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Direct Force Measurements between r-Alumina Surfaces with Adsorption of Anionic Surfactant/Polymer Mixtures Kenichi Sakai, Tomokazu Yoshimura, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received August 11, 2002. In Final Form: November 13, 2002 Direct measurements of interaction forces between R-alumina surfaces with coadsorbed anionic surfactant sodium dodecyl sulfate (SDS) and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) at pH 3.5 are carried out by colloidal probe atomic force microscopy. It is found that in aqueous PSS solutions the interaction forces are dependent on the initial PSS concentration. At low PSS concentration the electrostatic repulsion initially present is neutralized, resulting in a weak attraction between the surfaces, while as the concentration is increased further, repulsive forces reappear due to the additional adsorption of PSS on the surface. The adsorbed PSS adopts a relatively flat conformation at all PSS concentrations investigated. On the other hand, the interaction forces are dramatically different for the simultaneous addition of SDS and PSS: the force profile for the coaddition comes progressively close to that for the addition of SDS alone with increasing SDS concentration. This is attributed to the fact that SDS plays a key role for the coadsorption of SDS and PSS over a high SDS concentration region. The dispersion stability and ζ potential of R-alumina suspensions are evaluated for comparison with the resultant forces, and a good correlation is obtained between them. The effects of sequential addition of SDS and PSS are also investigated.
Introduction Direct measurements of interaction forces between solid substrates provide a useful insight for understanding the stabilization and flocculation mechanisms of dispersions. The development of the atomic force microscope has enabled such forces in colloidal systems and allowed a large variety of mineral surfaces to be used.1 This technique has been utilized to study the adsorption of surfactants,2-4 linear polymers,5-9 or dendritic macromolecules.4 Furthermore, a few recent publications describe the coadsorption behavior of ionic surfactants and oppositely charged polymers.10-13 These investigations mostly focus on the effects of the sequential addition of surfactants on interaction forces between the polyelectrolyte coated surfaces; however, the simultaneous addition of their adsorbates has been largely neglected until now. The knowledge base obtained here has been used in the development of various industrial products, including cosmetics, paints, detergents, and pharmaceuticals. In these areas surfactants are generally used to control the dispersion, flocculation, and wetting properties of suspen(1) (a) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (b) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (2) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (3) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1999, 146, 75. (4) Sakai, K.; Sadayama, S.; Yoshimura, T.; Esumi, K. J. Colloid Interface Sci. 2002, 254, 406. (5) (a) Biggs, S. Langmuir 1995, 11, 156. (b) Biggs, S. J. Chem. Soc., Faraday Trans. 1 1996, 92, 2783. (6) Braithwaite, G. J. C.; Howe, A.; Luckham, P. F. Langmuir 1996, 12, 4224. (7) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202. (8) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1998, 139, 199. (9) Giesbers, M.; Kleijn, J. M.; Fleer, G. J.; Cohen Stuart, M. A. Colloids Surf., A 1998, 142, 343. (10) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1999, 155, 1. (11) Muir, I.; Meagher, L.; Gee, M. Langmuir 2001, 17, 4932. (12) McNamee, C. E.; Matsumoto, M.; Hartley, P. G.; Mulvaney, P.; Tsujii, Y.; Nakahara, M. Langmuir 2001, 17, 6220. (13) Meagher, L.; Maurdev, G.; Gee, M. Langmuir 2002, 18, 2649.
sions, while polyelectrolytes serve to meet rheological requirements. The coadsorption behavior at the solid/solution interface depends on the molecular structure of the adsorbates.14 Certain surfactant-polymer combinations exhibit an interaction specifically in bulk solution. In a previous paper, we addressed interaction forces between R-alumina surfaces with coadsorbed anionic surfactant sodium dodecyl sulfate (SDS) and nonionic polymer poly(vinylpyrrolidone) (PVP) at pH 3.5.15 It is well-known16 that these adsorbates form polyelectrolyte-like complexes in the bulk phase, so that the simultaneous adsorption at the interface will be favored as in a familiar case. As was found in this work, the interaction forces are dramatically affected for the coaddition of SDS and PVP: electrostatic repulsion is observed above the critical association concentration, and virtually no steric hindrance due to adsorbed polymer chains can be detected. These coaddition effects are attributed to the flat conformation of the adsorbed PVP. On the other hand, the protrusion of the adsorbed polymer chains is only seen for the sequential addition of PVP to R-alumina pretreated with SDS hemimicellar layers. Accordingly, these differences corroborate the finding that complexation of SDS and PVP occurs in bulk solution for the simultaneous adsorption and on or near the solid surfaces for the sequential case. For the simultaneous addition of surfactants and polymers competitive adsorption has received much attention as well as the former type. For example, despite its high affinity to R-alumina, the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) adsorbed on positively charged R-alumina is replaced by SDS with increasing SDS concentration. By contrast, the adsorption of PSS has a negligible effect on the layered structure of (14) Otsuka, H.; Esumi, K. In Structure-Performance Relationships in Surfactants; Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 1997; Chapter 12. (15) Sakai, K.; Yoshimura, T.; Esumi, K. Langmuir 2002, 18, 3993. (16) Lindman, B. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Eds.; John Wiley & Sons Ltd.: New York, 2001; Chapter 20.
