Size Control of Styrene OxideâEthylene Oxide Diblock Copolymer

Biomacromolecules 2005, 6, 1438-1447

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Size Control of Styrene Oxide-Ethylene Oxide Diblock Copolymer Aggregates with Classical Surfactants: DLS, TEM, and ITC Study Emilio Castro, Pablo Taboada,* Silvia Barbosa, and Vıćtor Mosquera Grupo de Sistemas Complejos, Laboratorio de Fı´sica de Coloides y Polı´meros, Departamento de Fı´sica de la Materia Condensada, Facultad de Fı´sica, Universidad de Santiago de Compostela, Spain Received November 19, 2004; Revised Manuscript Received January 24, 2005

The interactions between the diblock copolymer S15E63 and the surfactants sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDeS), and sodium octyl sulfate (SOS) have been investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and isothermal titration calorimetry (ITC). The surfactants with the same headgroup differentiate in their chain length. At 20 °C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6 nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO or E) corona. The different copolymer/ surfactant systems have been studied at a constant copolymer concentration of 2.5 g dm-3 and in a vast range of surfactant concentrations, from 7.5 × 10-6 up to 0.75 M. When SDS and SDeS are added to the block copolymer solution, different regions are observed in the DLS data: at low surfactant concentrations (c < 1.0 × 10-4 M), single surfactant molecules associate with the copolymer micelle, probably the former being solubilized in the micelle core, leading to a certain disruption of the mixed micelle due to repulsive electrostatic interactions between surfactant headgroups followed by a stabilization of the mixed micelle. At higher concentrations (1.0 × 10-4 < c < 0.1 M), two types of copolymer-surfactant complexes coexist: one large copolymer-rich/surfactant complex and one small complex consisting of one or a few copolymer chains and rich in surfactants. At higher SDS and SDeS concentrations, complete disintegration of mixed micelles takes place. In contrast, SOS-S15E63 interactions are less important up to surfactant concentrations of 0.05 M due to its higher hydrophilicity, reducing the hydrophobic interactions between surfactant alkyl chains and copolymer micelles. At concentration larger than the critical aggregation concentration (cac) of the system, 0.05 M, disruption of copolymer micelles occurs. These regions have been confirmed by transmission electron microscopy. On the other hand, the titration calorimetric data for SDS and SDeS present an endothermic increase indicating the formation of mixed copolymer-rich-surfactant micelles. From that point, important differences in the ITC plot for both surfactants are present. However, the ITC curve obtained after titration of a SOS solution in the copolymer solution is quite similar to that of its titration in water. Introduction 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.1-15 The combination of hydrophilic and hydrophobic blocks confers to these block copolymers interesting and useful surface active and micellization properties in dilute aqueous solution and gelation ones in concentrated solutions. Variation of the hydrophobic block, the block length, and the block architecture allows close control of these properties. Most parts of the work done with this type of block copolymers have dealt with the selfassociation and gelation behavior of block copolymers whose hydrophobic block is formed by units of oxypropylene (-OCH2CH(CH3), denoted as PPO or P) or 1,2 oxybutylene * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 0034981563100 ext.: 14042. Fax: 0034981520676.

(-OCH2CH(C2H5), denoted as BO or B). It has been shown that single molecular species dominate at low temperatures and concentrations.1,2 Above a critical concentration or critical temperature, copolymer molecules form aggregates because of the limited and temperature-dependent solubility of the hydrophobic block.9,10 However, less attention has been paid to oxyalkylene block copolymers containing styrene oxide as the hydrophobic block (-OCH2CH(C6H5), denoted as SO or S), mainly due to its recent release onto the market by Goldsmischdt AG, Essen. At this respect, it has been demonstrated8 that the hydrophobicity of the styrene oxide block based on the molar critical micelle concentration (cmc) if compared with those of propylene and butylene oxide blocks is in a ratio of P/B/S ) 1:6:12.8 Moreover, it has been shown that the lower glass transition of poly(styrene oxide) (Tg ≈ 40 °C) as compared to poly(styrene) (Tg ≈ 100 °C) means that effects caused by immobility of blocks in the micelle core are less important in micellar solutions of EmSn copolymers, where E denotes oxyethylene and S

10.1021/bm049262+ CCC: $30.25 © 2005 American Chemical Society Published on Web 03/03/2005

Styrene Oxide-Ethylene Oxide Copolymer Aggregates Table 1. Molecular Characteristics of the Copolymera

S15E63

Mn/g mol-1 (NMR)

wt % S (NMR)

Mw/Mn (GPC)

