Langmuir 1999, 15, 7789-7794
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Surface Forces in Aqueous Polyvinylamine Solutions. I. Glass Surfaces E. Poptoshev, M. W. Rutland,* and P. M. Claesson Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden, and Institute for Surface Chemistry, P.O. Box 5607, SE-11486, Stockholm, Sweden Received March 18, 1999. In Final Form: July 1, 1999 A noninterferometric surface force apparatus has been used to measure interactions between glass spheres in dilute aqueous polyvinylamine solutions at two different salt concentrations. Close to the substrate charge neutralization point, an attractive interaction is present mainly because of bridging of the extending polymer tails. Additional adsorption leads to an overcompensation of the glass negative surface charge, and the interaction at this point is dominated by a long-range double-layer repulsion. The results from fitting the Derjaguin-Landau-Verwey-Overbeek theory to the measured force curves demonstrate that the degree of overcompensation increases with polyelectrolyte concentration and increasing ionic strength of the solution (addition of indifferent electrolyte). An increase in ionic strength results in the screening of the electrostatic forces which leads to: (i) a reduced free energy cost of creating a charged interface, (ii) a decreased repulsion between protonated amine groups along the polymer backbone and a corresponding increased chain flexibility, (iii) a reduced electrostatic attraction between the polyelectrolyte and the surface. The first effect is apparently is the most important in the present case.
Introduction Interactions between colloidal particles are influenced dramatically by adsorbing polyelectrolytes even at low bulk concentrations, a fact that has been exploited in many fields, such as paper making, the food industry, and water treatment. The demand for knowledge of polyelectrolyte adsorption to solid surfaces and its effect on particleparticle interactions has inspired extensive research, both theoretical and experimental, during the past few years. Experimental studies of interactions between surfaces bearing adsorbed layers of polyelectrolytes have mainly been carried out using the interferometric surface force technique.1-9 A large variety of polyelectrolytes have been used, ranging from biopolymers and their derivatives5,6,8,10 to purely synthetic polyelectrolytes having different linear charge densities.1-4 In most cases, muscovite mica was used as a substrate surface. The main driving force for adsorption of polyelectrolytes on oppositely charged surfaces is electrostatic. Hence, most studies have focused on how changes to the electrical environment affect adsorption and interactions. Such changes can be brought about by varying the polyelectrolyte charge density, ionic strength of the solution, or the charge density of the substrate surface. Some of the main results obtained from * Corresponding author. (1) Dahlgren, M. A. G.; Waltermo, Å.; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; Åkesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (2) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 166, 343. (3) Dahlgren, M. A. G.; Hollenberg, H. C. M.; Claesson, P. M. Langmuir 1995, 11, 4480. (4) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92, 6044. (5) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 865. (6) Kawanishi, N.; Christenson, H. K.; Ninham, B. W. J. Phys. Chem. 1990, 94, 4611. (7) Claesson, P. M.; Dahlgren, M. A. G.; Eriksson, L. Colloids Surf. A 1994, 93, 293. (8) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; Macnaughtan, W.; Chapman, D. Colloids Surf. 1987, 25, 263. (9) Dahlgren, M. A. G. Langmuir 1994, 10, 1580. (10) Blomberg, E.; Claesson, P. M.; Fro¨berg, J. C. Biomaterials 1998, 19, 371.