10.1021/la026388o CCC: $25.00 © 2003 American Chemical Society Published on Web 01/24/2003
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SDS adsorbed: even though the adsorbed amount of PSS increases, that of SDS remains constant. These results indicate that the adsorption of SDS is a more favorable energy state than that of PSS.17 In addition, a competitive adsorption behavior of a cationic surfactant, hexadecyltrimethylammonium bromide, and a cationic polyelectrolyte, polylysine, on silica by using optical reflectometry has been reported.18 Although these observations provide macroscopic explanations, the relationship between the coadsorption phenomena and dispersion stability of suspensions (or surface forces) has yet to be established at the nanoscopic scale. In the present work, the interaction forces between R-alumina surfaces in aqueous solutions of SDS and PSS were measured by colloidal probe atomic force microscopy (AFM). The effects of adding the surfactant before or after the polymer were studied to elucidate the complex phenomena that occur at the solid/liquid interface. The dispersion stability of R-alumina suspensions with the coadsorption of the adsorbates was assessed through sedimentation tests and ζ potential measurements. Experimental Section Materials. R-Alumina spheres with an average diameter of 20 µm were obtained from Showa Denko Co. (Japan). Singlecrystal sapphire windows, used as flat plates for AFM measurements, were kindly supplied by Saint-Gobain Crystals and Detectors Co. (France). The root-mean-square roughness of the plates, measured over an area of 1 µm2, was 0.25 nm. The R-alumina spheres and the sapphire windows were ultrasonicated for 10 min in concentrated H2SO4. The materials were then rinsed with water, immersed in an aqueous 0.1 mol dm-3 KOH solution overnight, and finally rinsed thoroughly with water. R-Alumina particles used in the sedimentation tests were also supplied by Showa Denko Co. Their specific surface area, average diameter, and purity are 8.9 m2 g-1, 2.51 µm, and 99.995%, respectively. SDS obtained from Nacalai Tesque Inc. (Japan) was recrystallized three times from ethanol. PSS (Aldrich Chemical Co. Inc.; Mw ) 70000) was used as received. All other reagents were of analytical grade. Water was deionized using a Milli-Q Plus system. Methods. The static surface tension of aqueous surfactant/ polymer solutions was measured using a Kru¨ss K122 tensiometer by the Wilhelmy plate technique. All solutions contained 1 mmol dm-3 NaCl as a background electrolyte. Surface force measurements were carried out using a TMX2100 atomic force microscope (TMmicroscopes Inc.). A detailed description of the force measurement technique is given elsewhere.19 Data were collected using commercial silicon nitride cantilevers (Digital Instrument Inc.) with a spring constant of 0.58 N m-1, modified by attaching an R-alumina sphere, as described by Ducker et al.1 Prior to force measurements, the colloidal probe attached to the cantilever was immersed in 0.1 mol dm-3 KOH overnight, followed by a thorough deionized water rinse. Sapphire windows were washed following the same procedure. After washing, the probe and the plate were both set at their fixed positions on the atomic force microscope and immediately immersed in the sample solution. The system was allowed to equilibrate for 30 min prior to the initial injection and force runs. In sequential adsorption, force acquisition was resumed only after a lapse of 120-150 min following the replacement of the preadsorbate solution with the postadsorbate solution. The temperature was maintained constant at 25 °C using a thermomodule controller (MT862-04C12, Netsu Denshi Co., Ltd., Japan). The dispersion stability of R-alumina suspensions was evaluated as follows: the suspension (0.01 g of R-alumina/50 cm3) was shaken in a water bath for 4 h at 25 °C and then transferred to a sedimentation tube. The absorbance at 600 nm of the top portion (17) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (18) Velegol, S. B.; Tilton, R. D. Langmuir 2001, 17, 219. (19) Sakai, K.; Torigoe, K.; Esumi, K. Langmuir 2001, 17, 4973.
