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Interaction Forces between r-Alumina Fibers with Coadsorbed Polyelectrolyte and Surfactant Ineke Muir, Laurence Meagher, and Michelle Gee* School of Chemistry, University of Melbourne, Victoria, 3010, Australia Received January 8, 2001. In Final Form: May 11, 2001 Surface interactions between two R-alumina surfaces, at the point of zero charge, with coadsorbed poly(styrene sulfonate) (PSS) and cetyltrimethylammonium bromide (CTAB) have been measured using the atomic force microscope. The interaction forces were found to be dependent on the order of addition of the polyelectrolyte and surfactant. When CTAB was sequentially added to a preadsorbed PSS layer, the surfaces were observed to jump into contact due to an attractive bridging force that was not present in the absence of CTAB. This indicates that the addition of CTAB alters the PSS adsorbed conformation such that there is significant tailing of the PSS chains away from the interface facilitating a bridging force. In contrast, when PSS and CTAB were coadded, no bridging attraction was measured, implying a more compact adsorbed layer and, most likely, a greater surface excess of the PSS-CTAB complex. The surfaces did exhibit an adhesive force when retracted due to PSS chain entanglement that occurred when the adsorbed layers were in contact. At an electrolyte concentration of 10-1 M KBr, sequential addition of CTAB to a preadsorbed PSS layer led to a marked reduction in the adhesion between the two surfaces, as compared to that measured under lower electrolyte conditions, and there was no attractive jump into contact. This implies that the PSS remained in a relatively flat surface conformation due to the small degree of PSS-CTAB complexation. In the coaddition case, the adsorbed species at high salt behaved like an uncomplexed PSS chain under the same solution conditions. These data show that a salt concentration of 10-1 M KBr is sufficient to restrict PSS-CTAB association significantly.
Introduction The adsorption of polyelectrolytes and surfactants to surfaces is of fundamental importance in a wide range of industrial, technological, and biological areas. In many systems, it is the coadsorption of polyelectrolytes and surfactants that is used to alter and control the colloidal forces and hence aggregation and dispersion, exploitation of which is vital in areas such as wastewater treatment, pharmaceutical preparations, blood compatibility enhancement, and oil recovery.1 Many colloidal polyelectrolyte-surfactant mixed systems are far more complex than those that consist of either single component. In polyelectrolyte-surfactant mixed systems, adsorption of both species at an interface is possible and the polyelectrolyte and surfactant may compete with each other for surface sites. Additionally, it is well established that, in bulk aqueous solution containing a polyelectrolyte-surfactant mixture, aggregates can form between both species.2-4 For oppositely charged polyelectrolyte and surfactant systems, binding of the surfactant headgroups to charged sites on the polyelectrolyte chain is due to electrostatic interactions, forming micelle-like aggregates of the surfactant along the polyelectrolyte chain. In these systems, replacement of the polyelectrolyte counterion with the surfactant ion occurs. The polyelectrolyte facilitates surfactant selfassembly by providing counterions for the surfactant headgroups. The surfactant concentration at which the surfactant starts to aggregate with a polyelectrolyte is termed the critical aggregation concentration (cac). In the * To whom correspondence should be addressed. (1) Hayawaka, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry; Surfactant Science Series 37; Marcel Dekker: New York, 1991. (2) Thalberg, K. Ph.D. Thesis, Lund University, Lund, Sweden, 1990. (3) Shubin, V. Langmuir 1994, 10 (4), 1093. (4) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476.
presence of an oppositely charged polyelectrolyte, the cac is often considerably lower than the critical micelle concentration (cmc).5 There exist only a few studies which have investigated the coadsorption of oppositely charged polyelectrolyte and surfactant on various substrates.6,7 These studies have shown that interactions between anionic polyelectrolytes and cationic surfactants are typically cooperative.8,9 The addition of a small amount of surfactant into a system containing oppositely charged polyelectrolytes could enhance polyelectrolyte adsorption.10,11 For example, Nievandt et al.8 found no adsorption of poly(styrene sulfonate) (PSS) onto silica unless in the presence of cetyltrimethylammonium bromide (CTAB). They found that a small amount of CTAB induces PSS adsorption onto silica, demonstrating clearly that even very low concentrations of oppositely charged surfactants have a large effect on polyelectrolyte adsorption. Studies of adsorption onto silica3 and haematite12 have reported that the order of addition of polyelectrolyte and surfactant is important in determining the polyelectrolyte conformation at the surface, the final adsorbed amounts, and hence the degree of aggregation. The determining factor in colloidal stability in systems containing both polyelectrolyte and surfactant is how the nature of the adsorbed polyelectrolyte-surfactant complex alters the interactions between two surfaces. Colloidal (5) Cleasson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (6) Goddard, E. D. Colloids Surf. 1986, 19 (2-3), 301. (7) Wei, Y.-C.; Hudson, S. M. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1995, C35 (1), 15. (8) Nievandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (9) Tadros, T. F. J.Colloid Interface Sci. 1974, 46 (3), 528. (10) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polym. Sci. 1994, 272, 1590. (11) Arnold, G. B.; Breuer, M. M. Colloids Surf. 1985, 13, 113. (12) Moudgil, B. M.; Somarsundaran, P. Colloids Surf. 1985, 13, 87.
