Interaction Forces on α-Alumina Surfaces with Coadsorbed Anionic

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Interaction Forces on r-Alumina Surfaces with Coadsorbed Anionic Surfactant and Nonionic Polymer 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 December 10, 2001. In Final Form: February 21, 2002 Direct measurements of interaction forces between R-alumina surfaces with coadsorbed anionic surfactant (sodium dodecyl sulfate (SDS) or lithium perfluorooctanesulfonate (LiFOS)) and nonionic polymer poly(vinylpyrrolidone) (PVP) at pH 3.5 are carried out by colloidal probe atomic force microscopy. In aqueous SDS solutions, the strength of interaction forces is found to depend on the initial SDS concentration, whereas in PVP solutions, the electric double layer interaction is completely screened due to the flat conformation of PVP on R-alumina. The interaction forces are dramatically different for the coaddition of SDS and PVP: electrostatic repulsion is observed above the critical aggregation 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 and the formation of SDS-PVP surface complexes as confirmed by surface tensiometry. The LiFOS-PVP system exhibits characteristics similar to those of the SDS-PVP system, except that the repulsion between adsorbed LiFOS layers is remarkably weak at concentrations above its critical micelle concentration. The effects of sequential addition of SDS and PVP are also investigated.

Introduction Identifying the adsorption characteristics of surfactantpolymer binary mixtures has been key to understanding the stabilization and flocculation mechanisms of dispersions.1 The knowledge base has been used in the development of various industrial products, including cosmetics, paints, detergents, and pharmaceuticals, and in brewing. Surfactants are generally used to control the dispersion, flocculation, and wetting properties of suspensions, while water-soluble polymers serve to meet rheological requirements. The coadsorption behavior of surfactants and polymers at the solid/liquid interface depends on their interactions in bulk solution. Certain surfactant-polymer combinations exhibit a very weak interaction, while others interact strongly, in a more specific manner. In the former case, the surfactant and polymer may compete for adsorption. For example, despite its high affinity to R-alumina, the anionic polymer sodium poly(styrene sulfonate) (PSS) adsorbed on positively charged R-alumina is replaced by SDS with increasing SDS concentration.2 By contrast, in more strongly interacting surfactant-polymer combinations, the simultaneous adsorption of the anionic surfactant and neutral polymer will be favored, as in the familiar case of SDS-PVP mixtures. The driving force for the complexation of SDS-PVP in bulk solution is believed to be either hydrophobic interaction between the SDS hydrocarbon chain and the PVP polyethylene backbone,3-5 or electrostatic attraction between the SDS headgroup and the PVP polarizable pyrrolidone side group (pearlnecklace model6-11). (1) Otsuka, H.; Esumi, K. Structure-Performance Relationships in Surfactants; Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 1997; Chapter 12. (2) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (3) Gila´nyi, T.; Wolfram, E. Colloids Surf. 1981, 3, 181. (4) Sesta, B.; Segre, A. L.; D’Aprano, A.; Proietti, N. J. Phys. Chem. B 1997, 101, 198. (5) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276. (6) Chari, K.; Lenhart, W. C. J. Colloid Interface Sci. 1990, 137, 204. (7) Chari, K. J. Colloid Interface Sci. 1992, 151, 294.

Adsorption of PVP from SDS-PVP binary solutions on positively charged titanium dioxide,12,13 iron oxide,14,15 R-alumina,16,17 and hydroxyapatite18 is heavily dependent on SDS concentration. PVP itself does not readily adsorb onto these surfaces. However, the amount of adsorbed PVP increases rapidly with SDS concentration, particularly at low SDS concentrations, up to a maximum and thereafter decreasing sharply. This behavior has been attributed to the formation of surface complexes between SDS and PVP. At low SDS concentrations, PVP binds via a hydrophobic interaction to the adsorbed SDS, the hydrocarbon tail of which extends into the solution. As the SDS concentration increases, SDS and PVP form polyelectrolyte-like complexes in bulk solution concomitantly with the formation of SDS bilayers on solid surfaces. The adsorption of PVP is inhibited by the resultant electrostatic repulsion between the solution complexes and bilayers. This implies that, in the presence of excess SDS, the chemical potential of SDS-PVP complexes formed on the solid surface rises above that of the complexes in solution.18 Although these observations provide macroscopic explanations, the relationship between coadsorption phenomena and dispersion stability of suspensions (or surface forces) has yet to be established at the nanoscopic scale. (8) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512. (9) Norwood, D. P.; Minatti, E.; Reed, W. F. Macromolecules 1998, 31, 2957. (10) Sukul, D.; Pal, S. K.; Mandal, D.; Sen, S.; Bhattacharyya, K. J. Phys. Chem. B 2000, 104, 6128. (11) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677. (12) Ma, C. Colloids Surf. 1985, 16, 185. (13) Esumi, K.; Sakai, K.; Torigoe, K.; Suhara, T.; Fukui, H. Colloids Surf. A 1999, 155, 413. (14) Ma, C.; Li, C. J. Colloid Interface Sci. 1989, 131, 485. (15) Ma, C.; Li, C. Colloids Surf. 1990, 47, 117. (16) Esumi, K.; Mizuno, K.; Yamanaka, Y. Langmuir 1995, 11, 1571. (17) Esumi, K.; Iitaka, M.; Torigoe, K. J. Colloid Interface Sci. 2000, 232, 71. (18) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloids Surf. A 1997, 123-124, 283.

