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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