Nonionic Copolymer Interaction: A SLS, DLS, ITC, and

23760

J. Phys. Chem. B 2005, 109, 23760-23770

Surfactant/Nonionic Copolymer Interaction: A SLS, DLS, ITC, and NMR Investigation Pablo Taboada,* Emilio Castro, and Vıćtor 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: June 14, 2005; In Final Form: August 26, 2005

The interactions between an oxyphenylethylene-oxyethylene nonionic diblock copolymer with the anionic surfactant sodium dodecyl sulfate (SDS) have been studied in dilute aqueous solutions by static and dynamic light scattering (SLS and DLS, respectively), isothermal titration calorimetry (ITC), and 13C and self-diffusion nuclear magnetic resonance techniques. The studied copolymer, S20E67, where S denotes the hydrophobic styrene oxide unit and E the hydrophilic oxyethylene unit, forms micelles of 15.6 nm at 25 °C, whose core is formed by the styrene oxide chains surrounded by a water swollen polyoxyethylene corona. The S20E67/ SDS system has been investigated at a copolymer concentration of 2.5 g dm-3, for which the copolymer is fully micellized, and with varying surfactant concentration up to approximately 0.15 M. When SDS is added to the solution, two different types of complexes are observed at various surfactant concentrations. From SLS and DLS it can be seen that, at low SDS concentrations, a copolymer-rich surfactant mixed micelle or complex is formed after association of SDS molecules to block copolymer micelles. These interactions give rise to a strong decrease in both light scattering intensity and hydrodynamic radius of the mixed micelles, which has been ascribed to an effective reduction of the complex size, and also an effect arising from the increasing electrostatic repulsion of charged surfactant-copolymer micelles. At higher surfactant concentrations, the copolymer-rich surfactant micelles progressively are destroyed to give surfactant-rich-copolymer micelles, which would be formed by a surfactant micelle bound to one or very few copolymer unimers. ITC data seem to confirm the results of light scattering, showing the dehydration and rehydration processes accompanying the formation and subsequent destruction of the copolymer-rich surfactant mixed micelles. The extent of interaction between the copolymer and the surfactant is seen to involve as much as carbon 3 (C3) of the SDS molecule. Self-diffusion coefficients corroborated light scattering data.

1. Introduction A great deal of fundamental investigations have been generated to study the interaction between different polymers and surfactants as a consequence of the varied applications found in technical processes for this class of mixtures:1-4 paints, coatings, laundry detergents, cosmetic products, and pharmaceutical formulations. One group of interesting polymers is the nonionic, water soluble, low molecular weight block copolymers consisting of a hydrophilic poly(oxyethylene) block (PEO, (-OCH2CH2)) and a second hydrophobic block. 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 in concentrated solutions. Most studies on this type of block copolymer have been focused on those whose hydrophobic block is formed by units of oxypropylene (-OCH2CH(CH3), denoted as PPO or P) or 1,2-oxybutylene (-OCH2CH(C2H5) denoted as BO or B), which are commercially available from BASF and Chemical Dow, respectively. However, less attention has been paid to other oxyalkylene block copolymers. For example, we have recently studied the micellization and gel properties of block copolymers containing styrene oxide ((-OCH2CH(C6H5), denoted as SO or S),5,6 recently released onto the market by Goldsmischdt AG, and phenyl glicidyl ether ((-OCH2CHO(C6H5), denoted as GO or G),7 not commercially available. * To whom correspondence should be addressed. Phone: 0034981563100, ext. 14042. Fax: 0034981520676. E-mail: [email protected].

In many copolymer applications surfactants are present, which is likely to influence the self-assembly properties of the copolymers to improve the properties of this kind of material.8 The interaction mechanisms in these systems are dependent on surfactant type, polymer molecular weight, chemical structures of polymer and surfactant, hydrophobic content of the polymer, electrolyte, temperature, and solvent quality. The surfactantcopolymer interactions have been mainly focused on mixtures formed by PEO-PPO-PEO copolymers and classical anionic, cationic, and nonionic surfactants,9-13 as a result of their commercial availability. For this class of systems, formation of mixed surfactant-copolymer micelles was found accompanied with a progressive reduction of the number of copolymer molecules and subsequent size decrease of these mixed aggregates.9,10 On the other hand, some studies have been made to characterize the interactions between surfactants and other types of block copolymers, as the diblocks formed by polybutadiene-poly(ethylene oxide) (PB-PEO),14,15 and polystyrenepoly(ethylene oxide) (PS-PEO) blocks.16-18 In recent articles, we have analyzed the interactions between cationic and anionic surfactants with two diblock copolymers whose hydrophobic block is formed by styrene oxide units,19-21 for which up to four different regions in the behavior of the system could be defined as the surfactant concentration increased. In the present article, we extend these previous studies and analyze the effect of copolymer hydrophobic block length and surfactant-copolymer mixed micelle structure on the interactions between diblock styrene oxide-poly(ethylene oxide)

10.1021/jp0532061 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/16/2005

Surfactant/Nonionic Copolymer Interaction

J. Phys. Chem. B, Vol. 109, No. 49, 2005 23761

TABLE 1: Molecular Characteristics of the Copolymera -1

Mn/g mol (NMR) S20E67

5300

wt % S (NMR)

Mw/Mn (GPC)