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Atomic force microscopy (AFM), which acquires topological mappings of nonconductive surfaces with subnanometer resolution, has been widely used to clarify adsorption phenomena at the solid/liquid interface in situ. Direct observations of adsorbed surfactant layers on solid surfaces have provided new insights ever since a selfassembled hexadecyltrimethylammonium bromide (HTAB) layer on graphite was first imaged by Manne et al. in 1994.19,20 Various aspects of layered structures on hydrophilic substances have been studied systematically, including surfactant concentrations,21 surfactant headto-tail balance,22-24 and effects of co-ions.22,25-29 Most of these reports concluded that, on hydrophilic surfaces, surfactants self-assemble into either spherical or rodlike (fully cylindrical) aggregates. Thus, the traditional picture of bilayer formation is evidently too simplistic. Surfactant morphology is also influenced by the addition of polymers, and by the order in which the surfactant and polymer are added. For instance, globular admicelles form upon the addition of SDS to mica preadsorbed with a cationic polymer, suggesting that SDS forms spherical surface micelles that cannot be observed on a bare mica surface.30 Liu et al.31 investigated the coadsorption behavior of hexadecyltrimethylammonium chloride (HTAC) and poly(diallyldimethylammonium chloride) (PDADMAC) on a negatively charged silica surface. The adsorbed featureless PDADMAC layer changed to the hemispherical conformation upon the addition of HTAC. This suggests that HTAC self-assembled on the backbone of the adsorbed PDADMAC through a hydrophobic interaction. By contrast, the addition of PDADMAC had a negligible effect on a preadsorbed HTAC layer. The PDADMAC was electrostatically repelled by the densely packed HTAC admicelles, which left no sites on the silica surface for PDADMAC adsorption. As AFM images surface topology based on the force between the probe and the substrate, AFM can also be used to make direct measurements of such forces. In particular, the interaction forces between a colloidal probe attached to the cantilever and the sample surface can be measured directly using the colloidal probe technique developed by Ducker et al.32,33 The technique has been utilized to study the adsorption of not only surfactants,34,35 but also linear polymers.36-41 A few recent publications (19) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (20) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (21) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506. (22) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (23) Manne, S.; Schaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (24) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924. (25) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558. (26) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (27) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (28) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (29) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (30) Dedinaite, A.; Claesson, P. M.; Bergstro¨m, M. Langmuir 2000, 16, 5257. (31) Liu, J.-F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895. (32) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (33) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (34) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412. (35) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. A 1999, 146, 75. (36) Biggs, S. Langmuir 1995, 11, 156. (37) Biggs, S. J. Chem. Soc., Faraday Trans. 1 1996, 92, 2783. (38) Braithwaite, G. J. C.; Howe, A.; Luckham, P. F. Langmuir 1996, 12, 4224.

