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Forces between Silica Surfaces in Aqueous Solutions of a Weak Polyelectrolyte Simon Biggs* and Andrew D. Proud Department of Chemistry, The University of Newcastle, University Drive, Callaghan N.S.W. 2308, Australia Received May 27, 1997. In Final Form: September 29, 1997X The forces between negatively charged silica surfaces in the presence of a weak polyelectrolyte, poly (2-vinylpyridine), were measured as a function of polymer concentration, salt concentration, solution pH, and surface collision rates. The solubility of the polymer is highly dependent on the solution pH; that is, when the molar concentration of solution protons is equivalent to the molar concentration of pyridine groups the polymer is >70% protonated and is highly soluble. As the pH increases, the degree of protonation decreases and the polymer becomes insoluble and precipitates from solution. At low polymer concentrations, low salt concentration, and a low pH, the polymer adsorbs strongly with an essentially flat conformation. The forces during compression are well described by DLVO (Derjaguin-Landau-Verwey-Overbeek) theory with no steric forces apparent. During decompression, the adhesive forces are much greater than those between the bare silica surfaces, indicating a strong bridging between the surfaces after contact and a sub-monolayer coverage. At higher polymer concentrations and/or salt levels, a steric interaction is seen during the compression runs and a significant decrease in the adhesion is observed. Both of these results imply a more expanded conformation of the polymer at the surface and a higher surface coverage. Increased collision rates between the surfaces give rise to an increase in the magnitude of the observed steric forces. Such an increase is attributed to an increased apparent stiffness of the chains as the compression rate increases. Measurements in a poor solvent resulted in the appearance of a shallow long-range intersegmental attractive force.
Introduction Polyelectrolytes have been used for many centuries to control the stability of colloidal dispersions. For example, the ancient Egyptians are known to have used Gum Arabic to stabilize their inks.1 Despite this long history of exploitation, it is only comparatively recently that detailed and controlled studies on such systems have been attempted.2 In modern industry, polyelectrolytes primarily find application in the areas of water and sewage treatment,3 paper making,4 food products,5 medical science,6 and the mining industry.7 Such a broad base of applications has led to a large volume of research over the last 30 to 40 years. During this time, much of our current level of understanding about the role of polyelectrolytes in determining colloid stability has been constructed. Deciphering the role of any polymer in a colloidal system relies primarily on investigating the conditions under which the polymer will be adsorbed to the surface of interest. It is then important to gain information about the adsorbed amount and the conformation at that interface.8 The primary method employed in the majority * Author to whom all correspondence should be addressed: Tel, +2 49 215481; fax, +2 49 215472; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Hunter, R. J. Foundations of Colloid Science, Vol.1.; Oxford: Oxford 1989. (2) Luckham, P. F. Curr. Opinion Colloid Interface Sci. 1996, 1, 39. (3) Kulicke, W-M.; Kniewske, R.; Ho¨rl, H.-H. Angew. Makromol. Chem. 1980, 87, 195. (4) Encyclopaedia of Polymer Science and Engineering; Mark, H. F.; Bikales, N. M.; Overberger, C. G.; Menge, G., Eds.; Wiley: New York, 1987; Vol. 10, p 780. (5) Industrial Gums: Polysaccharides and their Derivatives; Whistler, R. L.; BeMiller, J. N., Eds.; Academic: San Diego, 1993. (6) Williams, H. R.; Fletcher, D. S.; Elbert, E. E.; Lin, T. Y. Biochim. Biophys. Acta 1983, 757, 69. (7) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Hanser: Munich, 1994. (8) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993.