Mw/g mol-1

4600

39.7

1.04

4780

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

oxyphenylethylene, respectively, and the subscripts the length of each block, providing sufficient mobility to readily solubilize aromatic drugs.16 On the other hand, studies of the interaction between nonionic water-soluble copolymers and ionic surfactants are of significant interest because polymer-surfactant mixtures can be found in many formulations such as skin care and cosmetic products and pharmaceutical compounds.17-21 Using copolymers is more advantageous than polymers as they undergo aggregation processes. Surfactant-copolymer mixtures may associate into different nanostructures, which can be designed by simply changing the composition of the species in the system or the medium conditions. Currently, as a result of their capability to form these nanostructures, the systems are also attracting great interest as drug delivery systems.22 Thus, a full knowledge of the conditions governing the formation of these structures to control their formation, size, and properties to improve their ability for this application seems to be a key point. The study of surfactant-copolymer interactions and regulation of their aggregates have been mainly focused on mixtures formed by PEO-PPO-PEO and classical anionic, cationic, and nonionic surfactants.23-29 It was demonstrated that the mechanism of interaction between copolymer and surfactant is system-specific and strongly dependent on temperature, and the binding of surfactants occurs above a critical surfactant concentration, which is lower than the cmc in aqueous surfactant solutions. To contribute to a better understanding of the forces involved in the surfactant/block copolymer complex formation and size, in the present work the effect of the surfactant chain length in controlling the complex size is studied. Thus, the interactions of sodium dodecyl, sodium decyl, and sodium octyl sulfates with diblock copolymer S15E63 are analyzed by light scattering, transmission electron microscopy, and isothermal titration calorimetry. Light scattering and transmission electron microscopy will allow us to follow the change in structure and size of the copolymer/surfactant mixtures, whereas isothermal titration calorimetry will be a sensitive technique to characterize the energetics involved in surfactant/block copolymer interactions. All techniques will provide us a full picture to obtain proper size control of the formed surfactant/copolymer aggregates. Experimental Procedures Materials. Sodium dodecyl (SDS), sodium decyl (SDeS), and sodium octyl (SOS) sulfates were 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 S15E63 was described in detail by Crothers et al.15 Table 1 shows the molecular characteristics of the copolymer. Solutions ob-

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served during the tube inversion tests for the copolymer remained clear to the eye throughout the temperature of 20 °C investigated in the present work. To detect the presence of impurities in the surfactant, surface tension measurements were previously performed at 20 and 40 °C (figure not shown). No minimum was detected at 20 °C, and just a very slight curvature was seen at 40 °C, without affecting the cmc values, which were 0.132, 0.033, and 0.008 M at 20 °C and 0.136, 0.034, and 0.0086 M at 40 °C for octyl, decyl, and dodecyl sulfate, in good agreement with literature values. Dynamic Light Scattering (DLS). Dynamic light scattering measurements were made at 20.0 ( 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 an 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 at 20 °C for at least 0.5 h. 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 (Γ). The decay rates gave distribution of the apparent diffusion coefficient Dapp )

Γ q2

(1)

with the scattering vector, q q)

4πn θ sin λ 2

(2)

n being the refractive index of the solvent. 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 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 being dried, electron micrographs of the sample were obtained 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 surfactants 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

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Castro et al.

Figure 1. Selected relaxation time distributions for a 2.5 g dm-3 S15E63 solution with varying SDS concentrations at 20 °C: (a) 5 × 10-6 M; (b) 2.5 × 10-5 M; (c) 7.5 × 10-5 M; (d) 2.5 × 10-4 M; (e) 7.5 × 10-4 M; (f) 5 × 10-3 M; (g) 1.0 × 10-2 M; and (h) 5.0 × 10 -2 M.

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 a syringe at 200 rpm, which ensured rapid mixing but did not cause foaming in the polymer-surfactant solution. All experiments were repeated twice, and the reproducibility was within (3%. Results and Discussion Dynamic Light Scattering. Surfactants remove nonionic polymers from surfaces through adsorption to the surface and hydrophobic binding to the polymer, and often the interaction leads to the formation of complexes that can disaggregate later.30 Thus, the purpose of the present work is to investigate the influence of the surfactant hydrophobicity on the copolymer surfactant complexes and to determine if this characteristic property of surfactants can be used as a mechanism of size control of the complexes with the aim of application as drug delivery systems. Thus, we focused on following the changes in complex formation when surfactants sodium dodecyl, sodium decyl, and sodium octyl sulfates interact with S15E63 block copolymer micelles using light scattering and isothermal titration calorimetry techniques.