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describe the coadsorption behavior of ionic surfactants and oppositely charged polymers.42-44 However, these mostly focus on the effects of the sequential addition of surfactants on interaction forces between the polyelectrolyte coated surfaces. The coaddition of adsorbates that can interact in bulk solution has been largely neglected up until now. In the present study, the interaction forces between R-alumina surfaces in aqueous solutions of anionic surfactant (SDS or LiFOS) and/or PVP were measured by colloidal probe AFM. The aggregation behavior of these adsorbates in bulk solution was assessed through surface tensiometry. Furthermore, 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. 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 a concentrated H2SO4. The materials were then rinsed with water and immersed in an aqueous 0.1 mol dm-3 KOH solution overnight, and finally rinsed thoroughly with water. SDS obtained from Nacalai Tesque Inc. (Japan) was recrystallized three times from ethanol. LiFOS (Dainippon Ink and Chemicals Inc., Japan) was recrystallized several times from a tetrachloromethane/acetone mixture. PVP (Aldrich Chemical Co. Inc., USA; Mw ) 40 000) 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 or LiCl as a background electrolyte. Surface force measurements were carried out using a TMX2100 atomic force microscope (TMmicroscopes Inc., USA). A detailed description of the force measurement technique is given elsewhere.45 It should be noted that, under the prevailing pseudoequilibrium conditions, force profiles can change depending on the velocity at which the surfaces are brought together.37 In the present study, the approach rate was held constant at 0.5 µm s-1, which is approximately equal to the Brownian motion in a simple aqueous 1 mmol dm-3 1:1 electrolyte solution, as estimated by the Stokes-Einstein equation. Data were collected using commercial silicon nitride cantilevers (Digital Instrument Inc., USA) with a spring constant of 0.58 N m-1, modified by attaching an R-alumina sphere, as described by Ducker et al.32,33 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 AFM 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 post(39) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202. (40) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. A 1998, 139, 199. (41) Giesbers, M.; Kleijn, J. M.; Fleer, G. J.; Cohen Stuart, M. A. Colloids Surf. A 1998, 142, 343. (42) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. A 1999, 155, 1. (43) Muir, I.; Meagher, L.; Gee, M. Langmuir 2001, 17, 4932. (44) McNamee, C. E.; Matsumoto, M.; Hartley, P. G.; Mulvaney, P.; Tsujii, Y.; Nakahara, M. Langmuir 2001, 17, 6220. (45) Sakai, K.; Torigoe, K.; Esumi, K. Langmuir 2001, 17, 4973.

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Figure 2. Normalized forces between R-alumina surfaces in aqueous SDS solutions containing 1 mmol dm-3 salt as background electrolyte at pH 3.5.

Figure 1. Surface tension of aqueous solutions of (a) SDSPVP and (b) LiFOS-PVP. Initial PVP (Mw ) 40 000) concentration ) 1 g dm-3; background salt concentration ) 1 mmol dm-3. adsorbate solution. The temperature was maintained constant at 25 °C using a thermomodule controller (MT862-04C12, Netsu Denshi Co., Ltd., Japan). Data Analysis. The empirically derived decay length was compared with the Debye length, which corresponds to the thickness of the electric double layer on one surface. The former length was determined experimentally from the force results, while the latter was calculated through the nonlinear PoissonBoltzmann equation, assuming a constant charge limit. Our calculations assume that (i) the surfactant behaved as a simple 1:1 electrolyte capable of dissociating fully and (ii) the PVP in the bulk phase did not influence the electrolyte concentration. If the decay and Debye length values agree with each other, the forces can be assumed to result from the electric double layer interaction.

Results and Discussion SDS/LiFOS and PVP Complexation in Bulk Solution. Prior to force measurements, we investigated the aggregation behavior of the anionic surfactants and PVP in bulk solution. Complexation of surfactant-polymer mixtures is known to influence adsorption behavior. Figure 1 shows the surface tensions of aqueous solutions of (a) SDS-PVP and (b) LiFOS-PVP as a function of surfactant concentration. Also plotted are the surface tensions of SDS and LiFOS alone. The surface tension of SDS alone decreased monotonically with increasing concentration, plateauing at about 6 mmol dm-3 (critical micelle concentration, cmc). By

