Langmuir 1991, 7, 1441-1446
1441
Forces between Hydrophobic Surfaces Coated with Ethyl(hydroxyethy1)cellulose in the Presence of an Ionic Surfactant P. M. Claesson,t M. Malmsten,*J and B. Lindmant The Surface Force Group, De artment of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, weden, The Institute for Surface Chemistry, P.O. Box 5607, 23-114 86 Stockholm, Sweden, and Physical Chemistry 1, Lund University, P.O. Box 124, 23-221 00 Lund, Sweden Received November 21, 1990 The forces between hydrophobic surfaces across an aqueous solution containing 0.25 w t 5% ethyl(hydroxyethy1)cellulose (EHEC) and 4 mM SDS have been studied and compared to those appearing in the absence of SDS. A long-range repulsive force (measurable at distances up to 1200 A) is present already after an adsorption time of 30 min. The range of the repulsive force increases during the first few hours, indicating that the adsorption process is rather slow. After 20 h equilibration, a repulsion was measurable at separations up to 2500 A. The force is rather insensitive to temperature and decays essentially exponentially at large separations, with a decay-length of approximately 300 A. The force measured on compression is always slightly larger than that observed on decompression. Hence, the forces are not measured at true equilibrium. The system behaves strikinglydifferent from the pure EHEC-water system, where less long-range and reversible forces are observed. The adsorbed amount in the presence of SDS was about 2mg/m2at adsorptionequilibrium,independent of temperature (20-35"C),which is considerably less than that found in the absence of SDS (5 mg/m2 at 20 "C, and 15 mg/mz at 37 "C). These effects are discussed in terms of polymer-surfactant aggregation and competitive adsorption. Comparison is made with ellipsometry results on hydrophobized silica surfaces.
Introduction In many practical situations, surfactants and polymers are used together. This is the case, e.g., in cosmetic products, drug formulations, paints, detergent liquids, foods, polymer synthesis, and formulations for crop decrease control. Partly with this motivation, but also due to the intriguing complexity of these systems, the field of polymer-surfactant interactions has grown rapidly in the last years. From these studies, a fairly consistent picture has begun to emerge+ Most nonionic polymers and ionic (especially anionic) surfactants interact fairly strongly in aqueous solutions, forming "complexes", with the surfactant molecules "adsorbed* onto the polymer molecules. In most cases, it is found, that the surfactant is adsorbed essentially as micelles, displaying, however, slightly lower aggregation numbers than what is found for the corresponding "free" micelles. Furthermore, the aggregation starts at a fairly well-defined concentration, Tl, which is lower than the critical micelle concentration (cmc) of the surfactant. Like cmc, TI is a function of, e.g., surfactant chain length and salinity. At concentrations above 2'1, the surfactant adsorbs to the polymer, forming clusters up to a surfactant concentration, Tz,which is influenced by the polymer concentration as well as by the long-range electrostatic interactions. Above Tz, the monomeric surfactant concentration increases once more, until it reaches the cmc, where free micelles form. + The Royal Institute of Technology and The Institute for Surface Chemistry. t Lund University. (1) Breuer, M. M.; Robb, I. D. Chem. Znd. 1972, 13,530. (2) Saito, S. In Nonionic Surfactants-Physical Chemurtry; Schick, M. J.. Ed.:Marcel Dekker: New York. 1987. ( 3 j Robb,I. D. In Anionic Surfactants-Physical Chemistry of Surfactant Action; Lucaesen-Fleynders, E. H., Ed.; Marcel Dekker: New York, 1981. (4) Goddard, E. D. Colloids Surf. 1986, 19, 255.