Therefore, it was necessary to choose a concentration and a temperature where the copolymer systems demonstrate an uncomplicated relaxation time distribution with micelles as the single scattering species. At this respect, copolymer S15E63 in water dilution at 25 °C has an average aggregation number of 140 monomers and an apparent hydrodynamic radius (radius of the hydrodynamically equivalent hard sphere corresponding to the apparent diffusion coefficient) of 11.8 nm.15 It is also known that the cmc of the EmSn or SnEm copolymers (the order in the formula indicates the sequence of polymerization) is not temperature dependent15,29 and that this kind of diblock copolymers have, at room temperature, very low values of the cmc (i.e., 5.4 × 10-6 M for E45S10 as an example), so we can consider that the block copolymer S15E63, which is more hydrophobic than the former, is fully micellized at the concentration used in the present study. Examples of relaxation time distributions from DLS measurements on the S15E63/SDS, S15E63/SDeS, and the S15E63/SOS systems at a block copolymer concentration of 2.5 g dm-3 and surfactant concentrations ranging between surfactant concentrations of 7.75 × 10-6 and 0.75 M are depicted in Figures 1-3. All systems display monomodal distributions, that is, single-exponential g(2)(t) functions at concentrations up to 10 mM for SDS and SDeS and 25 mM for SOS. For SDS and SDeS, this mode would correspond to the translational diffusion process of the large S15E63 micelle-surfactant complexes, consisting of a diblock copolymer micelle with associated surfactant monomers, as will be further discussed below. As noticed in Figures 1 and 2, the relaxation time for both surfactants shifts toward slightly faster times with a certain

Styrene Oxide-Ethylene Oxide Copolymer Aggregates


Figure 2. Selected relaxation time distributions for a 2.5 g dm-3 S15E63 solution with varying SDeS concentrations at 20 °C: (a) 2.5 × 10-5 M; (b) 2.5 × 10-4 M; (c) 5.0 × 10-3 M; (d) 1.0 × 10-2 M; (e) 2.5 × 10-2 M; and (f) 7.5 × 10-2 M.

Figure 3. Selected relaxation time distributions for a 2.5 g dm-3 S15E63 solution with varying SOS concentrations at 20 °C: (a) 2.5 × 10-5 M; (b) 2.5 × 10-4 M; (c) 5.0 × 10-3 M; (d) 2.5 × 10-2 M; (e) 7.5 × 10-2 M; and (f) 2.5 × 10-1 M.

broading of the distribution for SDS and SDeS in the concentration range of 0.01-0.25 mM indicating the binding of surfactant to copolymer. The peaks return again to the original relaxation times in the concentration range between 0.5 and 5 mM. For SOS, the relaxation times remain constant at both concentration regions. This points out that interactions

between block copolymer micelles with monomer SOS, if any, are small, as will be discussed further next. Thus, it might be assumed that for SOS the relaxation mode would correspond to the diffusion of block copolymer-only micelles. Between 10 and 100 mM surfactant, the relaxation time distributions for the S15E63/SDS and S15E63/SDeS systems

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change to a bimodal distribution. The slow mode in the bimodal distributions is due to diffusion of large copolymer micelle-surfactant complexes as demonstrated by the linear dependence of the decay time as a function of q2 (figure not shown), which corresponds to the single mode found at lower surfactant concentrations. The broadening of this mode can indicate the formation of a very low number of clustered surfactant-copolymer micelles that, however, contribute highly to the scattered light, as occurred for other polymersurfactant systems.32 The fast mode, which also shows a linear q2 dependence, has a relaxation time very close to that observed for pure surfactant micelle solutions and is, therefore, attributed to the diffusion of a small surfactantrich complex that consists of cooperatively associated surfactant molecules interacting with polymer unimers. These unimers arise from the disintegration of the larger S15E63surfactant complexes to give small surfactant-rich complexes. The formation of free surfactant micelles in this concentration range also contributes to this mode. These free surfactant micelles start forming at concentrations close to 10 and 50 mM for SDS and SDeS. Both phenomena lead to the relative amplitude for the fast mode to increase in favor of the amplitude of the slow mode with increasing surfactant concentrations. Thus, two types of complexes coexist. The large copolymer-rich complex resembles the corresponding pure copolymer micelle, while the small complex is of the size of surfactant micelles, as was commented before. Similar observations have been made for other polymer-surfactant systems such as HEUR (hydrophobically modified ethoxylated urethane)/SDS,32 F127 (Pluronic E96P69E96)/SDS,24,33 F127/TTAB (tetradecyltrimethylammonium bromide), and P103 (Pluronic E17P60E17)/SDS.34 Finally, at SDS and SDeS concentrations higher than ∼100 mM and up to 750 mM, the fast mode is only present. In this region, due to the low scattering intensity of the solutions, the worse definition of the correlation functions reflects an increase in the peak widths of the relaxation time distributions. Furthermore, the width of the fast mode may also be because the small complexes contain a varying number of copolymer unimers before saturation is reached as was observed for other polymer-surfactant systems.23 SOS shows a similar behavior than those of SDS and SDeS in the concentration range of 25-750 mM. The only difference in respect to the latter surfactants is that complete destruction of the copolymer micelles is not achieved as confirmed by the appearance of the slow mode at the highest SOS concentration. Figure 4 shows the apparent hydrodynamic radius, rapp,h, of a solution of 2.5 g dm-3 of copolymer S15E63 as a function of total sodium dodecyl, sodium decyl, and sodium octyl sulfate concentrations at a temperature of 20 °C. The apparent hydrodynamic radius of S15E63 micelles obtained for the surfactant-free solution was 11.6 nm, which is very similar to that obtained by Crothers et al.15 (11.8 nm). The values of rapp,h reported in the present work are apparent values because there is no extrapolation of the diffusion coefficient to infinite dilution of copolymer; hence, they will be affected by size changes and electrostatic interactions due to surfactant surface charge.

