they enable the determination of intrinsicviscosities, [η], an important parameter to adjudge thehydrophilic character. The representative profiles of reducedviscosities as a function of copolymer concentrations inwater as well as for copolymers-SDS mixed systems at 30◦Cas well as at elevated temperatures are shown in parts (a)and (b) of Figure 3. The plots are linear and fitted wellwith the Huggins equation, ηsp/C = [η]{1 + kH[η] C,where kH = Huggins constant. The intrinsic viscosities, [η],of the copolymer-surfactant complex systems were obtainedby extrapolation of reduced viscosities, ηsp/C, to zeroconcentration. The summary of intrinsic viscosities and kHfor the mixed systems is given in Table 5. The [η] valuesat 30◦C systematically increased with (i) the increase in theconcentration of any of the three surfactants and (ii) theincrease in temperature. B-2-surfactants mixtures have been


Ta

ble

4:M

icel

lar

para

met

ers

for

2%(w

/v)

B-1

-or

B-2

-su

rfac

tan

tsm

ixtu

reso

luti

ons

inD

2O

atdi

ffer

ent

tem

per

atu

res.

T/◦

CSu

rf.

Con

c.,

g·d

l−1

Rc Å

Rh

s

Åφ

NB

-1N

SDS

T/◦

CSu

rf.

Con

c.,

g·d

l−1

Rc Å

Rh

s

Åφ

NB

-2N

SDS

B-1

-B

-2-

Har

dsp

here

SDS

SDS

300.

001

3988

0.08

237

—30

114

320.

041

330.

0535

740.

0917

4—

1.00

2045

0.19

293

b Åa Å

b/a

NB

-1N

SDS

b Åa Å

b/a

NB

-2N

SDS

Pro

late

ellip

soid

al56

0.00

181

362.

341

673

0.00

0583

292.

925

7—

670.

0550

361.

425

375

0.05

7731

2.5

255

—70

181

372.

24

205

751

8129

2.9

214

5b Å

a Åb/

aN

B-1

ND

TAB

b Åa Å

b/a

NB

-2N

DTA

B

Pro

late

ellip

soid

alD

TAB

DTA

B30

0.05

6134

1.8

269

301

3121

1.5

175

0.1

4335

1.2

202

1.0

4125

1.6

113

055

0.05

147

364.

136

5—

730.

0005

109

303.

634

3—

610.

188

382.

337

1—

750.

180

312.

625

2—

701.

012

035

3.4

125

075

1.0

4425

1.8

115

9T

X-1

00N

B-1

NT

X-1

00T

X-1

00N

B-2

NT

X-1

00

306.

042

234

1.7

2319

330

6.0

3423

1.6

1615

348

6.0

5924

2.5

3428

357

6.0

7823

3.4

3524

550

0.01

9427

3.5

276

593.

094

243.

969

373

533.

010

526

4.0

105

480

720.

0194

293.

226

6—


Ta

ble

5:In

trin

sic

visc

osit

y[η

],an

dH

ugg

ins

con

stan

tk H

,for

copo

lym

er-s

urf

acta

nt

aqu

eou

sso

luti

ons

atdi

ffer

ent

tem

per

atu

res.

T◦ C

CSD

Sg·

dl−

1[η

]d

l·g−1

k HT◦ C

CD

TAB

g·d

l−1

[η]

dl·g

−1k H

T◦ C

CT

X-1

00g·

dl−

1[η

]Dl·g

−1k H

B-1

-SD

SB

-1-D

TAB

B-1

-TX

-100

300

0.03

66.

9

500

0.06

03.

4

300.

001

0.03

85.

530

0.05

0.03

76.

230

0.01

0.03

95.

8

500.

001

0.06

03.

351

0.05

0.05

85.

650

0.01

0.06

42.

0

300.

050.

042

4.3

300.

10.

042

4.5

303.

00.

054

2.9

520.

050.

064

2.4

550.

10.

071

1.8

483.

00.

071

1.7

301.

00.

066

1.7

301.

00.

062

2.0

306.

00.

068

1.8

B-2

-SD

SB

-2-D

TAB

B-2

-TX

-100

300

0.03

21.