10.1021/la010045t CCC: $20.00 © 2001 American Chemical Society Published on Web 07/17/2001
R-Alumina Fibers with Coadsorbed PSS and CTAB Table 1. Chemical and Physical Data on the r-Alumina Fibers purity density diameter grain size
>99% R-alumina 3090-3096 kg m-3 10-12 µm 60 nm
interactions between adsorbed, mixed polyelectrolytesurfactant layers have only recently been studied by direct force measurements. Some recent work has investigated the adsorption of cationic poly([2-(propionyloxy)ethyl]trimethylammonium chloride) (PCMA), adsorbed onto negatively charged mica with subsequent addition of sodium dodecyl sulfate (SDS).5 At low ionic strength, the polyelectrolyte adopted a flat conformation thus neutralizing the surface charge. Subsequent addition of SDS at 0.1 cmc resulted in a significant increase in the thickness of the adsorbed layer and a subsequent recharging of the surface. Claesson et al.13 conducted a similar experiment, where they adsorbed a cationic polyelectrolyte onto negatively charged mica and then subsequently adsorbed an anionic surfactant. They too found that the adsorbed amount increased upon addition of the surfactant. Work has also been carried out investigating the surface forces between mica surfaces immersed in solution containing Polymer JR-400 (cationic polyelectrolyte) and SDS (anionic surfactant).14 The addition of surfactant to this system resulted in an attractive force. The surface forces measured in these systems relate directly to the nature and conformation of the polyelectrolyte-surfactant complex present at the interface. In the present work, we have studied the adsorption of the anionic polyelectrolyte PSS and the cationic surfactant CTAB and how their coadsorption affects the surface forces between two R-alumina fibers. Direct force measurements were performed using the atomic force microscope (AFM). Two adsorption schemes were investigated: the effect of sequential addition (polyelectrolyte followed by surfactant) and coaddition (polyelectrolyte and surfactant equilibrated together in solution first) to determine how the history of the system affects intersurface interactions. The effect of varying the background salt concentration on the adsorption and surface forces between adsorbed layers was also investigated. All force measurements were carried out at the point of zero charge (pzc) of the R-alumina fibers where the interactions between both PSS and CTAB with the surface are relatively weak. Most work in this area to date has focused on interaction forces between two surfaces where the polyelectrolyte and the substrate to which it adsorbs experience very strong electrostatic attraction. Additionally, any modification of the adsorbed layer through the introduction of surfactant is dominated by polyelectrolytesurfactant association. Experimental Section Materials and Cleaning Methods. The R-alumina fibers (Nextel 610, 3M Corp., St. Paul, MN) used in this study were polycrystalline and granular. The properties of the fibers, according to the manufacturer and He and Clarke,15 are shown in Table 1. AFM imaging of the fibers yielded a root mean square (rms) roughness of 2.7 nm and a peak-to-trough roughness of 19.9 nm. The fibers were supplied in bundles held together with one percent poly(vinyl alcohol) (PVA). The PVA was removed by (13) Claesson, P. M.; Fielden, M. L.; Dedinaite, A. J. Phys Chem. B 1998, 102, 1270. (14) Ananthapadmanabhan, K. P.; Mao, G. Z.; Goddard, E. D.; Tirrell, M. Colloids Surf. 1991, 61, 167. (15) He, J.; Clarke, D. R. Proc. R. Soc. London, Ser. A 1997, 453, 1881.
Langmuir, Vol. 17, No. 16, 2001 4933 ultrasonicating in Milli-Q Plus water for 2 h. The water was changed every 20 min to dissolve as much PVA as possible. They were then fired in an oven at 700 °C for 30 min, soaked in one percent surfactant solution (RBS 35, Pierce USA), and rinsed with copious amounts of Milli-Q Plus water. The fibers were finally air-dried and stored in a laminar flow cabinet until required for an experiment. The water used for cleaning and preparing solutions was purified using a Milli-Q Plus water purification system. The water had a resistivity of 18.2 MΩ cm and low silica content. Analytical grade KBr, HBr, and KOH were used as received, with the latter two used to adjust the pH. Analytical grade ethanol was used for all cleaning procedures. The poly(styrene sulfonate) (Polysciences, PA) had a molecular mass of approximately 177 000 amu and a low polydispersity (Mw/Mn 1:1). Cetyltrimethylammonium bromide (Eastman Kodak, Rochester, NY) was further purified by double recrystallization from an acetone/ethanol mixture. All solutions were prepared and stored in polyethylene bottles for the duration of each experiment. Glassware was not used so as to prevent contamination of the R-alumina by adsorption of dissolved silica.16 The presence of trace amounts of silica, which has a pzc of pH 2-3, when adsorbed onto alumina can lower its pzc substantially.17 The bottles were cleaned by soaking in surfactant for 2-3 days, followed by rinsing with copious amounts of water, ethanol, and then water again. The same cleaning procedure was used for the AFM fluid cell, tweezers, AFM appurtenances (which were made of either Teflon or KelF), and syringe, except they were also soaked in ethanol for 2 days after cleaning with surfactant. Immediately prior to an experiment, all tools and equipment