Castro et al.

Figure 4. Apparent hydrodynamic radii, rh,app, against surfactant concentration, m, of 2.5 g dm-3 S15E65/surfactant mixtures at 20 °C. (a) SDS; (b) SdeS; and (c) SOS. Circles describe rh,app of copolymer rich-surfactant complexes, whereas squares describe rh,app for surfactant rich-copolymer complexes.

For the SDS/S15E63 and SDeS/S15E63 systems, four different concentration regimes can be identified in the semilog plot of rapp,h versus surfactant concentration. In contrast for the SOS/S15E63 system, just two different regions are present matching the most common behavior of scattered light by surfactant/Pluronic block copolymer mixtures.33,34 At very low surfactant concentrations, 0 e c e 1.0 × 10-5 M, the apparent hydrodynamic radii of the block copolymer micelles remain invariable for all systems. Thus, interactions of surfactant monomers with the copolymer micelles, if any, do not cause any disruption of their structure. At this respect, it will be shown in the ITC section that interactions of SDS monomers with S15E63 micelles, for example, occurred at concentrations lower than 6 × 10-6 M without a change in the apparent hydrodynamic radii of block copolymer micelles, confirming the previous statement. With further addition of SDS and SDeS, 1.0 × 10-5 e c e 1.0 × 10-4 M, the increasing amount of the surfactant in solution leads to a decrease of the copolymer hydrodynamic radius up to a minimum. This decrease is more pronounced in the case of the SDS/ S15E63 system. For SOS, the hydrodynamic radii remain almost constant at the same concentration range. The reduction of the hydrodynamic radius for SDS and SDeS might be related to the increase in the amount of negatively charged -SO4- headgroups in the copolymer micelle due to the necessity of SDS and SDeS molecules to avoid direct contact with water due to their hydrophobicity. This explanation would be also supported by the fact that the decrease of hydrodynamic radii for SDS is larger than for SDeS (see Figure 4), while no effect is seen for SOS. At


