(162n) The Electrostatic Force between Dielectric Bodies
Large colloidal particles immersed in a solution of smaller particles, such as polymers or micelles, experience an attractive depletion force which originates from the change in available space for the smaller particles. This additional force is driven primarily by excluded volume interactions between the large and small particles and may lead to phase separation. In the case where the small particles are ions, such as in an electrolyte system, one finds a similar force driven by electrostatic, rather than excluded volume, interactions. When the colloids have a lower dielectric constant than the solvent (e.g., water), the ions in the solution feel a repulsive force due to image charge effects. This force excludes the electrolyte from the colloids, in a similar way as in the polymer-colloid system, which leads to an attractive ``electrostatic depletion force'' between the colloid particles. When the colloids have a higher dielectric constant than the solvent, the force is repulsive. We present theoretical analysis of this phenomenon, based on a variational field theory approach, which systematically includes the effect of static van der Waals interactions as well as depletion effects induced by the image charge potential. The relevance of this effect to salting out is discussed.