surface force studies using negatively charged mica surfaces and cationic polyelectrolytes that are relevant for the present study are: (i) In low ionic strength solutions (e0.1 mM 1:1 electrolyte) charge neutralization is reached at a low polyelectrolyte concentration. The degree of overcompensation reached by increasing the polyelectrolyte concentration generally is small,2 which demonstrates the predominance of electrostatic forces. (ii) At higher ionic strengths the degree of overcompensation increases, suggesting that non-Coulomb forces also are important for the adsorption.9 (iii) Highly charged polyelectrolytes adsorb in very thin layers on strongly oppositely charged surfaces when the ionic strength of the solution is low. For instance, Dahlgren et al.1 showed that under such conditions a cationic polyelectrolyte having one charge per segment forms an adsorbed layer on mica that does not exceed 1 nm in thickness at the equilibrium separation, defined as the position of the attractive minimum in the force curve. However, upon increasing the inert salt concentration, additional adsorption and an increase in layer thickness were observed. (iv) Long-range steric forces are often dominant when the adsorption occurs from high ionic strength solutions (g10 mM 1:1 electrolyte).3 (v) At low ionic strength, the thickness of the adsorbed layer increases with decreasing charge density of the polyelectrolyte.11 Much less is known about how the charge density of the surface influences the forces generated by adsorbed polyelectrolytes. One reason for this is that the interferometric surface force apparatus is limited largely to mica as a substrate. However, recent developments in surface force measurements make it possible to use glass surfaces with a noninterferometric surface force apparatus.12 Glass has numerous important differences comparised with mica. First, glass has a lower charge density, and the mechanism of surface charging in solution is completely different from that of mica. The negative (11) Dahlgren, M. A. G.; Claesson, P. M. Nordic Pulp Pap. Res. J. 1993, 8, 62. (12) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.
10.1021/la990322k CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999
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Figure 1. Molecular structure of the vinylamonium monomer unit.
charge at the glass-water interface originates from dissociation of the surface silanol groups, whereas mica is charged because of isomorphous substitution of silicon atoms by aluminum. Second, it has been suggested that, in aqueous salt solutions, a gel-like layer is formed on the glass surface,13-15 thus creating a rather diffuse distribution of the charges just outside the surface. These specific features of the glass surface undoubtedly will influence the adsorption of cationic polyelectrolytes. In this study we have chosen to work with polvinylamine, a highly charged polymer at neutral and low pH. Despite its relatively simple molecular structure (see Figure 1) successful synthesis and purification procedures have been developed only recently.16 The reason is that direct synthesis from the vinylamine monomer is not yet possible.16 Currently, polyvinylamine preparation is carried out by hydrolysis of poly(n-vinyl-tert-butylcarbamate) in alkaline aqueous solutions.17 Materials and Methods The cationic polyelectrolyte polyvinylamine (PVAm) with an average molecular weight 90 000 (corresponding degree of polymerization, 2050), was obtained from BASF AG, Ludwigshafen, Germany as a 12% aqueous solution. The molecular structure of the vinylammonium repeating unit is shown in Figure 1. The PVAm is 100% charged at neutral and low pH. The polymer was purified by precipitation from an approximately 3% aqueous polymer solution by addition of spectrographic grade ethanol.16 The precipitate was dried in a vacuum. Sodium chloride suprapure was obtained from Merck KgaA and used without further purification. The water purification system comprised Millipore RiOs-8 and Milli-Q PLUS 185 purification units, and finally the water was passed through a 0.2-µm Millipak filter. A total organic carbon monitor (Millipore A-10) was added to the system to control the quality of outgoing water. In all cases the total organic carbon content did not exceed 10 ppb. Force measurements were conducted using the noninterferometric surface force apparatus developed by Parker.12 This device, commonly known as MASIF (Measurements and Analysis of Surface Interactions and Forces), is based on a bimorph force sensor. One of the surfaces is mounted on the end of the bimorph spring and the other at the end of a piezoelectric tube. The assembly is enclosed in a liquid cell (volume ca. 10 mL) and mounted on a translation stage. During a force measurement, the surfaces are driven together and apart by applying a triangular voltage wave to the piezo crystal. Simultaneously, a measurement of the charge produced upon bending of the bimorph is recorded. Once the surfaces come into hard-wall contact, the linear movement of the piezo is transmitted directly to the bimorph, thus enabling the force sensor to be calibrated against the known piezo crystal expansion and contraction. (The hysteresis in the piezo movement is taken into account by the use of an linear variable differential transformer sensor). It should be noted that the hard-wall assumption is not always valid, especially in the soft and compressible adsorption layers.18 (13) Hunter, R. J. Foundation of Colloid Science; Clarendon Press: Oxford, 1987. (14) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (15) Yaminsky, V. V.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3223. (16) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. Polyelectrolyres; Hanser Publishers: Munich, 1994; Vol. II. (17) Molyneux, P. Water-Soluble Synthetic Polymers Properties and Behavior; CRC Press: Boca Raton, FL, 1984; Vol. II. (18) Schillen, K.; Claesson, P. M.; Malmsten, M.; Linse, P.; Booth, J. J. Phys. Chem. 1997, 101, 4238.
