Surfactant-Induced Collapse of Polymer Chains and Monodisperse

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Langmuir 2003, 19, 230-237

Surfactant-Induced Collapse of Polymer Chains and Monodisperse Growth of Aggregates near the Precipitation Boundary in Carboxymethylcellulose-DTAB Aqueous Solutions Samuel Guillot,† Michel Delsanti,‡ Sylvain De´sert,‡ and Dominique Langevin*,† Laboratoire de Physique des Solides, Baˆ timent 510, Universite´ Paris-Sud, 91405 Orsay Cedex, France, and Service de Chimie Mole´ culaire, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France Received July 22, 2002. In Final Form: November 7, 2002 We have studied aqueous solutions of a polyelectrolyte, carboxymethylcellulose, which is an anionic cellulose derivative, and a cationic surfactant, dodecyltrimethylammonium bromide (DTAB). We have investigated the interactions between the two species, both at the air/water interface and in the bulk, for increasing DTAB concentrations. Mixed surfactant/polymer aggregates are formed at the air/water surface at extremely low surfactant concentrations, whereas bulk aggregates are formed later, above a critical aggregation concentration (cac). A small viscosity maximum at the cac reveals a small degree of bridging of the polymer chains by the surfactant. Above the cac, the viscosity drops, indicating that the polymer chains undergo a rapid collapse. At higher surfactant concentrations, light scattering shows the existence of larger structures, which are surprisingly monodisperse and whose size increases with surfactant concentration. At still higher surfactant concentrations, a classical strong associative phase separation takes place. During the evolution of bulk properties, the surface tension remains constant, suggesting that the surface aggregates remain unchanged.

1. Introduction Mixed polymer and surfactant solutions in water have attracted much interest1,2 due to their frequent occurrence in many areas, both in industry and in biology. Early studies of polymer/surfactant interactions in aqueous solutions were done in the 1980’s.3 Interactions strongly depend on the macromolecule and surfactant types, leading to different macroscopic behavior of the solutions. Several types of binding have been identified in surfactant/ hydrophobic polymers,4 neutral polymer/anionic surfactant, or polyelectrolyte/surfactant of opposite charge.5,6 In the latter case, the polymer electrical charges play an important role still to be understood. These solutions also have unusual surface properties. Mixed layers of polyelectrolytes and surfactants of opposite charges are thicker than the pure surfactant layer which could, for instance, strengthen the surface against deformation and stabilize foams. Another important feature is the strong tendency to induce associative phase separation. Significant theoretical progress has been made recently.7-9 In particular, the role of the polymer backbone * To whom correspondence should be addressed. † Universite ´ Paris-Sud. ‡ CEA Saclay. (1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Picullel, L.; Lindman, B.; Karlstroem, G. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; p 65. (4) Saito, S. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987. (5) Sokolov, E.; Yeh, F.; Kohkhlov, A.; Grinberg, V.; Chu, B. J. Phys. Chem. B 1998, 102, 7091. (6) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; John Wiley and Sons: New York, 1976; Chapter XII. Exerowa, D.; Kashchiev, D.; Platikanov, D. Adv. Colloid Interface Sci. 1992, 40, 201. Tsekov, R.; Ruckenstein, E. Langmuir 1993, 9, 3264. (7) Diamant, H.; Andelman, D. Macromolecules 2000, 33, 8050. (8) Diamant, H.; Andelman, D. Phys. Rev. E 2000, 61, 6740. (9) Hansson, P. Langmuir 2001, 17, 4161.

rigidity has been predicted to be very important. We have recently evidenced the important role of the polymer backbone rigidity on the properties of the mixed layers at the solution surface and on the stability of foam films made from the solutions.10 Our previous studies were done using the surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) and polyelectrolytes with very different backbone rigidity: xanthan which has a rigid backbone (intrinsic persistence length lpint ) 140 nm in the double-helix form11) and polyacrylamido-methylpropanesulfonate (PAMPS) which is more flexible (with a persistence length likely to be close to that of polystyrene sulfonate,12 lpint ) 1 nm). Let us recall that in water the actual persistence length is larger, because it contains a contribution of the electrostatic charges, which decreases when salt is added. In this work, we have used sodium carboxymethylcellulose (carboxyMC), an anionic polysaccharide with an intermediate backbone rigidity (lpint ) 5-16 nm13-16). CarboxyMC is a useful component of many food systems and is widely used as a thickener and binding agent in pharmaceutical applications. In this paper, we first present results on the adsorption of carboxyMC/DTAB mixtures at the air/water interface as a function of the surfactant concentration. We then describe results on the (10) Bhattacharryya, A.; Monroy, F.; Langevin, D.; Argillier, J.-F. Langmuir 2000, 16, 8727. (11) Milas, M.; Rinaudo, M.; Duplessix, R.; Borsali, R.; Lindner, P. Macromolecules 1995, 28, 3119. (12) Barrat, J. L.; Joanny, J. F. Adv. Chem. Phys. 1996, 94, 1. (13) Rinaudo, M. Cellulose and Cellulose Derivatives: Physicochemical Aspects and Industrial Applications; Kennedy, J. F., Philips, G. O., Williams, P. O., Piculell, L., Eds; Woodhead Publishing: Cambridge, U.K., 1995; p 257. (14) Kamide, K.; Saito, M.; Suzuki, H. Makromol. Chem., Rapid Commun. 1983, 44, 33. (15) Lavrenko, P. N.; Okatova, O. V.; Tsvetkov, V. N.; Dautzenberg, H.; Philipp, B. Polymer 1990, 31, 348. (16) Hoogendam, C. W.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H.; Smit, J. A. M.; van Dijk, J. A. P. P.; van der Horst, P. M.; Batelaan, J. G. Macromolecules 1998, 31, 6297.