The situation is much more unexplored when it comes
to the adsorption of polymers in the presence of surfactants, and vice versa, and to the surface interactions in these complex systems." Thus, we most frequently have a situation where both polymer and surfactant adsorb at the surface, at the same time as both solution and surface complexes form.616 Ethyl(hydroxyethy1)cellulose (EHEC) is a nonionic cellulose ether, consisting of a cellulose backbone, which is substituted with ethyl groups and oligo(ethy1eneoxide) chains. Like many other nonionic polymers and surfactants, EHEC shows a reversed temperature dependent phase behavior and a lower consolute temperature?-'6The lower temperature phase boundary is usually referred to as the cloud point (CP), CP depends on the molecular weight and the degrees of substitution. However, it is also sensitive to the presence of cosolutes, like inorganic salts, alcohols, and surfactants.1w14 In particular, a very strong synergistic effect in CP at additions of small amounts of ionic surfactants with small amounts of electrolyte has been studied e x t e n s i ~ e l y . ~ ~By J ~parallel -~~ studies, it was found that the origin of these remarkable (6) Ma, Ch.-M. Colloids Surf. 1985, 16, 185. (6) Ma, Ch.-M.;Li, Ch.-L. Colloids Surf. 1990, 47, 117. (7) Shincda, K. Principles of Solution and Solubility; Marcel Decker: 1978. -New _ York. -- - -(8) Schick,M. J., Ed. Nonionic Surfactants-Physical Chemistry;Marcel Dekker: New York, 1987. (9) Bailey. F. E.: Koleeke, J. V. Poly(ethylene oxide);Academic Preee: . . New York, -1976. (10) Lindman, B.; C a r h n , A.; KarlatrBm, G.; Malmaten, M. Ado. Colloid Interface Sei. 1990,32, 183. (11) Malmeten, M.; Lindman, B. Longmuir 1990,6,357. (12) KarlstrBm, G . ; C a r h n , A.;Lindman,B. J.Phys. Chem. 1990,94, 5005. (13) Carlsson, A.; KarletrBm, G.; Lindman, B. Langmuir 1986,2,536. (14) Carlason, A.; KarlatrBm, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988, 266, 1031. (15) Samii, A.; Lindman, B.; KarlatrBm, G. Prog. Colloid Polym. Sci. 1990,82,280. ---I
0743-7463/91/2407-1441$02.50/0 0 1991 American Chemical Society
Claesson et al.
1442 Langmuir, Vol. 7, No. 7, 1991 effects is t h e formation of surfactant-polymer com-
plexes.1°J6J7 Another interesting property of the aqueous EHEC/ionic surfactant systems is the formation of thermoreversible gels at higher t e m p e r a t ~ r e . ~ ~The J ~ Jmech~ anism of t h e formation of the gels is still investigated by different methods, but it is quite clear, that t h e increased degree of binding of t h e surfactant t o t h e polymer at elevated temperatures, which has been directly demonstrated t o occur, plays a major r01e.1°J6J7 Previously, the adsorption of EHEC at silica surfaces of different hydrophobicity has been studied with ellipsometry.ll From these studies it was found, that the main driving force for t h e adsorption in these systems is a hydrophobic attraction between the polymer and the surface (although the expulsion from solution per se is an important driving force for adsorption in these systems). It was found, that on increasing t h e temperature, t h e adsorbed amount increases strongly, at the same time as the polymer layer becomes more dense. Also at additions of different cosolutes (inorganic salts and alcohols), the adsorption, as well as the conformation, follows the expectations from solvency. In a series of studies, we have studied the interactions between surfaces (hydrophobic and hydrophilic) coated with EHEC, inter alia, as a function of surface separation It was found, that EHEC adsorbs and rather weakly at hydrophilic surfaces but strongly at hydrophobic surfaces. Well below CP, the force is monotonically repulsive in both cases, as expected. With increased temperature, there are marked changes in t h e force curves, t h e temperature dependence, however, being different at hydrophobic and hydrophilic surfaces. Moreover, t h e temperature dependence of the interaction force in t h e absence of constraints of the adsorbed amount is different from that a fixed adsorbed amount. At sufficiently high temperatures, however, the force is attractive at both hydrophilic and hydrophobic surfaces, whether the adsorbed amount is fixed during t h e experiment or
