Polyelectrolyte adsorption onto an initially-bare solid surface of opposite electrical charge
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We contrast the adsorption, over a wide range of pH and ionic strength, of polyelectrolyte chains with different fractions of charged segments but similar degree of polymerization. The system was a cationic polymer, poly(1,4 vinyl)pyridine (PVP), with 14%, 48%, and 98% quaternized repeat units, adsorbed from aqueous solution (D2O or H2O) onto a single silicon oxide substrate at 25C. Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). In the first phase of this study, we varied the surface charge density by changing pH and showed that attraction of PVP to the surface was electrostatic. The amount adsorbed of charged (quaternized) PVP segments was nearly the same regardless of the overall fraction of charged segments in the chain. In addition, polymer adsorption appeared to enhance the dissociation of silanol groups on the solid surface. In a second phase of this study, the ionic strength was varied systematically under conditions of high negative surface charge density (high pH), focusing on 98% quaternized PVP. Strong chemical specificity was found; the polyelectrolyte was insoluble in KI above a low salt concentration, but soluble in NaCl, signifying that the anions, Cl and I, competed with the negatively-charged surface for association with the polyelectrolyte. At the same time, the cations, Na+ and K+, competed with the polyelectrolyte for access to the limited surface area. The mass adsorbed increased strongly with increasing salt concentration and, for polymer in aqueous NaCl, passed through a maximum with subsequent decrease, reflecting a greater abundance of loops and tails at intermediate ionic strength and ultimately complete desorption of the chains when the salt concentration was very high. The maximum in mass adsorbed occurred at very high ionic strength (1 molar NaCl), indicating competitive adsorption of Na+ with charged segments of the polymer. Direct measurements of the infrared dichroism of pyridinium rings of the adsorbed PVP confirmed the presence of a relatively flattened state at low ionic strength and nearly isotropic orientation otherwise. In the third phase of this study, we studied the competitive adsorption to surfaces of high negative charge of Na+ and a monomeric analog of the PVP repeat unit, the 1,4-dimethylpyridinium ion (P+). A tentative quantitative estimate of the effective surface sticking energy of P+ relative to Na+ ions indicated a decrease from 7kBT in low-ionic-strength buffer solution to 4.5kBT in 0.5 M NaCl. These numbers appear to exceed the weak-adsorption limit in which facile equilibration of the adsorbed layer should be expected.