10.1021/la0206561 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/21/2002

Surfactant-Induced Behavior near Precipitation

Figure 1. Schematic illustration of the repeating unit structure of sodium carboxyMC: Glucose units (considered as monomers) are linked by β-1,4-linkages. R ) -H or -CH2COONa depending on the degree of substitution DS.

bulk properties of the solutions and relate them to the surface behavior. We finally propose a detailed picture for the association phenomenon before complete associative phase separation. 2. Materials and Methods 2.1. Materials. DTAB was chosen as a cationic surfactant. It was obtained from Aldrich (purity 99%) and then recrystallized three times before use. Sodium bromide (NaBr) was also from Aldrich and was used as supplied (purity better than 99%). CarboxyMC is a water-soluble, anionic, linear polymer. This cellulose derivative is prepared by an etherification of monochloroacetic acid17 (Williamson synthesis), leading to partial substitution of the 2, 3, and 6 hydroxyl groups by hydrophilic carboxymethyl groups; its structure is shown in Figure 1. It is known that the distribution of substituted residues is nonuniform18 because of the heterogeneous conditions used in the preparation similar to that of methylcellulose: carboxyMC is a random block copolymer.19 Polysaccharides are known to have different properties depending on chemical composition, structure, and molecular weight. Several types of carboxyMC are available and differ from one another by their substitution degree DS (0 < DS e 3), which is the average number of carboxymethyl groups per monomer unit. Different DS values imply different amounts of charge per monomer (on average). In this study, we used Blanose Sodium CarboxyMethylCellulose 12M31P kindly supplied by Aqualon Hercules for which DS ) 1.23 and the monomer length is b ≈ 5 Å.20 In aqueous solutions, the Bjerrum length is lB ≈ 7 Å and we have b/lB < DS, meaning that partial counterion condensation is expected. However, the degree of condensation is certainly not uniform because of the nonuniform substitution. Blanose Cellulose Gum is a highly purified type of carboxyMC because of its use in food, cosmetic, and pharmaceutical applications; its minimum purity is 99.5% (defined for the dry substance; carboxyMC is known to be highly hydrated21). No further purification was done before use. The polymer solution is prepared by dissolution in deionized water (Millipore Milli-Q system) under stirring for at least 8 h. CarboxyMC/DTAB solutions were obtained by adding polymer to the pure surfactant solutions. For the light scattering experiments, the solutions have been centrifuged at 2600g for 10 min to remove dust particles (5000 rpm, rotor Sigma 12156H). 2.2. Surface Tension Measurements. Surface tension experiments were performed with two methods. The first one is the Wilhelmy method with an open frame, made of soldered platinum wires.22 This method avoids the irreversible adsorption and wetting problems of the more classical Wilhelmy plate23 and (17) Nussinovitch, A. Cellulose Derviatives. In Hydrocolloid Applications. Gum technology in the food and other industries, 1st ed.; Blackie Academic & Professional: London, 1997; Chapter 6, pp 105124. (18) Bhattacharjee, S. S.; Perlin, A. S. J. Polym. Sci., Part C 1971, 36, 509. (19) Arisz, P. W.; Kauw, H. J. J.; Boon, J. J. Carbohydr. Res. 1995, 271, 1. (20) Meyer, K. H.; Misch, L. Helv. Chim. Acta 1937, 20, 234. (21) Koda, S.; Hasegawa, S.; Mikuriya, M.; Kawaizumi, F.; Nomura, H. Polymer 1988, 29, 2100. (22) Mann, E. K. Ph.D. Thesis, Universite´ Paris VI, Paris, 1992. (23) Mann, E. K.; Langevin, D. Langmuir 1991, 7, 1112.

