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Thin-Film Forces in Foam Films Containing Anionic Polyelectrolyte and Charged Surfactants Vance Bergeron* Groupe Colloids, Service de Chimie Moleculaire, Centre d’Etudes de Saclay, 91191 Gif sur Yvette, France
D. Langevin and Atef Asnacios Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600 Pessac, France Received August 2, 1995. In Final Form: December 1, 1995X Foam-film disjoining pressure and surface tension isotherms are measured for solutions containing mixtures of charged surfactants and an anionic random-block copolymer, acrylamide-acrylamidesulfonate. When combined with an anionic surfactant, dioctyl sulfosuccinate (AOT), the polyelectrolyte shows no indication of direct surface interactions and the surfactant behaves as a simple electrolyte. However, when copolymer is added to cationic dodecyltrimethylammonium bromide (C12TAB) solutions, strong polymer-surfactant interactions create an enhanced adsorption of both components to the air-water interface. The resulting polymer-surfactant surface layers subsequently produce long-range thin-film interactions that have not been seen before. For the C12TAB-polymer solutions, film stratification is observed 3 orders of magnitude below the critical micelle concentration and nearly two orders of magnitude below the critical aggregation concentration (cac). This new type of film stratification is consistent with recent Monte Carlo simulations of polyelectrolyte “bridging” in thin films. In addition, when the C12TAB concentration is increased to the cac, dense polymer-surfactant surface layers overlap inside the film and produce a gel-like network. The structure and properties of this network are strongly influenced by the rate of film formation and the long-range forces produced are crucial to the film’s stability.
Introduction Industrially, it has long been realized that the addition of polymers to a surfactant solution can significantly affect the stability of foams and emulsions.1 Similarly, proteins and polyelectrolytes can mediate vesicle interactions and adhesion in biological systems.2 Therefore a fundamental understanding of these systems and what role macromolecules play in them is an extremely important area of research. In foams and emulsions, polymer additives can affect both their dynamic and thermodynamic stability. The dynamic influence stems from the effect polymer has on the drainage rate of the thin films separating the discontinuous phase. Typically when soluble polymers are added to the continuous phase, they increase its bulk viscosity which in turn can dramatically slow down the drainage of the thin-liquid films. In addition, if the polymers are surface active, they can also have a strong effect on the film’s surface rheological properties and further decrease the rate of fluid flow out of the film.3,4 Furthermore, on the basis of film-thinning experiments above the critical micelle concentration (cmc), it has been reported that nonionic polymers interact with the surfactant aggregates present in the bulk (e.g., micelles) and change the driving force for film thinning (i.e., modify the depletion forces).5,6 Polymers can also have a profound effect on the metastable equilibrium state of the films after drainage * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed.; Wiley: New York 1980. (2) Eriksson, B.; Hardin, A. M. In Flocculation in Biotechnology and Separation Systems; Attia, Y. A., Ed.; Elsevier Science Publishers B.V.: Amsterdam, 1987. (3) Lionti-Addad, S.; di Meglio, J.-M. Langmuir 1992, 8, 324. (4) Bruinsma, R.; di Meglio, J.-M.; Que´re´, D.; Cohen-Added, Sylvie Langmuir 1992, 8, 3161. (5) Krichevsky, O.; Stavans, J. Physica A 1993, 200, 743. (6) Krichevsky, O.; Stavans, J. Phys. Rev. Lett. 1994, 73, 696.
