Oscillatory Forces from Polyelectrolyte Solutions Confined in Thin

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Oscillatory Forces from Polyelectrolyte Solutions Confined in Thin Liquid Films O. The´odoly,†,⊥ J. S. Tan,‡ R. Ober,† C. E. Williams,† and V. Bergeron*,§ Laboratoire de Physique de la Matie` re Condense´ e, CNRS URA 792, Colle` ge de France, 11 Place M. Berthelot, 75231, Paris, France, and Eastman Kodak Company, Rochester, New York 14650, and Laboratoire de Physique Statistique, CNRS UMR 8550, Ecole Normale Supe´ rieure, 24 rue Lhomond, 75231, Paris, France Received December 30, 2000 Several combinations of polyelectrolyte-surfactant mixtures are studied to probe the various structureproperty relationships these systems produce in thin liquid foam films. We show that either bulk solution properties or surface specific adsorption of the polymer-surfactant complexes formed can dominate the thin-film behavior. Above the polymer overlap concentration, Cp*, oscillatory force profiles are observed. In agreement with previous studies, we find that the oscillation period, ∆h, scales as the negative square root of the polymer concentration, ∆h ∼ Cp-1/2. In addition, by comparing directly the bulk polymer correlation length, ξ, to the oscillation period, we establish for the first time that ∆h ) ξ for these systems. These findings are compared to oscillatory force behavior seen with micellar solutions which display a scaling relation with micelle concentration, Cm, which follows ∆h ∼ Cm-1/3. The different scaling exponents are explained by the local cylindrical and spherical symmetry of the molecular structures seen in the corresponding systems. Finally, it is also shown that the adsorption of polymer and/or complexes to the interface can govern the thin-film properties and swamp out the influence of bulk structural effects in certain systems.

Introduction Polyelectrolytes play an important role in colloid science and are often exploited for their ability to act as colloidal stabilizers as well as flocculants. This wide range of utilization stems from both their bulk properties (e.g., rheology modifiers) and the numerous interactions they can have with surfaces and surface-active components. Particularly interesting is the long-range nature of these interactions and the possibility to modulate it with external parameters (e.g., salt, pH). In addition to providing a broad range of practical applications, the complexity of polyelectrolyte interactions also presents a significant challenge when attempting to describe these systems theoretically. Thus, there is a strong impetus for systematic studies aimed at understanding colloidal interactions in the presence of polyelectrolytes. Moreover, understanding the behavior of polyelectrolytes in confined environments is important to practical applications involving biological systems (adsorption of proteins, confinement in lamellar phases, etc.). Recently, a new and fascinating observation has been made when highly charged polyelectrolytes are confined within a thin liquid foam film.1 This study revealed for the first time oscillatory force interactions in foam films made from surfactant solutions containing low levels of polyelectrolyte. At nearly the same time, an important study by Milling independently showed that oscillatory forces are also observed between repulsive hard silica surfaces in the presence of fully charged polystyrenesulfonate without the presence of surfactant.2 By investigating various polymer concentrations (Cp), Milling also showed that the period of the oscillations observed followed * Corresponding author. † Colle ` ge de France. ‡ Eastman Kodak Co. § Ecole Normale Supe ´ rieure. ⊥ Currently at University of California, Berkeley, CA 94720. (1) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550. (2) Milling, A. J. J. Phys. Chem. 1996, 100, 8986.