contrast, in the presence of PVP, two transition points appeared, at 3 (T1, critical aggregation concentration) and 12 mmol dm-3 (T2). These points are explained as follows: T1 is the onset of association of SDS with PVP in bulk solution. As SDS concentration increases, complexation is initiated at the air/solution interface. The binding sites on PVP are saturated with SDS molecules at T2. Above this concentration, regular SDS micelles coexist with SDS-PVP complexes and SDS monomers in bulk solution. These findings have been verified through the analysis of 1 H NMR spectra.4 In the case of LiFOS-PVP mixtures, similar transition points were observed at 0.7 (T1) and 14 mmol dm-3 (T2). In general, fluorocarbon surfactants can self-assemble more easily than hydrocarbon surfactants due to the higher hydrophobicity and rigidity of fluorocarbon tails. Consequently, the complexation of LiFOS-PVP aggregates could occur at much lower concentrations compared to SDS-PVP, even though LiFOS and SDS have roughly the same cmc. This implies that LiFOS-PVP complexes represent a more favorable energy state than SDS-PVP complexes. Interaction Forces between r-Alumina Surfaces. I. Addition of SDS. Figure 2 shows the normalized force values between adsorbed SDS layers on R-alumina. Initial SDS concentrations were set to 0, 0.06 (0.01 cmc), 3 (0.5 cmc), and 12 mmol dm-3 (2 cmc); the pH of each sample solution was adjusted to 3.5 ( 0.1 using HCl. The total concentration of background salts was 1 mmol dm-3. In the absence of SDS, electrostatic repulsion was observed. The surface potential and decay length determined by best curve fitting were 13 mV and 9.7 nm, respectively. The decay length is in good agreement with the theoretical Debye length (9.6 nm). The surface potential of R-alumina was positive, which was expected since the isoelectric point of R-alumina suspensions is about 9 and the surface hydroxyl groups on R-alumina are mostly protonated at pH 3.5.46 Interaction forces were dramatically changed by the addition of SDS. In the 0.06 mmol dm-3 SDS solution, an attractive force was detected at a separation of approximately 25 nm. The surfaces then made adhesive contact. This suggests that the adsorption of SDS neutralized the positive charge on the surface, resulting in the formation of a hydrophobic layer. A similar attractive (46) Roy, P.; Fuerstenau, D. W. Surf. Sci. 1972, 30, 487.

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Table 1. Surface Potential and Decay Length Estimates Based on Force Curves in Aqueous SDS Solutionsa C (mmol dm-3) 0 0.06 3 12

κ-1 (nm)

κc-1 (nm)

13

9.7

-9.1 -22

5.1 4.1

9.6 9.3 4.8 2.7

Ψ0 (mV)

a C is the initial concentration of SDS. Ψ is the surface potential. 0 κ-1 is the decay length estimated from force curves. κc-1 is the calculated Debye length.

force was observed in one of our previous experiments, between the adsorbed cationic surfactant layers on a silica surface at well below the surfactant’s cmc.45 With increasing SDS concentration, the repulsive force reappeared. Debye lengths and empirical decay lengths are listed in Table 1. The values differed significantly at 12 mmol dm-3, but not at 3 mmol dm-3, suggesting that the decay length is affected by the SDS micelles in bulk solution. Since the binding of counterions to micelles causes the linear specific conductivity versus surfactant concentration plot to break at the cmc, the reduction of the effective electrolyte concentration induced by micelle formation results in an increase in the decay length, as quantified by surface force apparatus (SFA).47 Table 1 also shows the estimated surface potentials based on the force measurements. The surface potential increased with increasing initial SDS concentration. This can be attributed to charging of the adsorbed surfactant layer, as reported in several previous publications.45,48-51 The analysis of force curves does not always provide much information about the adsorbed layer. However, with the advent of the AFM imaging technique, it is now possible to focus on the self-assembly of surfactant molecules at the solid/liquid interface. Liu and Ducker investigated the layered structure of cationic surfactants adsorbed on oppositely charged amorphous silica and found that spherical aggregates are not organized in two ways.25 One would expect to find similar SDS aggregates on R-alumina also, given that silica and R-alumina are both characterized by ionizable surface hydroxyl groups. The fact that surface potential was higher at an SDS concentration of 12 mmol dm-3 than in the absence of SDS suggests that self-assembly of SDS molecules most likely does occur on R-alumina. II. Coaddition of SDS and PVP. Interaction forces between R-alumina surfaces for the coaddition of SDS and PVP are shown in Figure 3. Initial PVP concentration was 1 g dm-3, while SDS concentration was fixed at 0, 0.06 (0.01 cmc), 3 (0.5 cmc, T1), and 12 mmol dm-3 (2 cmc, T2). The electric double layer repulsion observed in the aqueous NaCl solution at pH 3.5 was completely screened by the addition of PVP, and no significant forces were detected. Due to the weak interaction between the two, PVP adsorbs on R-alumina in very small quantities,17,52 though significant on a nanoscopic level. However, it is difficult to observe the steric hindrance caused by the adsorbed polymer chains. Accordingly, we concluded that PVP adsorbs on R-alumina in a flat conformation. Biggs37 reports that PVP adsorbed on zirconia extends 2-3 Rg (Rg, radius of gyration) from the adsorption surface in a (47) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569. (48) Ke´kicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (49) Herder, P. C. J. Colloid Interface Sci. 1989, 134, 346. (50) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (51) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110. (52) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45.