not. In this study, we aim t o study t h e impact of an ionic surfactant (SDS)on the adsorption and interaction forces in aqueous EHEC solutions. For simplicity, only the case with no added electrolyte will be treated in t h e present study. In doing so, we use t h e previously used techniques ellipsometry11 and surfaces force Experimental Section Chemicals. The polymer used here, EHEC, is a nonionic cellulose ether, which was supplied by Berol Nobel AB, Sweden. The EHEC fraction used has an average molecular weight of 475 000, as determined by light scattering. The polydispersity of EHEC is generally very broad. The radius of gyration is 850 A. The molar substitution of ethyl groups and ethylene oxide groups is 1.7 and 1.0, respectively. The cloud point (CP)of this particular fraction is 35 "C on heating. Dry EHEC powder normally contains 3-5 w t % of NaCl (impurity from synthesis) and therefore the EHEC solutions were dialyzed against mem(16) Carlsson,A.; Karlstrom, G.; Lindman, B. J.Phys. Chem. 1989,93, 3673. (17) Carlsson, A.; Lindman, B.; Watanabe, T.; Shirahama, K. Langmuir 1989, 5, 1250. (18) Carlsson, A.; Lindman, B.; Karlstrom, G. In New Functionalisation Deoelopments in Cellulosics and Wood-Fundamentals and Applications; Ellis Horwood: Chichester, 1989. (19) Carlsson, A; Karlstrom, G.; Lindman, B. Colloids Surf. 1990,47, 147. (20) Malmsten, M.; Claesson, P. M. Langmuir, in press. (21) Malmsten, M.; Claesson, P. M.; Pezron, E.; Pezron, I. Langmuir 1990, 6, 1572. (22) Pezron, I.; Pezron, E.; Claesson, P. M.; Malmsten, M. Langmuir, in press.
brane filtered water (Millipore, USA) for 5 days. As a dialyzing membrane regenerated cellulose, with a molecular cutoff of 6000 (Spectrum Medical Industries, USA), was used. Sodium dodecy1sulfate (SDS)was of analytical grade and was further purified by recrystallization 3 times from water before it was freeze-dried. No maximum was found in the adsorption isotherm. Preparation of Hydrophobic Surfaces. In the ellipsometry experiment, oxidized and hydrophobized silica surfaces were used. These surfaces have an advancing contact angle of about 90°. The hydrophobization procedure has been described elsewhere.!! In the surface force experiments, hydrophobic surfaces(the advancingand receding contact angle of which equals 112O and 80°, respectively) were obtained from mica surfaces by a Langmuir-Blodgett deposition of a 1:lmixture of eicosylamine and eicosanol. For details of the preparation procedure, see refs 20 and 21. Ellipsometry. The ellipsometer used was a modified automated Rudolph thin film ellipsometer, type 43603-2003, controlled by a personal computer, with a white mercury lamp (filtered to give 400 nm) as the light source. Throughout, in situ experiments were made. The adsorption measurements involved two steps, viz. (i) determination of the complex refractive index of the substrateand (ii) additionof polymer (surfactant) solution, followed by the determination of adsorbed amount (calculated according to Cuypersllp"*u). The adsorption temperature was 25 OC. Stirring was performed by a magnetic stirrer at a rate of about 300 rpm. Surface Force Experiments. Measurements of the forces acting between two hydrophobic substrate surfaces across an aqueous EHEC/SDS solution were carried out with a surface force apparatus.s128 An apparatus of the modified design by Parker et 81.26 was used in this set of experiments. The two interacting surfaces are mounted in a crossed cylinder geometry. This geometry is experimentally suitable, and the measured force, F,, divided by the local geometric mean radius of the cylinders, R, is related to the free energy of interaction per unit area between two flat surfaces, Gf, as first demonstrated by DerjauginZ7
F , ( D ) / R = 2rGf(D) (1) provided that the radius of the cylinders (about 2 cm) is much larger than the surface separation (D).The distance between the surfaces is determined with an accuracy of 0.2 nm by using multiple beam interferometry. The force is measured from deflections of a double variable cantilever spring supporting the lower surface. The detection limit is about lo-' N. The experiments were performed in the following way: First, the contact position between the hydrophobic surfaces was determined in air. This position defines the zero separation. After this, a droplet containing 0.25 w t % EHEC and 4 m M SDS was placed between the surfaces, and the force was measured as a function of surface separation. After changing the conditions, the system was always allowed to adjust for several hours. After equilibration,the force wasmeasured duringseveral compression/ decompression cycles. The use of a droplet instead of filling the