the same scaling law as the bulk correlation length, ξ, of the polymer in semidilute solutions (e.g., ξ ∼ Cp-1/2). Milling’s findings suggest that the oscillatory forces are closely related to the macromolecular structuring in the bulk. In addition, this study revealed the electrostatic nature of these interactions by demonstrating how the oscillatory forces depend on added salt; the forces diminish and vanish with increasing ionic strength. Subsequently, studies following up on the initial foam-film observations showed that the same basic features observed by Milling occur in foam films, suggesting that although surfactant is present in the foam-film system, the phenomena have the same origin.3-6 Additionally, a wide range of experimental evidence concerning this type of force behavior has now been reported.7-10 Another recent and intriguing study concerning force oscillations between solid surfaces in polyelectrolytesurfactant systems has been reported by Claesson et al.11,12 As with the original foam-film studies, an oppositely charged polyelectrolyte-surfactant system is studied. By use of mica surfaces precoated with highly charged cationic polyelectrolytes, it is shown that oscillatory forces are observed when these surfaces approach each other in dilute (3) Asnacios, A.; Bergeron, V.; Langevin, D.; Argilier, J.-F. Rev. Inst. Fr. Pet. 1997, 52, 139. (4) Asnacios, A.; Espert, A.; Colin, A.; Langevin, D. Phys. Rev. Lett. 1997, 78, 4974. (5) v. Klitzing, R.; Espert, A.; Asnacios, A.; Hellweg, T.; Colin, A.; Langevin, D.; Colloids Surf., A 1999, 149, 131. (6) Kolaric, B.; Jaeger, W.; Klitzing, R. J. Phys. Chem. B 2000, 104, 5096. (7) Milling, A. J.; Vincent, B. J. Chem. Soc., Faraday Trans. 1997, 93, 3179. (8) Sharma, A.; Tan, S. N.; Walz, J. Y. J. Colloid Interface Sci. 1997, 191, 236. (9) Milling, A.; Vincent, B. Colloid-Polymer Interactions: From Fundamentals to Practice; Farinato, R. S., Dubin, P. L., Eds.; John Wiley & Sons: New York, 1999; p 147. (10) Milling, A. J.; Kendall, K. Langmuir 2000, 16, 5106. (11) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (12) Claesson, P. M.; Dedinaite, A.; Fielden, M.; Kjellin, M.; Audebert, R. Prog. Colloid Polym. Sci. 1997, 106, 24.

10.1021/la001820s CCC: $20.00 © 2001 American Chemical Society Published on Web 07/04/2001

Oscillatory Forces in Thin Liquid Films

anionic surfactant solutions (i.e., surfactant concentrations below the critical micelle concentration, cmc) which contain no free polymer in solution. In this case, the periodicity of the oscillations is found to be equivalent to the diameter of a surfactant micelle. Concentration scans are not reported in this work, and hence a scaling law similar to that developed by Milling cannot be inferred. Although long-range oscillatory forces have been observed in concentrated micellar solutions without polyelectrolytes,13,14 the studies by Claesson et al. differ significantly in that the surfactant concentrations are an order of magnitude lower (thus well below the cmc), and the periods of the oscillations are substantially smaller (i.e., 4.0 nm opposed to 10.0 nm). The different length scales are interpreted as being due to the fact that the entities responsible for the oscillations in a free micellar solution constitute micelles including their diffuse counterion atmosphere, while in the mixed polymer-surfactant system they are due to aggregates closely associated with polyelectrolyte chains adsorbed to the surface. Thus, the oscillations in this case appear to arise from specific macromolecular polymer-surfactant structuring at the surface. These structures are presumably similiar to the mesomorphous phases characterized by Antonietti and co-workers.15,16 Further clarification of these surface aggregates has recently been carried out.17 By comparison of the two studies performed with solid surfaces to those carried out on individual “soft” foam films, it appears that the polyelectrolyte-induced oscillatory force behavior seen in thin liquid films thus far has the same origin as that observed by Milling (i.e., close correspondence with bulk structures). Indeed, many of the previous thin-film studies have applied the same method of Milling, which compares scaling relationships with respect to the polymer concentration to infer the force-structure behavior. However, it should be noted that the preliminary schematic of the confined polymers portrayed in Milling’s original work is not likely to be correct if the film and bulk structuring are closely related. In contrast, the proposed macromolecular structures responsible for the force oscillations in the mica surfaceforce studies are deduced from a comparison of the oscillation period directly to the proposed macromolecular structures present (i.e., absolute size measurements opposed to scaling relationships). We note that the presence of strongly interacting polymer-surfactant mixtures in many of the foam-film studies, as used for the mica surface-force study, also suggests a possible analogy to the mica surface-force observations. Indeed, it is likely that in the presence of certain polymer-surfactant combinations the two types of force oscillations (bulk dominated versus adsorption layer controlled) are not mutually exclusive. Our goal is to study systems which eliminate ambiguities regarding the polymer-surfactant and surface interactions, by investigating various polymer-surfactant combinations and pure polyelectrolyte stabilized systems (i.e., no added surfactant). The objective is to simultaneously provide quantitative measurements of the oscillatory force characteristics, polymer-surfactant interactions, and bulk macromolecular structures within our systems, so that definitive models can be developed and compared with analogous systems. (13) Bergeron, V.; Radke, C. J. Langmuir 1992, 8, 3020. (14) Richetti, P.; Ke´kicheff, P. Phys. Rev. Lett. 1992, 68, 1951. (15) Antonietti, M.; Conrad, J.; Thuremann, A. Macromolecules 1994, 27, 6007. (16) Antonietti, M.; Maskos, M. Macromolecules 1996, 29, 4199. (17) Dedinaite, A. Ph.D. Thesis, Royal Institute of Technology, Stockholm, Sweden, 1999.

