Novel Organized Structures in Mixtures of a Hydrophobically Modified

One of the factors behind the intensive studies of aqueous solutions of ..... S.N. and B.L. thank the Center for Amphiphilic Polymers for financial su...
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Langmuir 2000, 16, 6825-6832

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Novel Organized Structures in Mixtures of a Hydrophobically Modified Polymer and Two Oppositely Charged Surfactants S. Nilsson,*,† M. Goldraich,‡ B. Lindman,† and Y. Talmon‡ Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, and Department of Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa 32000, Israel Received October 19, 1999. In Final Form: March 31, 2000 In solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC) and two oppositely charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium chloride (DoTAC), an organized structure of micelles has been observed by using cryogenic temperature transmission electron microscopy (cryo-TEM). On the cryoelectron micrographs the micelles appear to be thread on strings, arranged in a cell-like pattern. This organized structure is induced by the polymer when the average number of polymer hydrophobic tails per mixed micelle is about 3-5, allowing hydrophobic tails from different polymer molecules to bind to the same surfactant micelle. The organized structure was not observed either in the surfactant mixture alone or in a low-viscous solution with a low number of polymer hydrophobic tails per mixed micelle. The oppositely charged surfactant mixture was studied at a total surfactant concentration of 30 mm in the presence and in the absence of 1% w/w polymer. The number of polymer hydrophobic tails per mixed micelle was varied by decreasing the micelle concentration through induced micellar growth. The micelles grow when the mole fraction of DoTAC (X) on the SDS-rich side of the phase diagram is increased. At low X values, only micelles are found, but from X ) 0.29 upward an increasing fraction of vesicles is present. Pulsed field gradient NMR was used to measure the self-diffusion of water for determining the aggregate structure and self-diffusion of the surfactant for determining the micellar aggregation number and the concentration of free SDS molecules.

Introduction Water-soluble hydrophobically modified polymers find frequent use as viscosity modifiers in various industrial products, such as in waterborne paints, cosmetics, and pharmaceuticals. It is common for multicomponent industrial formulas to contain both hydrophobically modified polymers and surfactants of various types that are added to stabilize pigments, latex particles, or fillers. One of the factors behind the intensive studies of aqueous solutions of hydrophobically modified polymers and surfactants that have been carried out during the past decade is thus their industrial importance.1,2 In semidilute polymer solutions, a progressive increase in surfactant concentration may give rise to a pronounced viscosity maximum.3-6 This behavior is attributed to the formation of mixed micelles, consisting of surfactants and polymer hydrophobic tails, which act as cross-links between the polymer molecules. A network of weakly associating polymer molecules is formed even without the surfactant being added. At low surfactant concentrations, the mixed micelles reinforce the associations of the polymer hydrophobic tails, increasing the viscosity. As the mixed micellar concentration * Corresponding author. † Lund University. ‡ TechnionsIsrael Institute of Technology. (1) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998. (2) Polymers in Aqueous Media; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 1989; Vol. 223. (3) Gelman, R. A. In TAPPI Proceedings of International Dissolving Pulps Conference, Geneva, Switzerland, 1987; p 159. (4) Tanaka, R.; Meadow, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (5) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3555. (6) Thuresson, K.; Lindman, B.; Nystro¨m, B. J. Phys. Chem. 1997, 101, 6450.

increases, a rearrangement of the network takes place until, at sufficiently high surfactant concentrations, each micelle binds to only one polymer hydrophobic tail, resulting in a disruption of the network and in low viscosity. Thus, the viscosity is mainly determined by the number of hydrophobic tails per mixed micelle and the lifetime of the interpolymeric cross-links.7,8 Aqueous mixtures of cationic and anionic surfactants display interesting bulk solution and interfacial properties.9 One important characteristic of those mixtures is their strong synergistic interactions, manifested as low critical micelle concentration (cmc) and enhanced surface activity.10,11 In recent years there have been many studies of the phase behavior of cationic and anionic surfactant mixtures.12-19 Already at low concentrations, mixtures of (7) Annable, T.; Buscall, R.; Ettelaie, R.; Whittelstone, D. J. Rheol. 1993, 4, 695. (8) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307. (9) Khan, A.; Marques, E. In Specialist Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional, An Imprint of Chapman and Hall: London, 1997. (10) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 81, 150. (11) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (12) Marques, E.; Regev, O.; Khan, A.; Miguel, M. d. G.; Lindman, B. J. Phys. Chem. B 1998, 102, 6746. (13) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Shivkumar, C. J. Phys. Chem. 1993, 97, 13792. (14) Jokela, P.; Jo¨nsson, B.; Khan, A. J. Phys. Chem. 1987, 91, 3291. (15) Kamenka, N.; Chorro, M.; Talmon, Y.; Zana, R. Colloids Surf. 1992, 67, 213. (16) Malliaris, A.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (17) Filipovic´-Vincekovic´, N.; Bujan, M.; Dragcˇevic´, D.; Nekic´, N. Colloid Polym. Sci. 1995, 273, 182. (18) Bergstro¨m, M.; Pedersen, J. S. Langmuir 1998, 14, 3754. (19) So¨derman, O.; Herrington, K. L.; Kaler, E. W.; Miller, D. D. Langmuir 1997, 13, 5531.