is complete. When two polymer-coated surfaces are brought into close proximity, the adsorbed polymer layers start to overlap and interaction forces will develop between the surfaces. These “surface forces” can be attractive or repulsive and depend on the nature and quantity of polymer adsorbed to the interface. Thus polymers are used as both flocculants and stabilizers.7 Polyelectrolytes have a particularly rich behavior because both electrostatic and entropic forces operate simultaneously, and in certain cases this can lead to unique force behavior between the two interfaces.8-10 Although there has been recent experimental and theoretical work on forces between mica surfaces with adsorbed layers of oppositely charged polyelectrolyte, there is no fundamental work reported on forces in thin-liquid films (i.e., foam and emulsion films) containing polyelectrolyte. These systems in particular are of great industrial importance. Therefore, the objective of this work is to investigate the forces in single thin-liquid foam films made from solutions containing polyelectrolyte and different charged surfactants. First, surface tension isotherms at the air-water interface are measured to determine the adsorption characteristics of both polymer and surfactant at the airwater interface. Information from these isotherms is then combined with disjoining pressure measurements on single thin-liquid foam films to understand how polymer adsorbed to the interface affects interaction forces in the thin films. In particular, one high molecular weight anionic polyelectrolyte is studied with two different surfactants: an oppositely charged and hence strongly (7) Dickinson, E.; Eriksson, L. Adv. Colloid Interface Sci. 1991, 34, 1. (8) A° kesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91, 2461. (9) Miklavic, S. J.; Woodward, C. E.; Jo¨nsson, B.; A° kesson, T. Macromolecules 1990, 23, 4149. (10) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; A° kesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769.
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Figure 1. Monomeric structures for an acrylamide/acrylamidesulfonate copolymer.
interacting cationic surfactant, dodecyltrimethylamonium (C12TAB), and a similarly charged, weakly interacting anionic surfactant, dioctyl sulfosuccinate (AOT). Experimental Section Materials. The anionic random-block copolymer used in the this study is an acrylamide-acrylamidesulfonate (AAS) (see Figure 1) Mw ) 3 × 106, with approximate 25% charged sulfonate monomers. This polymer was supplied by the Institut Francais du Pe´trole and is purified by ultrafiltration for 2 days before use. The anionic surfactant dioctyl sulfosuccinate (AOT) was purchased from Sigma and the cationic surfactant dodecyltrimethylamonium bromide (C12TAB) was obtained from Kodak. The C12TAB is recrystallized twice from an ethyl acetate-ethyl alcohol (10:1) solution while the AOT is used as received. Neither surfactant displayed a minimum in the surface tension versus surfactant concentration isotherm, indicating a high level of purity. Sodium chloride (NaCl) was supplied by Aldrich and heated to 500 °C before use to drive off surface active impurities. Finally, all solutions are prepared with water taken from a Millipore MilliQ ultrapure water system. Surface Tension Measurements. Surface tension measurements at the air-water interface were measured by the Wilhelmy method using a rectangular (20 mm × 10 mm), “open frame” made from platinum wire (0.19 mm in diameter) attached to a sensitive, Hottinger Baldwin Messstechnik (HBM) Typ Q11, force transducer. The open-frame probe eliminates wetting anomalies seen when using other probe geometries and reproducibility with this system is better than (0.1 mN/m. Solutions where placed in Teflon dishes and measurements where made in a closed humidified environment. The Teflon dishes, platinum probe, and all glassware are cleaned with sulfochromic acid and rinsed with copious amounts of Millipore-MilliQ purified water. For mixed polymer-surfactant solutions great care is required to garner reproducible equilibrium values of the surface tension. When oppositely charged polymer-surfactant pairs are in the solution and particularly at the lower surfactant concentrations, rather long equilibration times are needed (e.g., in some cases greater than 3 h). Moreover, better results are obtained when the interface remains quiescent and undisturbed. All measurements where carried out at ambient temperature, 21 ( 1 °C. Disjoining Pressure Isotherms. To construct disjoining pressure isotherms, we utilize a modified version of the porousplate technique, first developed by Mysels.11-13 This device operates by maintaining a balance between capillary and thinfilm forces and is called a thin-film balance (TFB).14 Single, thin-liquid films are formed in a hole drilled through a fritted glass disk which is fused to a 3 mm diameter capillary tube. This film holder is enclosed in a 200-cm3 hermetically sealed Plexiglas cell with the capillary tube exposed to a constant reference pressure. The solution under investigation is placed in a glass container within the cell so as to prevent contact and possible contamination with the Plexiglas chamber. Once assembled, the cell is then mounted on a vibration-isolation system and pressure in the cell is regulated with a syringe pump manually operated through a precise screw drive. Manipulation of the cell pressure alters the imposed capillary pressure, Pc, on the film and sets the disjoining pressure. Once equilibrium is established, (11) Mysels, K. J.; Jones, M. N. Discuss. Faraday Soc. 1966, 42, 42. (12) Bergeron, V. PhD. Thesis, University of California, Berkeley, 1993. (13) Bergeron, V.; Radke, C. J. Langmuir 1992, 8, 3020. (14) Claesson, P.; Ederth, T.; Bergeron, V.; Rutland, M. Surface Force Techniques; Marcel Dekker: New York, 1996.