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Figure 1. Schematic representations of commonly encountered polymer-surfactant interactions and their qualitative adsorption/complexation behaviors. In addition to specific interactions, both absolute and relative concentration of the polymer and surfactant are important for determining which behavior is witnessed.

One complication which arises with studies of thin liquid foam films is the need to have surface-active components present in order to stabilize the films. Without adequate film stability, measurement of the interactions between the two air-water interfaces cannot be accomplished. These surface-active species provide film stability via surface elasticity and repulsive force interactions between the interfaces (i.e., Derjaguin-Landau-Verwey-Overbeek type interactions). Additionally, surfactants may interact with polymers added to the system, which can mediate and change the polymer configuration, surface adsorption, and thin-film interactions. Interestingly, it was found recently that random copolymers of styrene and styrene sulfonate spontaneously adsorbed at the free water surface and could stabilize thin films with no need for added surfactant.18 Therefore, to determine the role of a polyelectrolyte one must understand independently the various interfacial and polymer-surfactant interactions. The schematic in Figure 1 provides a summary of the four most commonly seen adsorption/complexation behaviors encountered for polymer-surfactant mixtures at the air-solution interface. This general outline is clearly not exhaustive and somewhat arbitrary, but as these situations often arise it is convenient for us to identify the general categories: (I) synergistic adsorption, (II) repulsion or no interaction, (III) surface depletion, and (IV) competitive or indifferent adsorption. Case I, is often encountered with oppositely charged polyelectrolytesurfactant mixtures, while case II can occur when the polyelectrolyte carries the same charge as an ionic surfactant in solution or when a noninteracting, nonadsorbing polymer is present. Case III often takes place for surfactants that have a strong hydrophobic interaction with the polymer which results in complexes that become less surface active as the polymer is solubilized by surfactant micelles. In the absence of surfactant, the hydrophobically modified polyelectrolytes used in this case can act as macrosurfactants and serve as film stabilizers, (18) The´odoly, O. Ph.D. Thesis, Universite´ de Paris VI, Paris, France, 1999.