Figure 3. Normalized forces between R-alumina surfaces in aqueous SDS-PVP solutions ([PVP] ) 1 g dm-3, Mw of PVP ) 40 000) containing 1 mmol dm-3 salt as a background electrolyte at pH 3.5. Table 2. Surface Potential and Decay Length Estimated from Force Curves in Aqueous SDS-PVP Solutions ([PVP] ) 1 g dm-3)a C (mmol dm-3) 0 0.06 3 12

Ψ0 (mV)

-7.6 -7.9

κ-1 (nm)

κc-1 (nm)

4.7 3.8

9.6 9.3 4.8 2.7

a C is the initial concentration of SDS. Ψ is the surface potential. 0 κ-1 is the decay length estimated from force curves. κc-1 is the calculated Debye length.

good solvent. Rg of PVP (Mw ) 41 000) is approximately 17 nm.53 Biggs also notes 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.36,37 That the zero distance is defined as a constant compliance between a cantilever deflection and a piezo movement is a limitation of the colloidal probe technique. The force curve for the solution containing 0.06 mmol dm-3 SDS and 1 g dm-3 PVP highly resembled that for PVP alone. The lack of an attractive component between the SDS layers neutralizing the surface charge indicates that the force curves are largely determined by R-alumina’s interaction with PVP, while SDS plays a comparatively minor role. The surface tension curves shown in Figure 1 suggest that SDS monomers and PVP coexist in bulk solution at 0.06 mmol dm-3 SDS, i.e., far below T1. These results indicate that SDS adsorbs on R-alumina with its hydrocarbon tail oriented toward the aqueous solution, following which PVP adsorbs to a certain extent on the SDS-coated layer through hydrophobic interactions. Repulsive forces were detected at both 3 and 12 mmol dm-3 SDS. The decay length was in good agreement with the Debye length, especially for an initial SDS concentration of 3 mmol dm-3 (Table 2). This suggests that the repulsion was induced by the electric double layer interaction and that the counterions are bound weakly to the SDS-PVP complexes in bulk solution. Conductivity measurements have confirmed that the degree of ionization of SDS-PVP complexes is less than that of SDS regular micelles.54 (53) Miller, L. E.; Hamm, F. A. J. Phys. Chem. 1953, 57, 110.

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No significant differences were observed between the surface potentials at 3 and 12 mmol dm-3 SDS. Thus, charging is not relevant in the coadsorption of SDS and PVP on R-alumina. Rather, a discussion of the conformation of adsorbed polymer chains would be more in order. Otsuka and Esumi report that, during the coadsorption of lithium dodecyl sulfate (LiDS) and PVP, the fraction of train segments in the adsorbed PVP on R-alumina steeply increases with LiDS concentration, reaches a maximum, and then remains constant despite a remarkable decrease in the adsorbed amount of PVP.52 Our results fit in with these data since the range of repulsion between the two surfaces was found to be about 15 nm regardless of concentration, i.e., significantly shorter than the theoretical Rg of PVP in a good solvent. We propose the following coadsorption model: PVP disorders the adsorbed SDS molecules, causing an electrostatic screening to occur. The SDS-PVP complexes in bulk and the surface aggregates in solution reach a pseudoequilibrium. Thus, it is very important to consider bulk properties when interpreting adsorption results. At T1, the solution complexes adsorb electrostatically, anchoring by a negatively charged segment. In this situation, the expected interaction between the surfaces is electrostatic repulsion since the headgroup of the adsorbed SDS is oriented toward the solution phase. The adsorption isotherms indicate that the complexes in solution hardly adsorb on the SDS admicelles at T2.17 However, our AFM investigation confirms that PVP adsorption occurs even at T2. This is probably due to the force measurements’ sensitivity to trace amounts of adsorbates. In addition, the conformation of adsorbed PVP is predominantly flat at both SDS concentrations. Another coadsorption model has been reported through direct observation of SDS-PVP layers on graphite in an aqueous solution.55 The acquired AFM images demonstrated that two domains coexist on graphite on a microscopic level: one consists of ordered SDS-rich hemicylindrical structures, while the other is disordered and PVP-rich. The driving force for the adsorption is the hydrophobic attraction between the adsorbates’ hydrocarbon segments and the graphite surface. However, this coadsorption model could not be applied to our results since the interactions are quite different in our case, namely, PVP hardly adsorbs on R-alumina by itself. In addition, no significant differences in force values were observed for a fixed initial PVP concentration of 0.01 g dm-3 (data not shown). This suggests that the interaction forces for the coadsorption of SDS and PVP are independent of PVP concentration within the resolution of our experimental setup. III. Addition of LiFOS. Figure 4 shows the normalized force data obtained for adsorbed LiFOS layers on R-alumina surfaces. The initial LiFOS concentrations were set at 0, 0.07 (0.02 cmc), 0.7 (0.2 cmc), and 14 mmol dm-3 (4 cmc). The pH of each sample solution was adjusted to 3.5 ( 0.1 by HCl. The total concentration of background salts was 1 mmol dm-3. The aqueous 0, 0.07, and 0.7 mmol dm-3 LiFOS solutions behaved similarly to the SDS solutions, but the repulsion in the case of the 14 mmol dm-3 LiFOS solution was much lower compared to its SDS counterpart. The explanation lies in their different aggregation behaviors. As described above, the hydrophobicity and rigidity of LiFOS are both higher compared (54) Zanette, D.; Ruzza, A ˆ . A.; Froehner, S. J.; Minatti, E. Colloids Surf. A 1996, 108, 91. (55) Fleming, B. D.; Wanless, E. J.; Biggs, S. Langmuir 1999, 15, 8719.