Mw/ g mol-1

44.4

1.05

5570

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

copolymers and SDS. Thus, we study the interactions of copolymer S20E67 with the surfactant through static and dynamic light scattering (SLS and DLS, respectively), isothermal titration calorimetry (ITC), and 13C and self-diffusion nuclear magnetic resonance (NMR) techniques. The results are compared with those previously obtained for copolymers S15E63 and S17E65. To check the strength of the possible repulsive interaction between the surfactant-copolymer complexes in the observed effects by SLS and DLS, as suggested in recent articles,22,23 experiments were also done in the presence of 0.1 M NaCl. The effect of the salt was also considered in the thermodynamics of the interaction. Finally, to shed more light about the nature and dynamics of the copolymer-surfactant complexes formed, C13 and self-diffusion NMR experiments were conducted and the results were compared with the data derived from light scattering. 2. Experimental Section Materials. Sodium dodecyl (SDS) was purchased from Merck (stated purity g 99%) and used as received without further purification. Water was double distilled and degassed before use. All experiments were made at 20 °C. The synthesis of the diblock copolymer S20E67 was described in detail previously.5 Table 1 shows the molecular characteristics of the copolymer. Static and Dynamic Light Scattering. The setup for static and dynamic light scattering measurements was an ALV/DLS/ SLS-5000F, SP-86 gonyometer system from ALV-GmbH, Langen, Germany. The light source was a CW diode-pumped Nd:YAG solid-state Compass-DPSS laser with a symmetrizer from Coherent Inc., Santa Clara, CA. The laser operates at 532 nm with an output power of 400 mW, which can be varied using an attenuator. All solutions were filtered through a Millipore filter with a 0.22-µm pore size and thermostated at 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 measured quantity in DLS experiments is the time correlation function of the scattered intensity, g(2)(t).24 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)

The apparent hydrodynamic radius, rapp,h, can be calculated via the Stokes-Einstein equation:

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) was set up using interactive software, and all data were stored to a hard disk. We present the results of the ITC experiments in terms of the enthalpy change per injection (∆Hobs) 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%. NMR Spectroscopy. 13C NMR spectra were recorded from D2O solutions with a Bruker AMX 300 spectrometer operating at 300 MHz for proton. All experiments were performed at 20 °C, and the temperature of the sample was controlled to within (0.1 °C. A standard single pulse sequence was used at a frequency of 75.43 MHz with a pulse of 12 µs, and a number of transients dependent on the solution concentration were used, from a minimum of about 2000 to a maximum of 12 000. The assignment of the SDS carbon signals was carried out according to literature.25 The diffusion measurements were performed on a Varian 750 operating at a proton resonance frequency of 750 MHz. Diffusion was measured with the pulsed field gradient spinecho pulse sequence26 utilizing two gradient pulses of strength g and duration δ. The distance between the leading edges of the pulses is denoted ∆. The parameters for surfactant diffusion were δ ) 1 ms, ∆ ) 20 ms, and g increasing in a linear sequence up to 3 T/m in 16 steps. The corresponding settings for copolymer diffusion were δ ) 2 ms, ∆ ) 20 ms, and g increasing in a linear sequence up to 4.5 T/m in 16 steps. The intensity I of the echo is then given by

I ) I0 exp[-γ2g2δ2D(∆ - δ/3)]

(3)

where I0 represents the echo intensity in the absence of gradients, γ is the magnetogyric ratio of the observed nucleus, and D is the self-diffusion coefficient. D is obtained by regressing eq 4 onto the experimental data using a nonlinear routine with D and I0 as adjustable parameters. A problem when applying eq 3 to copolymer diffusion is that often their spin-echo decays are not monoexponential on account of the copolymers being polydisperse in molecular weight. However, in the present case, dealing with a rather monodisperse block copolymer, its diffusion could be reasonably well-evaluated in terms of a single diffusion coefficient. 3. Results and Discussion

kT rapp,h ) 6πηDapp

(2)

where k is the Boltzmann constant and η is the viscosity of water at temperature T. Isothermal Titration Calorimetry. Heats of dilution were measured using a VP-ITC titration microcalorimeter from

Static and Dynamic Light Scattering. The self-association process of the block copolymer S20E67 has been previously studied.5 It has been shown that this copolymer has an average aggregation number of 189 monomers and an apparent hydrodynamic radius of 15.6 nm at 25 °C. The cmc of this copolymer is very low, and therefore it was not possible to measure it by conventional methods. In a recent study, we measured the cmc

23762 J. Phys. Chem. B, Vol. 109, No. 49, 2005

Figure 1. Relative intensity measured at θ ) 90° and at 20 °C as a function of SDS concentration in the presence of 2.5 g dm-3 of copolymer S20E67 at (9) 0 and (O) 0.1 M of NaCl.

of the copolymer E45S10 by surface tension,5 with a value of ca. 5.4 × 10-6 mol dm-3, and thus we can consider that the block copolymer S20E67, which is more hydrophobic than the former, is fully micellized at the concentrations used in the present study. The total intensity of the scattered light normalized with the scattered intensity of toluene (Itot θ ) was measured in the absence and in the presence of 0.1 M of NaCl at a copolymer concentration of 2.5 g dm-3 and varying SDS concentrations from 1 × 10-6 to 0.1 M at 20 °C, with a scattering angle of 90°, because no angular dependence of the static light scattering intensity was observed, as previously shown5,19 (see Figure 1). The SLS intensity of the copolymer-surfactant system in water can be divided into three different intervals: at very low surfactant concentrations (7.5 × 10-6 to 1.0 × 10-4 M) there is a strong decrease in SLS intensity with increasing surfactant concentrations as a consequence of interactions of SDS molecules to the S20E67 micelles giving rise to the formation of a copolymer-rich mixed micelles. At this stage, the intensity decrease may be composed of two different contributions: an effective size reduction of the mixed surfactant-copolymer micelle and a contribution of the effect related to the structure factor as a consequence of the repulsive interactions of the charged mixed micelles, as will be discussed later in this article. Upon increasing surfactant concentration (1.0 × 10-4 to 2.5 × 10-3 M), the SLS remains practically constant, in agreement with the same concentration range displayed in Figure 2 (see below). At higher surfactant concentrations (>2.5 × 10-3 M), a subsequent decrease in SLS intensity is observed until reaching a plateau region whose intensity corresponds to the formation of surfactant-rich-copolymer micelles and free surfactant micelles, as will be discussed later in this article. In contrast, in the presence of 0.1 M of NaCl, the SLS intensity initially slightly increases to become practically constant at very low surfactant concentrations (7.5 × 10-6 to 7.5 × 10-5 M), followed by an almost continuous decrease up to 0.025 M of SDS, from which the SLS intensity remains almost invariable as a consequence of total breakdown of the copolymer-rich surfactant complexes. The remaining intensity would correspond to free SDS micelles and SDS micelles bound to copolymer unimers. This intensity is larger than in the case of salt-free solution, because salt would increase the aggregation number of both free and bound SDS micelles, which therefore would contribute to a major extent to the scattering intensity.