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Figure 1. Surface tension of aqueous solutions of SDS and PSS. Initial concentrations of PSS are set to 0, 5, and 50 ppm, and the background salt concentration is fixed at 1 mmol dm-3. of the aqueous suspension in the tube was measured after 4 h of standing; the higher the absorbance, the higher the dispersion stability. The ζ potential of the suspensions was monitored with an electrophoretic apparatus (Pen-Kem Laser Zee Meter Model 500): ζ potentials were converted from electrophoretic mobilities using the Smoluchowski equation µ ) ζE/4πη, where µ is the electrophoretic mobility, is the dielectric constant, E is the electric field, and η is the viscosity of the mobile phase.
Results Prior to force measurements, we investigated the association behavior of SDS and PSS at the air/solution interface. Figure 1 shows the surface tensions of aqueous SDS-PSS solutions as a function of SDS concentration. Feed concentrations of PSS are 5 and 50 ppm, and all solutions contain 1 mmol dm-3 NaCl as a background electrolyte. The surface tension of SDS alone decreases monotonically as the SDS concentration increases and reaches a plateau value at about 6 mmol dm-3 (cmc). For the mixtures of SDS and PSS the surface tension also decreases with increasing SDS concentration in a manner similar to that above. However, as PSS concentration increases the surface tension becomes lower compared to that at the same SDS concentration. This result suggests that the complexation of SDS and PSS occurs at the air/ solution interface via the hydrophobic attraction between the SDS hydrocarbon chain and the PSS polystyrene groups. However, this attraction is found to be very weak since any transition points do not exist in the resultant surface tension data. In addition, it can be pointed out that the PSS molecules adsorbed on the air surface are progressively replaced by SDS with increasing SDS concentration due to the preferential SDS adsorption. Interaction forces between R-alumina surfaces with the adsorption of PSS are shown in Figure 2. The initial PSS concentrations are fixed at 1, 5, 50, and 500 ppm. All sample solutions contain 1 mmol dm-3 background electrolytes (NaCl + HCl), and the pH value of each solution is 3.5 ( 0.1. In the 1 ppm PSS solution a weak attraction is detected at a separation of 10 nm. By contrast, as the polyelectrolyte concentration is increased above 5 ppm, a repulsive interaction between the surfaces is once again observed. The magnitude of this repulsion is seen to decrease with increasing PSS concentration. To examine the origin of such an interaction, the compression force data for the adsorption of PSS are plotted in Figure 3 as the log of the normalized force versus apparent separation. It is found from this figure that an approximately linear
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Figure 2. Normalized forces between R-alumina surfaces in aqueous PSS solutions containing 1 mmol dm-3 1:1 background electrolytes (NaCl + HCl) at pH 3.5.
Figure 4. (a) Dispersion stability and (b) ζ potential of R-alumina suspensions with adsorption of PSS as a function of the initial concentration of PSS. Each solution contains 1 mmol dm-3 background salts, and the pH value is adjusted to 3.5. The solid lines are drawn to guide the eye. Figure 3. Interaction between R-alumina surfaces with adsorption of PSS plotted as the log of the normalized force versus apparent separation.
relationship exists at all surface separations and the slopes of these linear plots steepen as the polyelectrolyte concentration increases. Thus, one would expect that the repulsion is induced by the electrostatic interaction between the PSS adsorbed layers on the surfaces. Comparing the resultant forces with the dispersion stability of the suspensions is one of the aims of this investigation. Figure 4a presents the changes in the absorbance at 600 nm of R-alumina suspensions as a function of PSS concentration. Also plotted in Figure 4b are the corresponding ζ potential data for the same samples. A dispersion-flocculation-redispersion sequence of R-alumina suspensions is observed with increasing PSS concentration. Indeed, the positive ζ potentials decrease to zero in the 1 ppm PSS solution, and then a charge reversal occurs above this concentration. It is obvious that these results are in good agreement with each other, and the difference in the interaction forces between the adlayers leads to such a phenomenon on a macroscopic level. Figure 5 presents the interaction forces between R-alumina surfaces for the coaddition of SDS and PSS. The initial PSS concentration is 50 ppm, while SDS concentrations are set to 0 and 0.06 (0.01 cmc), 3 (0.5 cmc), and 12 (2 cmc) mmol dm-3. All solutions contain 1 mmol dm-3 background salts (NaCl + HCl), and the pH values are adjusted to 3.5 ( 0.1. As described in our previous paper,15 in aqueous SDS solutions the strength
Figure 5. Normalized forces between R-alumina surfaces in aqueous SDS and PSS (50 ppm) solutions containing 1 mmol dm-3 1:1 background electrolytes (NaCl + HCl) at pH 3.5.
