Behavior of a Styrene Oxide− Ethylene Oxide Diblock Copolymer

Co-micellization of the diblock copolymer oxyphenylethylene/oxyethylene (S17E65) with the anionic surfactant sodium dodecyl sulfate (SDS) was investig...
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5592

J. Phys. Chem. B 2005, 109, 5592-5599

Behavior of a Styrene Oxide-Ethylene Oxide Diblock Copolymer/Surfactant System: A Thermodynamic and Spectroscopy Study Emilio Castro, Pablo Taboada,* and Vı´ctor Mosquera Laboratorio de Fı´sica de Coloides y Polı´meros, Grupo de Sistemas Complejos, Departamento de Fı´sica de la Materia Condensada, Facultad de Fı´sica, UniVersidad de Santiago de Compostela, Spain ReceiVed: NoVember 16, 2004; In Final Form: January 17, 2005

Co-micellization of the diblock copolymer oxyphenylethylene/oxyethylene (S17E65) with the anionic surfactant sodium dodecyl sulfate (SDS) was investigated in aqueous solution using light scattering, transmission electron microscopy, isothermal titration calorimetry (ITC), and density measurements. Upon the addition of the surfactant, changes in the physicochemical properties of the micellized block copolymer take place due to interactions between the surfactant and the copolymer. Mixed micelles of copolymer and surfactant are formed and the size of the mixed aggregates changes in dependence of the amount of SDS. At a certain limiting concentration of SDS, only small rich-surfactant-copolymer aggregates and free surfactant micelles are observed in solution, as confirmed by the thermodynamic data obtained by ITC and transfer volumes. Thus, it seems that the presence of surfactant can be a tool to control the size and properties of block copolymer aggregates in solution.

Introduction In the past 40 years, the interaction between synthetic nonionic amphiphilic polymers and ionic surfactants dissolved in water has been extensively studied.1-4 Mixtures of amphiphilic polymers and surfactants are applied in a diverse range of applications using various formulations and colloidal dispersions of industrial importance such as paints, coatings, laundry detergents, cosmetic products, and pharmaceutical formulations.2 Among the different classes of nonionic polymers, block copolymers comprising a hydrophilic poly(oxyethylene) block and a second hydrophobic block have been extensively studied, as indicated by the large number of reviews and papers found in the literature.5-19 Block copolymers of this type form aggregates in selective solvents, in analogy to the well-known behavior of surfactants. The length scale of the formed block copolymer micelles is usually in the range 5-200 nm,20 whereas surfactant micelles have smaller sizes in the range of 2-10 nm.21 The combination of hydrophilic and hydrophobic blocks confers to these copolymers interesting surface active and micellization properties in dilute aqueous solution and gel behavior in concentrated ones. Variation of the hydrophobic block, the block length, and the block architecture allows close control of these properties. Block copolymers and their mixtures with surfactants are being nowadays studied for their use as templates in the preparation of nanostructured materials,22-24 nanoparticles,25,26 and controlled drug delivery systems.27-29 The surfactantcopolymer interactions have been mainly focused on mixtures formed by PEO-PPO or PEO-PPO-PEO copolymers and classical anionic, cationic, and nonionic surfactants,30-34 where PEO is poly(ethylene oxide) and PPO is propylene oxide, respectively. Surfactants were added to solutions of these block copolymers and formation of mixed micelles was observed. For triblock copolymers, a decomposition of the copolymer ag* To whom correspondence should be addressed: [email protected]. Tel: 0034981563100 ext 14042. Fax: 0034981520676.

gregate upon addition of surfactant was observed.30,31 After addition of surfactant, only single block copolymer molecules surrounded by surfactant molecules could be detected. It was also demonstrated that the mechanism of interaction between the copolymer and the surfactant is system specific and strongly dependent on temperature. On the other hand, some few studies have been made to characterize the interactions between surfactants and other types of copolymers as the diblocks formed by polybutadiene-poly(ethylene oxide) (PB-PEO),35-36 where a transition from cylindrical to spherical micelles was detected in a relative narrow surfactant concentration range, and polystyrene-poly(ethylene oxide) (PS-PEO) blocks, where superclusters formed by several copolymer chains were detected at high surfactant concentrations.25,26,37 Thus, to shed more light on the behavior of surfactant/block copolymer systems, in the present work we report the study of the influence of the anionic surfactant sodium dodecyl sulfate (SDS) on the micellized state of the diblock copolymer S17E65, where S denotes styrene oxide as the hydrophobic block (OCH2CH(C6H5)) and E the ethylene oxide block (OCH2CH2) as the hydrophilic block. The synthesis and micellization properties of the block copolymer have been previously reported,19,38 showing an extremely low critical micelle concentration and an aggregation number of 150-170 monomers per micelle in the temperature range 25-50 °C. To characterize the thermodynamics of the surfactant/block copolymer interactions, isothermal titration calorimetry (ITC) and densimetry measurements were performed. Light scattering and transmission electron microscopy (TEM) measurements were used to follow and directly visualize, respectively, the structural changes of the mixed aggregates with increasing surfactant concentration. 2. Experimental Section Materials. Sodium dodecyl (SDS) was purchased from Merck (stated purity g99%) and used as received without further purification. Water was double distilled and degassed before use. The synthesis of the diblock copolymer S17E65 was