Poptoshev et al. Providing that the deflection and the spring constant of the bimorph are known, the raw data are easily recalculated into force-distance curves by use of Hooke’s law. This technique does not allow determination of the absolute zero of surface separation, and therefore little information about the thickness of the adsorbed layer can be obtained. In some cases, a layer thickness can be obtained from the magnitude of the inward “jump” if the layer is pushed out from the contact area upon compression.19 However, this was not the case in the present study. The surfaces were prepared by melting one end of a borosilicate glass rod (diameter 2 mm, length ca. 25 mm) in a butane-oxygen burner until a droplet with radius of about 2 mm was formed. The radius of curvature was determined more accurately with a micrometer after each experiment. The mean radius of the interaction was then calculated according to the following expression R ) r1r2/(r1 + r2), where r1 and r2 are the radii of the spherical surfaces used in the experiment. The measured force, Fs, was recalculated into forces between cylinders, and related to the free interaction energy per unit area, Gf, via the Derjaguin approximation20:
F ) 2πGf R It has been shown21 that flame-polished glass surfaces are smooth enough to enable accurate measurements of surface forces down to molecular separations. All procedures for assembling the measuring chamber and preparing the solutions were carried out inside a laminar flow cabinet. At the beginning of each set of experiments, the interaction profiles were first determined in air to ensure that the system showed no signs of contamination (i.e., the surfaces jump into a strong adhesive contact from a distance consistent with the van der Waals interaction.) In the next step, a salt solution was introduced into the measuring chamber. The polyelectrolyte concentration was varied by draining the whole volume of the chamber and replacing it with a solution of the desired composition. During this procedure, a droplet from the previous solution always remained between the surfaces; hence, once wetted, the substrate surfaces were never exposed to direct contact with air. Thus, in these measurements, each new concentration adsorbs to a surface preequilibrated with the previous solution and not to a bare surface. The measured forces were analyzed using classical DerjaguinLandau-Verwey-Overbeek theory22,23 taking into account doublelayer forces and attractive van der Waals forces. The doublelayer force was calculated within the nonlinear PoissonBoltzmann model, using constant surface charge boundary conditions, because these generally better approximated the measured forces than those calculated at constant potential. The van der Waals force was calculated using a nonretarded Hamaker constant of 0.5 × 10-20 J, the value expected for silica-watersilica.24
Results and Discussion Forces in Absence of PVAm. At the beginning of each set of experiments the force-distance profile was recorded in absence of PVAm in 1:1 electrolyte solutions. The interaction curves in 1 mM and 0.1 mM NaCl at pH ) 5.6-6, together with the corresponding DLVO fits, are shown in Figure 2. The measured forces are in excellent agreement with DLVO predictions down to a separation of about 2-3 nm with a Debye length calculated from the ionic strength of the solution. However, below this separation an extra repulsion is present instead of the attractive van der Waals force, which DLVO theory (19) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110. (20) Derjaguin, B. Kolloid Z. 1934, 69, 155. (21) Ederth, T.; Claesson, P. M.; Lindberg, B. Langmuir 1998, 14, 4782. (22) Derjaguin, B.; Landau, L. Acta Physiochem. 1941, 14, 633. (23) Verwey, E. G. W.; Overbeek, J. T. G. The Theory of the Stability of Liophobic Colloids; Elsevier: Amsterdam: 1948. (24) Bergstro¨m, L. Adv. Colloid Interface Sci. 1997, 70, 125.