Taboada et al. Parallel to SLS measurements, DLS data were also collected. These data provide complementary knowledge to establish mechanisms of interactions between the surfactant and the copolymer. To qualitatively take into account the role played by electrostatic repulsion between surfactant-copolymer complexes on the DLS data, measurements were also made in the presence of 0.1 M NaCl. An example of relaxation time distributions from DLS measurements is shown in Figure 2, corresponding to a copolymer concentration of 2.5 g dm-3 at different SDS concentrations and 20 °C in the absence of NaCl. This figure clearly shows that unresolved bimodal distributions are obtained for SDS concentrations between 7.5 × 10-6 and 5 × 10-4 M. Although bimodal, there is a single predominant scattering species with faster relaxation times, which we think is formed by copolymer-rich surfactant mixed micelles derived from the interaction between copolymer micelles and associated surfactant, as occurs for other block copolymers of similar structure.19-21 A second and larger scattering species with slower relaxation times is also observed, although it only contributes less than 1% to the scattered intensity (on a % mass basis). Similar results have been found for other surfactant-copolymer systems, as Pluronic-gemini surfactants.27,28 This species would be composed of a very little proportion of insoluble block copolymer resulting from polymer synthesis. In the absence of surfactant, the presence of this large material is a little more reduced; therefore the presence of surfactant molecules can favor the formation of insoluble material. On the other hand, this second mode has not been found for structurally related S15E63 and S17E65 block copolymer analyzed in previous works5,19-21 in the absence and in the presence of SDS. In the surfactant concentration 7.5 × 10-6 and 5 × 10-5 M the second larger species (insoluble copolymer) slightly increases its importance, favored by increasing amounts of the surfactant, whereas the faster mode corresponding to the main scattering species slightly shifts to faster relaxation times. At slightly higher surfactant concentrations (5.0 × 10-5 to 2.5 × 10-4 M), the mode corresponding to the larger species decreases until it disappears while the mode corresponding to the copolymer-rich-surfactant complexes increases its height until a monomodal distribution is reached at 2.5 × 10-4 M of surfactant in solution. Thus, the insoluble copolymer would resolubilize to form relatively uniform copolymer micelles in the presence of small amounts of surfactant. The mode corresponding to the copolymer-rich-surfactant mixed micelles would correspond to a translational diffusion process, as deduced from the linear dependence with respect to q2 (not shown). New increases of surfactant concentration involve first a narrower distribution of this mode and a shift to faster relaxation times. From a 2.5 × 10-2 M SDS concentration, a new faster relaxation mode appears, making the distribution perfectly bimodal (Figure 2). This faster relaxation mode comprises relaxation times close to those characteristic of surfactant micelles. Thus, we attribute this mode to two different species: On one hand, surfactantrich-copolymer mixed micelles that consist of cooperatively associated surfactant molecules interacting with polymer unimers, with a relaxation time that resembles that of a typical surfactant micelle. The amount of this kind of mixed micelles grows at the expense of the larger copolymer-rich-surfactant mixed micelle as a consequence of repulsive interactions between SDS headgroups. This fact is reflected by the increasing contribution to the scattering light intensity of the forming species at the expense of a reduction for the latter. In this respect, the predominance of one species over the other in solution in equilibrium would depend on the magnitude of their respective



Figure 2. Selected relaxation time distributions for a 2.5 g dm-3 S20E67 solution with varying SDS concentrations at 20 °C: (a) 0; (b) 7.5 × 10-6; (c) 2.5 × 10-5; (d) 5.0 × 10-5; (e) 7.5 × 10-5; (f) 2.5 × 10-4; (g) 1.0 × 10-3; (h) 2.5 × 10-2, and (i) 7.5 × 10-2 M.

equilibrium constants. Finally, at the highest SDS concentrations, the fast mode is only present (not shown). In this region, due to the low scattering intensity of the solutions, the correlation function resolution reflects an increase in the peak width of the relaxation time distribution, which is also influenced by the small surfactant-rich-copolymer micelles containing a varying number of copolymer unimers before saturation is reached, as observed for other polymer-surfactant systems.19,20,29 On the other hand, formation of pure free surfactant micelles would also contribute to this mode. The relaxation time distributions in the presence of salt are quite similar to those in its absence (figure not shown). The main differences arise from the slightly more important intensity of both modes corresponding to copolymer-rich-surfactant complexes and copolymer clusters at low surfactant concentrations and the appearance of the mode corresponding to SDS micelles at lower concentrations if compared with the no-salt case. The apparent diffusion coefficients obtained from both the relaxation time distributions are plotted as a function of SDS concentration for a 2.5 g dm-3 copolymer solution at 20° C in the absence and in the presence of NaCl in Figure 3. To derive the diffusion coefficients for unresolved bimodal distributions, CONTIN analysis could not be applied because of the two overlapping peaks arising from the coexistence of a very low amount of copolymer clusters, which contributes extensively to the scattered light intensity, and the copolymer-richsurfactant complexes. The CONTIN method was unable to

