POLYELECTROLYTE ADSORPTION ONTO AN INITIALLY-BARE SOLID-SURFACE OF OPPOSITE ELECTRICAL CHARGE

We contrast the adsorption, over a wide range of pH and ionic strength , of polyelectrolyte chains with different fractions of charged segmen ts but similar degree of polymerization. The system was a cationic pol ymer, poly(l,4 vinyl)pyridine (PVP), with 14%, 48%, and 98% quaternize d repeat units, adsorbed from aqueous solution (D2O or H2O) onto a sin gle silicon oxide substrate at 25 degrees C. Measurements were based o n Fourier transform infrared spectroscopy in attenuated total reflecti on (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 th e surface was electrostatic. The amount adsorbed of charged (quaterniz ed) PVP segments was nearly the same regardless of the overall fractio n of charged segments in the chain. In addition, polymer adsorption ap peared to enhance the dissociation of silanol groups on the solid surf ace. In a second phase of this study, the ionic strength was varied sy stematically under conditions of high negative surface charge density (high pH), focusing on 98% quaternized PVP. Strong chemical specificit y was found; the polyelectrolyte was insoluble in KI above a low salt concentration, but soluble in NaCl, signifying that the anions, Cl- an d I-, competed with the negatively-charged surface for association wit h the polyelectrolyte. At the same time, the cations, Na+ and K+, comp eted with the polyelectrolyte for access to the Limited surface area. The mass adsorbed increased strongly with increasing salt concentratio n and, for polymer in aqueous NaCl, passed through a maximum with subs equent 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), indicati ng 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 stat e 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 change of Na+ and a monomeric analog of t he PVP repeat unit, the 1,4-dimethylpyridinium ion (P+). A tentative q uantitative estimate of the effective surface sticking energy of P+ re lative to Na+ ions indicated a decrease from 7k(B)T in low-ionic-stren gth buffer solution to 4.5k(B)T 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. (C) 1998 American Institute of Ph ysics. [S0021-9606(98)51840-5].