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Chemical Physics 325 (2006) 492–498

Characterization of triblock copolymerE67S15E67–surfactant interactions

Emilio Castro, Pablo Taboada *, Vıctor Mosquera

Grupo de Sistemas Complejos, Laboratorio de Fısica de Coloides y Polımeros, Departamento de Fısica de la Materia Condensada,

Facultad de Fısicas, Universidad de Santiago de Compostela, Campus Sur, 15782 Santiago de Compostela, Spain

Received 2 December 2005; accepted 25 January 2006Available online 20 February 2006

Abstract

The interactions between the triblock copolymer E67S15E67 and the surfactant sodium dodecyl sulfate (SDS) have been investigated bystatic and dynamic light scattering, and isothermal titration calorimetry. Upon the addition of the surfactant, changes in the physico-chemical properties of the micellised block copolymer take place due to interactions between the surfactant and the copolymer. Mixedmicelles of copolymer and surfactant are formed and the size of the mixed micelles changes depending on the amount of SDS as seen byDLS data. These results are confirmed with ITC measurements. Comparison of the behavior displayed by the triblock copolymer in thepresence of SDS is also compared with that of the structurally related diblock copolymer, S15E63.� 2006 Elsevier B.V. All rights reserved.

Keywords: Block copolymer; Surfactant; Mixed micelle; Interactions

1. Introduction

In aqueous solution, polyoxyalkylene block copolymersbehave like surfactant molecules and build a variety ofaggregates as a consequence of their amphiphilic character.The combination of hydrophilic and hydrophobic blocksconfers to these polyoxyalkylene block copolymers inter-esting and useful surface active and micellization propertiesin dilute aqueous solution, and gelation ones in concen-trated solutions. Spherical aggregates are common, withthe hydrophobic block forming the micelle core and thehydrophilic blocks being solvated by water, although othergeometries are possible depending on concentration, tem-perature, block length and block architecture [1]. Thisbehavior allows block copolymers to be suitable for theiruse in different technological applications as paints, coat-ings, laundry detergents, cosmetic products and pharma-ceutical formulations [2–5].

0301-0104/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2006.01.027

* Corresponding author. Tel.: +34981563100x14042; fax:+34981520676.

E-mail address: fmpablo@usc.es (P. Taboada).

Neutral aqueous soluble block copolymers are used inmany applications in combination with ionic surfactants,because these systems offer numerous potential applica-tions in different processes [6–10], e.g., industrial: foaming,floating; pharmaceutical: drugs solubilization, oil recoveryand as a medium for metal nanoparticle formation. Thephysicochemical and thermodynamic properties of mix-tures of block copolymer with surfactants have been exten-sively studied in the last years, mainly on those copolymersdenominated Pluronics, triblock copolymers whose hydro-phobic block is formed by oxypropylene units [2,4,5,11].For this class of systems, formation of mixed surfactant–copolymer micelles was found accompanied with a progres-sive reduction of the number of copolymer molecules andsubsequent size decrease of these mixed aggregates[12,13]. On the other hand, some few studies have beenmade to characterize the interactions between surfactantsand other types of copolymers as the diblocks formed bypolybutadiene–polyethylene oxide (PB–PEO) [14,15],where a transition from cylindrical to spherical micelleswas detected in a relative narrow surfactant concentrationrange, and polystyrene–polyethylene oxide (PS–PEO)

E. Castro et al. / Chemical Physics 325 (2006) 492–498 493

blocks, where superclusters formed by several copolymerchains were detected at high surfactant concentrations[6,16]. However, to our best knowledge, no attention hasbeen paid to the mixtures of surfactants and block copoly-mer comprising a hydrophilic poly(oxyethylene) block anda second hydrophobic poly(oxyphenylethylene) block. Inrecent papers, we have studied the micellization and gela-tion properties of both diblock and triblock architecturesof this class of copolymers [17–19] (denoted as EmSn orSnEm for diblocks depending on polymerisation sequence,and EmSnEn for triblocks, where E denotes the oxyethyleneunit, OCH2CH2, and S denotes the oxyphenylethylene,OCH2CH(C6H5), and the subscripts m and n denote thenumber-average block lengths in repeat units). In a recentpaper, we have described the interactions between adiblock (styrene oxide–ethylene oxide) copolymer with cat-ionic and anionic surfactants by different experimentaltechniques [20–22]. In the present work, we extend the for-mer study with the aim of elucidating the effect of themolecular architecture on the copolymer–surfactant inter-actions. Thus, we have examined the interactions betweena triblock copolymer, E67S15E67, which presents the samehydrophobic block length than the diblock S15E63 previ-ously studied [20–22], with the anionic surfactant sodiumdodecyl sulphate, SDS, by using static (SLS) and dynamiclight scattering (DLS) and isothermal titration calorimetry(ITC) techniques.