10.1021/jp044766n CCC: $30.25 © 2005 American Chemical Society Published on Web 02/26/2005

Styrene Oxide-Ethylene Oxide Diblock System

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TABLE 1: Molecular Characteristics of the Copolymera -1

Mn/g mol (NMR) S17E65

wt % S (NMR)

Mw/Mn (GPC)

Mw/ g mol-1

42.1

1.04

5140

4940

Estimated uncertainty: Mn to (3%; wt % S to (1%, Mn/Mw to (0.01. Mw calculated from Mn and Mw/Mn. a

described in detail by Crothers et al.19 Table 1 shows the molecular characteristics of the copolymer. Solutions observed during the tube inversion tests for the copolymer remained clear to the eye throughout the temperature of 20 °C investigated in the present work. Dynamic Light Scattering (DLS). Dynamic light-scattering measurements were made at 20.0 and 40. 0 ( 0.1 °C and at a scattering angle of θ ) 90° to the incident beam, using an ALV 5000 laser light-scattering instrument equipped with a 500 mW solid state laser (Coherent Innova) with vertically polarized incident light of wavelength λ ) 532 nm in combination with a ALV SP-86 digital correlator with a sampling time range of 25 ns to 40 ms. All solutions were filtered through a Millipore filter with a 0.22 µm pore size and thermostated the desired temperature for at least 30 min. Experiment duration was in the range of 5-10 min, and each experiment was repeated two or more times. The correlation functions were analyzed by the constrained regularized CONTIN method to obtain distribution decay rates (τ). The decay rates gave distribution of the apparent diffusion coefficient

Dapp )

τ q2

(1)

with the scattering vector, q

q)

4πn θ sin λ 2

(2)

with n being the refractive index of water. The apparent hydrodynamic radius, rapp,h, can be calculated via the StokesEinstein equation

rapp,h )

kT 6πηDapp

(3)

where k is the Boltzmann constant and η is the viscosity of water at temperature T. Transmission Electron Microscopy (TEM). Samples for transmission electron microscopy were prepared by evaporation of a 2.5 g dm-3 aqueous micellized copolymer solution in several SDS solutions of different concentrations under air. A drop of copolymer solution was placed on an electron microscope copper grid. After drying, electron micrographs of the sample were recorded with a Phillips CM-12 electron microscope. Isothermal Titration Calorimetry (ITC). Heats of dilution were measured using a VP-ITC titration microcalorimeter from MicroCal Inc., Northampton, MA. In ITC experiments, one measures directly the enthalpy changes associated with processes occurring at constant temperature. Experiments were carried out first titrating monomeric and micellar surfactant into water and then into an aqueous solution containing a known amount of polymer. An injection schedule (number of injections, volume of injection, and time between injections) is set up using interactive software, and all data are stored to a hard disk. We present the results of the ITC experiments in terms of the

