Molecular weight fractionation in the adsorption of polyacrylic acid salts onto BaTiO 3

Abstract

The molecular weight fractionation resulting from the adsorption of polyacrylic acid (PAA) salts onto BaTiO 3 depends on the molecular weight distribution (MWD) of the PAA salt. Preferential adsorption of the longest chains is found if the entire MWD is below a certain chain length. If the MWD exceeds this limit while still including a sufficient amount of smaller chains, preferential adsorption of an intermediate molecular weight fraction occurs. The effect is quite pronounced for PAA salts of relatively low molecular weight (<20 000). Adsorption over a wide molecular weight range but with a higher proportion of short chains is observed if the MWD of the PAA salt is mainly above the aforementioned limit. The molecular weight fractionation can be understood using a sequential adsorption process in which the smallest chains of the PAA salt tend to adsorb first. These initially adsorbed chains overcompensate for the positive surface charge and thereby generate an electrostatic barrier to the PAA salt chains in the solution. The height of this barrier depends (at a constant charge of the PAA salt covered surface) on the total charge on the polyelectrolyte chain in solution, which is directly related to the chain length at high pH. The length of the chains that displace the shorter ones is limited by the electrostatic barrier. With an increased salt concentration the barrier is lower, allowing longer chains to reach the surface and displace the shorter ones. Consequently, the preferential adsorption from solutions with higher salt concentration is shifted to higher molecular weights.