this respect, from the aggregation number and the hydrodynamic radius values of the block copolymer micelle indicated previously, it can be easily calculated that the dry micellar density is 0.16 g dm-3, which would involve a very open structure that should be full of solvent. However, it has been demonstrated15,35 that the hydrophobic block of the EmSn copolymer is significantly coiled even in the unimer state, so it is expected that S blocks would be tightly coiled in the micelle core, with the oxyethylene blocks loosely packed around it, giving rise to the low value of the micellar density. Thus, a larger surfactant chain length involves larger surfactant hydrophobicity, so surfactant molecules that avoid direct contact with water molecules would penetrate in the copolymer micelle, reaching the micelle core, which provides a more favorable environment for the long surfactant alkyl chains. This phenomenon would be related to the displacement to the left of the relaxation time distribution observed for SDS and SDeS in the mentioned concentration range. The inclusion of surfactant molecules in the micelle core has been proposed as a possible interaction mechanism for other block copolymer/surfactant systems, as the PS-b-PEO/ SDS and PS-b-PEO/CTAC (cetyltrimethylammoniun chloride) systems17,36,37 or PEG (poly(ethylene glycol))/sodium perfluoroalkanoate38 and L64 (Pluronic E13P30E13)/sodium alkanoate39 mixtures. Repulsive interactions between surfactant headgroups in the micelle core would expand it allowing water penetration and, thus, giving rise to a less dense packing of the micelle, resulting in a decrease in the aggregation number that, in turn, is reflected in a decrease in the mixed micelle size. This also would cause a displacement of some styrene oxide chains from the micellar core, producing small amounts of unimers and block copolymer micelles with some part of the hydrophobic block exposed to the water phase, as has occurred for other polymer/ surfactant systems as HEUR/SDS.32 Meanwhile, SOS, which is less hydrophobic due to its shorter chain length, would remain in the micelle corona and would not penetrate into the micelle core, so electrostatic repulsions of headgroups would not affect the compact packing of the micellar core and, therefore, do not involve appreciable changes in micellar size. Between 1.0 × 10-4 e c e 1.0 × 10-2 M of SDS and SDeS, the further addition of surfactant leads to an increase of the hydrodynamic radius from 4.2 to 8.2 and 8.7 to 10.8 nm for SDS and SDeS, respectively, maybe as a consequence of an increase in the aggregation number in the mixed surfactant/copolymer micelles due to the incorporation of more surfactant monomers and free copolymer unimers to the mixed micelle. This incorporation occurred perhaps as a consequence of the screening of the repulsive interactions between surfactant headgroups due to the increasing amount of counterions surrounding the micelle corona, as occurred in other block copolymer surfactant systems.17,32,36,37,40 In contrast, the hydrodynamic radii of the SOS/S15E63 system do not change in the same concentration region. At higher surfactant concentrations, between 1.0 × 10-2 and 7.5 × 10-1 M for SDS and SDeS, almost all mixed surfactant/copolymer micelles disintegrate, giving rise to small surfactant-copolymer complexes present in solution


with a size that resembles a surfactant micelle as has indicated the decrease up to ∼1 nm of the hydrodynamic radii of the surfactant/S15E63 systems. This hydrodynamic radius is on the order of or even lower to that of a free micelle. A possible explanation is that the calculation of the apparent hydrodynamic radius is made under the assumption of the complex being a hard sphere, and the geometry of the complex could not accommodate this structure, being more similar to a flexible rod of radius of 1 nm. Moreover, in this region, there is also an increase in the number of free surfactant micelles. Both phenomena contribute to the increase in importance of the fast relaxation mode shown previously. This last result has been also found for other block copolymers as Pluronics interacting with surfactants.24,28,33,34 The width of the fast mode can be related to the small complexes containing a varying number of S15E63 units before saturation is reached as occurred for different Pluronic/surfactant systems.23,34 On the other hand, for both systems, a little part of the surfactant-copolymer mixed micelles tend to associate to give small micelle clusters increasing their size as shown in Figure 4 for SDS, although their number is very low as was previously commented. In contrast, for the SOS/S15E63 system, in the concentration region 2.5 × 10-2 to 0.75 M, micelles are not completely disintegrated due to surfactant micelle binding as shown in Figure 4, and coexisting copolymer/surfactant mixed micelles of an approximate radius of ∼5 nm and surfactant micelles bound to copolymer unimers with a size very close to the surfactant free micelles, as was commented previously. Transmission Electron Microscopy. To corroborate the results obtained by DLS, here we present a series of TEM images (Figure 5) that follows the evolution of mixed SDSblock copolymer micelles with changes in the SDS concentration, for which the size variation is larger. Most images of the aggregates are in agreement with size measurements by dynamic light scattering. However, one has to bear in mind that by TEM we image single particles, while DLS gives an average size estimation, which is biased toward the larger-size end of the population distribution. TEM images seem to follow the same trends as DLS results, displaying the four different regions for SDS-block copolymer micelle size found by the former technique. At this respect, Figure 5a shows the image for block copolymer-only micelles in the absence of SDS. The average size of the aggregates has been estimated around 25 nm, calculated from the extremeto-extreme distance of the spheres (over an average of 100 particles). However, it is necessary to mention that the differing electron densities between the block domains forming the core and the corona of the micelles, respectively, can result in different contrasts of the respective domains, thus altering the measured size.41 Adding SDS to the solution, there is a little plateau region observed by DLS followed by a decrease in the micelle size as the SDS concentration is increased, as seen in Figure 5b (SDS concentration 2.5 × 10-5 M), where the diameter of the particles has been reduced to 17 nm, although polydispersivity can be observed. This decrease carries on until the mixed copolymer-SDS aggregates reach a minimum size of 10 nm at 1 × 10-4 M SDS (see Figure 5c). The granules of this picture would

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Castro et al.