Figure 2. Surface tension versus AAS copolymer concentration.
Figure 3. Surface tension versus surfactant concentration for AOT and AOT + 0.1 M NaCl solutions. the film thickness is measured using Scheludko’s microinterferometric method.15,16 In this thickness measurement a homogeneous thin film with a refractive index of the bulk solution, n ) 1.33, is assumed in our optical model. Hence we report the so-called equivalent-water-thickness, hw. Corrections to this thickness are not attempted because a priori knowledge of the internal structure and refractive index of these polymer-laden films is unknown. A video camera is also mounted on the microscopic to permit visual inspection of the film. Further experimental details can be found elsewhere.11-14
Results Surface Tension Isotherms. Although the copolymer used in this study has a relatively high molecular weight, its electrostatic features make it fairly soluble in water. Furthermore, over a broad range of polymer concentrations, AAS is not surface active at the pure air-water interface. This is deduced from the surface tension versus polymer concentration isotherm pictured in Figure 2. The surface tension remains constant and equal to that of pure water up to polymer concentrations in excess of 2000 ppm. In Figure 3 the surface tension isotherm for AOT with and without 0.1 M added NaCl is shown. From the figure we can see that the addition of salt strongly influences this system. The effect is witnessed by the change in slope of the isotherm, and the lowering of both, the surface tension and surfactant concentration at the critical micelle concentration. These changes arise from the increased ionic strength of the solution which permits screening of the electrostatic repulsion between surfactant molecules thus allowing them to increase their adsorption density at the interface. We also note that there are two highly (15) Scheludko, A. Kolloid Z. 1957, 155, 39. (16) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391.
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Figure 4. Surface tension versus surfactant concentration for AOT and AOT + 750 ppm AAS copolymer solutions.
Figure 5. Surface tension versus surfactant concentration for C12TAB and C12TAB + 750 ppm AAS copolymer solutions.
reproducible abrupt changes in the pure AOT isotherm, while only one is present in the isotherm which contains added salt. We speculate that these changes correlate with structural changes of the surfactant aggregates in the bulk. In both isotherms when the curve plateaus to produce a zero slope, vesicles can be observed in the solution. Therefore, for the pure AOT solution, we designate the cmc as the surfactant concentration at which the first abrupt change in the curve appears at 2.5 mM. Similar to Figure 3, a comparison is made in Figure 4 between the surface tension isotherm for a pure AOT solution to one that contains a background polyelectrolyte concentration of 750 ppm. In this case the difference between the two is not as dramatic as that seen in Figure 3. Changes in the isotherm upon addition of polymer are again simply due to an increased ionic strength: however, the low concentration of polyelectrolyte amounts to a change in ionic strength that is much lower than the 0.1 M NaCl addition in Figure 3. Thus, the AOT adsorption is only moderately affected by the presence of copolymer. Changes in the surface tension isotherm for mixtures of cationic surfactant with anionic polyelectrolyte are much different than those observed for anionic surfactantanionic polyelectrolyte blends. This is of course due to the strong Coulombic interactions between the polyelectrolyte and the oppositely charged surfactant headgroups. These interactions produce a synergistic adsorption between surfactant and polymer which is apparent in the surface tension isotherms shown in Figure 5. This figure compares the surface tension isotherm of pure C12TAB solutions (open circles) to that of a solution containing 750 ppm added polyelectrolyte (closed diamonds). The strong synergistic effect can be seen below a surfactant
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Figure 6. Disjoining pressure isotherms for 5 mM AOT solutions, with and without 0.1 M NaCl added.