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anionic and cationic surfactants precipitate at an equimolar mixing ratio. When one of the surfactants is in excess, the precipitate redissolves forming micelles and, in some systems, also vesicles. The vesicles have attracted considerable interest and have been observed by direct visualization using transmission electron microscopy (TEM).12,13,15,19 Adding a hydrophobically modified charged polymer to a vesicle solution of oppositely charged surfactants induces structural changes in the vesicle mixture.20 Another feature of cationic and anionic surfactant mixtures is that they form mixed micelles having larger aggregation numbers than either surfactant would give at corresponding concentrations singly. The partial neutralization of the micellar surface charge results in a strong release of counterions with a resulting entropy increase that drives the aggregation. In the present study, the effect of the nonionic polymer hydrophobically modified hydroxyethyl cellulose (HMHEC)21 on an oppositely charged surfactant mixture of sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium chloride (DoTAC) is investigated. By changing the molar fraction of the two surfactants the ratio between polymer hydrophobic tails and surfactant micelles is altered. The polymer hydrophobic tail consists of hexadecyl chains grafted onto the polymer backbone at a low density, 1.2% w/w. In an earlier study of the system, we reported a remarkable increase in viscosity as the molar fraction of DoTAC in the SDS/DoTAC mixture was increased.22 The increase in viscosity was attributed mainly to the stoichiometry between the hydrophobic tails of the polymer and the concentration of mixed micelles, causing an optimal cross-link density in the polymer-surfactant network. It was also indicated that the size and shape of the aggregates influenced the viscosity. To further investigate the changes in the solution structure underlying the viscosity increase, the microstructural changes are followed here by using cryogenic temperature transmission electron microscopy (cryo-TEM). Cryo-TEM is a direct method, but it images only a minor part of the sample. Accordingly, the system is also studied here by means of the more quantitative NMR self-diffusion technique. Both techniques are used to study the mixtures of oppositely charged surfactants in the presence and absence of polymer. Throughout the article, the molar fraction of DoTAC in the surfactant mixture is denoted by X. If the solution contains only surfactant, it is denoted by Xs, and if 1% w/w polymer is also present, it is denoted by Xp. Experimental Section Materials. Hydrophobically modified hydroxyethyl cellulose (HMHEC), Natrosol Plus, grade 330, was obtained from Aqualon. According to the manufacturer, the molecular mass is about 250 000 and the degree of substitution of the hydroxyethyl groups per repeating anhydroglucose unit is 3.3. According to a previous investigation, HMHEC contains hexadecyl chains amounting to 1.2% w/w of the dry sample mass,22 which indicates there to be about 13 alkyl chains per polymer molecule. This corresponds to 0.54 millimolal (mm) alkyl chains in a 1% w/w aqueous polymer solution. Sodium dodecyl sulfate (SDS), “specially pure”, obtained from BDH in England, and dodecyltrimethylammonium chloride (DoTAC) from TCI Tokyo Kasei (purity >97%) were used as supplied. D2O from Dr. Glaser AG, Switzerland (99.8% pure) was employed as the solvent for the NMR measurements. Preparation of Solutions. Prior to use, HMHEC was washed extensively with acetone, so as to remove unreacted alkyl chains, dried, dissolved in water to a concentration of 1% w/w, and (20) Regev, O.; Marques, E. F.; Khan, A. Langmuir 1999, 15, 642. (21) Landoll, L. M. J. Polym. Sci., A: Polym. Chem. 1982, 20, 443. (22) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 7099.