were rinsed with distilled AR grade ethanol and blown dry with nitrogen gas. Force Measurements. Surface forces were measured with a Nanoscope III atomic force microscope (Digital Instruments Inc., Santa Barbara, CA) using a modified version of the colloid probe method of Ducker et al.18,19 In the present study, a welldefined geometry was obtained by using cylindrical R-alumina fibers mounted at 90° to each other.20 Epikote 1004 adhesive (Shell) was used to glue the fibers to the cantilever and a flat substrate. The resulting crossed-cylinder geometry is mathematically identical to the sphere-flat geometry typically used in AFM force measurements.20 Figure 1 is a photograph showing a fiber directly glued onto the AFM cantilever spring. This photograph is typical of the setup used in all experiments. Standard v-shaped silicon nitride cantilevers were used. The spring constant of the cantilever was measured using the resonance method of Cleveland et al.21 An average value of 0.056 Nm-1 was calculated from a sample of 10 cantilevers and used to scale the raw data obtained from AFM force measurements. A high-powered optical microscope connected to a video camera and display were used to measure the radii of the two fibers used in each experiment, and a geometric mean value for the radius was used to scale the force data. The AFM cantilever and substrate (with R-alumina fibers) and the cell were placed under a low-wavelength UV light in the presence of water vapor for 2 h before installation in the AFM, to ensure that they were clean and completely hydrophilic.22 We measured the water contact angle on single-crystal alumina after UV irradiation and found it to be zero. After mounting in the instrument, the R-alumina surfaces were left to equilibrate at about 30 µm separation following the injection of a KBr solution into the AFM fluid cell. Equilibration time for the initial injection was 30 min before any force runs were obtained. Subsequent pH-adjusted salt solutions were left for 15 min to equilibrate. Force versus separation measurements as a function of pH were (16) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Langmuir 1997, 13, 3. (17) Furlong, D. N.; Freeman, P. A.; Lau, A. C. M. J. Colloid Interface Sci. 1981, 80, 20. (18) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (19) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (20) Meagher, L.; Franks, G. V.; Gee, M. L.; Scales, P. J. Colloid Surf., A 1999, 146, 123. (21) Cleveland, J. P.; Manne, S.; Bocel, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64 (2), 403. (22) Vig, J. R. J. Vac. Sci. Technol. 1985, A3, 1027.
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Figure 1. Scanning electron micrograph of a Nextel 610 R-alumina fiber attached to an AFM cantilever.
Figure 2. Scaled force (F/R) versus surface separation plot for the interaction between bare R-alumina fibers in 10-3 M KBr at various pH values: (0) pH ) 3.85, ()) pH ) 4.90, (O) pH ) 6.80, and (4) pH ) 7.95. The solid curves are the DLVO fits to the data. The upper and the lower curves correspond to constant charge and constant potential, respectively. obtained at constant electrolyte concentration. One set of data is presented in Figure 2. These data were used to obtain the pzc of the R-alumina fibers (see Figure 3) by fitting the data to Derjaguin-Landau-Verwey-Overbeek (DLVO)23,24 theory and hence obtaining the surface potentials of the fibers as a function of pH. The pzc of the R-alumina fibers was thus determined at the beginning of each experiment as part of the surface characterization. Two different adsorption protocols were carried out using PSS and CTAB. The first (sequential addition) involved the adsorption of 100 ppm PSS, which was left to equilibrate between the R-alumina surfaces before any force versus separation measurements were taken. Any nonadsorbed PSS was then flushed from the cell and replaced with a solution containing 5.5 × 10-5 M (23) Derjaguin, B.; Landau, L. Acta Physicochim. URSS 1941, 14, 633. (24) Verwey, E. G. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
Figure 3. Surface potential of R-alumina fibers as a function of pH in 10-3 M KBr. Each data point of surface potential at a specific pH was obtained from the DLVO fits to the force curves shown in Figure 2, where surface potential was used as a fitting parameter. CTAB. The second scheme (coaddition) involved the injection of a solution that contained both PSS and CTAB, which had been equilibrated in solution together prior to injection. Both adsorption schemes were carried out at 10-3 and 10-1 M electrolyte concentrations. The pH of the polyelectrolyte and surfactant solutions was adjusted to the pzc of the R-alumina fibers before injection into the AFM and left for 12 h prior to any force measurements being obtained. At least five force runs were obtained for any given solution. An approach rate of about 0.3 µm s-1 was used so that repulsive hydrodynamic forces could be neglected.25 All the data presented here correspond to the more reproducible forces obtained after the first or second approach. The reason for this is that it is sometimes difficult to get firstapproach information from an AFM experiment.
Results It is important to note that it is not possible with the AFM to determine absolute zero surface separation. (25) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83 (10), 5311.
