Surface Interactions during Polyelectrolyte Multilayer Build-Up. 2. The

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Surface Interactions during Polyelectrolyte Multilayer Build-Up. 2. The Effect of Ionic Strength on the Structure of Preformed Multilayers Eva Blomberg,*,† Evgeni Poptoshev,† and Frank Caruso‡ Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44 Stockholm, Sweden and Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden, and Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Victoria 3010, Australia ReceiVed NoVember 2, 2005. In Final Form: February 8, 2006 Interactions between surfaces bearing multilayer films of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) were investigated across a range of aqueous KBr solutions. Three layer films (PAH/PSS/PAH) were preassembled on mica surfaces, and the resulting interactions were measured with the interferometric surface force apparatus (SFA). Increasing the ionic strength of the medium resulted in a progressive swelling of the multilayer films. Interactions in solutions containing more than 10-3 M KBr were dominated by a long-ranged steric repulsion originating from compression of polyelectrolyte segments extending into solution. In 10-1 M KBr, repeated measurements at the same contact position showed a considerable reduction of the range and the strength of the steric force, indicating a flattening of the film during initial approach. Furthermore, this flattening was irreversible on the time scale of the experiments, and measurements performed up to 72 h after the initial compression showed no signs of relaxation. These studies aid in understanding the dominant interactions between polyelectrolyte multilayers, including polyelectrolyte films deposited on colloidal particles, which is important for the preparation of colloidally stable nanoengineered particles.

Introduction Multilayer assemblies constructed by the sequential adsorption of oppositely charged species (i.e., the layer-by-layer (LbL) technique) have attracted growing research interest during the past decade.1,2 The main advantage of the LbL technique is its simplicity and flexibility. It allows for a range of charged materials, such as polyelectrolytes, nanoparticles, proteins, and dyes, to be assembled into coatings with controlled structure and functionality.3-5 Furthermore, the outcome is largely independent of the size and shape of the substrate to be coated, thus allowing the fabrication of surface-modified colloidal particles and hollow colloidal capsules.6,7 However, successful preparation of coated colloids necessitates retaining colloidal stability during both coating and subsequent storage of the dispersions,8 which requires an understanding and control of the interparticle interactions. Direct measurements of surface interactions between multilayercoated surfaces are therefore critical for understanding the mechanisms governing the stabilization of such colloids. In a preceding publication,9 we reported interactions between mica surfaces bearing PAH-PSS multilayers in dilute electrolyte solutions. It was found that in 10-4 M monovalent electrolyte * Corresponding author. E-mail: [email protected]. † Royal Institute of Technology and Institute for Surface Chemistry. ‡ University of Melbourne. (1) Iler R., K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (2) Decher, G. Science 1997, 277, 1232. (3) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (4) Decher, G., Schlenoff, J. B. Multilayer Thin Films Wiley-VCH: Weinheim, Germany, 2003. (5) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (6) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (7) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem. 1998, 37, 2202. (8) Kato, N.; Schutz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780. (9) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20, 5432.

the interactions at long distances are dominated by electrostatic double-layer repulsion. It was concluded that this repulsive barrier is responsible for the observed stability of coated colloids upon storage in pure water and dilute salt solutions.10,11 However, the coating itself is usually carried out under higher ionic strength conditions. Typically, 0.1-0.5 M monovalent electrolyte is added to the adsorption solutions to ensure uniform coating. Under these conditions, electrostatic interactions are effectively screened down to very short distances (10-20 Å). The DLVO theory12,13 predicts that low to moderately charged colloidal suspensions should be coagulated when the salt concentration exceeds the critical coagulation concentration (ccc). Despite that, colloidal stability is usually retained, and the coated suspensions can easily be redispersed after centrifugation. Therefore, the stabilizing mechanism involves interactions not accounted for by the DLVO theory. It is well established that polyelectrolyte adsorption gives rise to a range of non-DLVO surface interactions, such as steric repulsion, bridging, and depletion forces.14 Several studies on interactions between single polyelectrolyte layers adsorbed onto oppositely charged surfaces have shown that increasing the ionic strength of the medium results in the appearance of a longranged steric repulsion.15-18 Steric forces between polymer layers are induced when segments extending into solution (loops and tails) are confined in the gap between the approaching surfaces. (10) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (11) Gao, C.; Leporatti, S.; Moya, S.; Donath, E.; Mo¨hwald, H. Langmuir 2001, 17, 3491. (12) Derjaguin, B.; Landau, L. Acta Physiochem. 1941, 14, 633. (13) Verwey, E. G. W.; Overbeek, J. T. G. The theory of the stability of lyophobic colloids; Elsevier: Amsterdam, 1948. (14) Claesson, P. M.; Poptoshev, E.; Blomberg, E.; Dedinaite, A. AdV. Colloid Interface Sci. 2005, 114, 173. (15) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 865. (16) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; Macnaughtan, W.; Chapman, D. Colloids Surf. 1987, 25, 263. (17) Dahlgren, M. A. G. Langmuir 1994, 10, 1580. (18) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; A° kesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769.