of the interaction forces depends on the initial SDS concentration: in the 0.06 mmol dm-3 SDS solution an attractive force is detected at a separation of approximately 25 nm, while in the 3 and 12 mmol dm-3 SDS solutions repulsive forces reappear. Interaction forces are substantially altered by the coaddition of PSS. The force curve for the solution containing 0.06 mmol dm-3 SDS and 50 ppm PSS highly resembles that for PSS alone. The lack of an attractive component between the SDS adlayers neutralizing the surface charge indicates that the force curve is
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Figure 7. Sequential addition of SDS and PSS to the preadsorbed PSS at pH 3.5. Initial SDS and PSS concentrations are set at 3 mmol dm-3 and 50 ppm, respectively. Solutions contain 1 mmol dm-3 background salts.
Figure 6. (a) Dispersion stability and (b) ζ potential of R-alumina suspensions with coadsorption of SDS and PSS as a function of initial SDS concentration. Each solution contains 1 mmol dm-3 background salts, and the pH value is adjusted to 3.5. The solid and dashed lines are drawn to guide the eye. Open symbols: adsorption of SDS alone. Closed symbols: coadsorption of SDS and PSS (50 ppm).
largely determined by the interaction of R-alumina with PSS, while SDS plays a comparatively minor role. On the other hand, in aqueous 3 mmol dm-3 SDS and 50 ppm PSS solution the force data match the data for PSS alone only above the surface separations of 3 nm; however, the surfaces then come into adhesive contact. Moreover, as the SDS concentration reaches 12 mmol dm-3 an extra repulsion is observed, which is quite different from the force profile for the addition of PSS alone. These interesting results show that the adlayer-adlayer interaction consisting of SDS and PSS is dominated by the preferential SDS adsorption at high SDS concentration. In the case of the coaddition of SDS and PSS the dispersion stability of R-alumina suspensions also agrees with the corresponding force data. Figure 6 illustrates the dispersion stability and ζ potential of the suspensions as a function of SDS concentration. Initial PSS concentrations are fixed at 50 ppm in a manner similar to that for force measurements. The absorbance at 600 nm of the suspensions slightly increases with SDS concentration, reaches a maximum, and then decreases sharply to a deep minimum at 3 mmol dm-3. As SDS concentration increases further the absorbance steeply increases again. It is notable that the resultant ζ potential data are almost -50 mV over the whole SDS concentration region. Accordingly, the dispersion stability of the suspensions is not determined by the electrostatic interaction but rather
the structure of the adsorbed layers. More details will be given in the Discussion. Figure 7 shows the effect of the sequential addition of SDS (3 mmol dm-3) and PSS to a system containing preadsorbed PSS. The initial PSS concentrations of both solutions are set to 50 ppm to prevent desorption of preadsorbed PSS. All solutions are adjusted to a pH of 3.5 ( 0.1. The resultant force curve for the sequential addition is purely repulsive from 15 nm, which is significantly shorter than the range of repulsion between the two surfaces in the 50 ppm PSS solution. As a result, the magnitude of the former’s repulsion is much smaller than that of the latter’s repulsion. Since it seems likely that these changes are attributed to the difference in the effective electrolyte concentration in bulk solution, the measurement of interaction forces between adsorbed PSS layers across the 4 mmol dm-3 1:1 simple electrolyte (NaCl + HCl) solution was carried out. The result is also provided in Figure 7. The force curve for the sequential adsorption is very similar to that for the adsorption of PSS with 4 mmol dm-3 background salts, as expected. Therefore, it is concluded that the addition of SDS to preadsorbed PSS layers causes electrostatic screening to occur due to the increment of the electrolyte concentration. The sequential addition of PSS (50 ppm) to the system containing the preadsorbed SDS (3 mmol dm-3) was investigated as well. One can expect that PSS may not be adsorbing via the electrostatic repulsion between the SDS admicelles formed on R-alumina and the negatively charged PSS in bulk solution. However, our force measurements confirm that the force curve (not shown) fits in with that for the coaddition of SDS and PSS over all surface separations, rather than that for the SDS alone. Discussion Interaction between r-Alumina Surfaces with Adsorption of PSS Alone. Adsorption examination of PSS on positively charged R-alumina particles has shown a high-affinity isotherm with a plateau region that is reached at an equilibrium concentration of 50 ppm.17 Thus, the driving force for the adsorption is believed to be electrostatic attraction between the PSS side groups and the positively charged sites on R-alumina. This view is also supported by our present data: the charge reversal of the suspensions proceeds by the addition of PSS, resulting in the formation of adlayers with a net negative charge (Figure 4b).