enthalpy change per injection (∆Hi) as a function of surfactant concentration. Small aliquots of a stock solution of surfactant at a concentration either below or above the cmc were injected into a known volume of water or polymer solution (ca. 1 cm3) held in the cell of the calorimeter, initially to produce solutions below the surfactant cmc. Repeated additions of the stock solution gave the heat evolved (Q) as a function of copolymer concentration. The solution in the cell was stirred by the syringe at 200 rpm, which ensured rapid mixing but did not cause foaming on the polymer-surfactant solution. All experiments were repeated twice, and the reproducibility was within (3%. Although the Krafft point of SDS is reported at 16 °C,39 fairly concentrated solutions made at room temperature are normally easy to keep in solution for some period of time. To avoid the possibility of SDS precipitation during ITC experiments, solutions of SDS alone and SDS and polymer at the same concentrations used in the ITC experiments were prepared to monitor the time at which precipitation took place. This was determined via turbidity measurements. Thus, the duration of ITC experiments was chosen to carry out the experiments in the intervals at which precipitation was negligible. Density. Measurements were carried out using a commercial density apparatus Anton Paar DSA 5000 densimeter equipped. The temperature control was maintained by Peltier effect with a resolution of 0.01 °C, giving rise to uncertainties in density of ca. ( 1 × 10-6 g cm-3. Solutions were prepared with double distilled water by weighting the components to (0.001 mg, using a balance METTLER AT20. The calibration of the densimeter was made with water. The apparent molar volume of volumes (Vφ,i) of either the copolymer or the surfactant in the binary or ternary solutions were calculated from experimental densities using the relationship

Vφ,i )

Mi 1000(F - Fr) F miFFr

(4)

where Mi and mi are respectively the molar mass and the molality of the solute in the binary solution (water + the other solute), F and Fr the densities of the solution and of a binary solution, respectively. In ternary solutions, the measurements were carried out following different procedures. When the polymer is taken as the solute, its molality is kept constant and small, whereas the concentration of SDS in the binary reference solution is varied. In this case, the variations of the apparent properties of the block copolymer as a function of the surfactant concentration are found. Conversely, when the surfactant is considered as the solute, the apparent properties of SDS are determined as a function of its concentration in binary aqueous copolymer solutions at a fixed concentration of block copolymer. The deduced transfer quantities (∆Yi) from water to binary solutions, relative to a solute taken at the same concentration in both solutions, clearly show the effect of the solute on the structure of the solution

∆Yi ) Yφ,i(binary) - Yφ,i(water)

(5)

When the molality of the solute is sufficiently low, the solutesolute interactions can be neglected. It ensues that the values of transfer quantities are mainly representative of solute-solvent interactions. They characterize the distribution of the solute between the micellar and the aqueous phases. When their variations are plotted against the surfactant concentration, they

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Castro et al.

Figure 1. Selected relaxation time distributions for a 2.5 g dm-3 S17E65 solution with varying SDS concentrations at 20 °C: (a) 0 M; (b) 5.0 × 10-5 M; (c) 2.5 × 10-4 M; (d) 5.0 × 10-3 M; (e) 1.0 × 10-2 M, and (f) 2.5 × 10-2 M.

clearly show the changes in micellar solution, especially when a transition occurs. 3. Results and Discussion Dynamic Light Scattering. Solutions and micelle properties of the pure S17E65 block copolymer were studied previously by light scattering, densimetry, and ultrasound velocity measurements.19,38 Copolymer S17E65 in water at 25 °C has an average aggregation number of 150 monomers and an apparent hydrodynamic radius (radius of the hydrodynamically equivalent hard sphere corresponding to the apparent diffusion coefficient) of 12.7 nm. It is also known that the cmc of EmSn or SnEm copolymers (the order in the formula indicates the sequence of the polymerization) is not temperature dependent19,40 and that these kinds of diblock copolymers have, at room temperature, very low values of the cmc, i.e., 5.4 × 10-6 mol dm-3 for E45S10 as an example, so we can consider that the block copolymer S17E65, which is more hydrophobic than the former, is fully micellized at the concentrations used in the present study. DLS experiments were done for two systems containing constant copolymer concentrations, 2.5 and 10 g dm-3, respectively, and varying the SDS content at 20 and 40 °C. We have focused on following the changes in complex formation when the surfactant sodium dodecyl sulfate interacts with the S17E65 block copolymer. An example of a relaxation time distribution from DLS measurements is shown in Figure 1 corresponding to a copolymer concentration of 2.5 g dm-3 at different SDS concentrations and at 20 °C. Similar plots for the other conditions were obtained (not shown). One mode could be detected for SDS concentrations between 7.5 × 10-6 and 5 × 10-3 M. This mode would correspond to the translational diffusion process of the large S17E65 micelle-surfactant complexes, consisting of a diblock copolymer micelle with associated surfactant. In the SDS concentration range 1.0 × 10-5-2.5 × 10-4 M, the relaxation time shift toward little faster times with a certain broadening

