Effective interaction between charged colloidal particles near a surface

The effective pair potential between two charged spherical colloidal particles near a planar wall, separating the aqueous dispersion from a different dielectric medium, is calculated within linearized Poisson-Boltzmann theory. Due account is taken of image charges and of the finite size of the colloidal polyions. The effective interaction reduces to the standard DLVO potential far from the surface (i.e., in the bulk), but the effective electrostatic repulsion is enhanced for polyion pairs close to the surface, particularly at larger distances R parallel to the wall, when it decays like R-3 rather than exponentially. The calculation shows that confinement alone cannot account for the attractive interaction which is observed experimentally.