Figure 5. TEM image demonstrating the evolution of the SDS/S15E63 block copolymer micelle size in the presence of (from left to right) (a) 0 M; (b) 2.5 10-5 M; (c) 1.0 10-4 M; (d) 1.0 10-2 M; and (e) 5.0 10-2 M SDS. Block copolymer concentration is 2.5 g dm-3.

represent the smaller mixed micelles. With further SDS addition, mixed micelles begin growing again as seen in Figure 5d (SDS concentration 1 × 10-2 M), in which some larger spherical aggregates with a size almost similar to that pure of block copolymer micelles can be observed, together with smaller mixed micelles similar to those seen in the previous image as a consequence of the increase in sample polydispersivity. The existence of these two different types of aggregates then may cause the increase in the apparent hydrodynamic radii as commented previously. Finally, Figure 5e shows a copolymer solution with 0.05 M added SDS, in which almost no aggregates can be detected, in agreement with the complete disruption of SDS/S15E63 mixed micelles, as shown by DLS. Finally, the small micelle clusters detected by DLS in this last region have not been observed by TEM. Isothermal Titration Calorimetry. Isothermal titration calorimetry measurements were carried out at 20 °C with the S15E63 concentration fixed to 2.5 g dm-3. Figure 6 shows the ITC data of the titration of monomeric SDS, SDeS, and SOS in a S15E63 solution and in water. For both SDS and SDeS, it is observed that at very low concentrations (between 6.0 × 10-6 to 2.5 × 10-4 M and 3.0 × 10-5 to 3.5 × 10-4

M for SDS and SDeS, respectively), these surfactants clearly interact with the copolymer, confirming that these interactions at the lowest surfactant concentrations do not involve appreciable changes in hydrodynamic radius as observed in the light scattering results (see the previous section). In contrast, Figure 6c shows that the titration curve for SOS in S15E63 is close to that in water, suggesting that interactions between this surfactant and the block copolymer are small in such concentration ranges. Figure 7 shows the results for the addition of micellar SDS, SDeS, and SOS to the block copolymer solution in addition to the curve for just adding surfactant to water. The enthalpy curves for SDeS and SOS in the presence and in the absence of the block copolymer are the same up to the cac, ∼25 and ∼50 mM for SDeS and SOS, respectively, the concentration where surfactant micelles are formed onto the copolymer surface. Both concentrations are the starting point from which hydrodynamic radii obtained by DLS begin to decrease due to complete mixed micelle disruption, thus indicating interactions between surfactant and copolymer micelles.42 With the further addition of surfactant, differences between ITC curves for the surfactant/S15E63 and surfactant/water


Figure 6. Enthalpy change as a function of surfactant concentration due to the titration of monomeric (a) SDS (2 mM); (b) SDeS (8 mM); and (c) SOS (30 mM) in the absence (O) and the presence (9) of 2.5 g dm-3 copolymer S15E63 at 20 °C.

Figure 7. Enthalpy change as a function of surfactant concentration due to the titration of micellar (a) SDS (500 mM); (b) SDeS (600 mM); and (c) SOS (2400 mM) in the absence (O) and the presence (9) of 2.5 g dm-3 copolymer S15E63 at 20 °C.

systems occur. Meanwhile, for SDS, the titration curve into the polymer solution is different to that in water even at the lowest surfactant concentration. The enthalpy curve corresponding to the titration of micellized SDS shows an endothermic increase, giving rise


to a first endothermic maximum at a concentration of 9 × 10-3 M. A slight increase of SDS concentration leads to a shallow minimum followed by a second smooth endothermic maximum. With subsequent increases of surfactant concentration, ∆Hi decreases going through an exothermic minimum. Figure 6a shows that with a further increase of SDS concentration, ∆Hi again increases passing through a third very broad endothermic peak that approaches and becomes coincident with the profile obtained for the binary surfactant system. Similar ITC profiles has been observed for the interactions of other nonionic polymers, mainly polaxamers, and single-tailed43 and gemini surfactants.44 Possible contributions to the endothermic increase and, thus, to the first endothermic peak would include the dissociation of surfactant micelles from the injected concentrated solution, dilution effects, conformational changes in the polymer, dehydration of polymer micelles, and formation of mixed surfactant/copolymer micelles. When the differences between the enthalpy profiles of the ternary and binary solutions are considered, only the latter three contributions mentioned previously are important. This endothermic increase and the subsequent maximum has been attributed to interactions between surfactant and copolymer micelles that leads to the formation of mixed surfactantpolymer micelles and, therefore, to a partial disruption of block copolymer-only micelles.44 This would lead to the solubilization of the surfactant molecules, the end groups, and the dehydrated PEO segments from the water phase into the hydrophobic core of the mixed polymer-SDS micelles, as was previously demonstrated.33,45 This would induce stabilization of the hydrophobic core (the oxyphenylethylene groups) by replacing water at the core-corona interface.46,47 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.48 The second endothermic peak would correspond to the existence of interactions of SDS with some single polymer molecules resulting from the disruption of mixed polymersurfactant micelles as well as those corresponding to polymer micelles. It has been established33 that the thermodynamic condition to allow the formation of bound surfactant aggregates onto the polymer molecules resulting from micelle disruption is to reach a critical surfactant monomer concentration that is very similar in meaning to the critical aggregation concentration of the surfactant in the presence of only non-aggregated polymers in solution. As more SDS is added, the disruption of the mixed surfactant/polymer micelles would arise from the increase of SDS in these mixed micelles, which would lead to a decrease of the binding rate due to electrostatic repulsions of SDS headgroups, resulting in a decrease in the enthalpy up to an exothermic minimum. This exothermic minimum is attributed to rehydration of S15E63 segments due to the dissociation of copolymer micelles to monomeric copolymer.49 It has been established32 that the driving force for the rehydration process of the PEO segments is ion-dipole association between SDS headgroups and EO segments. Thus, after rehydration, these PEO segments wrap around