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alleviating the need for an added surfactant in thin-film studies. Finally, case IV is less common and requires noninteracting polymer-surfactant systems that adsorb independently to the interface. In all cases, the absolute and relative concentrations of the polymer and surfactant are important in determining the actual behavior and one system can fall into different categories depending on these concentrations. For further general information, Goddard and Ananthapadmanablan provide an excellent introduction to the various types of polymer-surfactant interactions that can occur.20 Thus far, the majority of the observations concerning polyelectrolyte-induced oscillatory forces in thin liquid films have focused on case I systems in Figure 1. Unfortunately, because of the strong polymer-surfactant interactions, this is one of the most complicated cases for decoupling the effects of the polyelectrolyte from polymersurfactant complexes adsorbed at the interface. In an effort to simplify and expand our understanding of polyelectrolyte-induced oscillatory forces in thin liquid films, this work presents data on systems that fall into cases II, III, and IV of Figure 1. For case II, we have chosen a nonionic surfactant-anionic polyelectrolyte system, hexaethylene glycol monododecyl ether (C12E6) and poly(2-acrylamido2-propane sulfonate) (PAMPS), which are noninteracting, and an anionic surfactant-anionic polymer, sodium dodecyl sulfonate (C12SO3-Na+) and PAMPS, which displays a repulsive interaction. Case III is studied by using a nonionic surfactant (C12E6) with a slightly hydrophobic anionic polyelectrolyte (polystyrene sulfonate (PSS), random copolymer at different degrees of sulfonation). Last, a simplified version of case IV is investigated by eliminating the surfactant and adding only the hydrophobic polyelectrolyte PSS, known otherwise to be surface active (all polymers were used as the sodium salt).18,19 In addition to investigating these various polymer-surfactant combinations, we also probe the effect polymer molecular weight and concentration of the nature of the oscillatory forces in thin liquid films. Experimental Section Materials. Highly pure nonionic surfactant C12E6 was purchased from Nikko Chemicals Co. Confirmation of its purity was established by measuring the surface tension isotherm which showed no anomalies and displayed a cmc of 6.8 × 10-5 M in agreement with literature values. Anionic surfactant sodium dodecyl sulfonate (C12SO3-Na+) was obtained from Fluka. Again, surface tension isotherms were used to verify the final purity. Using this anionic surfactant has an advantage over the more traditional sodium dodecyl sulfate (SDS), as it does not undergo hydrolysis in solution to produce surface-active dodecanol. The poly(2-acrylamido-2-propane sulfonate) used was specially synthesized and fractionated to obtain 100% charged, highly monodisperse samples of three different molecular weights: 70K, 500K, and 1700K. Further details concerning the characteristics of these polymers can be found in ref 21. Samples of poly(styreneco-styrene sulfonate) of different degrees of charge (charge fractions between 30% and 100%) and molecular weight were individually obtained by postsulfonation of polystyrene, and a complete description of these molecules can be found elsewhere.18,19 Sodium chloride (NaCl) was obtained from Aldrich and heated to 500 °C for several hours to drive off any organic surface-active impurities. Finally, all solutions were prepared with water taken from a Millipore MilliQ purification system (resistivity > 18 MΩ). (19) The´odoly, O.; Ober, R.; Williams, C. Eur. Phys. J. E 2001, 5, 51. (20) Goddard, E. D.; Ananthapadmanablan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (21) Fisher, L. W.; Sochor, A. R.; Tan, J. S. Macromolecules 1977, 10, 949.

The´ odoly et al. Surface Tension Measurements. Surface tension measurements were performed both with a standard Wilhelmy balance using a platinum-plate probe and via the drop-weight method. Measurements from both techniques were compared to ensure reproducibility and equilibrium values. For our equilibrium measurements, agreement to within 0.5 mN/m was found. As with all our measurements, the experiments were performed at 23 ( 2 °C. Disjoining Pressure Measurements. The disjoining pressure isotherms were obtained with a thin-film balance (TFB), which is described in detail elsewhere.13,22 Briefly, individual horizontal foam films are exposed to a controlled capillary pressure. Subsequently, thin-film interferometry is used to evaluate the film thickness, which together with the capillary pressure measurements allows us to deduce the repulsive force interactions in the film. In addition, valuable supplementary observations are obtained by monitoring the films with a CCD camera mounted to the interference microscope. We used both porous glass frits and Scheludko-cell film holders for all solutions tested. Moreover, careful attention was placed on cleaning the glass frits by controlled heat treatment to remove organic contaminants, followed by copious rinsing to eliminate extraction of free borosilicate macroions during measurements with our various chemical systems. Last, at least two unique film holders were constructed for each solution tested so as to guarantee fresh reproducible conditions. Small-Angle X-ray Scattering (SAXS) Measurements. SAXS measurements were performed using a conventional 0.9 m Kratky camera equipped with a rotating cathode Cu KR source, of 0.1 × 0.1 mm2 effective size, and a 1-D position sensitive detector. The data were corrected for the scattering of the empty sample holder (1 mm diameter capillary) and sample absorption. Data are obtained as scattered intensity, I, as a function of the scattering vector, q, where q ) (4π/λ) sin θ/2, with λ as the wavelength and θ as the observation angle. Semidilute solutions of highly charged flexible polyelectrolytes are characterized by a single broad peak at low ionic strength. The position of the peak, q*, provides a measurement of the average mesh size of the transient network formed by the overlapping chains, also called correlation length ξ, as ξ ) 2π/q*.