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Below the concentration at which the overcompensation of surface charges would be expected, the interaction force becomes attractive as shown in Figure 2. In addition, the dispersion stability of the R-alumina suspension with the adsorption of PSS from this concentration is significantly low, accompanying the zero ζ potential (Figure 4). These results indicate that the electrostatic component of the force is minimized due to the neutralization of the positive surface charge through the adsorption of the oppositely charged polyelectrolyte. Furthermore, the electrostatic patch theory20,21 is relevant, where bridging between surfaces could occur even when the overall charge is neutral because patches of positive and negative sites coexist and may attract each other at small surface separations. A similar attraction was observed for the adsorption of cationic polyelectrolyte on mica and glass surfaces and could be explained reasonably by the patch mechanism.8 However, a bridging between the PSS adsorbed layers is not rationalized in our case due to their flat conformation at the interface. The basis for this will be shown below. Contrary to this concentration region, the repulsive forces are observed in the range of 5-500 ppm. The origin of this repulsion is the electric double-layer interaction indicated by the linearity between the log of the normalized force and apparent separation (Figure 3) and by the serious effect of the background salt concentration on the interaction forces as seen in Figure 7. In addition, the gradient of the plots in Figure 3 increases absolutely with increasing PSS concentration, suggesting that the repulsion is affected by the ionic strength in bulk solution. It is notable that the decay length of the resultant force might not agree with the expected one in a polyelectrolyte-free solution. However, our result is consistent with previously reported results for cationic polyelectrolytes adsorbed on mica22 or silica7 from low ionic strength. It seems likely that these phenomena are caused by the binding of counterions to the polyelectrolyte for reducing the electrostatic repulsion between the PSS anionic groups. All the results obtained from each experimental technique in this study show the same trend for the adsorption of PSS alone. The magnitude of repulsion decreases with increasing PSS concentration; the dispersion stability of R-alumina suspensions from the 5 ppm PSS aqueous solution is higher than that from the 50 ppm PSS aqueous solution; in particular, that from the 500 ppm PSS aqueous solution is quite low. These results corroborate the finding that the electrostatic force is predominant for the adlayeradlayer interaction, as a result, for the stability of the suspensions on a macroscopic level. In a concentration region of 5-500 ppm, it is difficult to observe the steric hindrance caused by the adsorbed polymer chains. Accordingly, we conclude that PSS adsorbs on R-alumina in a flat conformation and extends only a small distance from the surface. The flat layer with few loops and tails is formed via the strong attractive interactions between the positive surface charges on R-alumina and the oppositely charged polyelectrolyte. It is noted, however, that the probe’s point of zero separation for a bare surface does not correspond exactly with the zero point for an adsorbed surface due to the incompressible polymer layer sandwiched between the solid surface and the probe in the latter case.5 The fact that the zero distance is defined as a constant compliance between a (20) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448. (21) Leong, Y. K. Colloid Polym. Sci. 1999, 277, 299. (22) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 166, 343.