provide an exact “two-peak” solution corresponding to these two different species in solution, and the approximate apparent diffusion coefficient of both classes of particles was derived by fitting the experimental data to a two-exponential function. In the absence of salt, the DLS data showed the predominance of a single scattering species characterized by a diffusion coefficient, Dapp1, which increases in this surfactant concentration range at very low surfactant concentrations (7.5 × 10-6 and 1.0 × 10-4 M). This increase has been related to the interaction of SDS molecules with the copolymer micelle, establishing a repulsive force between their polar headgroups that would facilitate water penetration into copolymer micelle, leading 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 the mixed micelle size,15,16,19,30 as has been shown by TEM.19-21 In the presence of electrolyte, Dapp1 remains almost constant in the same SDS concentration range (Figure 3b). This constancy would be related to the screening of the electrostatic repulsive forces between SDS molecules bound to the copolymer due to the presence of electrolyte, which also diminish the interactions between charged copolymer-richsurfactant mixed micelles. Moreover, differences in the values of Dapp1 in the presence and in the absence of salt would arise from both the change in the electrostatic repulsion between the charged complexes and the increase in size of the block copolymer micelles due to the addition of salt.31,32 Recent articles22,23 have exposed the possibility that the translational diffusive motion of the copolymer-rich-surfactant mixed mi-


Figure 3. Apparent diffusion coefficients, Dapp, against surfactant concentration in the presence of 2.5 g dm-3 of S20E67 at 20 °C (a) in the absence and (b) in the presence of 0.1 M NaCl. (+) Dapp,1, (9) Dapp,2, and (O) Dapp,3 (see definition in the text).

celles is affected by repulsive interactions and becomes faster. This would help to explain the strong increase of Dapp1 at low SDS concentration in the absence of added NaCl. However, these works also consider a size reduction of the mixed micelle as an additional reason for this increase. In our case, the electrostatic repulsion must be present, thus affecting the profile of Dapp1, but its strength should be moderate due to the less severe shift of the relaxation time distribution of the copolymerrich-surfactant mixed micelles to faster relaxation times, as commented previously. Moreover, it has been also shown22,23 that if electrostatic interactions between charged surfactantcopolymer mixed micelles are very strong the relaxation distribution finally splits, with the mode corresponding to the copolymer-rich-surfactant mixed micelles shifting to much slower times than in the present case, together with the appearance of a new mode corresponding to surfactant-richcopolymer micelles at faster relaxation times. However, this is not the case. The relaxation time distribution of the copolymerrich-surfactant micelles continuously decreases to faster times as SDS concentration grows, and neither splitting nor subsequent appearance of a new mode occurs, as seen for other copolymersurfactant systems for which a reduction in micelle size is also claimed.28,30 Nevertheless, we also consider that effects due to electrostatic repulsion between copolymer-rich-surfactant mixed micelles must be present as the presence of electrolyte partially shield the interactions between mixed micelles, but not being the predominant factor. On the other hand, at this very low surfactant concentration Dapp1 is slightly lower for the S20E67-SDS system than that of S15E63-SDS and S17E65-SDS as a consequence of the larger copolymer micelle formed by the former system. Moreover, a second, slower diffusion species exists in this range, which would correspond to insoluble copolymer cluster material, as commented previously, with a characteristic diffusion coef-

Taboada et al. ficient, Dapp2. This coefficient remains almost constant in this surfactant concentration range and is a little lower in the presence of salt with respect to the no-salt case. With addition of surfactant (1.0 × 10-4 to 2.5 × 10-3 M), Dapp1 decreases in water solution. This decrease might be related to the screening of the repulsive interactions between surfactant headgroups in the copolymer-rich-surfactant mixed micelles as a consequence of the increasing amount of counterions surrounding the micellar corona, which may also diminish the electrostatic repulsion between these mixed micelles, as occurred in other related systems.16,17,30,33,34 TEM studies have shown an increase of size polydispersity in the presence of similar amounts of surfactants for other styrene oxide-ethylene oxide block copolymers-surfactant systems,19-21 which might be also associated with this decrease in Dapp1 because DLS gives an average estimation of the diffusive species in solution. In the presence of electrolyte, Dapp1 shows an opposite trend in this SDS concentration range. This behavior arises because the amount of electrolyte is not enough now to avoid the repulsion between the SDS molecules in the mixed micelle, leading to a similar behavior as in the case of free salt at lower surfactant concentrations commented previously. Regarding Dapp2, it disappears at 5.0 × 10-4 M SDS because the insoluble copolymer resolubilizes as commented earlier in the presence and in the absence of salt. Finally, with further addition of SDS (>5.0 × 10-3 M), Dapp1 steeply increases in water solution as a consequence of the progressive reduction in size of the copolymer-rich-surfactant mixed micelles and, thus, the existence of less large diffusive objects in solution. At 2.5 × 10-2 M SDS, a third diffusive species appears with a characteristic diffusion coefficient, Dapp3. This diffusion coefficient would correspond to the surfactantrich-copolymer mixed micelle formed at the expense of copolymer-rich-surfactant mixed micelles, composed of surfactant micelles interacting with copolymer unimers expelled from the last type of mixed micelles. In the presence of salt, Dapp3 is also present at lower SDS concentrations (7.5 × 10-3 M) as a consequence of the screening of electrostatic repulsions between surfactant headgroups due to the electrolyte, which leads to an earlier formation of slightly bigger surfactant micelles. Moreover, Dapp3 slightly increases as the surfactant concentration does in the absence and in the presence of salt. The slightly lower values of Dapp3, if compared with those of free SDS micelles, can be due to the preponderance of surfactant aggregates bound to copolymer monomers over pure free surfactant micelles. However, when the surfactant concentration exceeds a certain value, the energetically favored formation of regular free SDS micelles can become the predominant process and the diffusion coefficient increases to approximately that of free SDS micelles. Isothermal Titration Calorimetry. Isothermal titration calorimetry measurements were carried out with the S20E67 concentration fixed at 2.5 g dm-3. In Figure 4a, the titration of a 300 mM SDS solution onto the copolymer and water solution, respectively, in the absence of NaCl are depicted (the transfer enthalpy of the mixed surfactant-copolymer system is present as Supporting Information). It can be seen that both curves are different even at the lowest surfactant concentration, indicating that interactions between the surfactant and the copolymer take place at much lower concentrations (figure not shown), as occurred for structurally related surfactant-copolymer systems,19-21 and thus a critical aggregation concentration could not be detected. The magnitude of the enthalpy of interaction, ∆Hobs, increases sharply at low surfactant concentrations, giving