concentration of 1 mM; neither component alone (surfactant or polymer) is surface active, however, together they dramatically lower the surface tension. This arises from coadsorption of polymer-surfactant complexes at the interface. For the copolymer-doped isotherm in Figure 5, the onset of the first plateau observed at a C12TAB concentration of 1 mM is typically termed the “critical aggregation concentration” (cac) and presumably corresponds to the formation of polymer-surfactant aggregates in the bulk.17 Further addition of surfactant beyond this primary plateau in the curve (i.e., >8 mM C12TAB), causes precipitation of the polymer, at which point the two surface tension isotherms coincide and pure surfactant solution behavior is recovered. Disjoining Pressure Isotherms. Anionic Surfactant-Anionic Polyelectrolyte. Disjoining pressure isotherms are measured for the same surfactant and surfactant/polymer combinations used in our surface tension measurements. Thus for example, Figure 6 displays the disjoining pressure isotherms for foam films formed from AOT solutions that are relevant to the surface tension isotherms in Figure 3. The open circles in Figure 6 represent data obtained for pure 5 mM AOT solutions while the closed circles signify 5 mM AOT solutions with 0.1 M NaCl added. Curves are provided in the figure to guide the eye. In both cases the disjoining pressure isotherms reveal strong common black films (CBF) stabilized by an electrostatic double layer. For the pure AOT foam films this double layer begins near 30 nm, whereas films made from solutions containing added salt indicate a much shorter range force, 10 nm. This length scale is set by the ionic strength of the solution, and when the ionic strength is increased the charge at the interface is further screened resulting in shorter range interactions. Simple exponential fits to the data allow us to estimate a Debye length of 44 Å for the 5 mM AOT solution and 3.5 Å for 5 mM AOT + 0.1 M NaCl. These values are in fair agreement with classical calculations of the Debye length which yield 33 and 2.8 Å, respectively.18 Again, matching the solution conditions with those relevant to our surface tension isotherms, we add 750 ppm of polyelectrolyte to the pure 5 mM AOT solution and determine the disjoining pressure isotherm. In Figure 7 it is clear that the copolymer has very little effect on the equilibrium interaction forces in the film. However, there is a 4 nm shift to slightly thinner films, this is likely due (17) Goddard, E. D.; Ananthapadmanablan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (18) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Kitchener, J. A., Ed.; Consultants Bureau: New York, 1987.
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Figure 7. Disjoining pressure isotherms for 5 mM AOT solutions, with and without 750 ppm AAS copolymer added.
Figure 8. Disjoining pressure isotherms for 0.05 mM C12TAB solutions containing 750 ppm AAS copolymer.
to the increased ionic strength of the solution. In fact, like the surface tension isotherms in Figure 4, the differences seen in Figure 7 can arise from the small change in ionic strength accompanying polymer addition rather than direct polymer interactions within the film. We also note that both isotherms in Figure 7 represent rather thin films, of order 20 nm, which suggests that polymer does not actually reside in the film. Cationic Surfactant-Anionic Polyelectrolyte. Although the addition of polymer shows little influence on AOT film behavior, foam films made from C12TABcopolymer mixtures are strongly affected by the addition of polymer. This results from the coadsorption of polymer-surfactant complexes at the interface. Figure 8 displays the disjoining pressure isotherm for a 0.05 mM C12TAB solution with 750 ppm added copolymer. This isotherm contains two repulsive branches which we accentuate further by sketching in lines between the data points. The first branch is encountered at rather thick films, hw ) 125 nm, and is stable until capillary pressures near 500 Pa are reached. Imposing a higher pressure results in a discrete transition to a new metastable equilibrium thickness of 75 nm. The transition process is pictured in Figure 9, where the lightly colored region corresponds to a film thickness of 125 nm and the darker expanding portion is the thinner 75 nm region which eventually covers the entire film. In thin-liquid films this type of transition, which is analogous to a spinodal decomposition, is termed “film stratification” and has been fully described by Bergeron et al.13,19 Both of the repulsive branches in the isotherm are reproducible, and hysteresis along a branch upon lowering the imposed capillary pressure is not observed. Transitions in the opposite direction, from thin to thick films, are impossible to investigate because we are unable to probe the negative region of the disjoining pressure isotherm with this
Figure 9. Stratification event for a foam film formed from a solution containing 0.05 mM C12TAB and 750 ppm AAS copolymer.