2. Experimental

2.1. Materials

Sodium dodecyl sulfate (SDS), was purchased fromMerck (stated purity P99%) and used as received withoutfurther purification. Water was double distilled anddegassed before use. To detect the presence of impuritiesin the surfactant, surface tension measurements were previ-ously performed at 20 �C (figure not shown) for which nominimum was detected, with a cmc value of 0.0081 M inagreement with literature ones [23].

The synthesis and experimental characterization to deter-mine purity and stoichiometry of the triblock copolymerE67S15E67 was described in detail by Yang et al. [18]. Table1 shows the molecular characteristics of the copolymer.

2.2. Light scattering

Light scattering measurements were made at20.0 ± 0.1 �C and at a scattering angle of h = 90� to theincident beam, using an ALV 5000 laser light-scattering

Table 1Molecular characteristics of the copolymer

Samples Mn (g mol�1) (NMR) Mw (g mol�1) (b) wt% E

E67S15E67 7700 8100 77

instrument equipped with a 500 mW solid state laser(Coherent Innova) with vertically polarized incident lightof wavelength k = 532 nm in combination with a ALVSP-86 digital correlator with a sampling time range of25 ns to 40 ms. All solutions were filtered through a Milli-pore filter with a 0.22 lm pore size and thermostated thedesired temperature for at least 30 min. Experiment dura-tion was in the range of 5–10 min, and each experimentwas repeated three or more times.

For dynamic light scattering, the correlation functionswere analyzed by the constrained regularized CONTINmethod to obtain distribution decay rates (C). The decayrates gave distribution of the apparent diffusioncoefficient

Dapp ¼Cq2

ð1Þ

with the scattering vector, q,

q ¼ 4pnk

sinh2

ð2Þ

being n the refractive index of water, h the scattering angleand k the wavelength. The apparent hydrodynamic radius,rapp,h, can be calculated via the Stokes–Einstein equation:

rapp;h ¼kT

6pgDapp

ð3Þ

where k is the Boltzmann constant and g the dynamic vis-cosity of water at temperature T in Kelvin.

2.3. Isothermal titration calorimetry

Heats of dilution were measured using a VP-ITC titra-tion microcalorimeter from MicroCal Inc., Northampton,MA. In ITC experiments, one measures directly theenthalpy changes associated with processes occurring atconstant temperature. Experiments were carried out firsttitrating monomeric and micellar surfactant into waterand then into an aqueous solution containing a knownamount of polymer. An injection schedule (number ofinjections, volume of injection, and time between injec-tions) is set up using interactive software, and all dataare stored to a hard disk. We present the results of theITC experiments in terms of the enthalpy change per injec-tion (DHi) as a function of surfactant concentration. Smallaliquots of a stock solution of surfactant at a concentrationeither below or above the cmc were injected into a knownvolume of water or polymer solution (ca. 1 cm3) held in thecell of the calorimeter, initially to produce solutions belowthe surfactant cmc. Repeated additions of the stock solu-tion gave the heat evolved (Q) as a function of surfactantconcentration. The solution in the cell was stirred by thesyringe at 300 rpm, which ensured rapid mixing but didnot cause foaming on the polymer–surfactant solution.All experiments were repeated twice and the reproducibilitywas within ±3%.