whole box is convenient because of the low volume needed and because of simple cleaning of the apparatus after the experiment (EHEC adsorbs strongly to moat surfaces). The change in surface separation during force measurements is small compared to the radius of the droplet. Consequently, the radius of the droplet and the Laplace pressure can be regarded as constant during the measurement. Evaporation from the droplet was minimized by covering the bottom of the apparatus with the EHEC/SDS solution. The temperature was varied by directing a hot airstream from a heating fan toward the apparatus. The temperature of the air surrounding the droplet inside the box was measured with a ther~~
~
(23) Arnebrant, T.;Bickstrijm, K.; Nylander, T. J. Colloid Interface Sci. 1989, 128,303. (24) Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. Th.; Hemker, H. C. J. B i d . Chem. 1983,258, 2426. (25) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 1, 1978, 74, 975. (26) Parker, J. L.;Christenson, H. K.; Ninham, B. W. Reo. Sci. Instrum. 1989, 601, 3135. (27) Derjaugin, B. V. Kolloid-Z. 1934, 69, 155.
Langmuir, Vol. 7, No. 7, 1991 1443
Hydrophobic Surfaces Coated with EHEC
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Results As can be seen in Figure 1, the cloud point of EHEC increases drastically with increasing SDS concentration. This effect, which is due to complex formation between EHEC and SDS,introducing intra- and intermolecular electrostatic interactions, is well-known and has been extensively discussed p r e v i ~ u s l y . ~ O J ~ - ~ ~ In Figure 2, the effects of adding SDS to a system with EHEC preadsorbed at hydrophobized silica are shown. At addition of SDS, the effective adsorbed amount decreases slowly. At the same time, there is a decrease in the average apparent adsorbed layer concentration (as = I'/d, d being the adsorbed layer defined by (aapp) thickness). Inserted graphs show the kinetics of the (28) de Feijter, J. A.; Benjamins,J.; Veer, F. A. Biopolymers 1978,17, 1759. (29) Tartar,H. V.; Lelong, A. L. M. J. Phys. Chem. 1956,59, 1185.
(30)Holmberg, K.;Nileson, S.; Singh, S. S.; Sundel6f. L.-0. J. Phys. Chem., in press.
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adsorption of SDS and EHEC. As can be seen in this figure, the adsorption of SDS is quick ("seconds), whereas the adsorption of EHEC is much slower (-hours). The force acting between hydrophobized substrate surfaces across a solution containing 0.25 wt 96 EHEC and 4 mM SDS,after two different adsorption times, is shown in Figure 3. A long-range repulsive force, measurable at distances smaller than approximately 1200 A, is already acting between the surfaces after an adsorption time of 30 min. The magnitude of the repulsive force increases with time the first few hours, showing that the adsorption process is slow, just as in the case without SDS.11*20121 After an adsorption time of 20 h, a repulsion is observed at separations smaller than approximately 2500 A. The repulsion observed on approach is always slightly larger than that found on separation. This is illustrated for the case of an adsorption time of 20 h in Figure 4. A similar hysteresis is observed at shorter adsorption times. This is contrary to the surfactant-free case, where the force is reversible on approach and separation.20*21The adsorbed amount is about 2 mg/m2, which is more than a factor of 2 lower than that found in the absence of SDS.20 The magnitude as well as the range of the force measured in the presence of SDS is larger than that observed without any SDS.20 This is illustrated in Figure 5. Moreover, the presence of SDS results in a slightly lower decay rate of the force (Figure 5, insert). On increasing the temperature to 35 "C (roughly equal to CP in the absence of SDS), only moderate changes in the adsorbed amount (I? = 2 mg/m2) and the force curve are observed in the presence of SDS (Figure 6). This is in large contrast to the situation in the absence of SDS, where a dramatic increase in the adsorbed amount (I' = 15mg/m2 a t 37 "C) results in a more long-range repulsive force at higher temperatures (see Figure 6, insert).20 The forces observed a t 35-37 "C in the presence of SDS are compared in Figure 7. Note that, a t this temperature, the repulsive force is weaker and decaying much slower in the presence of SDS. Discussion State of the Adsorbed Layer. The first obvious result from this ellipsometry and surface force study is that the
Claesson et al.