Glass Surface Forces in Aqueous PVAm Solutions
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Figure 2. Force normalized by radius as a function of surface separation across aqueous NaCl solutions. The solid lines represent calculated DLVO interaction under constant surface charge boundary conditions.
predicts should dominate the short-range force. Such a short-range repulsive force between glass or silica surfaces in aqueous solutions has been reported in numerous publications14,15,25-27 and often attributed to either dehydration of polar silanol groups (a hydration force)26 or to compression of short polysialic acid chains (a steric repulsion).14 The apparent surface potentials and area per surface charge extracted from the fitting were -63 mV and 90 nm2 in 10-4 M NaCl, and the corresponding values were -60 mV and 27 mm2 in 10-3 M NaCl. The surface charge density of glass clearly increases with increasing ionic strength. This is a consequence of the reduced free energy cost of creating the electrical double layer, which results in an increasing degree of dissociation of surface silanol groups. The surface potential values obtained are consistent with previously published data based on force and electrokinetic measurements on silica and glass substrates.25,28 Forces Between PVAm Layers in 0.1 mM NaCl. Addition of as little as 1 ppm of PVAm to the solution resulted in dramatic changes in the surface forces. The force-distance profiles for glass surfaces across a 1-ppm PVAm solution are shown in Figure 3. The long-range repulsive double-layer force observed before addition of the polyelectrolyte had disappeared 30 min after introducing PVAm to the solution. This demonstrates that the surfaces are nearly neutralized by the adsorbed polyelectrolyte. At separations below 30 nm an attractive force dominates the interaction. This attraction, which is substantially stronger than the van der Waals force at separations larger than 1-2 nm, is exponential and has a decay length of 6.6 nm (see inset). This decay length is much shorter than that expected for an attractive doublelayer force (30 nm), and we suggest that the measured force is due to bridging by extended tails. These forces are discussed further in the next section. The upper curve displayed in Figure 3 shows the interaction across the 1-ppm PVAm solution after 24 h of incubation. The bridging attraction is no longer present and the curve is more complex. At large separations between 30 and about 7 nm a very weak repulsive force emerges. The magnitude of the force (which is just above the detection limit) does not allow any reasonable DLVO fitting, but it is likely that additional adsorption during the prolonged incubation has led to a charge reversal even at this low bulk polyelectrolyte concentration. However, (25) Ducker, W. A.; Senden, T. J. Langmuir 1992, 8, 1831. (26) Chapel, J.-P. Langmuir 1994, 10, 4237. (27) Fro¨berg, J. C. PhD Thesis, Royal Institute of Technology, Stockholm, 1998. (28) Podgornic, R. J. Chem. Phys 1989, 91, 5840.
Figure 3. Force normalized by radius as a function of surface separation across 1 ppm PVAm solution also containing 0.1 mM NaCl; filled circles, after 30 min of incubation; open squares, after 24 h of incubation. The solid line represents the theoretical van der Waals interaction. The inset shows the attractive curve multiplied by -1 and plotted on a semilogarithmic scale.