technique.13 Finally, once the pressure exceeds 1000 Pa the film ruptures. In contrast to the these 750 ppm polymer-doped solutions, films made from pure 0.05 mM C12TAB are completely unstable and we are unable to measure a disjoining pressure isotherm for the pure surfactant solution. Holding the polymer concentration constant at 750 ppm and increasing the surfactant concentration to the cac (1 mM C12TAB) produces quite different behavior than that seen at the lower C12TAB concentrations. In this case stable thin-liquid films can again only be formed once polymer is added, but the resulting film thickness becomes very heterogeneous. This phenomenon is pictured in the series of photos shown in Figure 10. These images are taken from a VCR recording of the film and correspond to observations made with a color CCD camera mounted on the microscope. All four images are from the same film but correspond to different imposed capillary pressures. The series begins with Figure 10a at an imposed pressure of approximately 50 Pa followed by progressively higher capillary pressures (Figure 10d, Pc > 5000 Pa). As can be easily seen, the film pictured in Figure 10 is thick enough to produce iridescent colors from thin-film inter-
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Figure 10. Sequence of photos taken from a single foam film containing 1 mM C12TAB and 750 ppm AAS copolymer. Each frame corresponds to a different capillary pressure imposed on the film: (a) 50 Pa; (b) 250 Pa; (c) 1000 Pa; (d) >5000 Pa.
Figure 11. Typical cross section of a foam film made from 1 mM C12TAB + 750 ppm AAS copolymer solutions.
ference under white light illumination. Each color corresponds to a different film thickness and it is evident that the thickness is highly heterogeneous across the film. In fact there are two types of heterogeneities: microscopic, on the order of 20 µm diameter, which appear as spots scattered throughout the film, and much larger irregularly shaped macroscopic domains. A schematic diagram of a typical film cross section deduced from Figure 10 is shown in Figure 11. From the photos in Figure 10, it can also be observed that as the pressure is increased, each macroscopic domain becomes thinner (as evident by the
changes in color), yet the shape integrity of the domains remains intact. Lastly, we note that at these high C12TAB concentrations (i.e., cac) the films are very sensitive to the rate of film formation. For example, the macroscopic thickness heterogeneities can be minimized by forming the films very slow, >1 h; however, the microscopic points consistently remain at approximately the same number density. Conversely, if the films are generated quickly, very thick, highly heterogeneous films are formed. Moreover, in all cases pronounced hysteresis of the film thickness behavior is observed when the pressure is lowered. Discussion Anionic Surfactant-Anionic Polyelectrolyte. The surface tension and disjoining pressure isotherms both indicate that for the AOT/AAS solutions tested, polymer is not present at the interface and does not directly participate in the thin-liquid film interactions. However, a secondary effect caused by addition of the polyelectrolyte is observed. Once in solution the charged sulfonate monomers dissociate and liberate their counterions, thereby increasing the solution’s ionic strength. Since the ionic strength determines the screening length for the electrostatic interactions, an increase enhances AOT