R-Alumina Fibers with Coadsorbed PSS and CTAB
Separation distances are calculated relative to a hard wall contact position, which occurs when the surfaces can no longer be compressed, that is, further compression only results in deflection of the cantilever. The relative positions of the hard wall obtained from different force runs can only be compared qualitatively and do not give accurate information with regard to the thickness of the adsorbed layers. Determination of the pzc of the r-Alumina Fibers. The force versus distance profiles for the interaction between two R-alumina fibers in 10-3 M KBr as a function of pH (see Figure 2) are typical of a double layer interaction. They show a clear reduction in the double layer force, in both range and magnitude, as pH is increased from 3.85 to 6.80. As the pH passes through the pzc of the surfaces, the strength of the intersurface repulsion then increases as pH is further raised. Each of the force profiles shows good agreement with DLVO theory for intersurface separations greater than 8 nm. The additional repulsion measured at smaller surface separations is in part due to the roughness of the R-alumina fibers. AFM imaging of the fibers yielded an rms roughness of 2.7 nm and an average peak-to-trough roughness of 19.9 nm, thereby reducing the magnitude of the van der Waals attraction. It is also likely that the short-range repulsion is due to a hydration force, as has been measured previously.20 Desset et al.26 found that even after complete drying of alumina particles, there remained a monolayer of water. The DLVO theoretical predictions were calculated using a numerical solution to the nonlinear Poisson-Boltzmann equation via the algorithm of McCormack et al.27 incorporating the Derjaguin approximation.28 The Debye length calculated for a 1:1 electrolyte of the appropriate concentration and a nonretarded Hamaker constant of 5.1 × 10-20 J20 were used to obtain the DLVO theoretical curves. This Hamaker constant is for the interaction of bare alumina surfaces across water and is therefore a slight overestimate since our surfaces consist of alumina containing adsorbed polyelectrolyte. The roughness of the R-alumina fibers contributes to the inaccuracy of surface potential measurements, and therefore all quoted values for surface potential obtained from our DLVO fits to the data are for the purpose of comparison only. Surface potentials thus obtained are plotted as a function of pH in Figure 3. From such a plot, the pzc can be extracted since it corresponds to the point of intersection of the plot with the x-axis. Over a range of experiments, the pzc of the R-alumina fibers was found to occur at a pH of 6.7 ( 0.2. There is little agreement in the literature15,29,30 regarding the pzc of polycrystalline R-alumina fibers. For example, Horn et al.31 using two single crystals of alumina found that the pzc ranged between 6.5 and 10. Meagher et al.20 reported a pzc of 5.6 and 6.5, depending on the cleaning procedure of the R-alumina fibers and the composition and relative roughness of their surface. What is important is that our data is within range of that previously reported and is consistent from experiment to experiment. PSS Adsorption at 10-3 and 10-1 M KBr. Figure 4 contains two typical force versus surface separation profiles for the interaction between two R-alumina fibers (26) Desset, S.; Spalla, O.; Cabane, B. Langmuir 2000, 16, 10495. (27) McCormack, D.; Carnie, S. L.; Chan, D. Y. C. J. Colloid Interface Sci. 1995, 169 (1), 177. (28) Derjaguin, B. V. Kolloid-Z. 1934, 69, 155. (29) Veeramasunei, S.; Yalamanchili, M. R.; Miller, J. D. J. Colloid Interface Sci. 1996, 184, 594. (30) Butt, H. J. Biophys. J. 1991, 60 (6), 1438. (31) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3 (3), 413.
Langmuir, Vol. 17, No. 16, 2001 4935
Figure 4. Semilog plot of the scaled force (F/R) versus surface separation for the interaction of two R-alumina fibers in the presence of 100 ppm PSS, at the determined pzc of R-alumina in (0) 10-3 M KBr, and (4) 10-1 M KBr. The solid curves are the DLVO fits to the data in 10-3 M KBr. The upper and the lower curves correspond to constant charge and constant potential, respectively.
after the addition of 100 ppm PSS in 10-3 and 10-1 M KBr, respectively. The data are presented as a semilog plot. Note that upon preparation, all solutions were adjusted to the pzc of R-alumina. Hence, any double layer interactions originate from the adsorbed polyelectrolyte since the R-alumina fibers are net neutral. The PSS was left to adsorb for 12 h before measurement of a force curve to allow the system to equilibrate. Note the existence of a significant repulsive interaction at both 10-3 and 10-1 M background salt. Without any added PSS, there is no repulsive interaction between two R-alumina fibers when the pH is adjusted to the pzc (see Figure 2). The solid curves in Figure 4 are the DLVO theoretical fits to the force data for constant charge and constant potential for a 1:1 electrolyte at a concentration of 1 × 10-3 M. After the addition of PSS in 10-3 M KBr, the magnitude of the surface potential increased to 29.0 mV, as determined from the DLVO fits. The forces between R-alumina fibers with adsorbed PSS deviate from DLVO theory at a surface separation of 7.0 nm. There was no adhesion measured upon retraction of the surfaces during any of these force measurements. In 10-1 M KBr, the interaction between the surfaces is more short-ranged than that at the lower salt concentration, beginning at 20 nm and increasing monotonically until the surfaces come into contact. At an electrolyte concentration of 10-1 M, the force profile was not fitted to DLVO theory because the roughness of the R-alumina fibers is comparable to the Debye length, that is, 0.96 nm. Since the repulsive force has a range much greater than the Debye length, this repulsion must be primarily steric due to adsorbed PSS. Sequential Addition of CTAB to Preadsorbed PSS. Figure 5 shows the force versus surface separation profiles on a semilog scale when CTAB has been added to the system containing previously adsorbed PSS (i.e., sequential addition) for both 10-3 and 10-1 M KBr. Recall that, upon preparation, all solutions were adjusted to the pzc of R-alumina. Sequential addition of a 5.5 × 10-5 M CTAB solution to the system containing adsorbed PSS in 10-3 M KBr caused a significant decrease in the magnitude of the fitted
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Figure 5. Semilog plot of the scaled force (F/R) versus surface separation for the interaction of two R-alumina fibers after the sequential adsorption of PSS at 100 ppm, followed by CTAB at 5.5 × 10-5 M. The pH was set to the determined pzc of R-alumina. Force curves were obtained at (O) 10-3 M KBr and (4) 10-1 M KBr. The solid curve is the DLVO fit at constant potential to the data obtained at 10-3 M KBr. The arrow indicates the separation from where the surfaces were observed to “jump in” to contact between the two adsorbed layers.