1444 Langmuir, Vol. 7, No. 7, 1991 2oooo
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Figure 5. A comparison of the (compressional) forces acting between hydrophobic surfaces in a solution containing 0.25 wt % EHEC with (filledcircles) and without%(open squares)4 mM SDS. The insert shows the same forces on a logarithmic scale. The temperature was 20 "C. adsorbed amount is considerably smaller in the presence of SDS than in the SDS-free EHEC system. The adsorbed amount on hydrophobized mica in the absence of SDS is 5 mg/m2 at 20 "C and 15 mg/m2 at 37 "C. In the presence of 4 mM SDS (which is above Tim), however, the adsorbed amount is only about 2 mg/m2 (independent of temperature in this temperature range). Clearly, SDS reduces the adsorption of the nonionic polymer to the hydrophobic surface significantly. This is clearly shown with ellipsometry as well (cf. Figure 2). (The fact that the adsorbed amount obtained with the surface force technique (5mg/ m2 in the surfactant-free case) is much higher than the adsorbed amount obtained from ellipsometry (3 mg/m2) has been discussed previously.2l) It has previously been shown that the driving force for
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(A) Figure 7. A comparison of the forces acting between hydrophobic surfaces across a solution containing 0.25 w t % EHEC in the presence (filled triangles) and absence% (open triangles) of 4 mM SDS. The insert shows the same forces plotted on a logarithmic scale. The temperature was 37 OC in the surfactantfree case and 35 OC in the presence of SDS. DISTANCE
the adsorption of EHEC to hydrophobic surfaces is a hydrophobic interaction between the surface and the polymer (vide supra.)" Adsorption of SDS to the surface results in a competition with the EHEC segments for the surface sites and, consequently, in a reduced number of direct surface-segment contacts (SDS acts as a displacer for EHEC, cf. Figure 2). Moreover, since EHEC and SDS form complexes in solution10J2-1*~30it is likely that this will happen at interfaces as well. This results in the formation of polymer-surfactant complexes that are more hydrophilic than the polymer molecules themselves. Hence, SDS reduces the hydrophobicity of both the polymer and the surface, which, in turn, decreases the
Hydrophobic Surfaces Coated with EHEC
attractive interaction between the polymer and the surface. At the same time, the “polymer”-“polymer” interaction becomes more repulsive (the surfactant stabilizes the polymer solution, cf. Figure l), which further destabilizes the adsorbed layer. The results presented above, however, show that SDS does not prevent EHEC adsorption completely. It is obvious that SDS is included in the adsorbed layer both from the large difference in the adsorbed amount and from the very different forces observed with and without SDS present in solution (Figure 5-7). It is equally obvious that EHEC is included in the adsorbed layer from the distance dependence of the measured forces (vide infra) and from the slow adsorption process (the adsorption from the EHEC/SDS mixture, as well as the desorption/expansion process a t addition of SDS to preadsorbed EHEC is slow (-hours; cf. Figures 2 and 3), while the adsorption of SDS per se is a very fast process indeed (-seconds; cf. Figure 2, insert)). However, the quantitative aspects of the composition of the adsorbed layer cannot be deduced from our results. In several studies, Ma et al. have studied the effects of sodium alkyl sulfates (e.g., SDS) on the adsorption of PVP (to Ti02 and Fe2O3) and vice ~ e r s a In . ~these ~ ~ studies it was found, that surface complexes are formed in certain concentration regions (below approximately 4 mM for SDS), while solution complexes dominate at higher surfactant concentrations. Thus, the finding of the presence of both surfactant and polymer in the adsorbed layer in the present study (at alow surfactant concentration) agrees qualitatively with these previous findings. Surface Interaction. In the absence of SDS, EHEC adsorbs strongly at hydrophobic surfaces, giving rise to reversible interaction forces on approach and separation.2092l In the presence of SDS, however,the force curves are not reversible on approach and separation. A possible explanation for this is that a fraction of the adsorbed EHEC-SDS complexes desorbs on compression, which is also compatible with the lower adsorbed amount in the presence of SDS. Another possibility is that a fraction of the adsorbed surfactant molecules desorbs on compression. It could be argued, that the latter mechanism is less likely, since the adsorption rate of the surfactant molecules is high, compared to the speed of decompression (cf.Figure 2, insert). However, this refers to the bare surface, and it is unclear to what extent this represents the case with polymer included in the adsorbed layer. Moreover, although