Langmuir, Vol. 19, No. 2, 2003 231 was used for surfactant concentrations below the critical aggregation concentration (cac). For concentrations close to or above the cac, the film drawn from the solutions is too stable and does not break. We then used the axisymmetric drop shape analysis method, with a pendant drop tensiometer from IT Concept24 (Longessaigne, France). When using the first method, we worked at room temperature, typically 20 °C. In the second case, the temperature was controlled at 25 ( 0.5 °C. It must be stressed that the behavior of the solutions does not depend on temperature, at least in the range where the experiments have been done (the typical surface tension variation is 0.1 mN/m/°C). 2.3. Light Scattering and Refractometry Measurements. Quasi-elastic and static light scattering have been performed on an Amtech spectrometer SM200 using a vertically polarized argon ion laser operating at λ ) 514.5 nm. The sample temperature was controlled at 25 ( 0.5 °C. The scattered light was detected by a photomultiplier at scattering angles θ ranging from 30 to 150°, corresponding to scattering wave vectors q ) (4πn/λ) sin(θ/2) between 8.4 × 10-3 < q < 31.4 × 10-3 nm-1, where n is the solution refractive index. The output from the photomultiplier is fed into a Brookhaven correlator BI2030AT, which counts the photons and calculates the normalized intensity autocorrelation function g2(q,t). If the scattering field has a Gaussian statistic, the autocorrelation function g2(t) is related to the normalized scattered electric field autocorrelation function g(t) through the Siegert25 relation g2(q,t) ) 1 + B|g(q,t)|2, where B e 1 is an instrumental parameter. Dynamic light scattering is a convenient method to study the dynamics of complex fluid solutions, g(t) being proportional to the concentration fluctuation autocorrelation function. A diffusion coefficient D(q) ) 1/τq2 can be deduced, where τ is the characteristic decay time of g(t). When D is independent of q, it can be postulated that the relaxation observed is diffusion controlled. When particles are present and if their concentration is small enough (negligible interactions), a hydrodynamic radius RH of the particles can be deduced from the Stokes-Einstein equation D ) kT/(6πη0RH), η0 being the solvent viscosity, T the absolute temperature, and k the Boltzmann constant. In static light scattering experiments, the “absolute value of the scattered intensity”, that is, the Rayleigh factor of the sample, Rsample, is obtained by calibrating with a benzene standard sample: Rsample ) Rbenzene(nsample/nbenzene)2Isample/Ibenzene, where n is the refractive index and I is the scattered intensity in arbitrary units. The value of Rbenzene used is 3 × 10-5 cm-1. To measure the refractive index of the solution, we used a Knauer differential refractometer working at λ ) 950 nm. The solutions being nonabsorbing, the differences between the refractive index at 950 and 514.5 nm are extremely small and are not expected to introduce significant errors in the data analysis. The first cell is filled with water, which is the reference; the second cell is filled with the solution whose refractive index n is to be determined. The refractometer gives a potential difference ∆V linearly related to the difference of refractive index ∆V ) K(n - nwater); K is determined by a calibration with standard salt solutions (NaCl, KCl). 2.4. Viscosity. Shear viscosity of the solutions was measured by using capillary glass tubes: Cannon-Fenske routine glass viscometers by Ever Ready Thermometer Co. Different viscometers were used with different flow rate ranges: type 25 (range 0.5-2 mm2/s, that is, 0.5-2 cP for dynamic viscosities considering the density of water) and type 75 (range 1.8-8 cP). After a fixed amount of solution was poured into the viscometer, the solution and water flow times, t and t0, were measured. Knowing water viscosity η0, we can deduce the viscosity of our solutions with η ) η0(t/t0), neglecting corrections due to input and output effects.26 The relative viscosity, ηrel, is defined as ηrel ) η/η0 ) t/t0, and the specific viscosity is ηsp ) (η - η0)/η0. Measurements were carried out at 26 ( 1 °C. This method can be used here because of the (24) Labourdenne, S.; Gaudry-Rolland, N.; Letellier, S.; Lin, M.; Cagna, A.; Esposito, G.; Verger, R.; Riviere, C. Chem. Phys. Lipids 1994, 71, 163. (25) Chu, B. Laser Light Scattering. Basic Principles and Practice, 2nd ed.; Academic Press: San Diego, 1991. (26) Whorlow, R. H. Rheological Techniques; J. Wiley & Sons: New York, 1980. Bagley, E. B. J. Appl. Phys. 1957, 28, 624.

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Figure 2. Equilibrium surface tension of carboxyMC at the air/water interface and at room temperature as a function of polymer concentration. The dashed line corresponds to the water value.

Guillot et al.

Figure 3. Relative viscosity of carboxyMC aqueous solutions at 25 °C.

quite low viscosity of the solution which is close to that of water. The effect of shear is probably weak, especially at the low velocities and shear rates used here, less than 0.01 cm/s and 0.04 s-1, respectively: only objects of micrometric sizes could be affected, and as we will see later, the aggregates found in the solutions are much smaller.

3. Results and Discussion 3.1. Surfactant-Free Solutions. Surface tension measurement is informative in mixed systems when one of the two components is non-surface-active. First, we measured the variation of the surface tension γP of the polymer solutions with the polymer concentration CP. The surface tension equilibrium values for carboxyMC aqueous solutions as a function of CP are gathered in Figure 2. Below 7 g/L, the surface tension is constant and quite close to the water value, meaning that carboxyMC has no surface activity at concentrations below 7 g/L. Also, when those polyelectrolyte solutions are vigorously shaken, no foam appears. Therefore, in our study, we used polyelectrolyte concentrations below 7 g/L to ensure that the polymer alone did not adsorb at the air/water interface. Above 7 g/L, the surface tension of the polyelectrolyte drops, probably because of self-screening of electrostatic interactions, as observed for other polyelectrolyte solutions.27 In addition, polyelectrolyte solutions are known to exhibit high viscosities even at low concentrations and to display different viscosity regimes.28 To clarify this point, we have performed viscosity measurements as a function of CP for carboxyMC solutions. Relative viscosity data are shown in Figure 3 where two different power laws as a function of Cp can be clearly seen. In the first regime, below Ce ≈ 1.74 g/L, ηrel follows a power law, with a slope of 0.54, close to the well-known behavior for dilute polyelectrolyte solutions, ηrel ∼ Cp1/2; polymer chains overlap but do not entangle, and the chain dynamics is Rouse-like. Above the concentration Ce (determined as the intersection of the two power laws), we enter an entangled regime, characterized by another power law with a slope of 1.55, close to the theoretical prediction of the scaling theory of polyelectrolytes in semidilute solution ηrel ∼ Cp3/2.29 We notice that the viscosity of 4 g/L solutions is already 40 times larger than water viscosity. To work (27) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412. (28) Boris, D. C.; Colby, R. H. Macromolecules 1998, 31, 5746.