surface potential which now ranged from 17.5 to 15.0 mV, compared to 29.0 mV in the presence of PSS alone. The force profile is monotonically repulsive, fitting DLVO theory until a surface separation of 14.6 nm, at which point the interaction becomes attractive, the surfaces experiencing an attractive jump into contact from 14.6 to 3.3 nm. At surface separations below 3.3 nm, a steric repulsive force dominates the interactions. An adhesion of 0.62 mN m-1 was measured on retraction of the surfaces. Recall that no attractive forces were measured in the absence of surfactant when only PSS is adsorbed (see Figure 4). The force versus separation data following the sequential addition of a 5.5 × 10-5 M CTAB solution to the system containing adsorbed PSS but now in 10-1 M KBr is very different to that obtained at the lower salt concentration. There is no attractive jump into contact, and the force is monotonically repulsive, but a small adhesion of 0.09 mN m-1 was measured upon retraction of the surfaces. The addition of CTAB to a preadsorbed PSS layer under high salt conditions has resulted in an increase in the range of the repulsive interaction from 20 nm in the absence of CTAB to 70 nm after CTAB was added. Coaddition of CTAB and PSS. Figure 6 contains typical force profiles between two R-alumina surfaces after the coaddition of PSS and CTAB. One plot shows the surface interactions in 10-3 M KBr, and the other is that in 10-1 M KBr. The coaddition was achieved by allowing a solution containing both PSS and CTAB (at the appropriate salt concentration) to equilibrate prior to injection into the AFM, as detailed above. Here again, upon preparation, all solutions were adjusted to the pzc of R-alumina prior to the addition of the solution containing pre-equilibrated PSS-CTAB complex. For coaddition in 10-3 M KBr, there is no measurable force at intersurface separations greater than 30 nm. The force profile fits the DLVO theory until the surfaces are 8.9 nm apart, where the force becomes electrosteric rather than purely electrostatic, due to adsorbed PSS-CTAB
Muir et al.
Figure 6. Semilog plot of the scaled force (F/R) versus surface separation for the interaction of two R-alumina fibers after the coadsorption of PSS and CTAB at 100 ppm and 5.5 × 10-5 M, respectively. The pH was set to the determined pzc of R-alumina. Force curves were obtained at (9) 10-3 M KBr and (2) 10-1 M KBr. The solid curves are the DLVO fits to the data in 10-3 M KBr. The upper and the lower curves correspond to constant charge and constant potential, respectively.
complex. There was no jump into contact and no adhesion upon retraction of the surfaces, as were observed in the sequential addition of PSS and CTAB (see Figure 5). The effective surface potential was approximately 19 mV. For the coaddition of PSS and CTAB in 10-1 M KBr solution, interaction between the surfaces begins at separations below 26.5 nm, shorter in range compared to the sequential addition data at 10-1 M KBr. Data points at surface separations above 26.5 nm correspond to the force versus separation baseline on a linear scale, that is, zero force. Again, at this salt concentration, the force is purely repulsive and, interestingly, is similar to that obtained at low salt for the same adsorption protocol. However, despite this comparable range, the Debye length is only 0.9 nm at 10-1 M KBr, so the force can be attributed only to a steric rather than a double layer repulsion. There was no jump into contact and no adhesion upon surface retraction. Again, DLVO theory could not be applied when the KBR concentration is 10-1 M since the surface roughness is of a similar magnitude to the Debye length at this salt concentration. Hence, no surface potential has been determined. CTAB Adsorption Alone. An experiment to determine how CTAB alone affects the surface interactions between two crossed R-alumina fibers was carried out in a 5.5 × 10-5 M CTAB solution at 10-3 M KBr and a pH of 6.7, that is, the pzc of R-alumina. The resulting force versus surface separation data are shown in Figure 7. This figure also contains the force profile in the absence of CTAB under the same solution conditions (salt concentration and pH) for comparison. With CTAB present, the interaction forces are weakly repulsive at surface separations above 7.6 nm indicating some CTAB adsorption. In the absence of CTAB, there is little or no repulsive force over this range of surface separations. The small repulsion resulting from the adsorption of CTAB must be due to the positive charges from the adsorbed CTAB molecules. A DLVO fit to the data confirms this; at a surface separation of 7.6 nm, an attractive jump in was observed. A similar jump in is detected in the absence of CTAB. Upon retraction of the
R-Alumina Fibers with Coadsorbed PSS and CTAB
Figure 7. Semilog plot of the scaled force (F/R) versus surface separation for the interaction of two R-alumina fibers at the determined pzc in 10-3 M KBr with ()) no added surfactant and (O) added surfactant, i.e., 5.5 × 10-5 M CTAB. The solid curves are the DLVO fits to the data in 10-3 M KBr. The upper and the lower curves correspond to constant charge and constant potential, respectively. The arrows indicate the separation from where the surfaces were observed to “jump in” to contact between the two adsorbed layers for each force curve.