494 E. Castro et al. / Chemical Physics 325 (2006) 492–498

3. Results and discussion

3.1. Static and dynamic light scattering

We have focused on following the changes in complexformation when the anionic amphiphilic compound SDSinteracts with E67S15E67 block copolymer micelles usinglight scattering and isothermal titration calorimetry tech-niques. Therefore, it was necessary to choose a concentra-tion and a temperature where the copolymer systemsdemonstrate a well-defined relaxation time distributionwith micelles as the single scattering species. At this respect,copolymer E67S15E67 in water at 25 �C has an averageaggregation number of 25 monomers and an apparenthydrodynamic radius (radius of the hydrodynamicallyequivalent hard sphere corresponding to the apparent dif-fusion coefficient) of 8.3 nm [18]. It has been also shownthat the cmc of this copolymer is almost temperature inde-pendent, with a value of ca. 0.026 g dm�3 at 25 �C, so wecan consider the block copolymer is fully micellized atthe concentration used in the present study.

The total intensity of the scattered light normalized withthe scattered intensity of toluene ðI tot

h Þ was measured at acopolymer concentration of 2.5 g dm�3 and varying SDSconcentrations from 7.5 · 10�6 to 0.1 M at 20 �C, with ascattering angle of 90�, because no angular dependence ofthe static light scattering intensity was observed, as previ-ously shown [17,20] (see Fig. 1). The SLS intensity of thecopolymer–surfactant system in water can be divided intothree different intervals: At very low surfactant concentra-tions (7.5 · 10�6–5.0 · 10�4 M) there is a strong decrease inSLS intensity with increasing surfactant concentrations asa consequence of interactions of SDS molecules to thecopolymer micelles giving rise to the formation of a copoly-mer-rich-mixed micelles. At this stage, the intensitydecrease may be composed of two different contributions:

1E-5 1E-4 1E-3 0.01 0.10

1

2

3

4

5

6

7

8

I θ,t

lato.u.a( )

C (M)

Fig. 1. Relative intensity measured at h = 90� and at 20 �C as a functionof SDS concentration in the presence of 2.5 g dm�3 of copolymerE67S15E67.

an effective size reduction of the mixed surfactant–copoly-mer micelle and a contribution from the structure factor,which mirrors the repulsive interactions between themicelle–copolymer complexes as they become more andmore charged when surfactant monomers bind to copoly-mer micelles. The existence of both contributions has beenelucidated in previous works adding excess electrolyte toscreen repulsive interactions between complexes [24–26].Increasing surfactant concentration (5.0 · 10�4–2.5 ·10�3 M), the SLS remains practically constant, in agree-ment with the same concentration range displayed inFig. 2 (see below). At higher surfactant concentrations(>2.5 · 10�3 M) a subsequent decrease in SLS intensity isobserved until reaching a plateau region whose intensitycorresponds to the formation of surfactant rich-copolymermicelles and free surfactant micelles, as will discuss below.

Parallel to SLS measurements, DLS data were also col-lected. These data provide complementary knowledge inorder to establish mechanisms of interactions between thesurfactant and the copolymer. Fig. 2 shows the apparentdiffusion coefficient, Dapp, of solutions of 2.5 g dm�3 ofcopolymer E67S15E67 as a function of total SDS concentra-tion. A similar profile to that obtained for SLS data isderived. On the other hand, the hydrodynamic radius ofE67S15E67 micelles obtained for the surfactant-free solutionwas 8.1 nm, which is very similar to that obtained by Yanget al. [18] (8.3 nm).

The values of Dapp reported in the present work areapparent values because no extrapolation of the diffusioncoefficient to infinite dilution of copolymer is made and,hence, Dapp will be affected by size changes and electro-static interactions. As one can see for the copolymer/SDSsystem in Fig. 3, the DLS data showed the predominanceof a single scattering species characterized by an apparentdiffusion coefficient, which increases in this surfactant con-centration range at very low surfactant concentrations, thesystem being rather monodisperse at this SDS concentra-

1E-5 1E-4 1E-3 0.01 0.12

3

4

5

6

7

8

9

0111

Dpap

m(2 s

1-)

C (M)

Fig. 2. Apparent diffusion coefficient, Dapp, against surfactant concentra-tion, C, of 2.5 g dm�3 E67S15E67/surfactant mixtures at 20 �C.