Figure 4. Enthalpy change as a function of surfactant concentration due to the titration of micellized SDS at 20 °C (a) in the absence and (b) in the presence of 0.1 M NaCl. (O) Titrations in aqueous solution. (9) Titrations into the copolymer solution.

rise to a first endothermic maximum at a concentration of 8.2 × 10-3 M SDS. A slight increase of SDS concentration leads to a shallow minimum followed by a second smooth endothermic maximum. It has been shown that two endothermic processes are observed at high copolymer concentrations (>cmc) as a result of interactions between surfactants and either copolymer micelles or monomers.19,20,28 With subsequent increases of surfactant concentration ∆Hobs decreases, going through an endothermic minimum. Figure 4a shows that ∆Hobs again increases with further increase of SDS concentration, passing through a third very broad endothermic peak that approaches and becomes coincident with the plot obtained for the titration of the surfactant onto water. Similar ITC profiles have been observed for the interactions of other nonionic polymers, mainly polaxamers, and single-tailed35 and gemini surfactants,27,28 and between other oxyphenylethylene-oxyethylene block copolymers and SDS.19,20 The first endothermic increase and the subsequent maximum have been attributed to interactions between surfactant and copolymer micelles due to the difference between the titration curves of SDS in the presence and in the absence of block copolymer, leading to the formation of copolymer-richsurfactant mixed micelles,35 as shown previously by light scattering. The steep rise in the endothermic profile is likely dominated by the dehydration of the block copolymer micelle upon interaction with SDS, which more than compensates for the hydrophobic effect arising from the formation of surfactant aggregates replacing copolymer unimers in the copolymersurfactant mixed micelles. On the other hand, 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.36 In this respect, the SDS concentration at which the first endothermic maximum is present coincides fairly well with the

J. Phys. Chem. B, Vol. 109, No. 49, 2005 23765 cmc value obtained when titrating micellized SDS onto water. On the other hand, the concentration of this maximum differs from that corresponding to the final disruption of the surfactantcopolymer micelles seen by DLS (2.5 × 10-3). This difference may arise from the predominance of the endothermic heat evolved by the interaction between the surfactant and the copolymer micelles over the beginning of the copolymer-richsurfactant mixed micelle distortion which leads to copolymer rehydration, which is exothermic in nature. Beyond the first maximum (8.2 × 10-3 to 1.1 × 10-2 M SDS), the overall heat effects become less endothermic with increasing surfactant concentration as more SDS molecules take part in the SDS-copolymer mixed micelles. With further surfactant addition (between 1.1 and 1.5 × 10-2 M), a new slight endothermic increase with a subsequent shallow maximum occurs as a consequence of the interactions between SDS molecules and copolymer unimers expelled from the copolymerrich-surfactant mixed micelles, as commented earlier in this article. It has been established37 that the thermodynamic condition to allow the formation of bound surfactant aggregates onto the polymer molecules is to reach a critical surfactant monomer concentration which is very similar in meaning to the critical aggregation concentration of the surfactant in the presence of only non-aggregated polymer in solution. On the other hand, this smooth maximum partially masks the progressive disruption of the copolymer-rich-surfactant mixed micelles in this surfactant concentration range.27,36,38 With further surfactant addition (1.5-2.7 × 10-2 M SDS), a further increase of SDS in the mixed micelles increases electrostatic repulsion inside these, giving rise to a distortion of their structure (reflected in a decrease in their size as shown by DLS), which results in a decrease in the enthalpy up to an exothermic minimum. Thus, the exothermic minimum can be attributed to rehydration of S20E67 segments due to the progressive dissociation of copolymer-rich-surfactant micelles.39 At higher SDS concentration (>2.7 × 10-2 M), there is an enthalpy increment passing by a third broad endothermic maximum that intersects with the SDS/water curve and goes very close to it. According to 13C NMR data (see next section), weak endothermic effects would be expected for the enthalpy change for the formation of surfactant-rich mixed micelles because mainly the polar head and the C1 carbon of the surfactant are completely involved in the interactions with the block copolymer. Thus, this profile mainly arises from the fact that the enthalpy in the water+copolymer mixture will superimpose to that in water at high surfactant concentration, because in those conditions the additive is in the infinite dilution state. Therefore, interactions between the copolymer and the surfactant will be weak, and beyond the merging point, the copolymer no longer interacts with surfactant and the injections merely correspond to a dilution of micelles into solution containing free micelles. On the other hand, if we titrate a micellized SDS stock solution in the presence of 0.1 M NaCl onto the copolymer dissolved in the same salt concentration, the derived enthalpogram is rather different compared to the no-salt case, as seen in Figure 4b. In this plot we can observe that the difference between the titration curves of SDS onto copolymer-salt and onto only-salt solutions is more abrupt than that in the case of no salt, with a larger variation in the absolute values of the enthalpy. This can be originated from the copolymer-surfactant interactions to be more important due to the screening of the electrostatic repulsion between SDS headgroups, which facilitates the incorporation of more SDS molecules in the copolymer