at separations below 7 nm a steeply increasing steric repulsion is present which is overcome by an attraction at very short distance, evidenced by the inward step at about 0.8 mN/m. The fact that the steric force can be overcome shows that the additional repulsive force originates from just a few polyelectrolyte chains adsorbed in a more extended conformation than the rest rather than from the formation of a thick homogeneous layer. Similar force-distance curves have been reported to act between mica surfaces exposed to a dilute aqueous solution of the highly charged cationic polyelectrolyte poly((3methacrylamido)-propyl)trimethylammonium chloride for more than 12 h.1 The adsorption kinetics is rather slow in dilute polymer solutions. The first polyelectrolytes that reach the surface can easily adopt a flat conformation. However, the situation changes as the adsorption proceeds and the surface coverage approaches that needed to reach charge neutralization. Because of repulsive interactions with already adsorbed polyelectrolytes, the polyelectrolytes coming to the surface at this stage will find it more difficult to adopt a conformation where most of the segments are located close to the surface. Such an adsorption state would give rise to the type of force curve displayed in Figure 3. The forces acting between glass surfaces across a range of aqueous PVAm solutions are displayed in Figure 4, together with fits of DLVO theory. The system was incubated for at least 6 h with the surfaces far apart (1-2 mm) after each stepwise change in concentration. Apparently, the addition of PVAm to a concentration above 1 ppm led to a well-pronounced recharging of the surfaces. The surface potential and surface charge density increase with increasing polyelectrolyte concentration, indicating increasing adsorption at each new PVAm concentration and some adsorption even to a positively charged surface. The measured forces are in good agreement with DLVO theory at all separations (even though it is likely that bridging forces contribute to the strong adhesion force between the surfaces). The fitted apparent decay length is consistent to within 10% with the Debye length calculated from the corresponding 1:1 electrolyte concen-
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Figure 4. Force normalized by radius as a function of surface separation across PVAm solutions also containing 0.1 mM NaCl. The solid lines represent calculated DLVO interactions under constant surface charge boundary conditions. The fitting parameters are summarized in Table 1.
tration. At 10-ppm bulk concentration and 100% charge density, the PVAm counterion concentration would be about 0.22 mM, which implies that the polyelectrolyte and its counterions do not contribute significantly to the double-layer decay length. This is because the polyion is depleted from the region between the surfaces due to the strong electrostatic repulsion between the positively charged PVAm-coated surface and the polyion. This is consistent with the findings in other works concerned with forces in polyelectrolyte solutions.8 The concentrations are not sufficient, however, to give rise to a depletion force.29 The parameters obtained from fits of the DLVO theory to the measured forces are summarized in Table 1. Forces Between PVAm Layers in 1 mM NaCl. When PVAm is adsorbed from solutions containing the higher background electrolyte concentration (1 mM), the resulting forces between the adsorbed layers differ somewhat from those reported above. The force measured between the glass surfaces in the presence of 1 ppm PVAm and 1 mM NaCl after 30 min and 24 h of incubation are shown in Figure 5. After 30 min the force is purely attractive and has a decay length of about 4.4 nm clearly seen on a semilogarithmic scale (the inset of Figure 5). This is significantly less than the 9.6 nm expected for an attractive double-layer force. The only difference from the results obtained at the lower ionic strength is the shorter decay length of the attraction. After 24 h equilibration in the 1-ppm PVAm solution with 1 mM ionic strength the surfaces have become positively charged and the forces obey DLVO predictions at any separation. This is clearly different from the case at lower ionic strength and is discussed later. Figure 6 shows the forces in 1-10 ppm PVAm solutions, measured after allowing 6 h equilibration time after each change in polyelectrolyte concentration. The corresponding DLVO fits are also shown. It can be seen that for concentrations above 1 ppm the force at separations below about 5 nm is somewhat more attractive than predicted by DLVO theory. Adhesion Forces. The adhesion force normalized by radius is presented in Table 1. Generally, the adhesion (29) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Accademic Press: New York, 1991.