Figure 4. Variation of the equilibrium surface tension of carboxyMC/DTAB mixed layers (filled symbols) and DTAB (open symbols) only, as a function of DTAB concentration.

with fluid solutions, we have chosen a smaller polymer concentration: CP ) 0.127 g/L for which η127 ) 2 cP at 25 °C. 3.2. Carboxymethylcellulose/DTAB Mixed Solutions. 3.2.1. Surface Tension. Adsorption kinetics studies of carboxyMC/DTAB mixtures were carried out at fixed polymer concentration (CP ) 0.127 g/L) with varying surfactant concentration CS. Adsorption times can be quite long in these systems, up to hours, as in xanthan/ DTAB or PAMPS/DTAB systems.30 Equilibrium values are taken as equal to the measured value when no change is observed after a minimum elapsed time of 30 min. The equilibrium surface tensions of the mixed solutions are plotted in Figure 4 as a function of CS. We also show the surface tension of the pure surfactant solutions, where one sees that the critical micelle concentration (cmc) is about 15 mM. We note that the surface tension curve of the mixed solutions is well below the pure surfactant curve, underlining the strong interaction between DTAB and carboxyMC at the surface, which begins at very low surfactant concentrations. The synergistic surface tension lowering results from polymer and surfactant adsorption at the surface caused by electrostatic interactions between (29) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Macromolecules 1995, 28, 1859. (30) Ritacco, H.; Albouy, P.-A.; Bhattacharryya, A.; Langevin, D. Phys. Chem. Chem. Phys. 2000, 2, 5243.

Surfactant-Induced Behavior near Precipitation

the polyanion and the cationic surfactant.31 After an inflection point at a surfactant concentration of about CS ≈ 0.1 mM, the surface tension reaches a plateau at γcac ≈ 44 mN/m; this concentration is called the critical aggregation concentration. Electrostatic attractions become important in the bulk solution, where complexes also start to form; the cac is normally lower than the surfactant critical micelle concentration, and the gap between cac and cmc is a measure of the attraction strength between polymer and surfactant.32 With carboxyMC, the cac plateau is quite larger than for polyacrylamide sulfonates or DNA, almost two surfactant concentration decades as reported earlier.33 It is commonly accepted that in polymer-surfactant systems where a similar surface tension break point is observed (cac), micelles start to form and decorate the polymer chains. The existence of these micelles was first demonstrated in the poly(ethylene oxide) (uncharged)-sodium dodecyl sulfate (SDS) system.34 The association is driven there by the hydrophobic interactions between the surfactant chains and the hydrophobic regions along the polymer chain. CarboxyMC has a weak interaction with SDS as in other polymer-surfactant systems where the polymer and surfactant charges are of the same sign.35 This indicates that the hydrophobic interaction of carboxyMC with SDS chains should be strong enough to overcome the repulsive electrostatic forces. This could explain why the association is very strong when attractive electrostatic forces are present, because then the two forces have cumulative effects. The existence of micelles was also reported recently in DTAB/carboxyMC systems.33 We notice that the surface tension does not change in the plateau between cac and cmc. However, the solutions evolve from clear until 5 mM to cloudy above 7 mM. In addition, a macroscopic associative phase separation occurs at about CS ) 9 mM. At higher surfactant concentrations, it is likely that the polymer is less adsorbed at the surface and more incorporated in bulk aggregates. Indeed, the surface tension values approach that of the pure surfactant solutions meaning that the surface layer probably only contains surfactant.36 No polymer redissolution has been observed at 0.127 g/L even up to large surfactant concentrations (35.6 mM), as is usual for a polyelectrolyte with high charge density.37 3.2.2. Refractive Index. We first determined the refractive index38 of carboxyMC solutions (Figure 5) and measured the refractive index increment (dn/dC)950nm ) 0.144 cm3/g, in agreement with classical values for cellulose derivatives.39 We then measured the variation of the refractive index n of the mixed solutions with CS (Figure 6). Below 0.1 mM, n does not change much and remains equal to the value of the polymer alone in water (∆n)pol ) 1.782 × 10-5, within experimental accuracy. Until 1 mM, the DTAB contribution, nsol - npol, is the same as the pure DTAB value (∆n)surf ) nsurf - nwater (the surfactant refractive index increment was determined here as (dn/ dCS)950nm ) 0.093 cm3/g). This shows that the refractive (31) Asnacios, A.; Klitzing, R.; Langevin, D. Colloids Surf., A 2000, 167, 189. (32) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (33) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038. (34) Cabane, B. J. Phys. Chem. 1977, 81, 1639. Cabane, B.; Duplessix, R. J. Phys. 1987, 48, 651. (35) Schwuger, M. J.; Lange, H. Tenside 1968, 5, 257. (36) Jean, B.; Lee, L.-T.; Cabane, B. Langmuir 1999, 15, 7585. (37) Fundin, J.; Brown, W.; Iliopoulos, I.; Claesson, P. M. Colloid Polym. Sci. 1999, 277, 25. (38) Huglin, M. B. Light scattering from polymer solutions; Academic Press: New York, 1972. (39) Hirrien, M.; Desbrie`res, J.; Rinaudo, M. Carbohydr. Polym. 1996, 31, 243: dn/dC ) 0.136 cm3/g for methylcellulose.

