KINETIC ASPECTS IN THE ADSORPTION OF POLYACRYLIC-ACID SALTS ONTO BATIO3

The adsorption kinetics of two polyacrylic acid (PAA) salts, with different molecular weights, on BaTiO3 were studied from aqueous solutions. The pH values of the solutions were between 8.5 and 10.2: which is below the isoelectric point of BaTiO3. The adsorbed amount and the molecular weight fractionation were determined by size exclusion chromatography after mixing times varying from 15 min to 24 days. They initially changed rapidly (within 2 days) and then more gradually over longer periods of time. Changes occurred for up to 94 days at low KNO3 concentrations, The kinetics of the fractionation were highly dependent on the chain length, and the salt concentration. The chain-length effects were much more pronounced than would be expected on the basis of differences in diffusion rates. Theoretical calculations predict an electrostatic barrier between the negatively charged PAA covered surface and the PAA chains in the solution. This barrier strongly increases with the length of the PAA chains in the solution. Accordingly, the probability of reaching the surface decreases and the kinetics slows down with increasing chain length. At higher salt concentrations the barrier is lower, and a faster adsorption of the long chains is found. Owing to the presence of the barrier, the length of the chains that displace the initially adsorbed short chains is limited. An adsorption model, in which the packing on the surface and the rearrangement of the adsorbed chains depends on the salt concentration, is introduced. This model is used to discuss further results.