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This causes a loss of conformational entropy resulting in a repulsive interaction.19 With electrostatically adsorbed polyelectrolytes, increasing ionic strength has the effect of screening the polymer-surface attraction, which in turn increases the fraction of loops and tails extending from the surface.20 The repulsion generated in these systems is of electrosteric origin; that is, it is a result of the simultaneous action of both steric and electrostatic forces since the extending segments are charged. In the present investigation, we probe the response of preformed PAH-PSS multilayers to elevated ionic strength solutions. The main aim is to elucidate the mechanism responsible for stabilizing multilayer-coated particles under typical coating conditions. Materials and Methods Poly(styrenesulfonate sodium salt) (PSS) Mw ) 70 000 g mol-1 and poly(allylamine hydrochloride) (PAH) Mw ) 70 000 g mol-1 were purchased from Sigma. PSS was dialyzed for several days against deionized water and dried in a vacuum. KBr of spectroscopic grade was purchased from Merck. Water was purified by RiOS-8 and Milli-Q+ 185 units from Millipore. The outgoing water had a resistivity of 18.2 MΩ cm and total organic carbon content (TOC) not exceeding 10 ppb. For use in the SFA, thin muscovite mica sheets were silvercoated on one side and glued onto cylindrical silica disks using Epicote (1004) resin (Shell Chemicals). Polyelectrolyte multilayers were prepared ex situ by immersing the glued surfaces in polyelectrolyte solution (1 mg mL-1 polymer and 0.5 M KBr) for 20 min. KBr was used instead of NaCl (which is normally used) in order to avoid dissolution of the silver coating on the backside of the surfaces. The excess polyelectrolyte was removed after each adsorption step by rinsing three times in Milli-Q water for 1 min. In total, three layers were deposited on each surface (PAH/PSS/ PAH). After deposition, the surfaces were immediately sealed and allowed to dry in ambient air. This was preferred instead of drying in a stream of nitrogen since the pressure easily damages the thin mica sheets. The dried surfaces were then mounted inside the chamber of a Mark IV SFA.21 The chamber was filled with Milli-Q water. The ionic strength was adjusted by injecting a stock solution of KBr. The interferometric SFA has been described in a number of publications.21-23 Only a brief outline will be provided here. The surfaces are mounted in a crossed cylindrical geometry. The intersurface distance is controlled by means of a motor or by applying a voltage to a piezoelectric crystal to which the upper surface is attached. The distance between the surfaces is determined interferometrically using fringes of equal chromatic order (FECO). In the beginning of each experiment, the bare mica surfaces are brought in contact in dry air and the position of zero distance recorded. All measurements thereafter are referred to this zero separation. This allows the absolute distance between the surfaces as well as the thickness of the adsorbed films to be determined. The force is obtained from the deflection of a double cantilever spring holding the lower surface. Where appropriate the measured forces were compared with electrostatic double-layer forces calculated according to the nonlinear Poisson-Boltzmann (PB) model for conditions of constant surface charge density or constant potential. The calculations were performed following the algorithm of Chan et al.24 (19) Hasselink, F. T.; Vrij, A.; Overbeek, J. T. G. J. Phys. Chem. 1971, 75, 2094. (20) Dahlgren, M. A. G.; Hollenberg, H. C. M.; Claesson, P. M. Langmuir 1995, 11, 4480. (21) Parker, J. L.; Christenson, H. K.; Ninham, B. W. ReV. Sci. Instrum. 1989, 60, 3135. (22) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (23) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. AdV. Colloid Interface Sci. 1996, 67, 119. (24) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283.