Nilsson et al. centrifuged at 10000g to cleanse it of high molecular weight impurities (such as unreacted cellulose). Low molecular weight impurities (such as salt) were removed by dialysis against Millipore water in a Filtron Ultrasette device. The dialysis was performed until the expelled water showed a conductivity of less than 2 µS/cm. After being freeze-dried, the polymer was stored in a desiccator. The samples were prepared by weighing appropriate amounts of polymer or surfactant stock solutions into test tubes. Each sample had a polymer concentration of 1% w/w (1 g of polymer per 100 g of solvent), which for HMHEC is in the semidilute region and is well above the critical concentration (approximately 0.2%) above which intermolecular hydrophobic associations take place.23 The samples were mixed from stock solutions by a magnetic stirrer for at least a day and were stored at 25 °C for at least a week before measurements. Samples intended for NMR measurements were made using D2O. For water self-diffusion measurements, a mixture of 10% H2O and 90% D2O was employed as a solvent in order to obtain a sufficiently large H2O signal in the NMR spectra. Note that in the turbid solutions a precipitate was sometimes formed at Xs g 0.31 after more than 1 month. The complex salt involved, dodecyltrimethylammonium dodecyl sulfate, has a Krafft temperature of 308 K.14 When the samples were heated above the Krafft temperature, the precipitate immediately disappeared. Methods. Light Microscopy. The specimens were observed on glass slides (Clay-Adams) at 25 °C using an Olympus BH-2 light microscope and employing differential interference contrast (DIC). Images were recorded on a PC-based Cue-4 (Galai, Israel) image analysis system and were transferred to a Macintosh G3 computer. Image processing and contrast enhancement were done using the NIH Image 1.61 and Adobe Photoshop 5.0 software packages. Cryo-TEM. Specimens for transmission electron microscopy were prepared in a controlled environment vitrification system (CEVS) at 25 °C and 100% relative humidity, as described previously.24 In brief, a drop of the solution to be examined was applied to a perforated carbon film supported on an electron microscopy copper grid, held by CEVS tweezers. The sample was blotted and immediately plunged into liquid ethane at its freezing point (-183 °C). The vitrified samples were stored under liquid nitrogen (-196 °C) and were examined in a Philips CM120 microscope operated at 120 kV, using an Oxford CT-3500 cryospecimen holder equilibrated at about -180 °C. Low-dose imaging was employed to minimize electron beam radiation damage. Images were recorded digitally by a Gatan 791 MultiScan CCD camera using the Digital Micrograph 3.1 software package. To enhance phase contrast, an objective lens underfocus of 4-7 µm was employed. Image processing was performed using the Adobe Photoshop 5.0 package. PFG NMR Self-Diffusion Measurements. The self-diffusion coefficients of water and surfactant were obtained by the pulsed magnetic field gradient (PFG) technique, monitoring the 1H NMR spectra, following procedures described previously.25 All experiments were carried out at 25 °C in 5 mm NMR tubes. The water self-diffusion measurements were performed on a Bruker DMX 100 NMR spectrometer, with a Bruker DMX 200 NMR spectrometer being used for the surfactant self-diffusion measurements. Both spectrometers were equipped with a Bruker gradient probe providing a gradient strength of approximately 0.22 T m-1 A-1. For the self-diffusion of water, an ordinary spinecho (SE) pulse sequence (90°-τ-180°-τ-echo)26 with two pulsed magnetic field gradients of amplitude g and duration δ sandwiched around the 180° radio frequency (rf) pulse with a time separation of ∆ was employed. In these experiments g and ∆ were held constant at 0.0295 T m-1 A-1 and 140 ms, respectively, whereas δ was gradually increased in 20 steps to 16 ms. (23) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. (24) Talmon, Y. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; p 147. (25) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (26) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. (27) Tanner, J. E. J. Chem. Phys. 1970, 52, 2523.

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Figure 1. Complex viscosity (0.01 Hz) versus the molar fraction DoTAC at a total surfactant concentration (SDS + DoTAC) of 30 mm in solutions of 1% w/w HMHEC. The arrows indicate the compositions at which the cryo-TEM pictures were taken. A stimulated echo pulse sequence (STE)27 with gradients was used to measure the surfactant self-diffusion. Here, the surfactant signal intensity was followed as a function of the gradient strength g, while δ and ∆ were held constant. The experimental parameters were ∆ ) 70 ms, δ ) 1 or 3 ms, the maximum g being between 1.6 and 8 T m-1 A-1. The increase in g was carried out in 40 steps so as to obtain reliable biexponential fits of the methyl (CH3) and the main methylene (CH2) peaks, which contained contributions from both SDS and DoTAC. PFG NMR Evaluation. The PFG NMR technique has proven to be useful in studying dynamic and structural aspects of systems that self-assemble, such as surfactant systems.28 With PFG-SENMR or PFG-STE-NMR, the signal intensity in the case of free (Gaussian) diffusion is given by the Stejskal-Tanner equation26