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of studies is based on simple determinations of adsorption isotherms and phase diagrams from standard stability tests. Whereas such measurements have an undoubted value, they give no direct information about the polymer conformation at the interface. Techniques such as photon correlation spectroscopy9 and neutron scattering10 have been used previously to gain direct conformation data about the extension of polymers from the interface under various solution conditions. The development of the surface forces apparatus (SFA)11,12 and later the atomic force microscope (AFM)13 has enabled the direct study of how different solution conditions can affect polymer adsorption, adsorbed layer conformations, and intersurface forces in colloidal systems. Because it is these forces that ultimately determine the stability or otherwise of any dispersion, such measurements offer an extremely important route to understanding polymer stabilization of colloids. Over the last 20 years, there have been many reports of the role of factors such as polymer type, solvency, and molecular weight on interaction forces using the SFA.14-16 However, the great majority of these reports have been concerned with neutral polymers. Studies of polyelectrolytes are much less extensive.17-20 The adsorption of a polyelectrolyte will obviously be heavily influenced by the surface charge of the adsorbent. (9) Cosgrove, T. J. Chem. Soc., Faraday Trans. I 1990, 86, 1323. (10) Cohen-Stuart, M. A.; Waajen, F. H. W. H.; Cosgrove, T.; Crowley, T. L.; Vincent, B. Macromolecules 1984, 17, 1825. (11) Tabor, D.; Winterton, R. H. S. Proc. Royal Soc. 1969, A 312, 435. (12) Israelachvili, J. N.; Adams, G. E. JCS Faraday I 1978, 74, 975. (13) Binnig G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (14) Klein, J.; Luckham, P Macromol. 1984, 17, 1041. (15) Klein, J.; Luckham, P Macromol. 1985, 18, 721. (16) Israelachvili, J. N.; Tirrel, M.; Klein, J.; Almog, Y. Macromol. 1990, 17, 204. (17) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92, 6044. (18) Claesson, P. M.; Dahlgren, M. A. G.; Eriksson, L. Colls. Surfs. 1994, 93, 293. (19) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 166, 343. (20) Claesson, P. M.; Ninham, B. Langmuir 1992, 8, 1406.
© 1997 American Chemical Society
Silica Surfaces in Solutions of a Polyelectrolyte
An oppositely charged surface will tend to promote a rapid strong adsorption of a polyelectrolyte, whereas a surface carrying the same charge will tend to repel the polymer. Models of adsorbed layer conformations under differing solution conditions and at different polymer and surface charge densities have also been developed over the last few years.21,22 The main predicted features of these theoretical studies have largely been observed directly with force experiments using the SFA and cationic polymers.17-20 The main features of interest are that the adsorbed layers are essentially flat at low salt concentrations, and the adsorbed layer builds up until the surface charge is neutralized, whereupon further adsorption is not favored. The net result is that the bare surface charge is initially neutralized before a slight overcompensation resulting in a weak charge reversal. At higher salt concentrations, the charges are screened, which promotes a more coiled polymer conformation but also reduces polymer-surface attractions. The net result in most observed cases is a buildup of adsorbed polymer, resulting in an increased steric repulsion. In the present paper, we present the results of a recent study of the forces of interaction between silica surfaces in the presence of a weak polyelectrolyte poly(2-vinylpyridine) (P2VP). Use is made of the AFM technique for colloidal force measurements developed independently by Butt23 and by Ducker and co-workers.24 P2VP is interesting to study because its solubility is easily controlled through the solution pH. At low pH (10-3 M, a long-range electrostatic repulsion was not observed in any measurements here for separations of >25-30 nm. At these separations, the intersegment attractive forces operating will outweigh any longrange electrostatic repulsion and so no repulsion is observed. The decompression data indicate that for both P2VP concentrations there is a shallow adhesive minimum extending out around 20-30 nm before the surfaces separate spontaneously. At pH 6, there is a decrease in the magnitude These adhesive minima as the polymer concentration increases. This decrease in magnitude is most probably due to the increase in the adsorbed amount. Israelachvili et al.16 observed that the adhesion between polymer-coated surfaces in poor solvents may be due to a combination of both bridging and intersegmental attraction. The relative magnitudes of these two adhesive forces, however, are very hard to quantify. Nevertheless, it seems likely that as the adsorbed amount increases, the bridging component will decrease and the intersegmental term will increase. In this poor solvent, the data indicate a significant extension of the adsorbed layer and a fundamental change in the overall interaction potential. The results imply the existence of a shallow long-range intersegment attractive interaction between the surfaces. Once again, this result should be compared with the previous study of Marra and Hair17 in which the polymer was preadsorbed at a low pH and then the system was neutralized with an equivalence of NaOH. The data from their study, like ours, showed an increased steric interaction, but Marra and Hair also saw a long-range electrical doublelayer interaction. The latter interaction was explained by the lower binding efficiency of the polymer as the electrostatic interactions decreased and as the competition from Na+ ions for surface sites increased. The net result was an increase in the number of loops and tails. It was argued that this adsorbed layer was too dilute to interfere with the double layer that arose from the liberated negatively charged mica surface sites. However, it should also be recognized that a neutralized P2VP solution will have a significantly higher adsorption density (Figure 1) because there is a strong driving force for phase separation. An increased adsorbed amount, even at low charge densities on the polymer, would lead to increased levels of charge compensation. Marra and Hair17 argued that in their system the increased adsorption was hindered by the osmotic pressure gradient caused by the double layer through which the uncharged polymer had to pass. Thus, the polymers were effectively depleted from the surface.
