Charged colloids near interfaces
This article deals with the electric double-layer force between a charged colloidal sphere and a charged dielectric planar wall. To introduce the problem and to uncover the basic physics involved, we start by first reviewing the effective wall-colloid potentials that one obtains in linearized Poisson-Boltzmann theory. The important key concepts in this context are: charge renormalization, confinement effects, salty interfaces, and image-charge effects due to the dielectric discontinuity at the wall. Starting from the potentials derived in linear theory, we then come to approximate wall-colloid potentials that are valid also in the parameter regime where the non-linearity of the Poisson-Boltzmann equation becomes important. The range of validity of these potentials is systematically investigated by comparing them with potentials based on the exact numerical solution to the Poisson-Boltzmann equation. The important parameters of the calculation are the salt content of the electrolytic solution, the colloidal sphere radius, and the surface charge densities on both the wall and the colloid. We then briefly discuss what additional effect a concentrated suspension of such colloidal spheres has on the interfacial colloid, and close with a short report of an optical experiment that has recently been performed to measure the approximate wall-colloid potentials investigated here.