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cantilever deflection and a piezo movement is a limitation of the colloidal probe technique. Interaction between r-Alumina Surfaces with Adsorption of SDS and PSS. The behavior presented in Figure 6 is of interest in the relationship between the dispersion stability of R-alumina suspensions and the corresponding ζ potential data: despite a negative ζ potential, the dispersion stability is quite low for the solution containing 3 mmol dm-3 SDS and 50 ppm PSS. Apparently, this is not caused by the hydrophobic attraction between SDS hemimicellar layers formed on R-alumina particles, as was observed in the 0.06 mmol dm-3 SDS solution.15 Rather, a discussion of the layered structure of SDS and PSS adsorbed on R-alumina would be more in order. In aqueous 3 mmol dm-3 SDS and 50 ppm PSS solution the normalized force values increase monotonically with decreasing apparent separation in the range of 3-18 nm (Figure 5). This result fits in with the data for the adsorption of PSS alone at 50 ppm, and the origin of this repulsion is believed to be electric double-layer interaction. However, the surfaces then jump into adhesive contact from 3 nm, which is not experienced in the 50 ppm PSS solution but is experienced in the 3 mmol dm-3 SDS solution. Namely, the interaction for the coaddition of SDS and PSS is predominantly repulsive at long distances while is attractive below surface separations of 3 nm. It is worth recalling that the adlayer-adlayer interaction under this experimental condition is governed by attractive components at the macroscopic scale. In addition, it is noted that the dispersion stability is comparatively low in aqueous 3 mmol dm-3 SDS solution (Figure 6a), originating from the long-range weak repulsion and then the following attraction at small distances.15 Thus, it is very important to consider this attractive force when coadsorption results are interpreted. It is well-known3,4 that the discontinuity in a force curve corresponds to the fusion of adsorbed surfactant layers under a limiting compression, accompanied by the squeeze-out of the surface layer in SDS admicelles. In fact, such jumps were observed between the R-alumina surfaces in aqueous 3 and 12 mmol dm-3 SDS solutions.15 From this point of view, the fact that the obvious jumps are observed in the coaddition experiments suggests the formation of SDS patchy layers on R-alumina. These layers interact significantly with each other for the coaddition of 3 mmol dm-3 SDS and 50 ppm PSS, resulting in the low dispersion stability of the R-alumina suspensions. Here, it is useful to relate this phenomenon to the adsorption kinetics. In general, adsorption of polyelectrolytes takes longer to reach equilibrium compared to that of surfactants. It is likely that in the simultaneous addition PSS adsorbs on R-alumina after the formation of the adsorbed SDS layers. This situation is similar to the sequential addition of PSS to the preadsorbed SDS so that the following coadsorption model is relevant: two domains coexist at the solid/solution interface; one is the SDS-rich phase, and the other is the PSS-rich phase. As a result, the layered structure formed on R-alumina is mainly governed by SDS. This is confirmed by the surface tensiometry as shown in Figure 1: due to weak interaction between SDS and PSS, SDS progressively adsorbs on the air surface with increasing SDS concentration in a manner similar to that for SDS alone. Furthermore, the fact that a strong repulsion is observed for the solution containing 12 mmol dm-3 SDS and 50 ppm PSS (Figure 5) suggests that the growth of the SDS-rich domain occurs through the additional adsorption of SDS.
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On the other hand, the sequential addition of SDS to preadsorbed PSS layers has a serious effect on the force results (Figure 7): SDS screens entirely the electric doublelayer repulsion between PSS adlayers on R-alumina as well as a simple 1:1 electrolyte dissociating fully. Desorption or rearrangement of the preadsorbed PSS molecules does not take place although SDS adsorbs more preferentially than PSS. It is not hard to appreciate because a large number of anchor parts for adsorption must be removed at the same time for the adsorbed polymer chains to be desorbed from the surface.23 In addition, the result of the sequential addition of PSS to preadsorbed SDS corroborates the finding that a significant adsorption of PSS does occur even in the presence of SDS admicelles. Conclusion Coadsorption behaviors from binary solutions of SDS and PSS on positively charged R-alumina have been elucidated through surface force measurements. In the case of simultaneous addition, the observed forces are (23) Liu, J.-F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895.
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traced to the electrostatic repulsion between two R-alumina surfaces coadsorbed with SDS and PSS. It is expected from our present data that two domains consisting of the SDS-rich and PSS-rich phases spread out at the solid/ solution interface. In particular, in a high SDS concentration region, an attractive jump is observed in the range of 3 nm, indicating that the SDS-rich phases formed on R-alumina can interact with each other under a limiting compression. In sequential addition of SDS and PSS, the order of addition is found to be an important factor. The addition of SDS to the preadsorbed PSS results in the screening of the electrostatic repulsion between adlayers. Thus, SDS plays a role as a simple 1:1 electrolyte, and no replacement takes place under this experimental condition. On the other hand, adsorption of PSS does occur when PSS is exposed to the system containing the preadsorbed SDS layers with a net negative charge. These results are well correlated with the dispersion stability of the corresponding suspensions, so the colloidal probe technique provides a useful insight for understanding coadsorption phenomena on a nanoscopic scale. LA026388O