Results Case II(a): Noninteractive Systems. In this case, we aim to create two different model systems for which polyelectrolytes in solution do not complex with the added surfactant or adsorb to the interface. In a general sense, one system confines highly charged polyelectrolytes in solution between two neutral interfaces, C12E6/PAMPS (i.e., interface uncharged and surfactant noninteracting), and another confines the polyelectrolytes between equivalently charged repulsive interfaces, C12SO-3Na+/PAMPS (i.e., surfactant has the same sign as the nonadsorbing, noncomplexing polyelectrolyte). To ensure these conditions, characterization of the polymer-surfactant interactions in the bulk is accomplished by small-angle X-ray scattering, while adsorption behavior is monitored via surface tension measurements. Figure 2 displays the SAXS curves for bulk aqueous solutions of 0.06 M PAMPS, in pure water and in a solution containing 1.4 × 10-4 C12E6 and 10-3 NaCl. Low levels of salt are added to control the ionic strength of the solution. The scattering curves superimpose upon one another and are indistinguishable. This indicates that the polymer configuration in solution is not significantly influenced by the addition of C12E6 and NaCl at these concentrations, and if any polymer/surfactant interactions occur under these conditions, in the bulk they are too weak to be detected. The surface tension measurements presented in Figure 3 further support this finding. In Figure 3, the open circles correspond to solutions of C12E6 at twice the (22) Claesson, P.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119.

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Figure 2. SAXS data for 0.06 M PAMPS (MW ) 500K): (a) in water (filled symbols) and (b) in a solution containing twice the cmc of C12E6 and 10-3 M NaCl (open symbols). Figure 4. Disjoining pressure measurements for different concentrations of PAMPS (MW ) 500K) in a 2× cmc C12E6 + 10-3 M NaCl solution. Discrete film thickness transitions are emphasized by the oscillatory curves and solid bars (which mark the transition thicknesses) sketched in the figure. Single data points representing a transition indicate transitions detected in a dynamic mode (i.e., the metastable states are insufficiently stable to trace the isotherm).

Figure 3. Surface tension variation for a C12E6 solution at twice the cmc and with 10-3 NaCl, as a function of added polyelectrolyte. Filled circles represent PSS 100% charged, while open circles correspond to PAMPS; lines are drawn to guide the eye.

cmc and with 10-3 NaCl at various levels of added PAMPS (MW ) 500K), from zero to 50 000 ppm. The surface tension remains at the pure surfactant level of ∼35 mN/ m, regardless of the level of added polymer. Moreover, the polymer alone shows no surface activity at these concentrations. This behavior was also observed for the 70K and 1700K molecular weight samples of PAMPS used. Similarly, surface tension and bulk solution studies conducted with the equivalently charged anionic PAMPS/ C12SO3-Na+ polymer-surfactant mixtures again confirm that there are no significant surfactant-polymer complexes created in the bulk or at the interface with this system over the concentration ranges studied. In contrast, filled circles in Figure 3, which correspond to an equivalent series of measurements made with 100% charged PSS, display a rising surface tension as a function of polymer concentration. We interpret this rise as a depletion of surfactant at the surface, presumably because of hydrophobic interactions between the surfactant and the polymer backbone. Thus, with reference to Figure 1, the PAMPS/C12E6 and PAMPS/C12SO3-Na+ display case II behavior, noninteractive or repulsive behavior, while the PSS/C12E6 solutions manifest case III, surface depletion, behavior depending on the level of charge on the polymer backbone.18,19 In Figure 4, the disjoining pressure isotherms for four different solutions of PAMPS, at concentrations of 4000, 8000, 12 000, and 24 000 ppm (MW ) 500K), containing C12E6 at twice the cmc and 10-3 NaCl, are presented. Without added polymer, these C12E6 solutions produce only so-called Newton black films, as seen in other nonionic surfactant systems at similar solution conditions. The Newton black films observed in this study produce a monotonic steric repulsion between the interfaces at approximately 7.0 nm. In contrast to this, the isotherms shown in Figure 4 display an oscillatory force curve

extending to rather thick films. Solid lines connecting the data are sketched in the isotherms to accentuate this point. The negative sloping regions of the isotherms correspond to metastable films, which are separated by thermodynamically unstable states. This produces discrete stepwise film transitions commonly referred to as “film stratification”, analogous to the behavior seen with concentrated micellar solutions.13,23 We have also provided solid horizontal lines on the isotherms to indicate the characteristic separation distance, ∆h, between metastable film thicknesses. The number of metastable states increases with polymer concentration while the characteristic distances and peak heights decrease. In fact, at the higher polymer concentrations, although the metastable states are very well-defined, the forces are so low (3 × 10-2 M) no transitions are observed, regardless of the polymer (23) Bergeron, V.; Jime´nez-Laguna, A. J.; Radke, C. J. Langmuir 1992, 8, 3927.