23766 J. Phys. Chem. B, Vol. 109, No. 49, 2005 micelle. In this plot, the endothermic increase with the subsequent maximum also occurs (with a similar enthalpy than in the absence of salt, ∼1.5 kJ mol-1) due to the solvent reorganization in the transition from hydrated copolymer micelles to the formation of copolymer-rich-surfactant mixed micelles. However, in the presence of salt, this endothermic maximum extends over a larger SDS concentration, and there is no evidence of the exothermic-endothermic energetic balance determining the appearance of the second endothermic maximum, which is absent in this plot. The reason for this behavior might be the screening of the electrostatic repulsions due to the presence of salt, facilitating a slightly higher stability of these mixed micelles. This would involve a lower amount of copolymer monomers expelled to the solution, and thus interactions between these and surfactant monomers would be lower. Moreover, the exothermic decrease is rather shallow and the reached minimum possesses a lower energy if compared with the no-salt case, which indicates that the formation of the surfactant-rich mixed micelles at the expense of the copolymerrich ones takes place over a larger surfactant concentration range, which suggests a higher stability of this class of mixed micelles, in agreement with light scattering data. Finally, we want to mention that the titration curve onto the copolymer does not merge with that of water because the concentration of the stock solution was not high enough to reach this point. In this respect, we decided to use a 100 mM stock solution to enable the SDS micelles to disintegrate when diluting the first injections into the sample cell. If a 300 mM stock solution was used as in the case of no salt, this would not occur, and therefore the whole calorific effects after each injection would not have been the same. As in the case of no salt, the transfer enthalpy of the mixed surfactant-copolymer-electrolyte system is present as Supporting Information. Nuclear Magnetic Resonance Spectroscopy. To shed more light on the nature and structure of the mixed micelles formed between the surfactant and the copolymer, 13C and self-diffusion measurements were made. 13C. In Figure 5a, the curves of chemical shifts for carbons C1, C6, and C10 of SDS against surfactant concentration in the absence of copolymer are depicted. Other carbons show analogous curves. The assignment of the signals to the various surfactant carbons were taken from literature40 and can be seen in the Supporting Information. The discontinuities of the chemical shift plots are identified as the critical micelle concentration of the surfactant (∼8 mM). Usually, the chemical shift of the carbons of a surfactant tail neatly increase upon micellization because of both the change of the environment from polar to apolar and the increased amount of trans conformations in the chain.41 In SDS this happens for all the carbons except C1, whose shift decreases instead. Figure 5b shows the analogous curves relative to SDS chemical shifts in solutions containing 2.5 g dm-3 of copolymer S20E67. The presence of the block copolymer in solution does not modify the trend of the plots, but the SDS concentration value at which the discontinuity takes place is lower than in the case of no copolymer in solution (∼3 mM). The occurrence of the transition in this SDS concentration interval is also accompanied by a short displacement of the 13C signal of the copolymer. The micellization process of SDS to give free micelles seems to be almost quite similar in both the absence and presence of the copolymer. In Figure 6, to facilitate the comparison, some curves relative to SDS solution in the absence and in the presence of the copolymer are shown. As commented earlier, all carbons of

Taboada et al.

Figure 5. Comparison of the chemical shift of some SDS carbons in (a) D2O and in (b) D2O + 2.5 g dm-3 of copolymer S20E67. (b) C1; (9) C6; and (2) C10.

SDS, except C1, exhibit the same chemical shift trend in the presence and in the absence of copolymer. Therefore, if the copolymer molecules penetrated the core of the SDS micelles when forming the copolymer-rich mixed micelles, these would stay very near the hydrocarbon chains and experience a different environment. Consequently, the experimental chemical shift of the chain carbons should be rather different in corresponding solutions with and without copolymer. Our data (see Figure 6) suggest that, at the SDS concentrations analyzed, the surfactant seems to penetrate to C1 and to a lesser extent to carbons C2C3, and not more deeply into the hydrocarbon core, as seen for similar correlation in carbons C9-C11. Moreover, the interaction between the SDS and the copolymer is also revealed by the slight increase of 13C copolymer chemical shift for small SDS additions, remaining almost constant and independent of SDS concentration as more surfactant is added. The shift versus shift diagrams42,43 are further and very useful meansl for the study of situations in which chemical shift values are the result of an averaging process over various species in equilibrium. They offer the advantage of eliminating the possible error due to concentration uncertainties by relating data pertaining to the same sample. In the case of an equilibrium between only two species, one obtains just a straight line. For the SDS/D2O system at concentrations higher than cmc, the shift versus shift plot of any pair of carbon nuclei consists of a straight line, indicating that the SDS exchanges just among two situations: the monomer and the mixed micelle (plot not shown). The plot shift versus shift of the carbon pairs of the hydrocarbon tail C3-C11 for the SDS/S20E67/D2O system also gave straight lines, which coincide with the corresponding ones of the SDS/D2O system, suggesting that the interior of surfactant micelles in the presence of copolymer is quite similar to those in its absence. On the contrary, by plotting the shift of C1, C2, and C3 against any of the other carbons of the hydrocarbon tail, one obtains a trend that is different from mere micellization



Figure 6. Comparison of the chemical shifts of some SDS carbons in the absence (closed symbols) and in the presence (open symbols) of 2.5 g dm-3 of copolymer S20E67. (b) C1 and (9) C6.

micelles in the intermediate surfactant concentration region. The measured self-diffusion coefficient of the component i corresponds to the weight average44

Di ) pfi Dfi + pbi Dbi

Figure 7. (a) Chemical shift of the C10 carbon vs that of the C3 carbon and (b) chemical shift of the C1 carbon vs that of the C10 carbon for solutions of SDS in D2O in the presence of 2.5 g dm-3 S20E67 at 20 °C.

