Theoretical Investigation of the Effect of Interface Dielectric Properties on Electric Double Layer Interactions

Sammanfattning: Results for a simple model of interacting electric double layers are presented for the geometry of two parallel planar walls. The wall and solution media are described as dielectric continua, characterized by their frequency dependent dielectric functions. The effects of the dielectric discontinuities at the wall surfaces are taken into account by the method of images for the static polarization and by Lifshitz theory for the effects of dynamic polarization (i.e. dispersion forces.) The pressure between the walls is calculated within the anisotropic hypernetted chain approximation for moderate bulk electrolyte concentrations (in the order of 10^-1 M) for a range of surface charge densities. The effect of anion polarizability is investigated while the cation polarizability is taken to be zero. It is found that the influence of anion polarizability on the pressure is greatest for moderate to large positive surface charge density and intermediate separations, where wall-ion dispersion forces gives rise to a decrease in wall-wall repulsion. For neutral and negative surfaces the pressure depends only weakly on anion polarizability and then mainly for large separations, in these cases wall-ion dispersion forces gives rise to an increase in repulsion. The presence of image charges causes an attraction between walls for low surface charge densities but causes an increase in repulsion for more highly charged surfaces. All these effects are largely due to the effects of wall-ion interactions on the ionic concentration profiles. The results are discussed in the context of colloidal interactions and colloidal stability.