Mixed adsorption layers of uncharged polymers at particle/solution interfaces

Simultaneous competitive adsorption from binary mixtures of uncharged polymers on negatively charged colloidal dispersions and the structure of polymer layers formed at particle/solution interfaces were studied. From the adsorption isotherms determined in competitive adsorption at equal concentrations of two polymers in the bulk solution, preferential adsorption parameters for the polymers in pairs have been established, and used as a measure of the affinity of chemically different macromolecules for the solid surfaces. The diffusion coefficient of (monodisperse) particles with and without adsorbed polymer was measured by photon correlation spectroscopy, and the hydrodynamic thickness of adsorbed polymer layers has been calculated. The exchange of polymeric species adsorbed at the interfaces was detected by measuring the thicknesses of mixed layers after various contact times. The preferential adsorption parameters proved to be relevant markers for the various interfacial processes. Simultaneous adsorption onto particle surfaces of polymers with significantly different segment affinities resulted in considerable alterations in the structure of mixed adsorption layers. Close correlation was found between the affinity of the competing macromolecules and the spatial properties of mixed layers. From suitable polymer pairs, at partial surface coverages where enough surface sites were available for the various macromolecules, irregularly extended adsorption layers formed. At high surface coverages, the preferentially adsorbed species displaced the weakly adsorbed polymers from the interfaces.