Langmuir, Vol. 19, No. 2, 2003 233

Figure 5. Refractive index increment of carboxyMC aqueous solutions. The dotted line is a linear fit.

Figure 6. Refractive index increments (∆n)X ) nX - nwater as a function of DTAB concentration for carboxyMC/DTAB aqueous solutions. The full line corresponds to the surfactant-free value, and the dashed line to the polymer-free samples.

index is not sensitive to the interactions between polymer and surfactant. Above 1 mM, nsol - npol > nsurf - nwater, and the behavior of the refractive index is not additive anymore, indicating strong polymer-surfactant interactions. 3.2.3. Viscosity. The viscosity is also sensitive to complexation because it is sensitive to polymer conformation, although in the concentration range studied, the viscosity is not affected by the presence of the surfactant alone. The viscosity of polymer solutions depends in general on the dimensions and on the molecular weight of the polymer. Here, a change in viscosity reflects a change in polymer chain conformation, either extension or shrinkage. To separate electrostatic effects from hydrophobic effects, we have performed viscosity measurements not only in carboxyMC/DTAB solutions but also in carboxyMC/NaBr solutions; the data are shown in Figure 7. For both systems, the specific viscosity slowly decreases from the polymer solution viscosity ηsp,pol ) 1.23, but even at low concentrations (surfactant or salt) the effect on viscosity is different. Below cac, there is no salt effect, whereas with DTAB a small viscosity decrease occurs. Even there, the effect of surfactant addition to the polymer solutions is not equivalent to that of salt addition. For surfactant-free (or salt-free) polymer solutions, polyelectrolyte chains are in an extended conformation; the fact that the data with DTAB are below the NaBr data at low concentration shows that a small amount of DTAB is more

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Figure 7. Specific viscosity of carboxyMC aqueous solutions as a function of added DTAB or NaBr concentration at 25 °C. The dashed line corresponds to a 0.127 g/L carboxyMC solution without any surfactant or salt, and the full line to water.

efficient than salt to recoil the chains. This might suggest that the cac is smaller than indicated by the break point in the surface tension curve. However, binding isotherms lead to the same cac values, within experimental accuracies.33,40 Above 0.1 mM salt, the polymer/salt solution viscosity decreases: salt screens the electrostatic repulsions between carboxyMC charges, and the chains begin to collapse. The collapse is however less rapid than with DTAB. The reduction in size of the chains is such that they no longer overlap; the viscosity evolves from the semidilute to the dilute regime and decreases accordingly. In the case of DTAB addition, a small increase is clearly visible above 0.08 mM upon reaching the cac. This increase might be due to the presence of some surfactant micelles linking different polymer chains. This effect is known to lead to large viscosity increases in the case of polymers with hydrophobic monomers.41 Such a maximum has not been observed with PAMPS, but it has been measured in cationic starch/anionic surfactant systems.42 Above the cac, a sharp viscosity drop occurs for increasing surfactant concentration and reaches water viscosity at about 1 mM. This reduction in viscosity is much larger with surfactant than with salt. No turbidity arises during this sharp drop; the solution becomes cloudy only at about 7 mM. Then, this decrease is certainly due to a shrinkage of the carboxyMC chains and not to the precipitation of the complexes as in the case of PAMPS;43 precipitation occurs here at a much higher concentration (9 mM). The polymer configuration depends on the polymer persistence length lp. Most cellulose derivatives are known to have an intrinsic persistence length in the range of 5-20 nm,44 but lp is also affected by the electrostatic repulsions in the polyelectrolyte chain itself. In fact, the choice of a low polymer concentration allowed us to start from a semidilute polyelectrolyte solution and to reach easily a conformation of isolated polymer globules by adding surfactant or salt. It has also permitted us to enhance small details of the viscosity curve. Since the (40) Jain, N.; Guillot, S.; Turmine, M.; Letellier, P.; Langevin, D. Unpublished data. (41) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (42) Merta, J.; Stenius P. Colloids Surf., A 1997, 122, 243. (43) Asnacios, A.; Langevin, D.; Argillier, J. F. Eur. Phys. J. B 1998, 5, 905. (44) Yalpani, M. Polysaccharides; Elsevier: New York, 1988.