the kinetics give some support for the former mechanism, both the magnitude and the distance of convergence of the hysteresis are small, which supports the latter mechanism. Hence, further work concerning this is needed. As mentioned above, the interaction force at 20 OC is more repulsive in the presence of SDS than in the surfactant-free case, despite a much lower adsorbed amount. At large separations, the decay length of the force is approximately 300 A (insert Figure 5), which is considerably longer than the decay length of double-layer forces in an aqueous solution containing 4 mM completely dissociated 1:l electrolyte (50 A). (Note, that the adsorption of the surfactants to the surface does not markedly affect the free monomer concentration, due to the very small surface area (a few cmz).) However, the situation is complex, since the surfactant molecules aggregate, thus forming higher valence species. Although the detailed interpretation of these findings is difficult, it seems clear that the change of the forces on addition of SDS is not due to the introduction of double-layer forces per se (cf. e.g.
Langmuir, Vol. 7, No. 7, 1991 1445
Figure 2). Instead, conformational changes of adsorbed EHEC molecules induced by SDS cause the layer to swell and thus change the forces. Accordingly, the interaction force in the presence of SDS is of both electrostatic and “steric” origin. The steric interaction, in turn, originates from the increase in the polymer concentration on bringing the surfaces together.31~32However, since the introduction of electric charges induces conformational changes of the polymer, the steric and electrostatic contributions to the interaction force are interrelated. However, considering the strong increase in the “solvency conditions” (broadly speaking), which occurs in the presence of SDS (cf. Figure l),it is hardly surprising that the repulsive force is stronger in the presence of SDS than in the absence, despite the adsorbed amount being considerably lower in the former case. There are several ways in which SDS influences the conformation of adsorbed EHEC molecules and, thus, the long-range forces. The formation of EHEC/SDS complexes gives rise to strong electrostatic interactions in the adsorbed layer, which results in a more extended conformation. Furthermore, the adsorption of SDS at the hydrophobic surface reduces the number of binding sites for EHEC and thus promotes the formation of loops and tails. Other contributions to the large swelling arise from an electrostatic repulsion between the surfactant molecules adsorbed at the surface and the surfactant molecules associated with the polymer and from a repulsion between charges on the polymer/surfactant complex and the low dielectric constant surface (due to image effects). Obviously, the picture is complicated, since the charges are mobile and since SDS introduces not only electrostatic repulsions but probably “micellar” cross-links in the adsorbed layer as well. The latter effect is the likely reason why EHEC and ionic surfactants form gels under certain conditions. P3JWMO It is interesting to note that the EHEC/SDS complexes do not behave as ordinary polyelectrolytes. Thus, it is usually found that the polyelectrolytes adsorb in a rather flat conformation (depending, however, on the salinity) due to the intramolecular electrostatic repulsions, which suppress loop and tail for ma ti or^.^^-^^ This is true not only for surfaces containing the opposite sign of charge of that of the polymer but generally for uncharged surfaces as well. If, on the other hand, the surface and the polyelectrolyte have the same sign of charge, which is likely to be the case in the present study (surfactant adsorption at both polymer and surface), depletion generally occurs. In the present case, the situation is different, and the complexes do adsorb, despite the sign of charge of the polyelectrolyte and the surface probably being the same. The likely reason for this is that the complexes contain both adsorbing (i.e. hydrophobic) and nonadsorbing (i.e. charged) regions. Therefore, a direct analogue between the present system and ”conventional”polyelectrolytes is not expected. On the other hand, it has been observed that other heterogeneous polyelectrolytes, containing both charged and hydrophobic regions (e.g. gelatin), may adsorb (31) Napper, D.H.Polymeric Stabilization of Colloidal Dispersione; Academic Press: London, 1983. (32) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY,1953. (33) van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984,88,6661. (34) Papenhuizen, J.; van der Schee, H. A.; Fleer, G. J. J. Colloid Interface Sci. 1985, 104, 540. (35) BBhmer, M.R.; Evers, 0. A.; Scheutjens, J. M. H. M.Macromolecules 1990,23, 2288. (36) Cohen-Stuart, M. A.; Cosgrove, T.; Vincent, B. Adu. Colloid Interface Sci. 1986,24, 143.