I(k) ) I0 exp(-kD)

(1)

where D is the self-diffusion coefficient and k is defined as

( (3δ))

k ) (γgδ)2 ∆ -

(2)

Here γ is the magnetogyric ratio of protons (γ ) 2.6752 × 108 rad T-1 s-1), δ is the duration of the gradient pulses, and ∆ is the time between the leading edges of the gradient pulses, which is the same as the observation time for diffusion. The water signals investigated showed single-exponential decay when signal intensity was plotted against k in accordance with eq 1. For the surfactants, the non-overlapping peaks showed single-exponential decay. When the signals contained contributions from both SDS and DoTAC, a biexponential decay was observed.

Results Cryo-TEM. In the HMHEC/SDS/DoTAC system, the viscosity varies along a path on which X, the fraction of DoTAC, increases at a fixed total surfactant concentration of 30 mm and a fixed polymer concentration of 1% w/w.22 In an attempt to visualize the changes in solution microstructure leading to the increase in viscosity on the SDS-rich side, samples of differing viscosity, as indicated by the arrows in Figure 1, were studied by direct imaging, using the cryo-TEM technique. TEM images the microstructure without being model-dependent. Great care must be taken, however, to avoid introducing artifacts during preparation of the specimen.29 To minimize the likelihood of recording artifacts instead of real structures, different (28) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445. (29) Talmon, Y. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 364.

regions of the specimen and several duplicates need to be examined, and the results were found to be reproducible. Starting on the SDS-rich side of the phase diagram, spheroidal micelles are observed at Xp ) 0.10 in Figure 2A. As the fraction of DoTAC increases, a structure consisting of discrete micelles arranged in a cell-like pattern is seen at Xp ) 0.19, Xp ) 0.23, and Xp ) 0.29, in Figure 2B-D. Due to the low contrast of the polymer, only the surfactant micelles are visible in cryo-TEM. However, since the micelles are composed of SDS, DoTAC, and hydrophobic tails of the polymer, noting the position of the micelles is an indirect way of studying the polymer. Without the polymer, the position of the micelle appears random, as can be seen at Xs ) 0.23 in Figure 3, for example. It should be recalled that the TEM micrographs are two-dimensional projections of three-dimensional objects. Since the depth of the field is much greater than the thickness of the sample(10-300 nm), it is the entire thickness of the specimen that is observed, superimposed on the image plane. In thick sample films, the micelles appear to be randomly ordered, due to this overlap in projection. Thus, the cell-like structure of discrete micelles can only be observed in very thin areas of a sample film. In Figure 4 there is no film at all in the lower left corner of the micrograph; the border of the film is shown by arrowheads. As the film thickness increases, in looking diagonally upward and to the right of the image, the celllike structure becomes more difficult to detect. Whether the arrangement of the micelles is the result of a projection through a mesh in which necklaces of polymer molecules decorated with micelles pass over or connect to each other or of polymer molecules filling up the space in the cells and forming the structure is not distinguished on the micrograph (see discussion). In cryo-TEM, size segregation of the aggregates often appears in the film.24 At Xp ) 0.29, vesicles are found in thicker film areas (Figure 2E) and the cell-like structure of the micelles in thinner areas (Figure 2D). When the DoTAC fraction increases slightly, light is scattered by the solution, which appears whitish. At Xp ) 0.31, a broad polydispersity of the size and shape of the vesicle can be observed. Both multilamellar and unilamellar vesicles are seen (Figure 5A). Since TEM is limited to objects in the size range of some 1-500 nm, a light microscope with a resolution of about 0.5 µm was used to image larger objects. At Xp ) 0.31 giant vesicles of about 3-5 µm could be observed (Figure 5B). There was no difference in the micrographs between use of H2O and D2O as the solvent. Direct imaging also indicated the structures formed to be stable during storage. Thus, no changes in the Xp ) 0.23 sample examined both 1 week and 1 year after preparation could be detected. Water Self-Diffusion. Measurements of the selfdiffusion of water were performed to gain further knowledge of the structures in the solution. In the present situation, the self-diffusion of water is influenced by three factors that reduce the diffusion coefficient as compared with that of bulk water: (1) Hydration effects are caused by water that is bound to, e.g., the headgroup of surfactant, and as a result has dynamic properties that are different from those of the bulk water. (2) Obstruction effects are due to excluded volume. The obstruction depends on the size and, in particular, the shape of the aggregate. (3) The third effect is the confinement of water that is trapped in aggregates such as in vesicles. Since the total surfactant concentration and the polymer concentration are held constant, the changes in D/D0 (the actual diffusion coefficient obtained divided by the value for bulk water) reflect changes in solution microstructure