Figure 1. Force scaled by radius of curvature versus distance between two mica surfaces precoated with a PAH/PSS/PAH film in 10-4 M KBr. Filled symbols represent forces measured on approach, and unfilled symbols are forces measured on separation. The lines are fits to Poisson-Boltzmann theory with constant surface charge (upper line) and constant surface potential (lower line) boundary conditions.

Results and Discussion The interaction between two surfaces coated with PAH/PSS/ PAH films in 10-4 M KBr is displayed in Figure 1. From the figure, it is evident that the long-range part of the interaction is dominated by double-layer repulsion. As discussed in the preceding publication9 and in a number of electrokinetic studies,3,6,25 the surface charge is acquired via charge overcompensation upon adsorption of the outermost polyelectrolyte layer. In the present case, the outermost layer is cationic (PAH), which implies that the surfaces are net positively charged. When fitting the interactions in Figure 1, both the apparent double-layer potential and the decay length were used as adjustable parameters. The best fit to the PB theory gave a magnitude of 115 mV for the apparent double-layer potential, which gives an area per charge of 28.9 nm2, and a decay length of 303 Å (identical to the calculated Debye length for a 10-4 M monovalent electrolyte solution). A hard wall contact is not established until a compressive load of approximately 10 000 µN/m is applied indicating that the films are somewhat compressible under load. Figure 2 shows the interactions between the two PAH/PSS/ PAH films when the electrolyte concentration was raised to 10-3 M. Approximately 550 Å from contact an exponentially decaying repulsive force becomes detectable. Fitting PB theory to the experimental data gives values for the magnitude of the apparent double-layer potential of 100 mV, corresponding to an area per charge of 15.38 nm2. The best fit was obtained with a decay length of 118.7 Å. This is approximately 20% longer than the Debye length calculated for a 10-3 M monovalent electrolyte solution (95.7 Å). It appears that there is a weak nonelectrostatic contribution to the total interaction that shifts the decay length toward a slightly higher value. Furthermore, at a surface separation of about 400 Å, a slight deviation from the exponential decay is observed, which further indicates that the interaction at larger separations is not purely electrostatic. However, under these conditions, the nonelectrostatic component of the interaction cannot be clearly decoupled from the repulsive electrostatic double-layer force. (25) Decher, G., Schlenoff, J. B., Eds.; Multilayer Thin Films; Wiley-VCH: Weinheim, Germany, 2003; p 12.

Ionic Strength and Preformed Multilayers

Figure 2. Force scaled by radius of curvature versus distance between two mica surfaces precoated with a PAH/PSS/PAH film in 10-3 M KBr. Filled symbols represent forces measured on approach, and unfilled symbols are forces measured on separation. The lines are fits to Poisson-Boltzmann theory with constant surface charge (upper line) and constant surface potential (lower line) boundary conditions. The broken line corresponds to the calculated electrostatic doublelayer interaction for the given ionic strength, 1 mM 1:1 electrolyte (κ-1 ) 95.7 Å).