of the distribution, indicating the binding of the surfactant to the copolymer. With further SDS addition the shift backs again to the original relaxation times in the concentration range between 2.5 × 10-4-5.0 × 10-3 M. New increases of surfactant involve first a strong broadening of the relaxation time peak as a consequence of the overlapping of two different relaxation times, which emerge with further SDS in solution: the slow mode in the bimodal distributions is due to diffusion of some remaining mixed micelles in solution which disappears after total breakdown of these. The fast mode is attributed to two different species: On one hand, a small surfactant-rich complex that consists of cooperatively associated surfactant molecules interacting with polymer unimers with a relaxation time which resembles to that of a typical surfactant micelle. These unimers arise from the disintegration of the mixed block copolymer/surfactant micelles to give a small surfactantrich complex. On the other hand, the formation of pure free surfactant micelles would also contribute to this mode. As more free micelles are formed with increases of SDS, the height of the fast mode grows. Similar observations have been made for other surfactant-polymer systems as HEUR/SDS,41 F127/ SDS,42,43 F127/TTAB, and P103/SDS.44 Finally, at highest SDS, the fast mode is only present (not shown). In this region, due to the low scattering intensity of the solutions, the resolution of the correlation function reflects an increase in the peak width of the relaxation time distribution, which is also influenced by the small complexes containing a varying number of copolymer unimers before saturation is reached, as observed for other polymer-surfactant systems.45,46 Figure 2a shows the apparent hydrodynamic radius, rapp,h, of solutions of 2.5 and 10 g dm-3 of copolymer S17E65 as a function of total sodium dodecyl sulfate concentrations at 20 °C. The apparent hydrodynamic radius of S17E65 micelles obtained for the surfactant-free solution was 12.3 nm, which is very similar to that obtained by Crothers et al.19 (12.7 nm). As one can see, for the 2.5 g dm-3 copolymer/SDS system, a very short plateau

Styrene Oxide-Ethylene Oxide Diblock System

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Figure 3. TEM image demonstrating the evolution of the SDS/S17E65 block copolymer micelle size in the presence of (from left to right) (a) 1.0 × 10-5 M; (b) 2.5 × 10-4 M; (c) 5.0 × 10-3 M and (d) 5.0 × 10-2 M of SDS. Block copolymer concentration 2.5 g dm-3.

Figure 2. (a) Apparent hydrodynamic radii, rh,app, against surfactant concentration of (b) 2.5 and (0) 10 g dm-3 of S17E65/surfactant mixtures at 20 °C; and (b) at 40 °C.

region for rapp,h was observable at very low SDS concentrations, before the starting point of a transformation for which the value of rapp,h decreases suddenly from about ∼12 nm upon addition of very small amounts of SDS up to ∼4 nm at 2.5 × 10-4 M. This decrease might be related to the solubilization of SDS molecules in the interior of the copolymer micellar core, establishing a repulsive force between their polar headgroups which would facilitate water penetration and lead to a less dense packing of the micelle. This would result in a decrease in the aggregation number which, in turn, is reflected in a decrease in mixed micelle size. The inclusion of surfactant molecules in the micelle core also seems to explain similar hydrodynamic radius decreases for other block copolymer/surfactant systems, as those formed by the structurally related copolymer PS-bPEO with SDS and CTAC.25,36,47 Further addition of SDS leads to an increase in rapp,h up to a maximum of ∼10 nm at 5 × 10-3 M of SDS. The origin of this increase is not still clear. One hypothesis might be the incorporation of more surfactant monomers and some free copolymer unimers, expelled from the mixed micelle as a result of their size reduction, due to the screening of the repulsive interactions between surfactant headgroups as a consequence of the increasing amount of counterions surrounding the micellar corona, as occurred in other block copolymer surfactant systems.25,26,41,47,48 Finally, at higher surfactant concentrations, the mixed micelles start to disintegrate again as indicated the decrease up to ∼1 nm of the hydrodynamic radii. This fact leads us to think that the mixed surfactant/ copolymer micelles are destroyed to form small surfactantcopolymer complexes in solution with a size close to that of a surfactant free micelle. In this surfactant concentration region, there is also an increase in the number of free surfactant micelles, both species contributing to the fast mode of the relaxation time distributions as previously commented. Increasing the copolymer concentration up to 10 g dm-3 in the SDS/S17E65 system does not result in a significant change in the trend followed by the apparent hydrodynamic radius (see