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Figure 8. Electric field autocorrelation functions for a 4 mM DTAB/carboxyMC solution, at different scattering angles, as a function of tq2.

interaction between the polymer and surfactant is dependent on the surfactant concentration, the structure of the polymer-surfactant complex is also dependent on surfactant concentration. It was demonstrated that close to the cac, the polymer chains were decorated by micelles,34 but the collapse occurring afterward might cause structural changes. 3.2.4. Light Scattering. To have more information about the origin of the increase of turbidity of the solutions before the precipitation zone, we have performed dynamic and static light scattering experiments. We have checked that at DTAB concentrations equal to or below 7 mM, no multiple scattering occurs by checking the amplitude; B of g2(q,t) remains quite the same within instrumental accuracy. We know that when multiple scattering occurs, B drops. Dynamic light scattering experiments were used to study the region in the vicinity of precipitation in mixtures of polymers and surfactants of opposite charge.37,45-47 Typical normalized first-order field autocorrelation functions g(q,t) as a function of tq2 are depicted in Figure 8 for 0.127 g/L carboxyMC and 4 mM DTAB solutions in a semilogarithmic representation. Figure 9 shows the plot of D versus q for 5 mM DTAB. Similar results were obtained for all the DTAB concentrations above 2 mM; g(q,t) is a single-exponential function, and the same diffusion coefficient is deduced whatever the scattering angle is. Because of the small values of the viscosity, we know that the chains are no longer extended at these surfactant concentrations. Assuming that the scattering is due to polymer/surfactant aggregates and that the Stokes-Einstein formula is due to a dilute solution of polymer/surfactant aggregates, a hydrodynamic radius RH of these aggregates can be deduced. In Figure 9, between qRH ) 0.75 and 1.75, no q-dependence of the diffusion coefficient is observed; therefore no internal modes are present, and the objects are rigidlike, as are latex particles or methylcellulose temperature-induced aggregates48 for instance. The same behavior is observed at other concentrations. Figure 10 shows the evolution of the correlation functions when DTAB concentration increases. The dif(45) Wang, Y.; Kimura, K.; Huang, Q.; Dubin, P. L.; Jaeger, W. Macromolecules 1999, 32, 7128. (46) Wang, Y.; Kimura, K.; Dubin, P. L.; Jaeger, W. Macromolecules 2000, 33, 3324. (47) Wang, Y.; Dubin, P. L.; Zhang, H. Langmuir 2001, 17, 1670. (48) Guillot, S. Ph.D. Thesis, Universite´ Paris-Sud, Paris, 2001.

Surfactant-Induced Behavior near Precipitation

Figure 9. Diffusion coefficient for a 5 mM DTAB/carboxyMC solution as a function of the scattering wave vector.

Langmuir, Vol. 19, No. 2, 2003 235

Figure 11. Apparent hydrodynamic radius of the aggregates versus DTAB concentration. The full line corresponds to an exponential fitting. Table 1. Measured Polydispersity at 90° from a Cumulant Analysis of the Autocorrelation Function at Several DTAB Concentrations

Figure 10. Electric field autocorrelation functions of mixed DTAB/carboxyMC solutions with different DTAB concentrations. The lines are single-exponential fits.

fusion coefficient D decreases, suggesting growth of the aggregates. The rigorous procedure to obtain the exact radius is to dilute the solutions, but we cannot use it here because of the complexity of the phase diagram of the system. We have plotted in Figure 11 the evolution of the hydrodynamic radius RH as a function of DTAB concentration from 2 to 7 mM. It drastically increases from 12 to 160 nm in an exponential way. If we extrapolate the curve to the surfactant concentration for precipitation, we see that the aggregates reach micronic sizes, so that they should sediment, as observed. The smallest value corresponds to the size of the polymer chain in concentrated salt solution (1 M NaBr), that is, the size of the collapsed chain, which is also comparable to lpint. This confirms the collapse mechanism discussed before (section 3.2.3). We notice that for this polymer concentration no segregative phase separation (one polymer-rich and one surfactant solution) is found when salt is added. The remarkable monodispersity of the aggregates is not observed at the smallest DTAB concentrations: the autocorrelation function is not a single exponential, and the distribution of relaxation times is relatively broad. Upon increasing CS, the width of the distribution decreases until the regime of singleexponential decay of g(q,t) is reached. The polydispersity,

CS (mM)

polydispersity p

CS (mM)

polydispersity p

0.7 1 1.5 2 2.5 3

0.36 0.29 0.13 0.1 0.06 0.04

3.5 4 5 6 7

0.02 0.02 0.04 0.03 0.04

defined as p ) 〈D2〉/〈D〉2 - 1 and obtained from a cumulant analysis, is reported in Table 1 and is less than 0.1 for CS > 2 mM. The complexes are very stable. The measured size in the bluish 5 mM sample did not change during 9 months; the 4 mM sample did not change either during several months although a small change in size of about 3 nm was observed during the first 3 days. These results can be correlated with a recent work49 on DNA/DTAB complexes. Fluorescence microscopy allowed visualizing that compaction of single DNA is induced by DTAB. The polymer chain starts from an extended conformation at low surfactant concentration and evolves toward a compact globular state at high surfactant concentration. In addition, coexistence of coils and globules clearly appears at intermediate concentrations. This intermediate range could correspond to the range in which we observe broad size distributions and the strong lowering of viscosity. To check the assumption made for the origin of the scattering, we have also performed static light scattering measurements. Again, it is impossible to perform Zimm plots because the solutions cannot be diluted. Nevertheless, the angular variation of the scattered intensity, I(q), can be measured. In the case of scattering by objects of gyration radius RG, in the dilute regime this variation is given by the expression