At high compressive load, the surfaces reach a hard wall contact at a separation of 230 Å (zero separation is defined as the contact between two uncoated mica surfaces, measured before multilayer deposition on each surface). This corresponds to a compressed thickness of 115 Å per PAH/PSS/PAH film on each surface. Separating the surfaces from contact (open symbols in Figure 2) yields a slightly hysteretic force profile. The forces measured on separation are more repulsive than those measured during approach until a distance of about 400 Å is reached, where the curves merge. As discussed in the preceding publication,9 the extra repulsion is generated due to deformation of the surface contact region, and subsequent redistribution of the adsorbed material. Such deformations have been previously observed in systems with adsorbed protein and nonionic surfactant layers.26 This is due to pressure-induced lateral motion of molecules in the contact region. Under high loads, the adsorbed material is redistributed along the contact,26 with the adsorbate along the rim of the contact being expelled whereas the material inside the contact remains “trapped”. As a result, when the compressive pressure is further increased the surfaces move closer to each other at positions corresponding to the rim, but this does not occur at positions corresponding to the middle of the contact region. It is interesting to note that the interaction between PAH/ PSS/PAH films in 10-4 M KBr (Figure 1) did not show any hysteresis; that is, the forces measured on approach and separation were completely reproducible. Apparently, increasing the ionic strength 10-fold from 10-4 to 10-3 M KBr increases the mobility of the adsorbate in the contact region. Increasing ionic strength leads to screening of the electrostatic attraction between the oppositely charged polyions building the multilayers, as well as the attraction between the first PAH layer and the negatively charged mica surface. Since the films are held together mainly by electrostatic interactions, screening of these interactions reduces the structural stability of the coating.9,26,27 At the same (26) Blomberg, E.; Claesson, P. M.; Christenson, H. K. J. Colloid Interface Sci. 1990, 138, 291. (27) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736.

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Figure 3. Force scaled by radius of curvature versus distance between two mica surfaces precoated with a PAH/PSS/PAH film in 10-2 M KBr. Filled symbols represent forces measured on approach, and unfilled symbols are forces measured on separation. The solid line is a fit to Poisson-Boltzmann theory with a constant surface charge boundary condition and a decay length corresponding to the Debye length for the given electrolyte concentration. The dotted line represents the calculated interaction assuming a Debye length of 67.7 Å (corresponding to a monovalent electrolyte concentration of 2 × 10-3 M). This clearly illustrates that the measured force is not of an electrostatic origin.

time, when high compressive loads are applied, the films deform thereby producing the observed extra repulsion upon separation from contact. The measured forces after the ionic strength was increased to 10-2 M KBr are illustrated in Figure 3. The solid line represents the calculated interaction assuming an apparent double-layer potential of 60 mV and a decay length corresponding to the Debye length for the given ionic strength (30.3 Å). The dotted line, however, represents the fitted PB theory to the experimental data, which gives values for the magnitude of the apparent doublelayer potential of 60 mV, corresponding to an area per charge of 20.84 nm2. The best fit was obtained with a decay length of 67.7 Å (equivalent to a monovalent electrolyte concentration of 2 × 10-3 M). This clearly shows that the interactions in this case cannot be adequately modeled by using the Poisson-Boltzmann theory. There is a nonelectrostatic repulsive force that dominates the interaction at distances below approximately 550 Å. This extra repulsion is of steric origin and is attributed to a saltinduced swelling of the multilayer films.10 As mentioned earlier, increasing the ionic strength has the effect of screening the electrostatic attractions between oppositely charged polyions within the film. At low ionic strength, oppositely charged segments of PAH and PSS polymers are close together, producing compact and rigid multilayers with a high degree of internal compensation.9 When the ionic strength is increased, the electrostatic attraction is effectively screened. Furthermore, small ions compete with the polyelectrolytes in neutralization of the available charge.28 As a result, the adsorbed films undergo conformational changes, resulting in an increased fraction of the polyelectrolyte segments (loops and tails) extending into solution. This is often referred to as salt-induced swelling of adsorbed polyelectrolytes. For single polyelectrolyte layers electrostatically adsorbed on an oppositely charged surface, the phenomenon is well-documented experimentally15,16 and predicted theoretically.20 The main (28) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592.

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Figure 4. Force normalized by the local geometric radius as a function of surface separation between two mica surfaces precoated with a PAH/PSS/PAH film in 10-2 M KBr. Filled symbols represent forces measured on approach, and unfilled symbols are forces measured on separation. The forces at shorter range are shown at larger scale.