Figure 2a). As expected, the different zones defined in its behavior are shifted to higher surfactant concentrations due to the necessity of more surfactant to distort the increased amount of block copolymer micelles in solution. Moreover, an increase in temperature from 20 to 40 °C for the SDS/S17E65 system results in a similar behavior except for a little shift to lower surfactant concentration of the maximum in the DLS curve, with a decrease in the size of the mixed micelle up to ca. ∼8 nm at 2.5 × 10-3 M of SDS (see Figure 2b). From this concentration, the surfactant/copolymer micelle begins to disintegrate more slowly than at 20 °C up to 5 × 10-2 M of SDS, concentration which coincides with that of 20 °C and for which the mixed micelles are completely broken and surfactant-unimer complexes and free surfactant micelles are present, as previously commented. Transmission Electron Microscopy. TEM images of the 2.5 g dm-3 S17E65/SDS system at 20 °C were taken to confirm the trends in the micelle size obtained by DLS. These are shown in Figure 3. This figure shows that results obtained by TEM are in fair agreement with those by DLS, reproducing the same trend. Figure 3a shows the TEM image for a SDS concentration of 1.0 × 10-5 M, for which the micelle size did not change from that of a copolymer micelle in the absence of surfactant. This clearly indicates that the interactions between the copolymer micelle and the surfactant is not strong enough to disrupt the aggregate at this stage, as confirmed by ITC (see next section). When the surfactant concentration up to 2.5 × 10-4 M is increased, the disruption of the mixed micelle occurs as demonstrated in Figure 3b, where the diameter of the particles have been strongly reduced almost until the maximum sensitivity of the technique, so the particles are not completely well-defined in the figure, being identified as the granules displayed in the picture. With further increases of SDS, the size of the micelle increases, as shown in Figure 3c for a SDS concentration of 5.0 × 10-3 M, although an increase in the polidispersivity of the system is noted: coexistence of some larger mixed micelles with radius of ca. ∼12 nm with other with smaller size around 5-6 nm is seen in this figure. The existence of these two different types of aggregates then may cause the increase in the apparent hydrodynamic radii as commented above, so meanwhile DLS gives an average size estimation, which is biased toward the larger-size end of the population distribution,

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Figure 4. Enthalpy change as a function of surfactant concentration due to the titration of (a) monomeric (2 mM) SDS at 20 °C and (b) micellized (300 mM) SDS at different temperatures. In Figure 4a, open symbols (O) denote titration in water and closed symbols (9) into the polymer solution. In Figure 4b, measured temperatures are as follows: (9) 15 °C; (O) 18 °C; (2) 20 °C; (1) 30 °C and ()) 40 °C.

by TEM we image single particles. Finally with further SDS addition (0.05 M), the mixed micelles completely disintegrate as shown in Figure 3d, in which detection of any particle, either surfactant/copolymer unimers complexes or pure SDS micelles, is not possible because of their size exceeds the instrument resolution capability. Isothermal Titration Calorimetry. Plots of the enthalpy profile for the titration of different SDS stock solutions into aqueous solutions of S17E65 of 2.5 g dm-3, where the copolymer is completely micellized,19 as a function of surfactant concentration in the temperature range of 15-40 °C in the presence and in the absence of the copolymer are shown in Figure 4. Figure 4a shows the titration of 2 mM of SDS onto the block copolymer at 20 °C. Deviation from the copolymer titration curve in water indicates that the interaction between the surfactant and the polymer occurs even though at the lowest SDS concentration measured, 7.0 × 10-6 M. The continuous increase of ∆Hi in this measured concentration range (7.0 × 10-6-4.0 × 10-4 M) indicates the interaction between the surfactant molecules and block copolymer micelles, although the change in the DLS profile at 2.50 × 10-4M of SDS is not noted by this technique. To cover the whole surfactant concentration range seen by DLS, a titration of 0.3 M of micellized SDS onto the copolymer was also performed at different temperatures. A first general observation of these titrations shows that the titration enthalpy curves in the presence (Figure 4b) and in the absence (not shown for major clarity) of the block copolymer change their shape with temperature. It can be observed that an important contribution to the change of titration curve shape is the variation of the demicellization enthalpy of SDS from exothermic to endothermic with increasing temperature, being this effect stronger at 30 and 40 °C.49