2

700

0.06

91.

9

300.

0005

0.03

51.

030

0.00

050.

034

1.0

300.

010.

036

1.0

680.

005

0.07

21.

767

0.00

050.

069

1.8

690.

010.

049

0.2

300.

050.

038

0.8

300.

10.

040

0.7

303.

00.

050

0.4

710.

050.

074

1.6

750.

10.

085

1.2

563.

00.

072

0.8

301.

00.

055

0.4

301.

00.

053

0.4

306.

00.

061

0.3


0 1 2 3 4 5 6 7 80

0.05

0.1

0.15

0.2η

sp/C

(dL·g−1

)

CB-1 (g·dL−1)

(a)

0 1 2 3 4 5 6 7 80

0.05

0.1

0.15

0.2

ηsp/C

(dL·g−1

)

CB-1 (g·dL−1)

(b)

Figure 3: Reduced viscosity for B-1 + surfactant mixtures in aqueous solutions at different temperatures (in ◦C) (a) SDS (�) 0% 30◦C, (�)0% 50◦C, (�) 0.001 30◦C, (�) 0.001 50◦C, (©) 0.05% 30◦C, (•) 0.05% 52◦C, and (∇) 1% 30◦C and Reduced viscosity for B-2 + surfactantmixtures in aqueous solutions at different temperatures (b) (�) 0% 30◦C, (�) 0% 70◦C, (�) 0.0005% 30◦C, (�) 0.0005% 68◦C, (©) 0.05%30◦C, (•) 0.05% 71◦C, and (∇) 1% 30◦C. Line represents fitted values.

characterized by low [η] values at 30◦C as compared tothe B-1 containing mixtures. These trends are rationalizedby considering the following factors. The More hydrophilicthe copolymer-hydrocarbon surfactant mixture complexes,the more would be the hydrodynamic volume that is [η].At the same time, the overall magnitude of [η] dependsupon the number of copolymer molecules in the mixedsystem. Hence, the observed increase in [η] at 30◦C withthe increase in surfactant amounts can be rationalized basedon the first factor. Even though triblock B-2 itself has morehydrophilic character, its micelles get easily dissociated in themixed systems; therefore, its mixtures have smaller [η] overthe diblock B-1. The observed large [η] values at elevatedtemperatures in general can be explained by the fact that thenumber of copolymer molecules is considerable in spite ofthe destabilization effects by added surfactants.

4. Conclusions

The diblock E18B9 and triblock E13B10E13 are typical amphi-philic copolymers and their aqueous solutions displaycharacteristic turbidity and cloud points. The copolymersassociate into micellar structures, constituted by a densehydrophobic core surrounded by flexible hydrophilic corona.The copolymer micelles grow into large ellipsoids with highassociation numbers at elevated temperatures due to dehy-dration. The addition of ionic surfactants SDS or DTAB bothin unimeric or micellar forms leads to (i) increase of turbidityand cloud points, (ii) the formation of copolymer-surfactantcomplexes, (iii) dissociation of copolymer micelles, and(iv) suppression of copolymer micelle formation. Nonionicsurfactant, TX-100, addition to copolymer micelles, however,favors the formation of mixed micelles but with the decreased

proportion of copolymer molecules. As compared to thediblock copolymer, triblock E13B10E13 copolymer has moreE units attached at both the ends of the middle hydrophobicB units, and, therefore, the E chains interact strongly with thesurfactant molecules resulting easy dissolution of micellesof latter as compared to the former. The destabilizationand suppression of copolymer micelles was also observedat elevated temperatures indicating that the repulsive headgroup interactions among copolymer-surfactant complexesare more dominant than the usual dehydration and shrinkingof hydrophilic chains at high temperatures. The copolymer-surfactant complexes are characterized by large intrinsichydrodynamic viscosities and hence are hydrophilic innature.

Acknowledgment

The authors thank the UGC-DAE Consortium for thefinancial support under a collaborative research Grant no.IUC/AO/MUM/CRS M–108/03/2581.

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EffectofSurfactantsonAssociationCharacteristicsofDi-and ... · 2Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India 3UGC-DAE Consortium for Scientiﬁc