consequence of the swelling in terms of surface interactions is the appearance of a steric repulsion. When confined in the gap between the approaching surfaces, the extending loops and tails lose conformational entropy, thus generating the long ranged steric repulsion observed in Figure 3. At short distances, the force profile reveals some interesting features. In Figure 4, the short distance part of the force curves in 10-2 M KBr is replotted. At distances below 275 Å, the force curves show clear steps. Increasing the load yields a steeply increasing repulsion until, at some critical load, the surfaces spontaneously jump inward by 15 Å. Up to three distinct steps were observed before a final hard wall contact was reached at a separation of approximately 250 Å. The reason for the appearance of these steps is not clear. It was initially considered that a step is generated when a polyelectrolyte layer is expelled from the contact zone upon compression, in analogy with some surfactant bilayer systems.29 However, there are two observations that do not support this hypothesis. First, the force curves could be reproduced several times at the same contact spot (different symbols in Figure 4). If material were to be removed from the contact, subsequent measurements at the same position would generate a different force-distance profile. Furthermore, forces measured on approach and separation were also reproducible. This also cannot be explained by the assumption of layer-bylayer polyelectrolyte removal. Second, the final layer thickness does not decrease. If polyelectrolytes were indeed expelled, a corresponding decrease in the final layer thickness is to be expected. It should be remembered that the experiments were performed on preformed multilayers; that is, there were no polyelectrolytes present in the bulk solution, during measurements. Thus, any material removal from the surfaces is likely to produce irreversible structural changes in the films. We instead suggest that the observed discontinuous force profile is related to a stepwise collapse of the multilayer films under high compressive load. It appears that at a certain critical pressure, the swollen multilayer films suddenly collapse to the next stable state. We note that similar steps have been observed upon (29) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110.

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Figure 5. Force scaled by radius of curvature versus distance between two mica surfaces coated with a PAH/PSS/PAH film in 10-1 M KBr. Filled symbols represent forces measured on approach, and unfilled symbols are forces measured on separation. Squares: measured during the first approach and separation on a novel position. Diamonds: measured during second approach and separation shortly after. Triangles: measured during third approach and separation after 72 h of incubation.

compression of highly swollen cellulose Langmuir-Blodgett multilayer films deposited on mica.30 Figure 5 shows the interactions between PAH/PSS/PAH films in 10-1 M KBr solution. Filled symbols represent the interactions measured on approach, and unfilled symbols are the interactions measured on separation. The first approach on a novel contact position (filled squares) shows a long-ranged steric force, detectable at separations below 1200 Å. The onset of steric repulsion appears at approximately two times larger separation compared to the one measured in 10-2 M KBr (Figure 3). Increasing the salt concentration to 10-1 M causes further swelling of the multilayers. At this ionic strength, all electrostatic interactions are effectively screened down to very short distances (κ-1 ) 9.6 Å). The weakened electrostatic attraction between the PAH and PSS polyions promotes the formation of long loops and tails. From the onset of the steric force, it can be estimated that there is a small fraction (low density) of polymer chains extending as far as 460 Å (per surface) from the position of the hard wall contact. As discussed previously,9 optical techniques cannot readily detect such extended layers since these techniques (e.g., ellipsometry, surface plasmon resonance spectroscopy) determine the thickness on the basis of the assumption that the layer is homogeneous and has constant optical properties throughout its thickness. Separating the surfaces yields a different force profile (unfilled squares). Close to contact, the forces are more repulsive due to contact deformations as in the case of 10-3 M KBr. No stepwise discontinuities were observed. Another major difference between the approach and separation force curves appears at distances larger than 400 Å. The magnitude of the steric force measured on separation is considerably lower. A second measurement carried out immediately after the first one is represented with diamonds in Figure 5. In this case, there was no hysteresis between the approach and separation runs. Furthermore, the force profiles are essentially identical to the one measured during first separation (30) Poptoshev, E.; Carambassis, A.; O ¨ sterberg, M.; Claesson, P. M.; Rutland, M. W. Prog. Colloid Interface Sci. 2000, 116, 79.

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Table 1. Interaction Parameters of PAH/PSS/PAH Layers Adsorbed on Mica at Different Ionic Strength [KBr] (M)

compressed layer thicknessa (Å)

range of steric repulsion (Å)

10-4 10-3 10-2 10-1

200 ( 5 228 ( 5 250 ( 5b 225 ( 5

570 ( 50 1250 ( 50c

a The compressed layer thickness was evaluated at a load of 6000 µN/m and corresponds to the total compressed layer thickness between two mica surfaces, each coated with a PAH/PSS/PAH layer. b The thickness corresponding to the position of the last transition step. c Measured during first approach on a novel contact position.