surfaces in 10-3 M KBr, an adhesion of 0.06 mN m-1 was measured, whereas with CTAB present the measured adhesion was 0.14 mN m-1. This enhanced adhesion might be due to a limited hydrophobic interaction due to the small amount of adsorbed CTAB. The two interaction forces are indeed very similar at the isoelectric point of R-alumina, regardless of whether CTAB is present. However, although the differences are slight, they are significant in that they indicate that CTAB does adsorb onto R-alumina at the pzc but that the adsorbed amount is small. Discussion PSS Adsorption. Prior to the addition of PSS, the R-alumina fibers have a surface potential near zero in both 10-3 and 10-1 M KBr at pH 6.7. Addition and subsequent adsorption of PSS in 10-3 M KBr resulted in an increase in surface potential to 29.0 mV. See Figure 4. The surface potential was obtained from a theoretical DLVO fit to the data over the range of surface separations where the force is purely electrostatic. DLVO theory is not strictly applicable when the charge at an interface is distributed over a finite thickness, as is the case for an adsorbed polyelectrolyte layer. We can, however, interpret our data in terms of relative surface potentials and trends in their values. Therefore, the fact that the surface potential increases in magnitude to 29.0 mV upon the addition of PSS is an indication that PSS molecules have adsorbed, the surface charge originating from the ionizable groups along the polyelectrolyte chain. The Debye length used to fit the data corresponds well to that expected in a 1:1 electrolyte solution of concentration 10-3 M. This implies that the interaction forces at large surface separations are not due to steric or electrosteric interactions between overlapping polyelectrolyte layers but rather are a double layer repulsion. At an intersurface spacing of about 7.0 nm, the surface interaction deviates from DLVO theory. This surface separation marks the onset of electrosteric repulsion due to the overlap of the adsorbed polyelectrolyte layers at each
Langmuir, Vol. 17, No. 16, 2001 4937
interface. Previous studies have shown that polyelectrolytes of high charge density, such as PSS, tend to adopt a flat conformation when adsorbed onto solid surfaces due to the monomeric charges repelling neighboring monomeric charged groups.32 Our results are consistent with this, since the lack of any deviation from DLVO theory at separations greater than 7.0 nm implies that there is little or no extension into solution of adsorbed PSS. A comparison of the surface interactions resulting from the adsorption of PSS at 10-3 M KBr and 10-1 M KBr (Figure 4) shows that an increase in the salt concentration leads to a decrease in the range and magnitude of the double layer repulsion, as expected. In our PSS system at 10-3 M KBr, electrostatic interactions begin at 42.7 nm separation, whereas in 10-1 M KBr, no significant electrostatic interaction is measured. Recall that the Debye Length is only 0.96 nm in 10-1 M KBr. Note, however, that under high electrolyte concentration conditions, there is a significant repulsive force at surface separations as large as 14.8 nm. Since double layer interactions are negligible at this intersurface spacing, the repulsion can only be attributed to a steric or an electrosteric interaction between the adsorbed PSS layers. In comparison, under low electrolyte concentration conditions, the steric repulsion first occurs at a surface separation of 7.0 nm. This change in the range of the steric interaction can be attributed to a difference in conformation of adsorbed PSS. At low salt, PSS maintains an extended conformation due to segment-segment repulsion and so lies relatively flat when adsorbed at an interface. However, at the high electrolyte concentration, PSS is able to adopt a more coiled conformation at the surface since segment-segment electrostatic repulsion is reduced. Thus, the effective thickness of the adsorbed polyelectrolyte is increased. Not only is the steric repulsion longer in range, but the repulsion is that of a layer that is relatively soft and compressible since intersegment charge repulsion is decreased at high ionic strength. Indeed, small-angle neutron scattering data5 has shown that an increase in the ionic strength leads to a more coiled configuration of a polyelectrolyte at an interface and the possible development of loops and tails. As the ionic strength of a polyelectrolyte solution is increased, the major changes in the surface interactions/profile at large surface separations are from charge contributions as affected by the salt ions.33 The high ionic strength effectively screens out the charges along the polyelectrolyte chain, reducing the range and magnitude of double layer interactions. Thus, the system behaves more like that of a neutral polymer. There is also the possibility of an increase in surface excess of PSS as electrolyte concentration is increased. An increase in salt concentration results in an increased amount of adsorbed polyelectrolyte.5 This will also lead to a greater adsorbed layer thickness, simply because more PSS must be accommodated on the surface, and so as a consequence the steric repulsion becomes more longranged. Note, however, that using the AFM, zero surface separation is not absolute. This means that it is not possible to obtain an accurate adsorbed layer thickness. Sequential Addition of CTAB to Preadsorbed PSS. For the sequential addition protocol, replacement of the PSS solution in the AFM flow cell with the CTAB solution causes any nonadsorbed PSS to be flushed out from between the surfaces. Therefore, the only PSS available (32) Cosgrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986, 111, 409. (33) Woodward, C. E.; Akesson, T.; Jonsson, B. J. Chem. Phys. 1994, 101, 2569.