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force increases with increasing polyelectrolyte concentration. It is thus not largest at the charge neutralization point but increases with the degree of overcompensation. One important reason for the adhesion force is bridging. Once the surfaces are in contact the polyelectrolytes will have the possibility to rearrange and be attracted to both surfaces. How quickly this rearrangement takes place is not known. From similar studies to ours, which used polyelectrolyte-coated mica surfaces and the interferometric surface force apparatus, we know that the adhesion between polyelectrolyte-coated surfaces is independent of the contact time in the range of several minutes to about 1 h. Charge Regulation. The net charges of the glass surfaces are rather low with an area per charge of 90 and 27 nm2 in 0.1 and 1 mM NaCl, respectively. Clearly, the distance between charges on the glass surface is much larger than the distance between charges along the polyelectrolyte chain. Hence, one may then believe at first glance that each polyelectrolyte is electrostatically attached to the surface with only a few segments. This is not the case, however. The reason is that both the surface and the polyelectrolyte can regulate their charges in such a way that the net charge of the surface and the adsorbed layer becomes small. The surface silanol groups can release a proton to create a negatively charged group next to a positively charged polymer segment. Such a surface charge regulation is well-known from adsorption studies of cationic surfactant30 and polyelectrolytes31 onto silica. Similarly, it is well documented31 that it is possible for the primary ammonium groups of the polyelectrolyte to release a proton to reduce its charge next to an uncharged surface site. Hence, because of the charge-regulating capability of the surface and polyelectrolyte, most of the polyelectrolyte segments will be attracted to the surface and a flat adsorbed layer structure is expected. Surface force measurements32 using glass surfaces and cationic polyelectrolytes with permanently charged groups show that these polyelectrolytes also adsorb in a flat conformation, indicating that the charge-regulating ability of the surface is most important in this case. We further note that charge regulation also occurs when polyelectrolytes adsorb to mica surfaces,4,10,33 but in this case the polyelectrolyte replaces adsorbed inorganic ions at the surface. Buildup of the Adsorbed Layer. The adsorption is rather rapid until close to charge neutralization, as evidenced by the fact that charge neutralization is reached after less than 30 min in the 1-ppm solution. Next, a slower adsorption onto some unoccupied spots follows. The chains adsorbing to these spots are forced to adopt a conformation extending further into solution, because initially only part of the molecule contacts the glass surface. This situation results in the forces observed after 24 h adsorption in the 1-ppm PVAm solution at 0.1 mM ionic strength. Presumably, even slower changes in the adsorbed layer occur to allow all adsorbed polymers to adopt a flat conformation on the surface. Such force is not seen at slightly higher concentrations, so these changes clearly are facilitated by a higher bulk polyelectrolyte concentration, because it is considerably easier to replace one polymer on the surface with another polymer than it is to desorb the polymer.34 (30) Wa¨ngnerud, P.; Berling, D.; Olofson, G. J. Colloid Interface Sci. 1995, 169, 365. (31) Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (32) Dedinaite, A.; Poptoshev, E.; Claesson, P. M., to be published. (33) Rojas, O. J.; Claesson, P. M.; Muller, D.; Neuman, R. D. J. Colloid Interface Sci. 1998, 205, 77. (34) Cohen Stuart, M.; Fleer, G. J. Annu. Rev. Mater. Sci. 1996, 26, 463.
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Table 1. Interaction Parameters Extracted from Fitting DLVO Theory to the Measured Forces Together with the Measured Adhesion Force PVAm concentration (ppm)
NaCl concentration (mmol/L)
surface charge density (mC/m2)
surface potential (mV)
fitted apparent decay length (nm)
adhesion force (mN/m)
0 1 2 5 10 0 1 2 5 10
0.1 0.1 0.1 0.1 0.1 1 1 1 1 1
1.8 0.4 0.5 0.7 6.0 2.3 3.3 4.0 4.9
-63 16 23 28 -60 30 40 47 53
30.4 32.5 32.5 33.5 9.6 9.4 9.2 9.2 9.0
0 13.4 22.4 23.3 23.7 0 11.9 19.7 19.9 22.4
Figure 5. Force normalized by radius as a function of surface separation across 1-ppm PVAm solution also containing 1 mM NaCl; filled squares, after 30 min of incubation; open squares after 24 h of incubation. The solid line represents theoretical van der Waals interaction.The inset shows the attractive curve multiplied by -1 and plotted on a semilogarithmic scale.