(unfilled squares). Apparently, the swollen multilayer films are compressed during the initial approach and the chains extending far into solution are forced to compact closer to the surface. Retracting the surfaces thus produces a shorter-range steric repulsion. Consecutive force runs on the same flattened contact position also produce short-range steric repulsion, suggesting that no relaxation of the chains back into solution occurs. Clearly, in our case, there is no significant relaxation since the force profiles measured on first separation and on second approach and separation are identical. To examine the possibility of some long time relaxation effects, the surfaces were separated completely and incubated for 72 h prior to the next force run, represented with triangles in Figure 5. As can be seen, the forces obtained after this long incubation remain identical to those measured at short time; that is, the steric barrier is significantly reduced compared with the first approach. Thus, it appears that the compression of the films at high ionic strength is practically irreversible on the time scale of our experiments. The films flatten during the first approach, and their structure remains unchanged thereafter, which also is an indication that the undisturbed layers have a low density of polymer chains extending out into the solution. The fact that the films swell in contact with 10-1 M KBr but remain flattened after compression suggests that compression leads to an irreversible (on the time scale of the experiments) structural change within the films. Previously, such irreversible structural changes have been observed for single polyelectrolyte layers adsorbed on oppositely charged surfaces in high ionic strength solutions.15 It is important to note that this behavior was only observed at the highest salt concentration studied here, i.e., 10-1 M KBr. In all other cases, there was no significant difference in the long-ranged forces between initial and consecutive force runs. This indicates that the effect is related to the effective screening of the electrostatic interactions in the films.28 Finally, when the layer thickness is examined under high compressive load (Table 1), it can be concluded that there is a small increase in the thickness of the compressed layer with salt

concentration up to 10-2 M, whereafter the compressed layer thickness decreases again. These differences are likely due to different compressibilities of the films at different ionic strengths rather than due to differences in adsorbed mass. In the highest electrolyte concentration, no or very limited polyelectrolyte desorption occurs even after incubating the films for 72 h in 10-1 M KBr. This is an important finding concerning the application of PAH/PSS multilayer-coated substrates in high ionic strength media.

Conclusions Interactions between mica surfaces bearing preformed PAH/ PSS/PAH films were studied using the interferometric SFA. The interactions were measured in aqueous KBr solutions ranging in concentration from 10-4 to 10-1 M. At 10-3 M KBr and below, the interactions at large separations are primarily of electrostatic origin. Increasing the ionic strength causes the appearance of a steric repulsion, which increases in range with ionic strength. This may be attributed to salt-induced swelling of the multilayer films. However, at lower ionic strength (10-4-10-3 M), the strong double-layer repulsion makes the distinction between the electrostatic and the steric components difficult. Thus, the presence of some steric interactions cannot be ruled out even at low ionic strength. Interactions across 10-1 M KBr were characterized with an irreversible (on the time scale of the experiments) compression of the polymer chains protruding into solution during the initial surface approach. Thus, the range of the steric repulsion measured upon consecutive force measurements was considerably reduced. If the stability of polyelectrolyte multilayer-coated colloids is considered, it can be concluded that there are two main stabilizing mechanisms. At low ionic strength (up to approximately 10-3 M monovalent electrolyte), the colloids are primarily electrostatically stabilized. The net surface charge acquired via charge reversal upon polyelectrolyte adsorption gives rise to electrostatic double-layer repulsion. At elevated ionic strength, the dominating mechanism is steric stabilization. This is expected to play an important role particularly during the build up of each layer, which is usually carried out at high ionic strength. Since at high ionic strength the double-layer repulsion is effectively screened, the polymer-induced steric barrier is the dominant factor preventing flocculation of polyelectrolyte multilayer-coated particles. Acknowledgment. E.B. acknowledges the Swedish Research Council for financial support. F.C. acknowledges financial support from the Australian Research Council under the Federation Fellowship and Discovery Project schemes. Professor Per M. Claesson is acknowledged for helpful discussions during the course of this investigation. LA052946Y