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Figure 5. Direct comparison between film thickness transitions, ∆h (open circles), obtained from thin-film measurements, and bulk correlation lengths, ξ (filled triangles), measured by SAXS. The line represents an empirical fit to both data sets. Figure 7. Disjoining pressure measurements for different concentrations of PAMPS (MW ) 500K) in a 10-3 M C12SO3-Na+ solution. Discrete film thickness transitions are emphasized by the oscillatory curves and solid bars (which mark the transition thicknesses) sketched in the figure. Single data points representing a metastable film indicate transitions detected in a dynamic mode (i.e., the metastable states are insufficiently stable to trace the isotherm).

Figure 6. The molecular weight and concentration dependence of the film thickness transitions for PAMPS in solutions containing 2× cmc C12E6 + 10-3 M NaCl.

concentration. For these elevated ionic strengths, we recover the same Newton black film behavior witnessed when polyelectrolyte is not present. We have also conducted additional disjoining pressure measurements with PAMPS of differing molecular weights, 70K and 1700K, under the same solution conditions. Our results indicate that there are only slight differences in the disjoining pressure isotherms between these various molecular weights. For example, Figure 6 compares the thickness transitions as a function of molecular weight and polymer concentration. In all cases, we continue to observe the same concentration dependence; however, there is a slight increase in the characteristic distance with increased molecular weight. Again, for all molecular weights studied, direct SAXS measurements of the correlation length in the bulk solution are in excellent agreement with the thickness transitions measured using the thin-film balance (i.e., ξ ) ∆h). Although a slight increase in the magnitude of the forces with increased molecular weight is detected, the extremely weak forces involved, compared to the precision and statistical nature of the measurements, again prohibit us from conducting a quantitative analysis of these changes. Moreover, a noticeable increase in the viscosity of the high molecular weight solutions complicates an equilibrium interpretation of measurements on these systems. Case II(b): Repulsive Systems. Disjoining pressure isotherms for the equivalently charged PAMPS/ C12SO3-Na+ polymer-surfactant mixtures are provided in Figure 7. When polymer is not present in this system, we observe simple double-layer repulsion between the two interfaces, extending monotonically out to 40 nm film thicknesses. However, as with the nonionic surfactant system, oscillatory force curves are witnessed at PAMPS concentrations in excess of 2000 ppm. Although the absolute values of the film thicknesses are greater than those observed with the nonionic surfactant mixtures, the thickness transitions observed are identical for the same corresponding polymer concentration. That is, the period of the oscillatory forces and its dependence on PAMPS

concentration are the same regardless of whether nonionic or anionic surfactant is used to stabilize these films. The greater absolute thicknesses for the anionic surfactant system at the lower polyelectrolyte concentrations can be accounted for by the double-layer repulsion between the charged air-water interfaces which is superimposed on the oscillating structural forces. At higher ionic strengths (i.e., higher polyelectrolyte concentrations), screening of these double-layer forces produces isotherms identical to those witnessed with nonionic surfactant/polyelectrolyte mixtures. Cases III and IV: Surface Depletion/Competitive or Indifferent Adsorption. Recent surface tension measurements conducted with PSS having different degrees of charge reveal that these polyelectrolytes can adsorb to the air-water interface in the absence of added surfactant.18,19 Indeed, with a marked hydrophobic character, when surfactant is present competitive adsorption or surface depletion can occur depending on the charge density of the PSS and the type of surfactant used. Therefore, we have studied this polyelectrolyte under two conditions: (1) no added surfactant and (2) in the presence of nonionic, C12E6, and anionic, C12SO3-Na+, surfactants. PSS and Nonionic Surfactant. As noted earlier in Figure 2, the addition of PSS 100% (i.e., fully charged) to C12E6 solutions results in an increase of the surface tension, which seemingly arises from a depletion of surfactant from the surface because of hydrophobic interactions between the surfactant tails and the polymer backbone (see Figure 3). The kinetics at the interface of this behavior can be rather slow (∼ hours); as such, when measuring the disjoining pressure isotherms for these systems, we encounter difficulties in establishing equilibrium conditions. Both interface lifetimes before film formation and increasing and decreasing pressure scans can produce different absolute thickness results. However, oscillatory forces with the same qualitative features as those seen for the noninteractive systems above are observed, namely, an increase in the number of oscillations with polymer concentration and oscillation periods that scale with polymer concentration as ∆h ∼ Cp-1/2. Thus, for this system it appears that force oscillations due to bulk polyelectrolyte structures are apparent, but an additional influence on the measured forces is produced by polymer-surfactant complex formation on later timescales. Further quantita-

