Colloidal suspensions at dielectric interfaces

We study suspensions of charged mesoscopic spheres (colloids) near typical dielectric interfaces. Having a dielectric constant that is different from that of the suspension, the interface gives rise to image-charge-induced forces which compete with the usual double-layer forces acting between the colloids. Within the framework of Poisson-Boltzmann theory we calculate the total force acting on an interfacial macrosphere for a broad range of possible system parameters. These are: the ratio of dielectric constants of the two media, the charge and size of the macroion, its distance from the wall, the temperature and volume fraction of the suspension. To reduce the number of parameters, use is made of the scaling property of the Poisson-Boltzmann equation in the salt-free approximation. We have furthermore performed Monte Carlo simulations to investigate finite-size effects of the microions. We discuss, quite generally, how both confinement effects and image charges affect the structural properties of interfacial colloidal suspensions and explain how our results can be used for estimating their effects in experiments.