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the charged surface of SDS micelles to form the necklacelike SDS/copolymer complex, decreasing the contact between the exposed hydrophobic segments of SDS micelles and the water phase. At higher SDS concentrations, there is an enthalpy increment passing by a third broad endothermic maximum that intersects with the SDS/water curve and travels very close to it. This new maximum is associated with the tail end of the binding process involving the formation of SDS aggregates bound to single copolymer units (i.e., small surfactant/copolymer complexes with a size similar to a free surfactant micelle, which comes from the disruption of most part of the mixed surfactant-polymer micelles).47 The saturation concentration of copolymer monomer by SDS micelles corresponds to the merging point of the titration curve with the SDS dilution curve. Beyond this point, the polymer no longer interacts with surfactant, and the injections merely correspond to a dilution of micelles into solution containing free micelles. On the other hand, the enthalpy curve corresponding to the titration of micellized SDeS shows an endothermic increase with surfactant concentration, giving rise to an endothermic maximum at 40 mM SDeS. Further addition of SDeS leads to the disruption of most of the mixed surfactant-polymer micelles in the same way as commented for SDS, accompanied with a decrease in ∆Hi beyond the endothermic maximum up to an SDeS concentration of 50 mM. At higher SDeS concentrations, ∆Hi shows again an endothermic increase as a consequence of the tail end of the binding process involving the formation of SDeS aggregates bound to single copolymer units, as was commented previously. Formation of free SDeS micelles also takes place in this concentration range becoming the dominant process, as indicated by the important increase in the intensity versus relaxation time distribution function shown in Figure 2, contributing definitively to this second endothermic increase. Comparison of SDeS/S15E63 ITC data with that of SDS/S15E63 shows the inexistence of both the second smooth endothermic maximum and the exothermic minimum in the plot for SDeS. The absence of the second endothermic maximum can be a consequence of not achieving the necessary critical surfactant monomer concentration to allow the formation of bound surfactant aggregates onto the polymer monomers resulting from micelle disruption.33,43 On the other hand, the absence of the exothermic minimum might result from the predominance of the formation of free SDeS micelles and SDeS aggregates bound to single copolymer units (both endothermic processes in the ITC plot), masking the rehydration of the PEO chains of the copolymer previously expelled from the hydrophobic SDeS micellar core to the water after mixed micelle disruption, which seems to be a less important phenomenon for this surfactant than for SDS. In contrast, the ITC titration curve of SOS in S15E63 is quite similar in shape to that of the titration of SOS in water, departing the former from this at ∼50 mM as commented previously. Thus, it seems that the formation process of mixed copolymer/surfactant micelles and subsequent destruction of these mixed micelles would be much less important for this surfactant in agreement with the DLS results, in

Castro et al.