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for complexation with CTAB is PSS that has already adsorbed onto the R-alumina fibers. The concentration of CTAB used in this study was 5.5 × 10-5 M. We were unable to find a literature value for the cac of CTAB with PSS; however, we can estimate this based on the cac for DTAB with PSS published by Hansson et al.34 They obtained a cac of 1 × 10-5 M for DTAB with 100 ppm PSS in water with no added electrolyte. CTAB has a longer carbon chain than DTAB, tending, comparatively, to reduce the cac.1 However, added electrolyte is known to reduce the degree of polyelectrolyte-surfactant complexation.35 Therefore, in our study we expect the presence of electrolyte to offset somewhat the effects of surfactant hydrocarbon chain length on the cac. We are therefore working close to the cac of PSS and CTAB. The addition of CTAB to a preadsorbed PSS layer in 10-3 M KBr results in a marked change in the interaction forces (compare Figure 4 to Figure 5). The significant reduction in the magnitude of the fitted surface potential from 29.5 mV to around 16 mV, resulting from the addition of positively charged CTAB molecules, demonstrates clearly that CTAB molecules associate with the negative sites on the adsorbed PSS thereby reducing the net charge at the interface. Recall that, before the addition of CTAB, PSS is the only source of charge at the interface, as discussed above. Complexation between PSS and hexadecyltrimethylammonium chloride (HTAC) at an alumina surface has been seen in previous studies.36 Almgren et al.37 have shown that PSS and CTAB associate strongly. It is interesting to compare the range of the steric interaction between two surfaces that contain only adsorbed PSS to the steric interaction once CTAB has sequentially adsorbed. Complexation of CTAB with previously adsorbed PSS clearly leads to a reconformation of the adsorbed polyelectrolyte such that the range of the steric interaction doubles. We have discussed above how PSS adsorbs in a relatively flat conformation due to its high charge density and resulting segment-segment repulsion. The complexation of CTAB with PSS reduces the charge density of the polyelectrolyte and so too the degree of segment-segment repulsion. This allows the polyelectrolyte chain to become more coiled yielding a longer ranged steric component to the surface interactions. The appearance of an adhesion (i.e., a jump into contact) following the sequential addition of CTAB into the system containing preadsorbed PSS is evidence of the presence of PSS-CTAB complex that is not lying flat on the surface but rather tails into bulk solution. The adhesion suggests that polyelectrolyte adsorbed at one surface is intercalating with polyelectrolyte adsorbed at the opposite surface; that is, the chains now have some affinity for each other and a bridging attractive force is measured. This is very different from when PSS alone was adsorbed to the surface, when no adhesion was measured. Indeed, bridging attraction is not expected in a high charge density polyelectrolyte that adsorbs flatly on a surface.38 The magnitude of the fitted surface potential of the interface following the adsorption of PSS in 10-3 M KBr at the pzc of the R-alumina fibers is 29.5 mV, as discussed above. This potential is negative due to the sulfonate groups present at each monomer unit of the polyelectrolyte. The drop in the magnitude of the surface potential (34) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (35) Hayakawa, K.; Kwak, J. J. Phys. Chem. 1982, 86, 3866. (36) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (37) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (38) Åkesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91 (4), 2461.
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following the subsequent addition of CTAB might signify a reduction in surface potential due to partial neutralization of PSS charged groups by association with CTAB, and so the potential remains negative in sign. However, another possibility is that there is a charge reversal due to neutralization followed by excess adsorption of CTAB, resulting in the surfaces adopting a net positive charge. In the present study, there was enough CTAB injected into the AFM cell in the presence of preadsorbed PSS to possibly cause a charge reversal of the R-alumina surfaces. However, if the proportion of CTAB that complexed with PSS reached a maximum ratio of bound to unbound PSS similar to that achieved by Almgren et al.,37 no surface charge reversal could have occurred. A surface charge reversal can also be caused by CTAB adsorbing directly onto the R-alumina fibers, thereby displacing some preadsorbed PSS. It has been observed that in the coadsorption of a polyelectrolyte and surfactant, the surface competes with the polyelectrolyte for the surfactant.5 However, our results show unequivocally that CTAB adsorbs minimally onto the R-alumina at the pzc. This is illustrated clearly in Figure 7 where the interaction between two bare R-alumina surfaces is compared to the interaction after the addition of CTAB. There is little difference in the interaction forces on addition of CTAB indicating only a small amount of CTAB adsorption. Therefore, in our study, it is highly unlikely that there is any significant displacement of PSS by the sequentially introduced CTAB. CTAB is drawn to the interface by its attractive electrostatic interaction with PSS. Similar results have been obtained by Dedinaite et al.,39 who adsorbed SDS onto preadsorbed PCMA layers. They found that the addition of SDS to preadsorbed PCMA led to only very limited desorption of the polyelectrolyte. We can explain the surface interactions as affected by electrolyte concentration by considering the influence of electrolyte on the degree of PSS-CTAB complexation. There are several reports that investigate the effect of salt in dilute solutions of polyelectrolyte and oppositely charged surfactant.1,5,40 In general, the cac increases when salt is added since electrolyte screens the electrostatic attraction between the polyelectrolyte and surfactant, so the driving force to electroneutralization is reduced.41 Therefore, in our system at 10-1 M background electrolyte, we expect a reduction in the degree of association between CTAB with PSS compared to that in 10-3 M KBr. This is evident from a comparison of the sequential addition force curves in 10-3 and 10-1 M KBr (see Figure 5). A reduction in the degree of association between PSS and CTAB will limit the amount of tailing of PSS into solution and therefore reduce the intercalation of the PSS chains. Thus, in 10-1 M salt, we have measured a far smaller adhesion than was observed in 10-3 M KBr. Additionally the attractive jump into contact measured in 10-3 M KBr is no longer evident in 10-1 M KBr. Coaddition of CTAB and PSS. In the coaddition case, where PSS and CTAB were equilibrated in solution together, the formation of a complex occurs prior to injection into the AFM, and so it is the adsorption of this complex to the R-alumina surfaces that modifies the surface interactions. Figure 6 contains the surface interactions between two R-alumina fibers after the coadsorp(39) Dedinaite, A.; Claesson, P. M.; Bergstro¨m, M. Langmuir 2000, 16, 5257. (40) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87 (3), 506. (41) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004.