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Figure 9. Film thickness transitions for several different PSS polyelectrolytes with different charge fractions in a 10-3 M C12SO3-Na+ solution: open circles, 30%; filled squares, 42%; open squares, 55%; filled circles, 90%. Figure 8. Surface tension versus the concentration of free ions in solution for various polyelectrolytes in a 10-3 M C12SO3-Na+ solution. The solid curve represents the addition of a simple electrolyte, NaCl.

tive characterization of these complexation effects is significantly hindered by the slowness of the processes involved. PSS and Anionic Surfactant. Figure 8 contains surface tension measurements for solutions containing different PSS or PAMPS polymers with various charge fractions, in a 10-3 M C12SO3-Na+ surfactant solution (no added salt). Measurements were conducted for increasing polymer concentration, but to compare the results on a common basis the x-axis is reported as the concentration of free ions. This is necessitated by the fact that the surface tension of C12SO3-Na+ decreases with increasing ionic strength; thus, normalizing the data in this way conveniently allows us to separate purely ionic strength effects from specific polyelectrolyte interactions. Ionic dissociation of the polymer solutions without surfactant was determined by osmotic pressure measurements, which permitted us to establish the relationship between free ion concentration as a function of polymer concentration. The solid line sketched into Figure 8 represents the evolution of the surface tension with the addition of a simple monovalent salt. All measurements were taken at a surface age of about 5 min as in some cases extremely long times are required to establish near equilibrium conditions with weakly charged PSS polymers.19 Under these conditions, the data in Figure 8 indicate that we are unable to detect significant polymer-surfactant adsorption phenomena for the PSS/C12SO3-Na+ systems under study. In addition, it is clear that the 90% charged PSS behaves very similarly to the 100% charged PAMPS. Although our surface-tension measurements do not indicate any short-time PSS adsorption in C12SO3-Na+ solutions, thin-film measurements clearly reveal polymer adsorption effects on timescales greater than 5 min. That is, if we allow our interfaces to age for several minutes before forming a film, the film drainage becomes significantly slower and the final film thickness does not evolve to a plane-parallel state. This behavior is characteristic of gel-like polymer-surfactant complexes adsorbing onto the film interfaces and similar nonequilibrium phenomena observed in the original polymer-surfactant thin-film study referred to earlier.1 This type of complication again underlies the difficulty of working with polymer systems that adsorb to the interface and interact with surfactants in the solution. Nonetheless, PSS/C12SO3-Na+ films formed immediately with fresh air-solution interfaces manifest film stratification phenomena very similar to what was observed with the nonadsorbing, noninteracting PAMPS systems. That is, film thickness transitions are

Figure 10. Evolution of the thickness transitions at constant polymer concentration (0.02 M) as a function of PSS charge fraction.

