Charged colloids near interfaces

This article deals with the electric double-layer force between a charged c olloidal sphere and a charged dielectric planar wall. To introduce the prob lem and to uncover the basic physics involved, we start by first reviewing the effective wall-colloid potentials that one obtains in linearized Poisso n-Boltzmann theory. The important key concepts in this context are: charge renormalization, confinement effects, salty interfaces, and image-charge ef fects due to the dielectric discontinuity at the wall. Starting from the po tentials derived in linear theory, we then come to approximate wall-colloid potentials that are valid also in the parameter regime where the non-linea rity of the Poisson-Boltzmann equation becomes important. The range of vali dity of these potentials is systematically investigated by comparing them w ith potentials based on the exact numerical solution to the Poisson-Boltzma nn equation. The important parameters of the calculation are the salt conte nt of the electrolytic solution, the colloidal sphere radius, and the surfa ce charge densities on both the wall and the colloid. We then briefly discu ss what additional effect a concentrated suspension of such colloidal spher es has on the interfacial colloid, and close with a short report of an opti cal experiment that has recently been performed to measure the approximate wall-colloid potentials investigated here.