1446 Langmuir, Vol. 7, No. 7, 1991
at surfaces having the same (net) sign of charge as that of the p~lyelectrolyte.~~ Effects of Temperature. The difference between the adsorbed layer in the presence and absence of SDS becomes even more clear a t higher temperatures. In the absence of SDS, EHEC adsorbs more extensively at higher temperaturesem This results in a 3-fold increase in the adsorbed amount as the temperature is increased from 20 to 37 “C. Moreover, the adsorbed layer is very compact at the higher temperature, as seen from the steepness of the force-distance profile (Figure 7). The reason for this dramatic temperature effect in the absence of SDS is that the solvency conditions become progressively worse at higher temperature (CP = 35 “C). In contrast, an increase in temperature from 20 to 35 “C has hardly any effect on the adsorption, or on the interactions, in the presence of 4 mM SDS. Both the adsorbed amount and the structure of the adsorbed layer (the latter as inferred from the force behavior) are largely independent of temperature (in this temperature range). These findings are in accordance with those of a previous ellipsometry study of the adsorption to EHEC to hydrophobic silica surfaces.” In this study it was found that the adsorbed amount, as well as the average polymer concentration in the adsorbed layer, changes in good correlation with the proximity to the cloud point. Since CP is very high in the presence of SDS, a weak temperature dependence of both interfacial behavior and solution properties is expected. Hence, the weak variation of the adsorbed amount and of the interaction force, on increasing the temperature from 20 to 35 “C, is expected. (37) Kawaniahi,N.;Christeneon, H. K.; Ninham,B. W.; J.Phys. Chem. 1990,94,4611.
Claesson et al.
Conclusion The forces between hydrophobic surfaces coated with EHEC in the presence of SDS have been studied as a function of separation and temperature and compared to those appearing in the absence of SDS. It was found that SDS reduces the adsorbed amount at the same time as it “swells”the adsorbed layer. Furthermore, SDS eliminates the strong temperature dependence of the adsorption and the interaction force, found for the SDS-free EHEC system in the temperature range studied (20 to 35 “C), which is in good correlation with the phase behavior. Qualitative agreement was found between surface force measurements and ellipsometry, although one should be somewhat cautious not to draw the comparison too far. The reason for this (apart from the obvious reason that two different surfaces were used in the two sets of experiments) is that the two methods reflect rather different parts of the adsorbed layer. Hence, ellipsometry reflects essentially trains and loops (dilute tails are essentially “invisible” to ellipsometry), while force measurements emphasize the tail fraction. Despite this objection, it is clear that the main features of the different adsorption properties in the presence and absence of SDS, i.e., the lower adsorbed amount and the larger extension of the adsorbed layer, in the presence of SDS, are nicely illustrated by both techniques.
Acknowledgment. This work was financed by Berol Nobel AB, Sweden, Kabi Invent, Sweden, and the Swedish National Board for Technical Development.