Castro et al. Between 15 and 20 °C, the enthalpy curves corresponding to the titration of micellized SDS show an endothermic increase leading to a first endothermic maximum at a concentration ranging from 6.2 × 10-3 to 8.5 × 10-3 M. This endothermic increase and the subsequent maximum has been attributed to the dehydration of copolymer micelles, conformational changes in the copolymer and interactions between the surfactant and copolymer micelles, once the difference between the ternary and binary solutions is considered, for which the dilution effect and dissociation of surfactant micelles is unimportant. Therefore, interactions between surfactant and copolymer micelles would lead to the formation of mixed surfactant-copolymer micelles and, thus, to a partial disruption of block copolymer-only micelles.50 Due to their higher hydrophobicity, the core of these mixed micelles should be formed by oxyphenilethylene chains which might be stabilized by the solubilization of dehydrated PEO segments and surfactant end groups replacing water molecules in the core-corona interface as occurred for oxyethyelene-oxypropylene block copolymers.43,51 Furthermore, it was also shown that the maximum in the enthalpy curve can represent the surfactant concentration at which free micelles of the surfactant begin to form in solution.52 The SDS concentration at which the maximum is observed in DLS at 20 °C, around 5.0 × 10-3 M, is lower than that of this obtained by ITC, which takes place at 8.5 × 10-3 M of SDS. This difference may arise from the predominance of the exothermic heat evolved due to the interaction between the surfactant and the copolymer micelles over the beginning of the mixed micelle distortion which leads to copolymer rehydration, which is endothermic in nature. A slight increase of SDS concentration above the first endothermic maximum leads to a shallow minimum followed by a second smooth endothermic maximum at ca. 1.25 × 10-2 M. This second endothermic maximum has been related to the interactions between SDS molecules and copolymer unimers resulting from the initial disruption of the mixed surfactant/ block copolymer micelles. This smooth maximum partially masks the progressive disruption of the mixed micelles in this surfactant concentration range.50,53,54 As more SDS is added, the disruption of the mixed surfactant/ block copolymer micelles completely takes place due to an increase in the amount of surfactant in the mixed micelle which leads to a decrease of the binding rate due to electrostatic repulsions of SDS headgroups, resulting in a decrease of ∆Hi up to an exothermic minimum. This minimum, thus, reflects the complete breakdown of the mixed micelles and the rehydration of the S and PEO chains of the copolymer previously expelled from the micelle to the water. Moreover, it has been established41 that the driving force for the re-hydration process of the PEO segments is ion-dipole association between SDS headgroups and EO segments. Thus, after rehydration these PEO segments wrap around the charged surface of SDS micelles, decreasing the contact between the “exposed” hydrophobic segments of SDS micelles and the water phase, giving rise possibly to rich surfactant/copolymer unimer complexes. With further addition of SDS, rehydration slows down after the minimum of ∆Hi. A third smaller endothermic process reflected in ∆Hi occurs at higher SDS concentrations, which is associated with the tail end of the binding process involving the formation of SDS aggregates bound to single copolymer units, as seen in the DLS section, in which a relaxation mode corresponding to sizes very close to SDS micelles appears. Other contribution to this maximum is the formation of free SDS micelles in solution. Once the saturation of all copolymer

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Figure 5. Enthalpy change as a function of surfactant concentration due to the titration of micellar SDS (300 mM) in (9) 2.5, (O) 5.0, and (2) 10 g dm-3 of copolymer S17E65 at 20 °C.

unimers is reached, which coincides with the merging point of the curves of SDS titration in the presence and absence of copolymer, both titration curves superimpose indicating a dilution of micelles into a solution containing free micelles previously formed. For the titration curves of SDS onto the copolymer at 30 and 40 °C the thermodynamic profiles reflected in the ∆Hi behavior are masked by the demicellization of the SDS micelles, which becomes the predominant process. Thus, the first endothermic peak is not totally defined (see Figure 4b). On the other hand, there is no evidence of the second endothermic peak. This can be masked by the more energetic demicellization process or can be absent as a consequence of the non achievement of the critical surfactant monomer concentration needed for binding to monomeric polymer claimed in previous works.43,53 This critical concentration would not be reached due to a more compact conformation of the copolymer micelle as the temperature raises, presumably because the poorer hydration of the PEO outweighs any weak electrostatic repulsion at higher temperatures. However, the exothermic minimum and the third endothermic maximum are displayed, indicating that destruction of the mixed micelles and subsequent formation of free surfactant micelles and rich surfactant-unimer complexes take place, which confirms the results obtained by DLS at 40 °C previously shown. On the other hand, the increase and shift at higher temperatures of the first endothermic maximum with temperature in the ITC plots can be related to an increase in the number of SDS molecules interacting with the tightly packed block copolymer micelles, which would agree with the shift of the maximum observed in DLS with increasing temperature, as previously commented. The effect of copolymer concentration on the heat exchange upon interaction with titrated micellar SDS (0.3 M) was also evaluated. It is evident from Figure 5 that these plots are copolymer concentration dependent. As the copolymer concentration increases, the position of the first and third endothermic maximums and the exothermic minimum are shifted to larger surfactant concentrations. Moreover, the height of the first endothermic maximum increases whereas that of the second one diminishes with the copolymer concentration. The increase in the height and area of the first endothermic maximum with polymer concentration is related to the proportional increase in the interactions between the mixed surfactant/block copolymer micelles and the subsequent added surfactant molecules. This

