KINETIC REGIMES OF POLYELECTROLYTE EXCHANGE BETWEEN THE ADSORBED STATE AND FREE SOLUTION

We studied the exchange between the adsorbed state and free solution w hen polyelectrolyte chains, adsorbed to a solid surface of opposite ch arge, were displaced by chains of-higher charge density. Metastable st ates of surface composition were extremely long-lived (>2-3 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 s ame degree of polymerization); samples were exposed sequentially from aqueous D2O solution to a single silicon oxide substrate at pH where t he surface carried a large negative charge (pH=9.2 or 10.5). Measureme nts were based on Fourier transform infrared spectroscopy in attenuate d total reflection (FTIR-ATR). As a first conclusion, we found charge of adsorbed polymer to be conserved during extended exchange times, su ggesting that charge at the surface (not mass adsorbed) regulated the dynamics of adsorption and desorption. Except at the highest ionic str ength charge of polymer at the surface during the displacement process considerably exceeded that for the initially-adsorbed layer, suggesti ng an intermediate state in which newly-adsorbed chains were more exte nded from the surface and not yet equilibrated in their conformations. Second, we concluded that desorption was the rate-limiting step in ad sorption-desorption, since the desorption rate responded more to chang es of ionic strength than did the adsorption rate onto previously-adso rbed polymer. Ionic strength appeared to modulate the intensity of sti cking to the surface. Third, we found that the initial stages of desor ption obeyed a simple functional form, exponential in the square root of elapsed time. This is conclusively slower than a first-order kineti c process and suggests that desorption in this polyelectrolyte system was diffusion-controlled during the initial stages. It is the same fun ctional form observed for flexible polymers in nonpolar solvents. Four th, we concluded that at relatively low concentration of salts desorpt ion proceeded in two stages; one subpopulation of adsorbed chains deso rbed 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 i onic strength, the less the polymer was kinetically frozen and this ef fect disappeared entirely for the highest ionic strength. The interpre tation that the kinetically-frozen states reflected conformational het erogeneities within the adsorbed layer was supported by direct measure ments of the dichroic ratio of adsorbed pyridinium rings. Finally, a n ew kinetic regime was observed at the highest salt concentrations, in which the exchange was inhibited by worsened solubility of the displac ed molecules. It is significant that this regime began at salt concent rations 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 expecte d to be general. (C) 1998 American Institute of Physics. [S0021-9606(9 8)51940-X].