0.01 0.1 1 100.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.01 0.1 1 100.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.01 0.1 1 100.00

0.02

0.04

0.06

0.08

0.10

0.12

1E-3 0.01 0.1 1 100.00

0.01

0.02

0.03

0.04

0.05

Inytisnet

).u .a(

Iny tisnet

).u.a(

Inytis net

).u.a(

Decay time (ms)

Inytisnet

).u.a(

Decay time (ms)

a b

dc

Fig. 3. Selected relaxation time distributions for a 2.5 g dm�3 E67S15E67 solution with varying SDS concentrations at 20 �C: (a) 1.0 · 10�5 M; (b)1.0 · 10�4 M; (c) 1.0 · 10�3 M; (d) 1.0 · 10�2 M.

E. Castro et al. / Chemical Physics 325 (2006) 492–498 495

tion range (7.5 · 10�6–5.0 · 10�4 M). This increase hasbeen related to the interaction of SDS molecules with thecopolymer micelle to give copolymer-rich mixed micelles.A repulsive force between the surfactant polar headgroupswould facilitate water penetration into the copolymermixed micelle, leading to a less dense packing of thismicelle. This would result in a decrease in the aggregationnumber which, in turn, is reflected in a decrease in themixed micelle size and thus, an enhance mobility reflectedin the increase of Dapp [15,16,26]. Moreover, this increasein Dapp for the E67S15E67 extends in a slightly wider concen-tration range of SDS if compared to the related diblockS15E63 in the same concentration regime (see Ref. [21]).This fact might be related to the lower dry micellar densityof the triblock copolymer (0.14 g cm�3) if compared to thatof the diblock (0.16 g cm�3), which would involve an evenmore open micellar structure full of solvent in the formercase. Furthermore, the presence of two junctions in thecore forming block of the micelles does not allow a sotightly packing of the micelle core as in the case of thediblock copolymers, for which the micellization enthalpychange is null indicating that the hydrophobic block ofEmSn copolymer is significantly coiled even in the unimerstate [18,27]. However, for triblock EmSnEm copolymersthis enthalpy change is important, confirming a less densepacking of the S block which results in lower aggregationnumbers and hence, a slight reduction in hydrodynamicradii. Therefore, penetration of SDS molecules in the tri-block copolymer micelle might take place in a wider surfac-

tant concentration range. On the other hand, recent papers[24–26] have exposed the possibility that the translationaldiffusive motion of the copolymer-rich–surfactant mixedmicelles is affected by repulsive interactions and becomesfaster. However, these works also consider a size reductionof the mixed micelle as an additional reason for thisincrease. In our case, the electrostatic repulsion must bepresent, thus affecting the profile of Dapp, but its strengthshould be moderate due to a not severe shift of the relaxa-tion time distribution of the copolymer-rich–surfactantmixed micelles to faster relaxation times, as seen inFig. 3. Moreover, it has been also shown [24,25] that if elec-trostatic interactions between charged surfactant–copoly-mer mixed micelles are strong the relaxation distributionfinally splits, with the mode corresponding to the copoly-mer-rich–surfactant mixed micelles shifting to much slowertimes than in the present case, together with the simulta-neous appearance of a new mode corresponding to surfac-tant-rich–copolymer micelles at faster relaxation times,which is not the present case: The relaxation time distribu-tion of the copolymer-rich–surfactant micelles continu-ously decreases to faster times as SDS concentrationgrows and neither splitting nor subsequent appearance ofa new mode occurs, as seen for other copolymer–surfactantsystems for which a reduction in micelle size is also claimed[28,29].

With further SDS addition (5 · 10�4–2.5 · 10�3 M),Dapp slightly decreases. An increase in the polydispersityis also observed. This decrease in Dapp might be related

0 20 40 60 80 100-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

ΔHi

k( /Jm

ol)

[SDS] (mM)

Fig. 4. Enthalpy change as a function of surfactant concentration due tothe titration of micellar SDS (500 mM) in the absence (s) and thepresence (d) of 2.5 g dm�3 of copolymer E67S15E67 at 20 �C.

496 E. Castro et al. / Chemical Physics 325 (2006) 492–498

to the screening of the repulsive interactions between sur-factant headgroups in the copolymer-rich–surfactantmixed micelles as a consequence of the increasing amountof counterions surrounding the micellar corona, whichmay also diminish the electrostatic repulsion between thesemixed micelles, as occurred in other related systems [16,29].

