Depletion forces due to image charges near dielectric discontinuities

•Image charge interactions repel electrolytes from low dielectric colloids.•Electrolyte depletion leads to an attraction between low dielectric particles.•The force is due to osmotic attraction and reduced screening of dispersion forces.•The attraction strengthens with electrolyte concentration and its range is constant.•The electrostatic depletion force plays a role in salting-out phenomena.

The depletion force is an effective inter-particle attractive interaction that is entropically driven by the exclusion of co-solvent molecules. For large co-solvents, such as polymers, the exclusion is primarily driven by excluded volume interactions. However, the exclusion of co-solvents, such as electrolytes, can be caused by other mechanisms. In this review, we summarize the literature on inter-particle depletion forces that arise from repulsive image-charge forces between low-dielectric particles and electrolytes. In particular, we emphasize the results from a variational perturbation theory for describing the salting-out behavior observed in moderately concentrated salt solutions. The theory predicts an unscreened force with a range given by the Bjerrum length and a magnitude proportional to the osmotic pressure of the salt solution. The force becomes significant at the same salt concentration where salting-out behavior is typically observed.

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