observed which increase in number and decrease in thickness and strength with increases in polymer concentration. Owing to the reproducibility of the film stratification observations under dynamic conditions, we have chosen to focus on film drainage measurements for these PSS/C12SO3-Na+ systems. Therefore, instead of providing disjoining pressure isotherms for these systems, only film thickness transitions, ∆h, measured at an imposed capillary pressure of 150 Pa are reported. This allows us to obtain reliable quantitative data for comparison with our PAMPS studies. The characteristic film thickness transitions as a function of polymer concentration for the differently charged PSS polymers are plotted in Figure 9. The solid lines are provided to guide the eye. The correspondence with polymer concentration follows the same type of power law observed earlier, ∆h ∼ Cp-1/2, independent of the level of charge on the polymer. Therefore, as with the PSS in nonionic surfactant solutions under dynamic conditions, structural oscillations with the same features of our noninteracting systems are observed. However, upon equilibration of the surfaces we detect additional structural features in the film that evolve over a long (> hours) timescale. Last, in these PSS/C12SO3-Na+ systems, for the same polymer concentration, the thickness transitions increase as the level of charge on the PSS decreases. This later observation can be understood by the lower effective charge for the weakly charged polymers, resulting in a diminished ionic strength and concomitant increase in the length scale for electrostatic interactions (i.e., Debye length). In Figure 10, we verify that the dependence of the thickness transition at fixed polymer concentration varies linearly with the square root of the concentration of free ions, in accord with double-layer interaction theory. PSS (No Added Surfactant). Weakly charged PSS ( hours) make it difficult to obtain quantitative, reproducible data. In such cases, we have seen evidence for oscillatory forces originating from structural features in the bulk solution; however, these observations are superimposed on time-dependent adsorption phenomena that significantly alter the force behavior. Quantifying these later effects requires systematic solution and interface aging studies; otherwise, incomplete conclusions can be drawn. Our current focus is aimed at a more comprehensive understanding of these strongly interacting systems. Conclusions The bulk solution, interfacial tension, and thin liquid film properties of various polyelectrolyte/surfactant combinations have been studied to provide a deeper understanding of the complex structure-property relationships these systems display. In particular, the influence molecular structures in the solution and at the air-water interface have on the forces between thin liquid foam films is investigated. We show that either bulk solution properties or surface specific adsorption of polymer-surfactant complexes can dominate the forces in thin liquid films. For cases in which little to no polymer is adsorbed to the interface, we observe oscillatory forces in individual foam films when the polyelectrolyte concentration is in the semidilute regime. In agreement with previous studies, the oscillation period, ∆h, follows a power law that scales with the negative square root of the polymer concentration (i.e., ∆h ∼ Cm-1/2). Although it has been noted on several occasions that this scaling behavior is also seen for the

The´ odoly et al.

polymer correlation length in the bulk, ξ, we show for the first time that the two length scales are identical. That is, under these conditions ∆h ) ξ. This relationship is established by directly comparing independent measurements of the bulk solution correlation length via SAXS and the force oscillation period using a thin-film balance. We also compare the similarities and differences between the type of oscillatory force behavior observed for semidilute polyelectrolyte solutions and those seen with solutions of charged spherical micelles. As with polyelectrolyte solutions, macromolecular structures within the bulk micellar solution determine the period of the oscillatory force profile. Above a critical volume fraction, for which the characteristic distance is set by the molecular diameter of the micelles plus twice the electrostatic doublelayer interaction distance, micellar solutions display spherical packing behavior in thin liquid films. This spherical symmetry produces a corresponding oscillatory force profile with a period that decreases with the -1/3 of the micelle concentration (i.e., ∆h ∼ Cm-1/3). For both nonadsorbing polyelectrolyte and charged micellar solutions, force oscillations arise from confining macromolecular structures in a constricted space. The periodic distance of the force oscillations is set by the bulk correlation, by which upon breaking the symmetry of the system at this length scale, that is, by confining the solution between walls, we generate the corresponding oscillatory force interaction. This type of behavior is analogous to that seen at the molecular level for which force oscillations occur when a solvent is confined at length scales comparable to its molecular dimensions. It is also shown that when polyelectrolyte or surfactant/ polyelectrolyte complexes are adsorbed to the interface, the force interactions are dominated by the adsorbed layer, and bulk structural effects play a secondary role. Furthermore, the adsorbed layer containing these high molecular weight polymers develops slowly over time, thus creating a strong dependence of the thin-film properties on surface age and film history. As such, quantitative reproducible data are difficult to obtain for these systems. However, important qualitative observations such as microgel formation in these systems is reported. Quantitative evaluation of these gels is currently under way. Acknowledgment. We thank Rhodia Recherches for partial financial support and J. F. Joanny and P. G. de Gennes for helpful discussions. LPS de l’ENS is UMR 8550 of the CNRS, associated with the universities Paris 6 and 7. LA001820S