R-Alumina Fibers with Coadsorbed PSS and CTAB
tion of the PSS-CTAB complex, in both 10-3 and 10-1 M KBr at the pzc of R-alumina. The amount of background electrolyte affects the surface interactions when PSS-CTAB complex is adsorbed onto R-alumina in a similar way to that seen for the sequential addition experiments discussed above. In 10-3 M KBr, the electrostatic interaction begins at a surface separation of 26.5 nm, whereas in 10-1 M KBr, no purely electrostatic interaction was measured (Figure 6). This is due to the reduction in the Debye length to 0.96 nm at high electrolyte concentration. Hence, the repulsive interaction measured in 10-1 M KBr is steric or electrosteric and is more longranged than at 10-3 M KBr. It is interesting to compare the force profile of the coaddition experiment in 10-1 M KBr to the interactions between adsorbed PSS layers under the same electrolyte conditions with no surfactant present (Figure 4). Indeed, the force profiles are very similar in form, and the range of the steric interaction is comparable with neither system exhibiting an adhesive force. It appears that the coaddition of PSS and CTAB at high salt results in an adsorbed species that behaves similarly to an uncomplexed PSS chain, explainable because the large amount of electrolyte inhibits PSS-CTAB association, as discussed in detail above. Note that in 10-3 M background salt, a surface separation of 8.9 nm sees the onset of the steric or electrosteric repulsion. This is less than the 14 nm surface separation that marks the onset of the steric repulsion in the sequential addition case. This is consistent with a greater degree of complexation in the coaddition experiments, resulting in a reduced segment-segment repulsion and hence a greater packing density of the adsorbed layers. The lack of any adhesion on retraction of the compressed surfaces shows that even when the surfaces are in contact, the packing density is high enough that it prohibits significant intercalation between the PSS chains on the neighboring surfaces. Such behavior has been seen previously.7 Additionally, the absence of an attractive jump into contact when PSS and CTAB are coadded is a further indication of the difference in conformation between the coadded complex and the sequentially formed complex. The more compact coadded adsorbed layer has little extension of the polyelectrolyte chains away from the surface into solution, hence no bridging attraction. On the other hand, when CTAB is added to a preadsorbed PSS layer, the PSS chains that initially lie flat on the surface rearrange as CTAB associates with PSS already at the interface leading to a conformation conducive to a bridging force.
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Conclusions PSS is able to adsorb onto R-alumina at the pzc even though there is limited charge-charge interaction between the polyelectrolyte and surface, the PSS adopting a relatively flat conformation at the interface due to repulsive segment-segment interactions. Interaction forces between two such PSS layers were dominated by double layer interactions at 10-3 M KBr, the surface charge originating from the adsorbed polyelectrolyte. There was a short-range electrosteric interaction at small surface separations. An increase in electrolyte concentration to 10-1 M removed any double layer interactions but increased the amount of adsorbed PSS and hence the range of the steric repulsion. The interaction forces between two R-alumina surfaces with adsorbed PSS-CTAB complex were shown to be very sensitive to the adsorption protocol. Sequential addition of CTAB to a previously adsorbed PSS layer and the resulting association of PSS with CTAB led to a rearrangment of the PSS chains. PSS at the interface changed from a relatively flat conformation in the absence of CTAB to a more random coil tailing into solution on addition of CTAB. This resulted in the measurement of a bridging attractive force on approach of the two surfaces and an adhesion upon retraction. When PSS and CTAB were coadded, their mutual association was maximized. Thus, due to the increased hydrophobicity and reduced charge of the resulting PSSCTAB complex, the adsorbed layer was densely packed with little or no tailing of the PSS chains into solution, and so no bridging attraction was measured. An increase in electrolyte concentration to 10-1 M KBr reduced the extent of complexation between CTAB and PSS in both the coaddition and sequential addition studies. Any repulsive force measured was predominately steric due to suppression of the electrical double layers, and the range of the steric interaction increased due to an increased adsorbed amount. In the sequential addition case, no attractive jump into contact was measured and the adhesion on retraction of the surfaces previously measured in 10-3 M KBr was significantly reduced. In the coaddition case, the adsorbed species at high salt behaved like an uncomplexed PSS chain under the same solution conditions because of the reduction in PSS-CTAB complexation. For both experimental protocols, at high salt both the high packing density and the coiled conformation of the PSS prohibited a jump into contact and adhesion due to intercalation of polyelectrolyte chains. LA010045T