which the decrease in hydrodynamic radii at high surfactant concentrations is less pronounced. Therefore, the behavior in the ITC profiles for SOS and SDeS in comparison with that of SDS would confirm that hydrophobic interactions play an important role in the interactions between block copolymer S15E63 and sodium alkyl sulfates, as has occurred for other surfactant-copolymer systems38,39 since repulsive interactions between surfactant headgroups would be of similar magnitude because all surfactants possess the same polar headgroups. Conclusion When surfactant is added to a 2.5 g dm-3 S15E63 micellar solution, different concentration regions are observed in the semilog plot of rapp,h versus surfactant concentration. At very low concentrations (c < 1.0 × 10-4 M), a decrease in the hydrodynamic radius is observed as a consequence of the association of SDS or SDeS 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. The binding of both SDS and SDeS at both stages can be considered as the solubilization of the surfactant hydrocarbon chains into the styrene oxide core of the block copolymer micelles, and the relaxation time distribution function remains monomodal. Repulsive interactions between surfactant headgroups in the micelle core would expand it, allowing water penetration and, thus, giving rise to a less dense packing of the micelle, resulting in a decrease in the aggregation number that, in turn, is reflected in a decrease in mixed micelle size. In contrast, for SOS, no variation of the copolymer micelle size is detected in this concentration range, so we postulate that interactions of this surfactant with the copolymer micelles, if any, occur via interaction with the micelle PEO corona. With further SDS and SDeS addition (1.0 × 10-4 < c < 1.0 × 10-2 M), the size of the mixed micelle grows slightly due to an increase in its aggregation number because of the binding of more surfactant molecules and some copolymer unimers resulting from the previous copolymer micelle size reduction. No change is observed for SOS in the same concentration range. At higher surfactant concentrations (c > 1.0 × 10-2 M), DLS data for all surfactants changes from monomodal to bimodal distributions, and the hydrodynamic radii of the copolymerrich/surfactant mixed micelles abruptly decreases. We conclude that at this concentration range, the mixed micelles begin to disintegrate with the accompanying rehydration of the hydrophilic PEO blocks, which is masked in the ITC data for SDeS, if compared with those of SDS, due to the formation of small surfactant-rich/copolymer complexes and free surfactant micelles, which become the predominant processes. For SOS, the titration curve of the copolymer solution resembles that of it in pure water due to the less intense surfactant-copolymer interactions. In the small complexes formed, cooperative surfactant aggregates interact with hydrated copolymer unimers. The small complexes coexist with the large copolymer-surfactant complexes until all unimers have been expelled from the large complex. Finally, at 0.075 and 0.5 M for SDS and SDeS, respectively,


only free and small complexes are present in solution, whereas a bimodal distribution is still observed in the case of SOS. TEM images corroborate the change in size of the mixed SDS/block copolymer micelles as the SDS concentration is varied obtained by DLS. Thus, these results seem to indicate the ability of some surfactants to control the size of the polymer aggregates. Acknowledgment. The project was supported by the Ministerio de Ciencia y Tecnologıá through project MAT200402756 and Xunta de Galicia. P.T. and. E.C. thank the Ministerio de Eduacioń, Cultura y Deporte for the Ramoń y Cajal position and Ph.D. grant, respectively. We thank Profs. Attwood and Booth for the generous gift of the block copolymer. References and Notes (1) Chu, B.; Zhou, Z. Physical chemistry of polyoxyalkylene block copolymer surfactants. In Nonionic Surfactants, Vol. 60; Nace, V. M., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1996. (2) Alexandridis, P.; Lindman, B., Eds. Amphiphilic Block Copolymers: Self-Assembly and Applications; Elsevier Science BV: Amsterdam, 2000. (3) Tuzar, Z.; Kratochvil, P. AdV. Collloid Interface Sci. 1976, 6, 201. (4) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110. (5) Alexandridis, P.; Hatton, A. T. Colloids Surf. A 1995, 96, 1. (6) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (7) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (8) Kabanov, A. V.; Alakhov, V. Y. Crit. ReV. Ther. Drug Carrier Syst. 2002, 19, 1. (9) Brown, W.; Schilleń, K.; Hvidt, S. J. Phys. Chem. 1992, 25, 5434. (10) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. (11) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (12) Chu, B. Langmuir 1995, 11, 414. (13) Goldmints, I.; Yu, G.-E.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651. (14) Kelarakis, A.; Havredaki, V.; Viras, K.; Mingvanish, W.; Heatley, F.; Booth, C.; Mai, S.-H. J. Phys. Chem. B 2001, 105, 7384. (15) Crothers, M.; Attwood, D.; Collett, J. H.; Yang, Z.; Booth, C.; Taboada, P.; Mosquera, V.; Ricardo, N. P. S.; Martini, L. G. A. Langmuir 2002, 18, 8685. (16) Rekatas, C. J.; Mai, S.-H.; Crothers, M.; Quinn, M.; Collet, J. H.; Attwood, D.; Heatley, F.; Martini, L.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 4769. (17) Bronstein, L. M.; Chernyshow, D. M.; Timofeeva, G. I.; Dubrovina, L. V.; Valetsky, P. M.; Obolonkova, E. S.; Khokhlov, A. R. Langmuir 2000, 16, 3626. (18) Davidson, R. L. Handbook of Water-Soluble Gums and Resins; McGraw-Hill: New York, 1980. (19) Robb, I. Anionic Surfactants-Physical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981.

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