I)

I0 1 + q2RG2/3

when qRG e 1

(1)

We have measured the intensity I(q) for four DTAB concentrations, 1, 2, 3, and 4 mM. For the three first solutions, the angular variation was too small to allow (49) Miguel, M. G.; Marques, E.; Dias, R.; Mel’nikov, S. M.; Kahn, A.; Lindman, B. Prog. Colloid Polym. Sci. 1999, 112, 157.

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Guillot et al.

Table 2. Equivalent Radii R for the Volume Occupied by Surfactant and Polymer in the Aggregates, Together with the Hydrodynamic Radii RH and the Effective Volume Fraction Oeff ) O(RH/R)3 a CS (mM)

RTH (nm)

R (nm)

RH (nm)

φeff (%)

2 3 4

7 33 40

6.2 ( 0.1 8.0 ( 0.1 13.8 ( 0.1

13.9 ( 0.6 24.0 ( 1.9 35.8 ( 3.2

0.5 ( 0.1 2.0 ( 0.5 1.8 ( 0.5

a

RTH is the radius calculated after eq 4.

the determination of the gyration radius. We could therefore only use the 4 mM sample. Assuming that the solution is dilute enough to be able to use eq 1, we have obtained a radius of gyration RG ) 26 ( 3 nm at CS ) 4 mM. Here, RH ) 36 ( 3 nm and RH/RG ) 1.38 ( 0.27. This value larger than 1 and close to the value of 1.29 expected for spherical particles suggests that these structures are globular. In the case of scattering close to a critical point where concentration fluctuations of correlation length ξ are present, the scattered intensity is given by

I)

I0

(2)

1 + q2ξ2

In this case, the diffusion coefficient is given by D ) kT/ (6πηξ), where η is the viscosity of the solvent. We would have obtained ξ ) 15 nm from the intensity measurements, and ξ ) 36 nm from the inelastic scattering measurements. This rules out the fact that the scattering could come from critical concentration fluctuations. Let us note that the two ξ values are not the same in experiments made with polymer solutions. For polymers in good solvents, ξΗ/ξ varies with concentration: 1 < ξΗ/ξ < 3.50 In theta solvents, the situation is more complicated, two lengths are measured in quasi-elastic light scattering (QELS),51 a distance between entanglements and the hydrodynamic screening length ξΗ: ξΗ/ξ ≈ 4.52 It is therefore not possible to exclude the existence of scattering by polymeric objects, although this does not seem likely in view of the viscosity behavior. We have also extracted a characteristic size from the absolute value of the scattered intensity at zero scattering wave vector. Theory predicts for diluted objects of volume V

Rsolution(qf0) - Rsolvent )

4π2 dn 2 n φV λ4 dφ

( )

(3)

φ being the volume fraction of the objects. Assuming that the objects are spheres (V ) 4πR3/3), containing all the surfactant and the polymer but no water, we can use the dn/dφ of Figure 6 and calculate the radius R of this sphere. CarboxyMC and DTAB densities were taken as equal to that of water. The results are given in Table 2 together with the hydrodynamic radii and the effective volume fractions defined as φeff ) φ(RH/R)3. The effective volume fraction is a few times larger than that occupied by the surfactant and the polymer, indicating that there is a considerable amount of water in the aggregates. However, the effective volume fraction remains small, less than or around 2%, so our assumption of negligible interactions between aggregates is probably valid. Furthermore, this (50) Wiltzius, P.; Haller, H. R.; Cannell, R. Phys. Rev. Lett. 1984, 53, 834. (51) Adam, M.; Delsanti, M. Macromolecules 1985, 18, 1760. (52) Stepanek, P.; Perzynski, R.; Delsanti, M.; Adam, M. Macromolecules 1984, 17, 2340.

explains why the viscosity of the solutions is indistinguishable from that of water. If we use the Einstein formula for spheres, η ) η0(1 + 2.5φeff), we find that the viscosity should exceed that of water by at most 5%, which is comparable to the error bar. To explain the value of the radius found for the aggregates, we can propose the following simple model. If we assume that once neutralized by the charges of the surfactant, the charged monomers are insoluble in water, the chain will collapse and form hydrophobic aggregates, leaving its remaining charged monomers at the boundary of the aggregates. The area Σ occupied by the charged monomers will be determined by a balance between surface energy and electrostatic energy, as in surfactant micelles; this area is therefore expected to be comparable: Σ ∼ 0.5 nm2. The radius of the aggregates will then be given by

RTH )

