Molar mass selectivity in the adsorption of polyacrylates on calcite

Aqueous solutions of polyacrylates, with a wide molar mass distribution, have been equilibrated with aqueous dispersions of calcium carbonate particles. In mixtures with excess polyacrylates, it is found that only a fraction of the original collection of macromolecules adsorbs on the mineral surfaces. The adsorbed macromolecules have been recovered, and their molar mass distribution has been measured using size exclusion chromatography (SEC). It is found that a narrow fraction with intermediate molar mass is preferentially adsorbed. This selection results from a kinetic process where these macromolecules are the first to bind to the surface. Smaller macromolecules do not bind at all and larger ones could bind, but are repelled by the macromolecules that arrived first at the surface.