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Figure 4. Normalized forces between R-alumina surfaces in aqueous LiFOS solutions containing 1 mmol dm-3 salt as a background electrolyte at pH 3.5.

to SDS despite the former’s shorter tail length and correspondingly lower surfactant packing parameter, uniquely characteristic of all fluorocarbon surfactants.56 Therefore, LiFOS micelles can be more compact than SDS micelles. Time-resolved fluorescence quenching using pyrene reveals that the aggregation number of LiFOS is 6 at around its cmc.57 In addition, the degree of ionization of LiFOS micelles is higher than that of SDS micelles58 because the fluorocarbon core of the former packs more densely, increasing the head-to-head repulsion. As a result, a large number of micelles exist in the 14 mmol dm-3 LiFOS solution, screening the electric double layer repulsion. Bremmell et al. used the colloidal probe technique to investigate adsorption phenomena related to hydrocarbon surfactants on silica, and found that the magnitude of repulsion decreases above the surfactant concentration corresponding to the maximum in the adsorption isotherm.35 IV. Coaddition of LiFOS and PVP. The interaction forces observed between R-alumina surfaces for the simultaneous addition of LiFOS and PVP are shown in Figure 5. The initial PVP concentration was fixed at 1 g dm-3; LiFOS concentrations were 0, 0.07 (0.02 cmc), 0.7 (0.2 cmc, T1), and 14 mmol dm-3 (4 cmc, T2). The LiFOS-PVP results were similar to the SDS-PVP results: (i) at concentrations well below their cmc, neither SDS nor LiFOS had any effect on the interaction forces; (ii) the decay length determined experimentally was in agreement with the Debye length at T1, suggesting that the repulsion was induced by the electric double layer interaction; (iii) virtually no steric hindrance was observed at any of the surfactant concentrations. The interaction forces between the surfactant-polymer complex layers appear to be independent of whether the surfactant is SDS or LiFOS. V. Sequential Addition of SDS and PVP. Figure 6 shows the normalized force curves for the addition of PVP to a system containing preadsorbed SDS at 0.06 mmol dm-3 (0.01 cmc). All solutions were adjusted to a pH of 3.5 ( 0.1 and contained 1 mmol dm-3 NaCl as background electrolyte. Coadsorption results for the same adsorbates at this SDS concentration are provided for comparison. As described above, an attraction force was observed due (56) Lamont, R. E.; Ducker, W. A. J. Colloid Interface Sci. 1997, 191, 303. (57) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (58) Hoffmann, H.; Tagesson, B. Z. Phys. Chem. N. F. 1978, 110, 113.