Finally, with further addition of SDS (>2.5 · 10�3 M),Dapp increases again as a consequence of the progressivereduction in size of the copolymer-rich–surfactant mixedmicelles and thus, the existence of less large diffusiveobjects in solution. This size reduction originates fromthe disintegration of copolymer-rich mixed micelles dueto repulsive interactions as the micelle becomes richer inSDS molecules, until forming surfactant-rich copolymermixed micelles, with a size that resembles a surfactantmicelle, as indicated by the decrease up to �2.5 nm of thehydrodynamic radii of the mixed micelles. The increaseof the width of the relaxation time distribution in this con-centration region (see Fig. 3) can be related to the smallmixed micelles containing a varying number of triblockcopolymer unimers, as occurred for other Pluronic/surfac-tant systems[13,24,30]. In contrast with the behavior shownby the S15E63–SDS system, for which a coexistence of tworelaxation modes occurs between 0.01 and 0.05 M of SDSdue to the simultaneous existence of important populationsof copolymer-rich and surfactant-rich mixed micelles [21],only one relaxation mode going towards faster relaxationtimes is observed for the E67S15E67–SDS system. This is aresult of the smaller number of copolymer-rich surfactantmicelle present in solution, which makes no possible thepresence of simultaneous important populations of copoly-mer-rich and surfactant-rich mixed micelles, being the lat-ter formed from the disintegration of the former ascommented above. Moreover, in this region there is alsoan increasing number of free SDS micelles which also con-tributes to the relaxation time distribution. These freemicelles show a characteristic size close to the smallSDS–copolymer complexes, so their relaxation time distri-butions will be superimposed to those of the small surfac-tant–copolymer complexes. However, their formationseems to be corroborate provided that apparent diffusioncoefficients tend to the value of classical SDS micelles asthe SDS concentration in solution increases, indicatingthe predominant species in solution.

3.2. Isothermal titration calorimetry

Isothermal titration calorimetry measurements were car-ried out at 20 �C with the E67S15E67 concentration also fixedat 2.5 g dm�3 for different stock solutions of SDS. As in thecase of the diblock copolymer S15E63, the titration curve ofmonomeric SDS onto the micellised solution of the triblockcopolymer deviates from that of water indicating the inter-action between the surfactant and the polymer occurs evenat the lowest SDS concentration measured, 6.5 · 10�6 M soa critical aggregation concentration could not be derived(figure not shown). Fig. 4 shows the results for the addition

of micellar SDS to the triblock copolymer solution in addi-tion to the curve for just adding the amphiphilic compoundto water. In this way, combination of titration of mono-meric and micellized amphiphilic compounds allows us tocover the same concentration range than in light scatteringexperiments. For titrations of micellized SDS in water (seeFig. 4), we can derive its critical micelle concentration, withvalue 8.2 mM; and the enthalpy of micellization, DHmic,with value of 2.5 kJ mol�1. Both cmc and DHmic valuesagree well with the literature ones [23].

When titrating micellized SDS in an ITC experiment,interactions copolymer–surfactant seem to start at the low-est surfactant concentration (5.2 · 10�4 M). The enthalpycurve corresponding to the titration of micellized SDSshows an endothermic increase leading to a first endother-mic maximum at �1.0 · 10�2 M. This endothermicincrease and the subsequent maximum has been attributedto interactions between surfactant and copolymer micelleswhich leads to the formation of mixed surfactant–copoly-mer micelles and, therefore, to a disruption of blockcopolymer-only micelles [31]. The height of the maximumis lower for the triblock E67S15E67 than for the diblockcounterpart, S15E63, as a result of the lower concentrationand less compact structure of triblock copolymer micellesavailable to interact with the surfactant [20]. The concen-tration region corresponding to the increase and subse-quent endothermic maximum relates to the DLS regionof reduction of the mixed micelle size. Thus, the initialinteractions and subsequent initial decrease of size wouldbe comprised in the first injection of the ITC experiment.The steep rise in the endothermic profile is likely dominatedby the dehydration of the block copolymer micelle uponinteraction with SDS, which more than compensates forthe hydrophobic effect arising from the formation of sur-factant aggregates replacing copolymer unimers in thecopolymer–surfactant mixed micelles. Thus, due to theirhigher hydrophobicity, the core of these mixed micelles