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Figure 2. Cryo-TEM micrographs of mixtures of 1% w/w HMHEC and 30 mm SDS + DoTAC at different molar fractions of DoTAC, X. Randomly ordered micelles are seen in (A) at Xp ) 0.10, and micelles ordered in a cell-like structure in (B) at Xp ) 0.19, in (C) at Xp ) 0.23, and in (D) at Xp ) 0.29. At Xp ) 0.29, ordered micelles coexist with vesicles (E).

rather than hydration. The water self-diffusion coefficients in SDS/DoTAC and SDS/DoTAC/HMHEC mixtures remain basically constant up to Xs ) 0.30 and Xp ) 0.29, respectively (Figure 6). Since the diffusion coefficient is sensitive to hydration, the water self-diffusion coefficient is lower when the polymer is present than it is in a pure

surfactant solution, due to hydration of the polymer. An investigation of a similar polymer, ethyl(hydroxyethyl) cellulose, showed that, at the same concentration as here of 1% w/w, the polymer reduces the water self-diffusion by the same amount as is observed here (D/D0 without surfactant ) 0.97).30 The fact that D/D0 ≈ 1 implies the

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Figure 3. Cryo-TEM micrograph of a mixture of 30 mm SDS + DoTAC at Xs ) 0.23.

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vesicles are present. The cryo-TEM pictures, however, reveal the presence of vesicles at a lower DoTAC fraction than the water self-diffusion coefficient. Marques et al.12 were able to detect the presence of giant vesicles and determine the fraction they represented through analyzing the biexponential decays observed for the water signal by means of self-diffusion NMR when the exchange between the water entrapped inside the vesicle and the bulk water was sufficiently slow. However, no such behavior was detected in our investigation in the range of X ) 0 to X ) 0.29. This might be due of the fact that there is fast exchange between bulk water and trapped water or, if this is not the case, that the concentration of vesicles present is very small or nonexistent. From Xs ) 0.31 and from Xp ) 0.30, D/D0 decreased in a manner typical of an increase in the fraction of vesicles. Surfactant Self-Diffusion. When aggregates grow, they diffuse slower and their transverse relaxation time T2 becomes faster, leading to a broadening of the peaks in the NMR spectra. The rapid relaxation that results leads to the signal disappearing in the echo. This effect, together with the low sensitivity of NMR, was evident in the R-CH2 peak of SDS, which could only be monitored up to Xp ) 0.21 and Xs ) 0.29, respectively. At fractions lower than this, the fast component of a biexponential fit of the main CH2 and CH3 peaks gave the same diffusion coefficient as that of R-CH2. The CH2 and CH3 peaks were used then to determine the diffusion coefficient of SDS above Xp ) 0.21 and Xs ) 0.29, respectively. At Xp ) 0.25 the peak produced by the DoTAC headgroup (N-(CH3)3) started to overlap with the polymer peak, due to peak broadening referred to above. The diffusion coefficient of DoTAC was then determined in the same way as for SDS, except that it was computed for the slow component of the CH2 and CH3 peaks. The surfactant self-diffusion coefficients presented in Figure 7 are the means of the two or three values obtained from the different peaks. The observed diffusion coefficient is affected by micellar growth, by the presence of vesicles, and by the amount of free surfactant. From the observed self-diffusion coefficient, the concentration of the free surfactant monomers and the fraction of the aggregated surfactant can be calculated using a two-site model:

Dobs ) pbDmic + (1 - pb)Dfree

Figure 4. Cryo-TEM micrograph of mixture of a 1% w/w HMHEC and 30 mm SDS + DoTAC at Xp ) 0.23. The cell-like structure of the micelles is observed in thin films only. As the film becomes thicker, the structure disappears due to the superposition of several layers. Frost particles are denoted by f.