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Figure 5. Normalized forces between R-alumina surfaces in aqueous LiFOS-PVP solutions ([PVP] ) 1 g dm-3, Mw of PVP ) 40 000) containing 1 mmol dm-3 salt as a background electrolyte at pH 3.5.

Sakai et al.

layer. Muir et al. report that HTAB-PSS complex layers adsorbed on fibrous R-alumina surfaces adopt a flatter and denser conformation during coaddition of PSS and HTAB as compared to sequential addition.43 One possible reason for this difference is that complexation occurs in bulk solution in the case of simultaneous adsorption, while it occurs on or near the solid surfaces during sequential adsorption. The sequential addition of SDS-PVP complexes to preadsorbed SDS layers was investigated as well (data not shown). The initial SDS concentration of both solutions were set to 3 mmol dm-3 to prevent desorption of preadsorbed SDS. The force results were very similar to those obtained for bare surfaces, suggesting that SDSPVP complexes may not be adsorbing under these conditions. This is expected given the electrostatic repulsion between the SDS admicelles formed on the surfaces and the negatively charged complexes in bulk solution. Similar results were observed in the HTAC-PDADMAC-silica system.31 Finally, we investigated the effect of sequential addition of SDS to a system containing preadsorbed PVP. Significant adsorption of PVP occurs on a nanoscopic scale; thus the adsorption behavior of SDS on precoated PVP is of great interest. The obtained force data (not shown) matched the data for adsorption from the 3 mmol dm-3 SDS solution at long distances. In fact, the decay length was estimated as 4.8 nm, pointing to the purely electrostatic origin of the repulsion. This result also suggests that the headgroup of SDS orients toward the solution phase and that the driving force for surface complexation is the hydrophobic binding to the precoated PVP backbone. The conformational changes in the adsorbed polymer chains were evidently not significant given that no notable differences were observed between the PVP-precoated and bare surface results for any of the separations. Conclusion

Figure 6. Sequential addition of PVP to the preadsorbed SDS at pH 3.5. Initial SDS and PVP concentrations were set at 0.06 mmol dm-3 and 1 g dm-3, respectively. Solutions contained 1 mmol dm-3 background salt.

to surface neutralization and hydrophobic interactions at the 0.06 mmol dm-3 SDS alone. The sequential addition of PVP changed the resultant force curve dramatically: the repulsion with a monotonic increase was detected. The estimated decay length of the repulsion was 4.4 nm, which is quite different from the Debye length (9.6 nm) at this salt concentration. This implies that the observed repulsion is not due to the electrostatic interaction, but rather is due to the steric hindrance between the overlapping PVP layers on the surfaces. Indeed, PVP appears to adsorb on the preadsorbed SDS layers solely through hydrophobic attraction so that desorption or rearrangement of the preadsorbed SDS molecules does not take place. As a result, the adsorbed PVP orients toward the solution phase and the electrostatic effect is screened entirely. On the other hand, the range of repulsion was approximately 25 nm, corresponding to twice the “soft layer” thickness. This indicates a relatively flat conformation of PVP. It is worth recalling that the simultaneous adsorption of SDS and PVP on R-alumina leads to the formation of a much flatter

Phenomena related to coadsorption from binary solutions of SDS/LiFOS and PVP on R-alumina have been elucidated through surface force measurements. In the case of simultaneous addition, the observed forces were traced to the electrostatic repulsion between two R-alumina surfaces coadsorbed with surfactants and PVP. This repulsion originates from the head-to-head interaction of the complexed surfactants on the surfaces. The adsorbed polymer chains hardly caused any steric hindrance, suggesting that the adsorbed PVP or SDS/LiFOS-PVP complexes adopt a relatively flat conformation. Similar results were obtained for both surfactant concentrations (T1 and T2) and species (SDS and LiFOS). In sequential addition of SDS and PVP, the sequence of addition was found to be an important factor. Only steric hindrance was detected upon the addition of PVP to R-alumina pretreated with a hydrophobic surfactant (i.e., SDS). Moreover, the addition of SDS at T1 to preadsorbed PVP led to purely electrostatic repulsion, while no adsorption of SDS-PVP complexes occurred on negatively charged SDS admicelles. These results corroborate that complexation on R-alumina surfaces is predominantly governed by hydrophobic interactions between PVP and SDS/LiFOS. Thus, colloidal probe atomic force microscopy is a useful and a powerful technique for correlating surface forces and coadsorption phenomena on a nanoscopic scale. LA011786X