(see Figure 7 as an example). However, for C2 and C3 this change is rather weaker. At the lowest concentrations of SDS above the cmc, these plots show a descent region different than that for micellization, reaching a plateau as the SDS increases. This plateau can indicate the simultaneous presence of surfactant-rich mixed micelles and free surfactant micelle and, to a lesser extent, copolymer-rich mixed micelles. At still higher surfactant concentration the data points move toward the micellization plot, in agreement with the formation of pure surfactant micelles in solution. Self-Diffusion. The purpose of self-diffusion NMR measurements was to shed further light on the nature of the mixed

(4)

where the superscripts f and b refer to the free and bound states, pfi and pbi are the free and bound fractions of component i, respectively, and Dfi and Dbi are the self-diffusion coefficients of species i in the free and bound states, respectively. The fraction of free surfactant will be the fraction not bound to either the copolymer or free micelles. In this case, Dbi is the weighted average of the self-diffusion coefficients for the copolymerbound state and the micelle-bound state. Without additional information, one cannot separate these contributions. Experiments were performed at a copolymer concentration of 2.5 g dm-3 at varying SDS concentrations at 20 °C. The properties of the species in solution were followed by measuring the intensity decay for NMR peaks at different shifts corresponding to protons of different environments. Peak assignments were made from literature data (see Supporting Information for an example).45 Self-diffusion experiments were made on the peaks marked in this figure. These correspond to the proton signal corresponding to the copolymer (∼4.5 ppm), SDS (∼1.3 ppm), and water (∼4.8 ppm). In this way, it was possible to follow the diffusion of the different chemical components in the solution, and hence it will be possible to draw conclusions about the conformation of the complexes. Moreover, the self-diffusion of the copolymer micelles in the absence of SDS was also measured (Figure 8). From the selfdiffusion measurements a single exponential decay was obtained, allowing the extraction of a single diffusion coefficient for the copolymer, which involves the monomer-micelle equilibrium, and is clearly shifted to the micelle formation at the measured copolymer concentration. The concentration dependence of the self-diffusion coefficients was measured as a function of the volume fraction. The volume fraction was measured as a function of concentration, which was obtained by φ ) cν, where c is the concentration in g mL-1 and ν is the partial specific volume for S20E67 (ν ) 0.885 cm3 g-1).5 The diffusion coefficient (D) decreases for increasing concentration, because of an increase in obstructing objects in solution. This gives an


Taboada et al.

Figure 8. Self-diffusion coefficients against hard sphere volume fraction of S20E67 copolymer micelles in D2O at 20 °C.

Figure 9. Echo decay for 15 mM SDS as a function of x () γ2g2δ2(∆ - δ/3)) in the presence of 2.5 g dm-3 of S20E67 at 20 °C.

increase in friction coefficient (fc) with concentration from the expression

D ) kBT/fc ) D0(1 + kfφHS)

(6)

where φHS is the hard sphere volume fraction, D0 ) kBT/f0, and f0 is the friction coefficient at infinite dilution. φHS was calculated from the volume fraction of micelles by applying a thermodynamic expansion factor δt ) νt/νa, previously obtained from static light scattering measurements.5 νt is the thermodynamic volume of the micelles (i.e., one-eighth of the volume, u, excluded by one micelle to another), and νa is the anhydrous volume of the micelles (νa ) MwNa/Fa, where Na is the Avogadro’s number, Fa is the liquid density of the copolymer solute calculated from published data assuming mass additivity of specific volumes,46 and Mw is the molecular mass of the copolymer micelle, respectively). The concentration-dependent friction coefficient can be described by the linear relationship, fc ) f0(1 + kfφHS). For a dilute system with long-time diffusion coefficients, taking only pairwise hydrodynamic interactions into account, we found that the kf coefficient derived via several theoretical approaches is 2.1 for hard spheres.47,48 Therefore, from a linear fit of the experimental data we obtain a selfdiffusion coefficient of the copolymer micelle of 1.28 × 10-11 m2 s-1, with kf ) 5.5. The hydrodynamic radius can be obtained using eq 2, with a value of 16.7 nm, in great agreement with the value previously obtained by DLS.5 The kf value is larger than that for the hard sphere case. This difference may arise from the “hairy” surface of the spherical aggregates. Moreover, the high aggregation number of the S20E67 micelles may also indicate that these aggregates are not completely spherical and some elongation can be produced. It has been proved that micelles formed by copolymer E17S8 possess very large aggregation numbers and present a large elongated structure.49 However, in the last case the polyoxyethylene block is much shorter than our copolymer. Turning to data for copolymer-surfactant solutions, we present in Figure 9 the intensity decay for the copolymer peak at 0.015 M SDS. As seen, a single exponential again adequately fits the experimental data. The results obtained from fits of the intensity decays of the copolymer, the surfactant, and water (i.e., their diffusion coefficients) are plotted in Figure 10a. For the copolymer, its diffusion coefficient continuously increases as the surfactant concentration does, indicating an increase in the mobility of the block copolymer chains as a result of the progressive destruction of the copolymer-rich-surfactant mixed

Figure 10. (a) Self-diffusion coefficients of 2.5 g dm-3 of S20E67 vs SDS concentration at 20 °C. (b) Self-diffusion coefficients of SDS vs surfactant concentration (O) in the absence and (9) in the presence of 2.5 g dm-3 of S20E67 at 20 °C. (c) Self-diffusion coefficients of water vs SDS concentration (O) in the absence and (9) in the presence of 2.5 g dm-3 of S20E67 at 20 °C.

micelles due to the electrostatic repulsion of SDS molecules bound to them. This increment of the diffusion coefficient is stronger at SDS concentrations below 0.015 M, in fair agreement with the concentration for which the mixed surfactantcopolymer micelles were almost completely absent in DLS data. At higher SDS concentrations, the diffusion coefficient of the copolymer molecules is still increasing, but much more slightly. These coefficients would correspond to the diffusion coefficient of the surfactant-rich mixed micelles, and their concentration dependence might be a consequence of the different number of copolymer chains bound per surfactant micelle. The profile of this region points out the possible appearance of a plateau region,

