Kinetic regimes of polyelectrolyte exchange between the adsorbed state and free solution
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We studied the exchange between the adsorbed state and free solution when polyelectrolyte chains, adsorbed to a solid surface of opposite charge, were displaced by chains of higher charge density. Metastable states of surface composition were extremely long-lived (<23 days). The system was a family of poly(1,4 vinyl)pyridines (PVP) with different fractions of charged segments (14%, 48%, and 98% quaternized and the same degree of polymerization); samples were exposed sequentially from aqueous D2O solution to a single silicon oxide substrate at pH where the surface carried a large negative charge (pH=9.2 or 10.5). Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). As a first conclusion, we found charge of adsorbed polymer to be conserved during extended exchange times, suggesting that charge at the surface (not mass adsorbed) regulated the dynamics of adsorption and desorption. Except at the highest ionic strength charge of polymer at the surface during the displacement process considerably exceeded that for the initially-adsorbed layer, suggesting an intermediate state in which newly-adsorbed chains were more extended from the surface and not yet equilibrated in their conformations. Second, we concluded that desorption was the rate-limiting step in adsorptiondesorption, since the desorption rate responded more to changes of ionic strength than did the adsorption rate onto previously-adsorbed polymer. Ionic strength appeared to modulate the intensity of sticking to the surface. Third, we found that the initial stages of desorption obeyed a simple functional form, exponential in the square root of elapsed time. This is conclusively slower than a first-order kinetic process and suggests that desorption in this polyelectrolyte system was diffusion-controlled during the initial stages. It is the same functional form observed for flexible polymers in nonpolar solvents. Fourth, we concluded that at relatively low concentration of salts desorption proceeded in two stages; one subpopulation of adsorbed chains desorbed relatively quickly, with a rate exponential in the square root of time, and a second subpopulation was so much slower to be desorb that it appeared to be kinetically frozen at the surface. The higher the ionic strength, the less the polymer was kinetically frozen and this effect disappeared entirely for the highest ionic strength. The interpretation that the kinetically-frozen states reflected conformational heterogeneities within the adsorbed layer was supported by direct measurements of the dichroic ratio of adsorbed pyridinium rings. Finally, a new kinetic regime was observed at the highest salt concentrations, in which the exchange was inhibited by worsened solubility of the displaced molecules. It is significant that this regime began at salt concentrations significantly below the point of bulk insolubility. Since most organic polyelectrolytes may be considered to be a copolymer of polar charged units and hydrophobic uncharged units, this effect is expected to be general.