Figure 6. (a) Transfer volumes of SDS from water to aqueous copolymer solution and (b) transfer volumes of block copolymer S17E65 from water to SDS solutions, as a function of SDS molality at (O) 10, (9) 20, (+) 30, and (2) 40 °C.

gives rise to the displacement of the titration curves because more SDS molecules are necessary to disrupt the mixed micelles due to their increasing amount, then shifting the exothermic minimum as well; in fact, at a concentration of 1% of copolymer, the minimum is only partially defined. When the titrated surfactant concentration is increased up to 1 M, the minimum is already fully complete (plot not shown). Once the micelles are disrupted, the total number of block copolymer unimers in the titration cell is larger, and thus, a greater amount of SDS is needed to saturate the surfactant rich/copolymer unimer complexes, which explains the absence of a merging point between the titration curves in the presence and in the absence of 5 and 10 g dm-3 of copolymer. Transfer Volumes. Apparent molal volumes of SDS, Vφ,S, in the presence of 2.5 g dm-3 of S17E65 vs SDS molality, m, have been calculated in the temperature range 10-40 °C. For sake of consistency, the apparent molal volumes of SDS in pure water were calculated from separate runs at the same experimental conditions. In Figure 6a, transfer volumes of SDS, ∆VS, i.e., the difference between the values of Vφ,S in aqueous block copolymer solution and in water, are plotted against m at 10 and 40 °C. The range of analyzed SDS concentrations is the same than that discussed when titrating micellar SDS onto the copolymer by ITC, which also corresponds to the final decrease of hydrodynamic radii in the DLS curve. Lower SDS concentrations were not measured because of the intrinsic accuracy of measurements. An initial increase in ∆VS is observed at concentrations lower than 0.01 mol kg-1 of SDS where a maximum is located. A maximum in transfer volumes is characteristic of the displacements associated with the different equilibrium present near a structural transition. Formation of surfactant micelles at this stage seems the most possible phenomenon involving the presence of this maximum, which coincides fairly well with the first endothermic maximum detected by ITC, being also

5598 J. Phys. Chem. B, Vol. 109, No. 12, 2005 attributed to the formation of surfactant micelles in solution as previously commented. Moreover, a positive contribution to the surfactant transfer volume in the presence of the copolymer arising from the attractive interactions between SDS molecules and copolymer unimers resulting from the initial deaggregation of the mixed surfactant/copolymer micelles can be also considered. Thereafter, upon further addition of SDS up to a concentration of 0.035 mol kg-1 of SDS, the following decrease of ∆VS is assumed to result from the predominance of the stronger hydrophobic interactions between the oxyphenylethylene chains and SDS which involves the removal of bound water molecules from both the micellar surface and the copolymer chain. The interaction between the hydrophobic chains of the block copolymer are weakened to a large extent while the SDS binding leads to the breakdown of the mixed micelles, giving rise to smaller mixed aggregates of varying composition and with increasing number of SDS molecules until only block copolymer monomers are remaining. Thus, the binding of SDS to copolymer micelles leads to lower values of the apparent molar volumes of SDS, i.e., to negative transfer volumes, since the neighborhood of the hydrocarbon chain of SDS is comparatively more hydrophilic in copolymer micelles than in SDS aggregates. A similar behavior has been found for the interaction of SDS and propylene glycol and triblock pluronics.55,56 Finally, for SDS concentrations larger than 0.035 mol kg-1, the apparent molar properties of the surfactant are nearly equal to those in pure water, or else the transfer quantity tends to zero when the binding process is brought to close. The trends in the copolymer transfer volume, ∆VP, are shown in Figure 6b. At SDS concentration up to ca. 5 × 10-3 mol kg-1, ∆VP remains almost constant and positive confirming that the interactions between the S chains of the block copolymer and the SDS in the mixed micelle are stronger than the S-S interactions. With further addition of surfactant, an increase in the copolymer transfer volume occurs up to a maximum at a SDS concentration of ca. 0.007-0.009 mol kg-1 depending on temperature. This increase and the subsequent maximum we think reveals, as in the case of ∆VS, a contribution due to the strong interaction between SDS molecules and copolymer unimers resulting from the initial mixed micelle distortion. As the surfactant concentration increases, ∆VP starts to decrease until a minimum around 0.035 mol kg-1. This decrease is associated with the expulsion of the hydrophobic copolymer chains from the hydrophobic environment inside the mixed micelle core toward the aqueous phase where their hydrophobic hydration is restored.56 Consequently, this contribution appears to be related to the properties of rehydration of copolymer monomers and dependent on the equilibrium between monomers and micelles. When SDS is added beyond 0.035 mol kg-1 of SDS, only copolymer monomers remain present. This concentration is in good agreement with those obtained by DLS and ITC, at which small SDS aggregates are bound on the copolymer giving rise to surfactant rich/copolymer unimer complexes as commented in the DLS section. Therefore, the trend in ∆VP in transfer properties of the block copolymer has been interpreted as the result of simultaneous contributions of the two modes of binding of SDS and unimers and micelles of block copolymers. On the other hand, the effect of temperature on both ∆VS and ∆VP is not very important, maintaining both quantities the same profile at all temperatures. However, some little differences can be observed, mainly in the ∆VP plot: The maximum is shallower and slightly shifts to lower SDS concentrations, whereas the minimum is a little more pronounced as the

