The adsorption of small particles onto larger particles of opposite charge Direct electron microscope studies

Abstract

A new thin-film, freeze-drying/scanning electron microscopy technique for directly observing the adsorption of small colloidal particles onto larger colloidal particles of opposite charge, is described. Independent establishment of adsorbed amounts using a centrifugal/analytical technique gives confidence that no artefacts exist in the freeze-drying method. Adsorption isotherms are presented for small, negative latex particles adsorbed onto larger, positive latex particles, at varying background electrolyte concentrations, and in the presence and absence of preadsorbed layers of poly(vinyl alcohol—co-vinyl acetate) on both sets of particles. It is seen that the presence of this preadsorbed polymer layer only affects the adsorbed amount at electrolyte concentrations greater than some critical value. Kinetic data for the small particle adsorption are also presented. These give further support for the implication that long range interactions between the particles play a significant role in determining the adsorption rate coefficient, as well as the final amount adsorbed.