We propose that this is the reason a steric repulsion is observed only at the lowest PVAm concentration in 0.1 mM NaCl. The rearrangement in the layer is also facilitated by a reduced polyelectrolyte-surface affinity, e.g., due to a higher ionic strength. This may explain why the steric repulsion is seen after 24 h across the 1-ppm solution at 0.1 mM ionic strength but not at 1 mM ionic strength. We note that the changes in the adsorbed amount corresponding to the change in force profile with time and polyelectrolyte concentration are likely to be minor, and it is not certain that it could be picked up by, e.g., ellipsometric measurements. These changes nevertheless have a large influence on the measured interaction because, at a given ionic strength, the magnitude of the double-layer force is determined by the excess charges (Table 1), and a steric repulsion caused by compression of a few extending tails can easily dominate the forces close to the charge neutralization point. Recharging of the Surfaces. When comparing the forces in 0.1 mM and 1 mM NaCl one immediately notices that the recharging of the glass surfaces is much more pronounced (especially in very dilute PVAm solutions) at the higher ionic strength, indicating that the polyelectrolyte adsorption increases with the inert salt concentration. That this indeed is the case was shown in an experiment where the layer initially was adsorbed from a 10-ppm PVAm solution in 0.1 mM NaCl, and then the
Figure 6. Force normalized by radius as a function of surface separation across PVAm solutions also containing 1 mM NaCl. The solid lines represent calculated DLVO interactions under constant surface charge boundary conditions. The fitting parameters are summarized in Table 1.
Figure 7. Force normalized by radius as a function of surface separation for the case when the layer was preadsorbed from low salt concentration and then the ionic strength was increased without PVAm (lower curve) and with PVAm (upper curve) present in the solution.
solution was replaced with one containing only 1 mM NaCl. The measured force curve is shown in Figure 7. The decay length was 9.7 nm in accordance with the electrolyte
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concentration. The apparent surface potential and area per charge were 35 mV and 60 nm2, respectively. When 10-ppm PVAm solution was introduced in the measuring chamber the magnitude of the double-layer force increased considerably, resulting in an apparent surface potential and area per charge of 46 mV and 40 nm2, respectively. The fact that the adsorption increases with increasing ionic strength indicates that non-Coulomb forces also contribute to the adsorption energy. A similar sensitivity to the electrolyte concentration has been observed for the adsorption of the cationic polyelectrolyte poly((3-methacrylamido)-propyl)trimethylammonium chloride on mica,3 where it was demonstrated that adsorption from a 10ppm polyelectrolyte solution in 0.1 mM KBr resulted in a close-to-neutral surface, whereas adsorption from a 10ppm 1 mM KBr solution resulted in a very strong electrostatic double-layer force. The adsorbed layer also became somewhat thicker with increasing ionic strength. The Nature of the Long-range Attractive Force. The forces measured between the nearly uncharged surfaces obtained after adsorption of PVAm from a 1-ppm solution for 30 min were purely attractive and exponentially decaying with distance. The decay length decreased with increasing ionic strength from 6.6 nm in 0.1 mM NaCl to 4.4 nm in 1 mM NaCl. The distance dependence of this force rules out both van der Waals forces and attractive double-layer forces as the origin, whereas the low polymer concentration rules out depletion force. Let us consider two other mechanisms: interactions between net uncharged surfaces having net positive and net negative patches, and bridging attraction. The attraction between the patchy surfaces would arise from a correlation between positive patches on one surface and negative patches on the other. Provided the adsorbed molecules are mobile on the time scale of the experiment (an unlikely situation for adsorbed polyelectrolytes) one would expect an attraction that decays with half the Debye length,28,35 i.e., 15 nm in 0.1 mM and 4.8 nm in 1 mM NaCl. The distance dependence of the measured attraction is roughly consistent with the predictions at the higher concentration but not at the lower one. Bridging forces between surfaces coated with uncharged polymers arise