aggregates obstruct the water diffusion very little. Thus the aggregates present are mainly spherical or cylindrical micelles, since at low to moderate concentrations particles of these shapes produce only minimal obstruction effects.31 At higher fractions of DoTAC, there is a break in the curve, D/D0 decreasing. The onset of the decrease coincides with the onset of turbidity in the solutions. On visual inspection, the solution of surfactant alone appears bluish and that containing also the polymer whitish. The turbid solution, the decreased water self-diffusion coefficient and the cryoTEM pictures obtained lead us to the conclusion that (30) Carlsson, A.; Lindman, B.; Nilsson, P.-G. Polymer 1986, 27, 431. (31) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, p. Colloid Polym. Sci. 1986, 264, 77.

(3)

Since the exchange of surfactant molecules between the monomeric state and the micelles on the experimental diffusion time scale, ∆, is rapid, the observed self-diffusion coefficient, Dobs, is a population-weighted average of the fraction of micellized (pb) and monomeric (1 - pb) surfactant molecules. Dfree and Dmic are the monomeric and micellized diffusion coefficients of SDS. Because DoTAC is the minority surfactant, all the DoTAC molecules are assumed to be in the mixed micelles, which means that Dmic is the same as the observed diffusion coefficient of DoTAC. It has often been assumed that when oppositely charged surfactants are mixed, the concentration of free surfactant monomers is negligible, even at low mixing ratios, due to the very low cmc values obtained, as measured, for example, with surface tension. In contrast to this, we find a non-negligible amount of free SDS monomers (Figure 8). Values for the concentration of free SDS monomers which have been reported, obtained by the tracer self-diffusion measurement of DoTAB-SDS, are in close agreement with the values shown in Figure 8.32 (32) Chorro, M.; Kamenka, N. J. Chim. Phys. 1991, 88, 515.

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Figure 5. (A) Cryo-TEM micrograph of an SDS and DoTAC mixture of 30 mm at Xp ) 0.31 and 1% w/w HMHEC. Unilamellar and multilamellar vesicles are seen. (B) In the same sample, larger vesicles are imaged by a light microscope.

Figure 6. Reduced water self-diffusion coefficient as a function of the molar fraction of DoTAC at a total surfactant concentration (SDS + DoTAC) of 30 mm. The open circles denote solutions without polymer, solid triangles denoting solutions containing 1% w/w HMHEC. The hatched area indicates turbid solutions that without polymer are bluish and with polymer whitish in appearance.

We have assumed in our study that when HMHEC is present, all the micellized surfactant molecules bind to the polymer, since it is unlikely that free surfactant micelles are formed at this polymer concentration when the total surfactant concentration is this low.33 Under such conditions, the diffusion coefficient of the micelles, Dmic, is the same as the diffusion coefficient of the polymer. The (33) Piculell, L.; Nilsson, S.; Sjo¨stro¨m, J.; Thuresson, K. In Associative Polymers in Aqueous Media; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 2000; Vol. 765, Chapter 19.

Figure 7. Surfactant self-diffusion coefficients as a function of the molar fraction DoTAC at a total SDS + DoTAC concentration of 30 mm. Mixtures of surfactants and 1% w/w HMHEC are marked by solid symbols, mixtures simply of surfactants being marked by open symbols. In both cases, circles denote the self-diffusion of SDS, and squares the self-diffusion of DoTAC. The insert presents the same results on a semilogarithmic plot.

polymer self-diffusion coefficient is often at least 1 order of magnitude lower than that of the free surfactant.34 When Dobs . Dmic, the first term in eq 3 can be neglected; the fraction of bound SDS is easily being calculated. Adding a polymer to a surfactant solution lowers the relevant (34) Nilsson, S.; Thuresson, K.; Nystro¨m, B.; Lindman, B. Manuscript in preparation.

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our surfactant concentration they obtained slightly higher aggregation numbers. In Figure 9, one can note that the aggregation number of SDS is 44, as compared with the value of approximately 60 found for pure SDS micelles.36 It is well-known that polymers typically lower the aggregation number of the surfactant. For example, the aggregation number of anionic and cationic surfactants is lower in the presence of ethyl(hydroxyethyl) cellulose (EHEC) than without.37,38 Above the critical aggregation concentration, hydrophobically modified EHEC and unmodified EHEC give the same surfactant aggregation number, which is lower than that for the binary surfactant/ water system.39 Discussion Figure 8. Concentration of free SDS monomers as a function of the molar fraction of DoTAC at a total SDS + DoTAC concentration of 30 mm. Open circles denote SDS/DoTAC mixtures, solid squares SDS/DoTAC/HMHEC mixtures, the open triangle representing the assumed concentration of free SDS in a pure SDS solution as taken from its cmc value. The lines are guides for the viewer.