Castro et al. temperature increases. Both effects may arise from an increase in the number of free copolymer unimers expelled from mixed micelles as reflected the decrease of the hydrodynamic radius in the same surfactant concentration region (see Figure 2b). Conclusions Presence of surfactant sodium dodecyl sulfate in aqueous solutions of micellized diblock copolymer S17E65 involves important changes in its physicochemical parameters as a consequence of the strong interactions between the copolymer and the surfactant. This results in an abrupt change of the hydrodynamic radius of the block copolymer micelles at low SDS concentrations due to the association of SDS molecules with the block copolymer micelles to give a copolymer-rich/ surfactant complex (or mixed micelle), which becomes more charged as more surfactant is added. This binding can be considered as the solubilization of the surfactant hydrocarbon chains into the styrene oxide core of the block copolymer micelles.With further addition of surfactant, the size of the mixed micelle increases. The origin of this increase remains unclear, although a possible explanation might be an increase in its aggregation number due to the binding of more surfactant molecules and some copolymer unimers resulting from the previous copolymer micelle size reduction. At higher SDS concentrations, DLS data changes from monomodal to bimodal distributions and the hydrodynamic radii of the copolymer rich/ surfactant mixed micelles abruptly decreases. We conclude that at this concentration range the mixed micelles disintegrate to give small surfactant-rich/copolymer complexes and free surfactant micelles which become the predominant processes. TEM images corroborate the change in size of the mixed SDS/block copolymer micelles as the SDS concentration obtained by DLS. The change in temperature and in copolymer concentration affects the point where the disintegration of the mixed micelle begins as seen by DLS. ITC data also confirms this point. Moreover, this technique allows us to demonstrate that interactions between the copolymer and the surfactant are present even at SDS concentrations so low as 7.0 × 10-6 M. Titrations of micellar SDS onto the polymer seem to confirm the profiles of the DLS curves: an endothermic maximum is present at SDS concentrations where the increase in mixed micelle size is maximum, and an exothermic minimum exist at a SDS concentration where the complete breakdown of the mixed micelles takes place. Besides that, a special feature in the ITC profiles can be also seen between 15 and 20 °C: the existence of a second shallow endothermic maximum which we think is related to the interactions between the surfactant molecules and the copolymer monomers resulting from the initial destruction of the mixed surfactant/copolymer micelles. Finally, the initial increase in surfactant transfer volumes seems to confirm this hypothesis. The subsequent decrease of ∆Vs with further surfactant addition is related to the disintegration of the mixed surfactant-copolymer micelles, which agrees with the results obtained by the other techniques. Thus, it seems that the presence of surfactant can be a tool to control the size and properties of block copolymer aggregates in solution. Acknowledgment. The project was supported by the Ministerio de Educacio´n y Cultura through Project MAT2004-02756 and Xunta de Galicia. P.T. and E.C. thank Ministerio de Eduacio´n y Cultura for his Ramo´n y Cajal position and his PhD grant, respectively. We thank Professors Attwood and Booth for the generous gift of block copolymer. References and Notes (1) Goddard, E. D. Colloid Surf. 1986, 19, 255.

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