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adsorption to the interface and diminishes the length scale of the double layer forces acting in the film. This behavior is completely analogous to adding simple electrolytes to the solution and is clearly demonstrated in Figures 3 and 4 and 6 and 7. Cationic Surfactant-Anionic Polyelectrolyte. Films made from solutions containing complementary combinations of positively charged C12TAB and negatively charged copolymer behave much differently than those made with surfactant polymer pairs with the same charge. The strong attraction between C12TAB’s cationic headgroup with the anionic sulfonate monomers on the polymer backbone of AAS leads to the adsorption of a surfactantpolymer complex to the air/water interface. This complex affects both the surface tension isotherm and the molecular interactions within the film. Below the cac. The film stratification observed in Figure 9 is quite striking because it is the first observation of this phenomenon in foam films made from solutions well below the cmc (note: also well below the cac). All previous observations have been made in highly concentrated surfactant solutions or in solutions containing microscopic particles.13,19-22 However, the present solutions are very dilute in both surfactant and polymer, and light scattering from the bulk reveals no evidence of macromolecular structures in solution. Hence it is unlikely that the present stratification arises from micellar or lamella phases in the bulk. Alternatively, it may be possible that near the interface locally high concentrations of polymer and surfactant form macromolecular structures that cannot be detected in the bulk; however, we find no precedence for such a theory in the present case. Instead we turn to the recent work on electric double layer forces in the presence of polyelectrolytes.8-10 It is first important to realize that thin-film stratification does not require macromolecular structures in the film, simply an oscillation in the disjoining pressure isotherm will create the phenomena.13,19,20 Such force oscillations can clearly be generated by molecular structuring in a film;23,24 however, other forces can combine to produce an oscillatory force curve. In thin-liquid films the transition from a common black film (CBF) to a Newton black film (NBF) is a good example and can be considered a film stratification event. In this case double layer forces stabilize the CBF and steric/hydration forces support the NBF. A CBF to NBF transition occurs when there is a thickness range at which attractive van der Waals forces are stronger than the double layer yet insufficient to rupture the NBF (i.e., there is an oscillation in the curve). Details of the transition process can be found elsewhere.19 When polyelectrolytes are adsorbed to the interface, Akesson et al. have shown with Monte Carlo simulations that an oscillation in the force curve can be induced by “polymer bridging”.8-10 In their case bridging is defined by a polymer chain that is able to cross the midplane of the film and “sample the Coulomb well on both sides”; the chain does not have to occupy an adsorption site on each interface. Thus, thick films are first stabilized by double layer repulsion between the two adsorbed polyelectrolyte layers; however, once the film is thin enough, polymer bridging can create a strong attraction that will induce an abrupt decrease in the film thickness. After the (19) Bergeron, V.; Jime´nez-Laguna, A. I.; Radke, C. J. Langmuir 1992, 8, 3027. (20) Bergeron, V.; Radke, C. J. Colloid Polym. Sci. 1995, 273, 165. (21) Exerowa, D.; Lalchev, Z. Langmuir 1986, 2, 668. (22) Nikolov, A.; Wasan, D. T. Langmuir 1992, 8, 2985. (23) Pollard, M. L.; Radke, C. J. J. Phys. Chem. 1994, 101, 6979. (24) Israelachvili, J. In Intermolecular and Surface Forces, 2nd ed.; Academic Press Inc.: San Diego, CA, 1991.
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Figure 12. A single foam film made from a solution containing 1 mM C12TAB + 750 ppm AAS copolymer and produced extremely slow, >1 h formation time. The photo was taken 4 h after the film was generated.