Figure 9. Concentration of mixed micelles (open circles) and the surfactant aggregation number (solid circles) versus the molar fraction DoTAC at a total SDS + DoTAC concentration of 30 mm in solutions of 1% w/w HMHEC.

cmc. This phenomenon can also be seen in Figure 8, where the concentration of free SDS monomers is lower when HMHEC is present than it is for the surfactant solution alone. Note that at Xp ) 0 the value is close to the critical aggregation concentration of SDS in HEC solutions (critical aggregation concentration (cac) ) 6 mm SDS).33,35 The continuous decrease in the diffusion coefficient evident in Figure 7 reflects the growth of mixed micelles and a decrease in the concentration of free surfactant monomers. In a previous study it was shown that the concentration of mixed micelles decreases with increasing Xp, as can be seen in Figure 9.22 The surfactant aggregation number was calculated from the fraction of bound SDS (calculated from the data presented in Figure 8) and the concentration of mixed micelles (Figure 9). As the molar fraction of DoTAC increases, there is a large increase in the aggregation number, implying that the micelles are no longer spherical. Herrington et al.13 obtained similar values when they measured the aggregation number of the mixed surfactant system of SDS and DoTAB. At twice (35) Goddard, E. D.; Hannan, R. B. In Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2; p 835.

On the basis of the results just reported we may conclude that, at the concentrations studied, the addition of a hydrophobically modified nonionic polymer to a mixture of cationic and anionic surfactants induces only minor changes in phase behavior. At X ) 0, there are only micelles in the systems. As the molar fraction of DoTAC increases, the micelles grow to a nonspherical shape, as shown by the aggregation number. Vesicles are detected by water self-diffusion at Xp ) 0.30 and Xs ) 0.31. However, in cryo-TEM micrographs a small fraction of vesicles is also observed at significantly lower X values. Both the fraction of the vesicles and the polydispersity in vesicle size increase with X. Whereas the aggregate structure does not seem to change with addition of the polymer, the polymer apparently determines the position of the surfactant aggregates. What is observed in the cryo-TEM images of Figures 2 and 4 can be described as necklaces of surfactant micelles bound to the polymers. At Xp ) 0.10, the solution is of low viscosity and the polymer network is disconnected, most micelles are not shared by different polymer molecules. The repulsion between the micelles makes the polymer molecules rigid, similar to polyelectrolytes, and the micelles are evenly distributed due both to the repulsion and to the freely diffusing polymer strands. At Xp ) 0.19, when the number of polymer hydrophobic tails per mixed micelle is higher (the micelle concentration is half that found at Xp ) 0.10), the micelles have a higher probability of being shared by different polymer strands. On the basis of the data shown in Figure 9, the information that in the 1% w/w HMHEC solution there are 0.54 mm polymer hydrophobic tails, and under the assumption that all of the polymer tails are aggregated, there should be about 3 polymer tails/micelle at Xp ) 0.19 and about 4 at Xp ) 0.23. Although some of the hydrophobic tails within a micelle are from the same polymer strand, others must come from different polymer strands, because of the marked increase in viscosity. We can only speculate about the configuration of the polymer molecules in the cell-like pattern, but knowledge of the distances and the structure of the polymer should facilitate interpretation. We note that to observe the pattern using cryo-TEM, the sample film must be very thin, perhaps as thin as 15 nm. Because of the film being so thin, the two-dimensional picture we observe may be related to orientational effects at the surfaces and differ (36) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain, 1st ed.; VCH Publishers: New York, 1994. (37) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (38) Zana, R.; Binana-Limbele´, W.; Lindman, B. J. Phys. Chem. 1992, 96, 5461. (39) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909.