because segments of the polymer adsorb to both surfaces. The bridging polymer chains gain entropy when the surfaces come closer together, and this is the primary mechanism for the attraction. For adsorbed polyelectrolytes the situation is similar except that the chains do not need to bind directly to both surfaces. Because of the long-range nature of electrostatic forces it is sufficient for the polyelectrolyte to have segments close to both surfaces to be attracted to both of them and to give rise to an entropic bridging force.1 Monte Carlo simulations have shown that the length of the extended tails has to be at least half the distance between the surfaces to give rise to a bridging attraction of the type described above.1,36,37 At higher ionic strength the electrostatic potential is screened and the range of the bridging attraction is expected to decrease, consistent with the results presented here. The fact that the polyelectrolytecoated surfaces are net uncharged and no double-layer (35) Attard, P. J. Phys. Chem. 1989, 93, 6441. (36) ) Åkesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91, 2461. (37) Miklavic, S. J.; Woodward, C.; Jo¨nsson, B.; Åkesson, T. Macromolecules 1990, 23, 4149.
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force is observed means that there is a zero net potential at the midpoint between the surfaces when they are far apart. However, once the tails from the polyelectrolytes adsorbed on one surface penetrate into the layer associated with the other surface they will experience a net electrostatic attraction to the other surface and the bridging force will develop. Hence, if bridging is the reason for the long-range attraction, as we suggest, we can conclude that some tails extend at least 15 nm from the surface in 0.1 mM NaCl (Figure 3) and at least 7 nm from the surface in 1 mM NaCl. The Monte Carlo simulations1 also showed that for polyelectrolyte-coated surfaces a strong net attraction, due to bridging, is expected only close to charge neutralization. This is consistent with our experimental results. The simulations indicate that the bridging force between polyelectrolyte-coated surfaces decays roughly exponentially with surface separation. It is tempting to explain the slight shift in the force maximum toward larger separations observed when PVAm was adsorbed from 1 mM NaCl solutions (see Figure 6) as also being caused by a bridging attraction. However, we avoid drawing this conclusion because of limitations in the theoretical model used for the double-layer interaction. For instance, the Poisson-Boltzmann theory neglects charge-charge correlation effects that are important for polyelectrolyte systems. Further, all theoretical curves were fitted assuming that the plane of charge was located at the hard-wall contact, whereas in reality the charges in the polyelectrolyte layer are distributed in a more diffuse manner. Conclusions The noninterferometric surface force apparatus, using versatile glass surfaces is a useful tool for studying adsorption behavior and interaction forces between surfaces bearing adsorbed polyelectrolyte layers. Cationic PVAm adsorbs strongly onto negatively charged glass surfaces, causing a considerable charge reversal, dependent on NaCl concentration. The measured forces are well described by DLVO theory with decay length consistent with the Debye length calculated from the electrolyte concentration, which indicates that the polyion and its counterion do not contribute to the screening of the double-layer force. Two notable effects display strong time dependence. First, close to the charge neutralization point a long-range, exponentially decayng bridging attraction is present with the decay length decreasing with an increase of salt concentration. This effect disappeared with time as the surfaces became more highly charged and bridging became unfavorable. Measurements after such a short equilibration time would be impossible using conventional interferometric surface force apparatus. The second effect is the steric repulsion associated with adsorption to a nearneutral surface where solution conditions affect the ability of the polyelectrolyte to rearrange into an “equilibrium” conformation. Acknowledgment. E.P. gratefully acknowledges financial support from Bo Rydins Foundation for Scientific Research. The polyvinylamine sample was kindly provided by Dr. Ralf No¨renberg, BASF AG, Ludwigshafen, Germany. Thomas Ederth and Mikael Kjellin are acknowledged for the help with instrumentation. LA990322K