thickness transition, film rupture is prevented by the overlapping polymer adsorption layers. This mechanism is completely consistent with our stratification observations. Above the cac. The images presented in Figure 10 indicate that once the surfactant concentration is raised to the cac, dense polymer-surfactant complexes strongly adsorb onto the interface. In this case the high polyelectrolyte adsorption not only lowers the surface tension but also creates a gel-like network within the films as the two interfaces are brought together. The structure and properties of this network can be strongly influenced by the rate of film formation. When films are formed quickly, the polymer chains extending from the surface do not have time to rearrange into an equilibrium configuration and thus get trapped (quenched) into a complicated network of knotted overlapping adsorption layers. Moreover, unlike solid surfaces, thin-liquid films (i.e., foam, emulsion, etc.) have highly deformable interfaces that succumb to the local pressures generated by squeezing the polymer network. The surface tension in this case is not strong enough to prevent deformation; thus, these films develop a very heterogeneous thickness profile. This ability to dissipate energy through interfacial deformations has not been considered in previous surface force theories; hence, standard polymer interaction theories between completely flat surfaces cannot be applied to these systems. Furthermore, once formed, the network takes a very long time to relax to its equilibrium state. The imposed pressure on the film can be held constant for days without observing any significant changes in the heterogeneous thickness profile. Although fascinating and important to our understanding of film stabilization by mixed polymersurfactant systems, this complex behavior is extremely difficult to quantify. Films at the cac were also generated extremely slowly (>1 h generation times) to try and minimize the thickness heterogeneities by allowing the polymer chains to relax as the interfaces where slowly brought together. A typical example of the resulting film is shown in Figure 12. In
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this figure the lightly colored region has a fairly uniform thickness of approximately 125 nm, while, like the film in Figure 10, the microscopic heterogeneities are impossible to avoid. These films are much more uniform than their rapidly formed counterparts, and the persistence of the microscopic domains suggests adsorption sights of highly concentrated polymer-surfactant complexes. Similar complexes where not detected in the bulk with light scattering techniques; however, this may simply be a result of their low bulk number density at these polymersurfactant concentrations. In Figure 12 we also note the irregularity of the contact line between the film and the plateau border. Even when the pressure is held constant for over 24 h, these irregularities persist. Such slow relaxtion times are again a consequence of the polymersurfactant complex present at the air-water interface. More precise structural information concerning these surface complexes has been recently attempted using ellipsometric measurements from the air-solution interface.25 Finally, we find again that pure C12TAB films at this surfactant concentration (1 mM) are completely unstable. As with the lower surfactant concentration, there is not enough C12TAB adsorbed at the interface to produce the required electrostatic or steric barriers needed to overcome the van der Waals attraction within the film and prevent film rupture.18,24 Summary The addition of polyelectrolyte to a charged surfactant solution can have ether a large or negligible effect on the forces in individual foam films. When anionic acrylamide-acrylamidesulfonate copolymer is added to AOT solutions (i.e., an anionic surfactant) the polymer has no direct influence on the surfactant adsorption or the thinfilm forces. Instead the polyelectrolyte behaves as if it where a simple electrolyte, and when added to the AOT solutions, changes in the surface tension and disjoining pressure isotherms can be accounted for by considering changes in the solutions ionic strength. (25) Asnacios, A.; Langevin, D. Submitted for publication in Macromolecules.
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On the contrary, there are dramatic affects in both the surface tension isotherm and thin-film forces when anionic copolymer is added to cationic, C12TAB, surfactant solutions. In this case, there is a synergistic adsorption between the surfactant and polymer to the interface. The resulting polymer-surfactant complex at the interface lowers the interfacial tension and generates long-range forces in the resulting foam films. In fact our C12TAB films are not stable without these additional forces. For the copolymer-C12TAB solutions at extremely low surfactant and polymer concentrations (3 orders of magnitude below the cmc and nearly 2 orders of magnitude below the cac), we observe long-range repulsive forces and a discrete film-thickness transition (i.e., stratification event) from 125 to 75 nm. This is the first report of stratification below the cmc, and the nature of the transition is somewhat different. Instead of macromolecular structuring, it is more likely produced by the interplay between repulsive electrostatic forces and attractive “polyelectrolyte bridging” as predicted by Monte Carlo calculations.8-10 At higher C12TAB surfactant concentrations (e.g., cac) a dense polymer-surfactant layer is adsorbed to the air/ water interface. As a consequence of this, gel-like networks form in the foam films made from these solutions. These networks are sensitive to the rate of film formation and produce films with very nonuniform thicknesses. Furthermore, these gel-like structures can be very important for the enhanced long-term stability of foams and emulsions made from solutions containing high molecular weight polymers. Unfortunately, the complicated nature of these gel-like systems makes it extremely hard to quantify their disjoining pressure isotherm. Acknowledgment. V.B. wishes to acknowledge Rhoˆne Poulnec for partial financial support of this work. Likewise the authors acknowledge Institut Francais du Pe´trole for their financial support and for the polymer samples they donated. LA950654Z