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Figure 8. Normalized force versus log (apparent separation) for the compressive interaction between a silica sphere (radius 10 µm) and a silica plate at a pH of 3.7 with an added electrolyte (NaCl) concentration of 10-3 M and P2VP concentration of 10 ppm. Data were collected at four scan rates and at a single scan size of ( 120 nm Key: (O) 0.1 Hz; (4) 0.5 Hz; (0) 1.0 Hz; (b) 2.0 Hz.
In the system examined here, these problems were not apparent, perhaps because of the generally lower surface potentials for the silica surfaces that would tend to give a weaker double-layer interaction, and hence a smaller osmotic force. Alternatively, these problems may not have been apparent simply because the binding affinity of the polymer to the silica is higher. Thus, there is a bigger driving force for surface adsorption and more is adsorbed, giving increased charge compensation. Whatever the reason for this charge compensation, these data show clearly the presence of a shallow long-range attractive force. Intersegmental forces of this type have not been demonstrated previously with an AFM. Scan Rate-Dependent Polymer Forces. Data for the compression force-separation curves for 10 ppm P2VP in 1 mM NaCl at various scan rates (particle-surface collision rates) are shown in Figure 8 as normalized force versus logarithm of the apparent separation. These data were collected at a single scan size of (120 nm. The scan rates indicated correspond to minimum and maximum scan rates of 0.024 and 0.48 µm s-1, respectively. For a steric layer size of ≈5 nm, this range of scan rates will correspond to the layer being compressed in a time of between 10 and 208 ms. The compression force-separation data indicate that as the rate is increased, the steric force observed at close separations increases in magnitude. This phenomenon has been observed recently by us for adsorbed poly(vinylpyrrolidone) layers.29 The increasing steric repulsion was attributed to a decreased possibility for segmentsegment rotation (or chain relaxation) as the time scale of compression gets faster, which causes an increased effective rigidity of the polymer chains and hence an increased force. Previously, as well as here, the longer range components of the compression did not have any
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Figure 9. Normalized force versus apparent separation for the decompressive interaction between a silica sphere (radius 10 µm) and a silica plate at a pH of 3.7 with an added electrolyte (NaCl) concentration of 10-3 M and P2VP concentration of 10 ppm. Data were collected at four scan rates and at a single scan size of ( 120 nm. Key: (O) 0.1 Hz; (4) 0.5 Hz; (0) 1.0 Hz; (b) 2.0 Hz.
significant rate dependency. Also, it should be noted that at these scan rates and with these probe sizes, hydrodynamic forces are expected to be insignificant. Therefore, rate-dependent forces such as these can be ascribed to conformation effects of the adsorbed polymer layers. The decompression force-separation curves shown in Figure 9 also exhibit a clear trend; that is, the scan rate
Biggs and Proud
increases, the adhesive force decreases. One possible explanation for this relationship is that a finite time is required for the formation of polyelectrolyte bridges and, at increased scan rates, the surfaces are in contact for a shorter time period and may become separated before substantial bridging can occur. In our previous study,29 it was also found that a decreasing time between compression and decompression led to a decreasing incidence of bridging interactions and, consequently, the adhesive force was found to decrease. Conclusions. The interaction forces operating between colloidal particles in the presence of the weak polyelectrolyte P2VP are strongly influenced by the solution pH. This polymer is highly soluble at any pH < 5, where it is present as a highly charged rigid rod in solution. Above this pH, the solution conformation of the polymer is that of a highly compact collapsed globular coil. This differing solubility strongly affects the adsorption properties of P2VP, both in terms of the adsorbed amount and surface conformation. Changes in the adsorbed amount of P2VP have been measured directly and have also been inferred from direct measurements with an AFM with a colloid probe. The two sets of data both indicate a charge reversal of the silica between 10 and 20 ppm polymer. Conformational changes for the adsorbed polymer can be inferred from the measured interaction forces. At a pH of 6, the adsorbed amount increases greatly over that seen at a pH of 3.8. The polyelectrolyte, which is only poorly soluble at this pH, is present in solution in a globular form. Direct force data indicate a weak intersegmental attraction between the surfaces under these conditions, which is attributed to the precipitation of globular polymer on the surface. Acknowledgment. Acknowledgments. We acknowledge the financial assistance of the Australian Research Council through the provision of a Large Research Grant (A29601539). LA970548S
