Effective interaction of a charged colloidal particle with an air-water interface

Within the framework of linear and non-linear Poisson-Boltzmann theory, we study the effective interaction of a single charged colloidal sphere in an aqueous electrolytic solution with an air-water interface. The effects of varying the salt concentration and the colloidal surface charge density on the effective interaction are being investigated, with a view to understanding some physical phenomena, which include electrostatic adsorption and trapping at the air-water interface. Results show an electrostatic double-layer barrier to the colloid's approach to the interface which can be lowered considerably by increasing the salt concentration. For enough added salt, the charged colloid should be able to suddenly pop up at the air-water surface, an effect which has actually been observed in recent experiments. We discuss the relevance of our results to other experimental observations, and emphasize the close analogy between the problem considered here and the classical problem of the interaction of two colloids in a bulk suspension.