E. Castro et al. / Chemical Physics 325 (2006) 492–498 497

should be formed by oxyphenylethylene chains whichmight be stabilized by the solubilization of dehydratedPEO segments and surfactant groups replacing water mol-ecules in the core–corona interface, as occurred for oxyet-hyelene–oxypropylene block copolymers [13,32]. Inparticular, recent NMR data have shown that carbonsC1–C3 of SDS are those involved in the interactions withthis class of copolymers, so a deep penetration of the sur-factant inside the mixed micelle structure is not expected[33]. On the other hand, the concentration of this endother-mic maximum differs from that corresponding to the finaldisruption of the surfactant–copolymer micelles seen byDLS (2.5 · 10�3). This difference may arise from the bal-ance between the endothermic heat from the interactionbetween the surfactant and the copolymer micelles overthe beginning of the copolymer-rich–surfactant mixedmicelle distortion which leads to copolymer rehydration,which is exothermic in nature.

Further SDS addition leads to an exothermic decreaseup to a minimum at 3.5 · 10�2 M of SDS. In the presentcopolymer–SDS system the endothermic maximum is notfollowed by a shallow minimum and a second smoothendothermic maximum, which are present for the S15E63–SDS mixture. This would involve that the concentrationof triblock copolymer monomers expelled from the mixedmicelles as SDS binds to the copolymer-rich mixed micellesdoes not reach the necessary critical value to allow the for-mation of an important amount of SDS micelles bound tothese copolymer unimers, as previously stated [20,34].

The exothermic decrease and subsequent minimumwould be originated from the additional disruption of themixed surfactant/copolymer micelles derived from theincrease of SDS in these. This exothermic minimum isattributed to re-hydration of E67S15E67 segments due tothe dissociation of copolymer micelles to monomericcopolymer [34], and is lower in the case of the triblockcopolymer if compared with the diblock one as a conse-quence of a major rehydration of the triblock copolymerunimers due to the less tightly structure of the mixedmicelles formed, in agreement with light scattering data.

At higher SDS concentration (>3.5 · 10�2 M), there isan enthalpy increment passing by a third broad endother-mic maximum that intersects with the SDS/water curve,and goes very close to it. The enthalpy in the water pluscopolymer mixture will superimpose to that in water athigh surfactant concentration because, in those conditions,the surfactant is in the infinite dilution state. Therefore,interactions between the copolymer and the surfactant willbe weak, and beyond the merging point, the copolymer nolonger interacts with surfactant molecules and the injec-tions merely correspond to a dilution of micelles into solu-tion containing free micelles.

4. Conclusions

Presence of surfactant sodium dodecyl sulfate in aque-ous solutions of micellised triblock copolymer E67S15E67

involves important changes in the physico-chemical param-eters of the block copolymer as a consequence of the stronginteractions between this and the surfactant. This results inan abrupt change of the hydrodynamic radius of the blockcopolymer micelles at low SDS concentrations due to theassociation of SDS molecules with the block copolymermicelles to give a copolymer-rich/surfactant complex (ormixed micelle), which becomes more charged as more sur-factant is added. Under progressive destruction of thecopolymer rich-SDS mixed micelles due to electrostaticrepulsion between surfactant headgroups the formationof a new type of complex occurs, denote as surfactantrich-copolymer mixed micelles, which would be formedby SDS molecules in the micellised state bound to one orvery few copolymer unimers. This behavior of the presentsystem is corroborated by ITC data, from which the dehy-dration and rehydration of the copolymer chains followingformation of copolymer rich-surfactant mixed micelles andtheir distortion are observed.

Acknowledgements

The project was supported by the Ministerio de Edu-cacion y Ciencia through project MAT2004-02756 andXunta de Galicia. P.T. and. E.C. thanks Ministerio deEducacion y Ciencia for his Ramon y Cajal position andhis PhD grant, respectively. We thank Professors DavidAttwood and Colin Booth for generous gift of the blockcopolymer sample.

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