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Figure 10. Schematic view of a possible structure of the polymer surfactant network. The winding polymer chains are decorated with micelles, which are acting as cross-links between different polymer molecules.

from the microstructures in bulk more distant from a surface. However, if the average cell diameter is about 70 nm, then each cell contains 1.4 polymer molecules. Each polymer carries on the average 13 hydrophobic tails, the distribution of which is not known. The polymer might either fill up the space in the cell or follow the string of micelles. In the former case the polymer must be rather flexible and turn its hydrophobic tails outward to form mixed micelles with the surfactants. Cellulose ethers are, however, semirigid polymers with a persistence length of about 10 nm.40 We believe that the string of micelles are necklaces of polymer molecules decorated with micelles. The micelles cross-link different polymer molecules, and the bundles of cross-linked polymer molecules possibly diverge at the cell junctions; see Figure 10. In certain respects there are clear similarities between our system and the systems studied by Cabane et al.41 They assumed that in their polymer-surfactant systems a single polymer chain wraps around several micelles such as a necklace. Their early work was performed on poly(ethylene oxide) PEO-SDS,41 but poly(N-isopropylacrylamide) (PNIPAM)-SDS42 and ethyl(hydroxyethyl) cellulose (EHEC)-SDS 43systems have been studied more recently by means of neutron scattering. In the EHECSDS system, different polymer-surfactant necklaces formed an extensive network in which the SDS micelles were shared by several macromolecules, which was not the case in the PEO-SDS system. For the PEO-SDS system, Cabane et al. investigated whether the polymer changed the positions of the micelles. They found one PEO/ SDS composition (with a high polymer/micelle ratio) in which the polymer adapted to the micelle array and one composition (with an intermediate polymer/micelle ratio) in which the distance between the micelles on the same polymer molecule was shorter than the distance between micelles on different polymer molecules. This was possible because of the rather low polymer concentration (although the concentration was above c*) and the low surfactant (40) Kroon-Batenburg, L. M. J.; Kruiskamp, P. H.; Vliegenthart, J. F. G.; Kroon, J. J. Phys. Chem. 1997, 101, 8454. (41) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1987, 48, 651. (42) Lee, L.-T.; Cabane, B. Macromolecules 1997, 30, 6559. (43) Lindell, K.; Cabane, B. Langmuir 1998, 14, 6361.

Nilsson et al.

concentration (above cac). At a low polymer/micelle ratio, the surfactant micelles were in excess and showed the same spatial distribution as without polymer. The cell-like pattern referred to is observed at Xp ) 0.19-0.29, where the number of polymer hydrophobic tails per micelle, for this system, is high and the network is more constrained than that at lower X values. Under such conditions, the stoichiometry resembles the composition that Cabane found, with the intermediate polymer/micelle ratio described above. In analogy, the cell-like pattern might be understood as consisting of a mesh of polymersurfactant necklaces that are connected to each other. To be able to observe the cell-like structure by means of cryo-TEM, the solution must have a certain number of polymer hydrophobic tails per mixed micelle and the micelles must be large. When these conditions are fulfilled, the solution is often rather viscous. Since solutions that are too viscous are difficult to work with in cryo-TEM, solutions that are suitable for investigating the HMHEC/ SDS/DoTAC system are rather limited. Using cryo-TEM to investigate other systems, varying the substitution degree of hydrophobic modification of the polymer, and comparing different flexible polymers would provide more information of the generality of the observations here. Conclusions A highly organized structure of the micelles, forming a cell-like pattern, was found in the HMHEC/SDS/DoTAC system. It appears that the polymer induces this structure at a “high” number of polymer hydrophobic tails per micelle, with different polymer strands sharing the same mixed micelle. This cell-like pattern coincides with the region of high viscosity. At a low number of polymer hydrophobic tails per micelles, and in the SDS/DoTAC system without HMHEC, no such structure was observed. In the area investigated the micelles grew continuously as the fraction of the minority, oppositely charged surfactant was increased in amount. On further addition of the surfactant, the micellar phase was found to coexist with vesicles, the fraction and the polydispersity of the vesicles gradually increasing. The concentration of free SDS monomers in the aqueous solution decreased with an increase in the molar fraction DoTAC and was throughout lower than the cmc of SDS. Acknowledgment. We thank Bjo¨rn Håkansson and Olle So¨derman for valuable discussions and technical assistance on NMR self-diffusion. Yuliya Melnikova is gratefully acknowledged for taking some of the cryo-TEM pictures. We also thank Eduardo Marques for interesting discussions on catanionic surfactant mixtures. S.N. and B.L. thank the Center for Amphiphilic Polymers for financial support. The work at the Technion was supported in part by a “Center-of-Excellence” grant from the Israel Academy of Science and Humanities and by the Fund for the Promotion of Research at the Technion, the Technion V.P.R. Fund-C. Welner Research Fund. LA991379+