3φeff c-Σ

(4)

where c- is the number of charged monomers per unit volume: c- ) cprec - cs, where cs is the number density of surfactant ions and cprec is the number density such that c- is zero at the precipitation boundary (in mM units, cprec ) 9 mM). This expression is found easily by writing the total volume and total area of the spherical aggregates. The values of the calculated radii are given in Table 2. The agreement with the simple model is fair, as can be seen, especially in view of the poor accuracy on the volume fraction φ. 4. Discussion The surface tension measurements show that surface complexation between DTAB and carboxyMC begins at extremely small surfactant concentrations. Bulk complexation begins at larger concentrations, above a cac. The viscosity measurements show that this bulk complexation is accompanied by a partial chain collapse. All these phenomena, including the collapse of the chains above the cac, are similar to that found for other polyelectrolytes, DNA in particular. The collapse is much more effective with surfactants than with (monovalent) salts, probably because of the additional hydrophobic effects. In the case of carboxyMC, it was previously found that close to the cac, surfactant micellar-like aggregates form and stick to the polymer chains.33 Upon an increase of surfactant concentration, when the viscosity reaches the water value, the polymer chains are sufficiently collapsed so that they do not overlap any more. When the surfactant concentration increases further, larger aggregates appear, in which several chains are associated with surfactant, either in the form of micelles or with a liquid crystalline arrangement. The more striking observation is the monodispersity of the aggregates despite the broad polymer chain distribution. The simple model presented above deserves further confirmation (in particular, measurements of the electrical charge of the aggregates). Well-defined aggregates, controlled by electrostatic interactions, have recently been described: disks made from mixed solutions of anionic and cationic surfactants, whose borders are covered by the excess surfactant once charges have been neutralized in the flat parts,53 and DNA-lipid aggregates, where DNA is sandwiched be(53) Zemb, T.; Dubois, M.; Deme´, B.; Gulik-Krzywicki, T. Science 1999, 283, 816.

Surfactant-Induced Behavior near Precipitation

tween a well-defined number of lipid bilayers.54 It is unlikely that carboxyMC/DTAB aggregates are cylindrical like the giant micelles decorated by polymers observed by Merta et al. with surfactant-potato starch mixtures.55 Indeed, the polydispersity will be high in this last case. The microstructure might resemble that recently found for neutral (hydrosoluble)/polyelectrolyte diblock copolymers and oppositely charged surfactants.56 In this case, monodisperse particles are also observed, with a core made of densely packed surfactant micelles linked by the polyelectrolyte blocks (the corona being made of the neutral hydrosoluble blocks). Because we have only the polyelectrolyte parts, such a core structure may occur in our system. Up to now, we do not have enough information about the internal structures of our aggregates to propose a precise model for the internal organization of the aggregates observed here. Electron microscopy and neutron scattering experiments are in progress on this system and on others containing DNA or polyacrylamide sulfonate, where similar monodisperse aggregates were found close to the precipitation boundary. 5. Conclusions We have evidenced strong interactions between carboxyMC, a polyanion initially in the unentangled semidilute regime, and DTAB, an oppositely charged surfactant. At the interface, a synergistic surface tension lowering due to coadsorption occurs, although carboxyMC is non-surface-active at the concentration used. The (54) Pitard, B.; Aguerre, O.; Airiau, M.; Lachage`s, A. M.; Boukhnikachvili, T.; Byk, G.; Dubertret, C.; Herviou, C.; Sherman, D.; Mayaux, J. F.; Crouzet, J. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 14412. (55) Merta, J.; Garamus, V. M.; Kuklin, A. I.; Willumeit, R.; Stenius, P. Langmuir 2000, 16, 10061. (56) Herve´, P.; Destarac, M.; Berret, J.-F.; Lal, J.; Obersisse, J.; Grillo, I. Europhys. Lett. 2002, 58, 912.

Langmuir, Vol. 19, No. 2, 2003 237

polymer forms surface complexes with the surfactant even at very low surfactant concentrations. In the bulk and also at low surfactant concentrations, a slight decrease in viscosity is observed due to the binding of the two species and to a partial coiling of the initial extended polyelectrolyte conformation. A stronger and more cooperative binding process of DTAB and polymer in bulk occurs at the cac, here about 0.1 mM, as seen from several measurements: surface tension and viscometry. As usual, the cac is lower than the critical micelle concentration of the surfactant alone. At this point, one may assume that the complexes are polymer chains decorated by surfactant micelles. During the intermediate range of 0.1-1 mM, the surfactant induces first partial bridging and then a rapid collapse. Between 1 mM and precipitation (9 mM), a different aggregation process takes place; the size of the complexes increases from the size of the collapsed polymer chain of 12 nm up to 160 nm at 7 mM. These complexes are remarkably monodisperse. When extrapolated to the precipitation boundary, the size becomes micronic, suggesting that precipitation might be the point where the aggregates are too big to remain in solution and start to sediment. Adding an ionic surfactant to a polyelectrolyte of opposite charge dramatically alters the physical properties of the solutions, which in turn allows one to follow the microscopic details of the polymer collapse process. Further investigations are in progress using polymers with backbones of different rigidity. Acknowledgment. We thank Aqualon France, a division of Hercules S.A., for kindly supplying the carboxyMC Blanose type and Claude Germain (LPS-ORSAY) for DTAB recrystallization. We also thank Patricia Lixon (SCM/CEA-SACLAY) for her assistance in refractive index measurements. LA0206561