Surfactant/Nonionic Copolymer Interaction which would correspond to the diffusion coefficient of the single copolymer molecules bound to SDS micelles. Finally, the diffusion coefficients obtained by NMR are slightly slower than those obtained by DLS. The difference in diffusion coefficients obtained from both techniques is an effect of the different concentration dependences of the two methods: In DLS, the mutual diffusion is measured, which usually increases with increasing volume fraction, whereas with NMR the self-diffusion is measured, which decreases with increasing volume fraction.47,50 Looking at the self-diffusion coefficients of the surfactant species in the solution in the absence and in the presence of the block copolymer, it can be seen that the diffusion of the pure surfactant is faster. This indicates that at least part of the SDS molecules when the copolymer is present in solution interacts with it. But the SDS component still has faster diffusion coefficients than that of the copolymer. Thus, there is a fraction of SDS that is not part of the surfactant-copolymer mixed micelles. Therefore, depending on the SDS concentration, the unbound SDS can stay as monomers, forming free micelles or binding to copolymer unimers. Thus, this would relate to the predominance of the various species in solution in equilibrium, which depends on the magnitude of their respective equilibrium constants. Moreover, from Figure 10b we can observe the SDS self-diffusion coefficient decrease with lower values as the SDS concentration increases, as commented before. This would indicate that the surfactant is progressively being bound to the block copolymer micelle, forming mixed surfactant-copolymer micelles (which would confirm why the diffusion coefficient of the copolymer increases in this concentration range, as commented previously), although the existence of free surfactant micelles would be also expected. However, with further surfactant increase the surfactant self-diffusion coefficient remains almost constant, but with slightly lower values than in the absence of copolymer, as a consequence of the binding of SDS micelles to copolymer unimers expelled to solution from copolymer-rich mixed micelles. This behavior would be in agreement with that of the diffusion coefficient obtained by DLS previously shown. Finally, to gain insights into the microscopic “viscosity” of aqueous solutions, we examined water mobility via self-diffusion data. These results are shown in Figure 10c. Addition of only SDS to water leads to a slight increase in the water self-diffusion coefficient in the SDS concentration range analyzed, in agreement with previous studies.51 This increase has been related to a little disruption of the self-structuring water. On the other hand, addition of block copolymer also leads to a little increase in the absolute values of water self-diffusion coefficients, which are higher if compared with that of SDS-water solutions. In contrast, in the surfactant-copolymer solutions, the mobility of the water molecules decreases slightly at low SDS concentrations to become almost constant as SDS concentration increases. This might indicate that little more ordered hydration layers are created in the mixed system around copolymer molecules as a consequence of the expulsion of the copolymer chains from the hydrophobic environment inside the mixed micelle core toward the aqueous phase, where their hydrophobic hydration is restored, due to the progressive disruption of the mixed surfactant-block copolymer micelles.52 Thus, these data would be in agreement with data derived from density measurements for other structurally related styrene oxide block copolymers interacting with SDS.19,20

J. Phys. Chem. B, Vol. 109, No. 49, 2005 23769 Conclusions In this study, dilute solutions of the S20E67 diblock copolymer mixed with the anionic surfactant SDS were investigated using light scattering, isothermal titration calorimetry, and 13C and self-diffusion nuclear magnetic resonance techniques. It was found that at very small SDS concentrations the SDS molecules interact with the copolymer micelles to give copolymer-richsurfactant mixed micelles. These mixed micelles reduce their size, but the interaction between these mixed micelles upon charging of copolymer micelles upon binding of SDS molecules also affects the reduction of light scattering intensity. Under progressive destruction of the copolymer-rich-SDS mixed micelles due to electrostatic repulsion between surfactant headgroups the formation of a new type of complex occurs, denoted as surfactant-rich-copolymer mixed micelles, which would be formed by SDS molecules in the micellized state bound to one or very few copolymer unimers. This behavior of the present system is corroborated by ITC data, from which the dehydration and rehydration of the copolymer chains following formation of copolymer-rich-surfactant mixed micelles and their distortion are observed. Nuclear magnetic resonance experiments also confirmed the aforementioned behavior. 13C data allowed us to observe that carbons C1-C3 of SDS are those involved in the interactions with the copolymer chains. From self-diffusion data, we can see that even at the lowest SDS concentrations, the self-diffusion coefficient of the surfactant is lower than in the absence of the copolymer, confirming the interaction, and its profile sustained the hypothesis of the disintegration of the copolymer-rich-surfactant complexes to give the surfactant-rich-copolymer mixed micelles. Acknowledgment. The project was supported by the Ministerio de Educacioń y Cultura through project MAT2004-02756 and Xunta de Galicia. P.T. and. E.C. thank the Ministerio de Educacioń y Cultura for his Ramoń y Cajal position and his PhD grant, respectively. We thank Professors David Attwood and Colin Booth for the generous gift of the block copolymer sample. Supporting Information Available: Figures showing enthalpy of transfer as a function of surfactant concentration due to titration of micellized SDS, 13C NMR spectrum of an aqueous solution of SDS and S20E67, and 13C NMR spectrum of an aqueous solution of SDS and S20E67. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Goddard, E. D.; Ananthapadmanaban, K. P. Interactions of Surfactants with Polymer and Proteins; CRC Press: Boca Raton, FL, 1993. (2) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley & Sons: New York, 1999. (3) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series 77; Marcel Dekker: New York, 1998. (4) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (5) 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. (6) Yang, Z.; Crothers, M.; Ricardo, N. M. P. S.; Chaibundit, C.; Taboada, P.; Mosquera,V.; Kelarakis, A.; Havredaki, V.; Martini, L.; Valder, C.; Collett, J. H.; Attwood, D.; Heatley, F.; Booth, C. Langmuir 2003, 19, 943. (7) Taboada, P.; Castro, E.; Mosquera, V. J. Phys. Chem. B 2004, 108, 3030. (8) Taboada, P.; Velasquez, G.; Barbosa, S.; Castelletto, V.; Nixon, S. K.; Yang, Z.; Heatley, F.; Hamley, I. W.; Ashford, M.; Mosquera, V.; Attwood, D.; Booth, C. Langmuir 2005, 21, 5263. (9) Dai, S.; Tam, K. C.; Li, L. Macromolecules 2001, 34, 7049.

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Nonionic Copolymer Interaction: A SLS, DLS, ITC, and

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