The interaction between two surfaces separated by an electrolyte (an electrical double layer) is a fundamental determinant of the behaviour of colloids. Direct measurement of this interaction revealed a strong, approximately exponential, short-range repulsion that has been interpreted as an effect of the hydration structure surrounding the ions. This exponential consists of a series of steps (at high electrolyte concentrations) and oscillations (at lower concentrations). There is still no generally accepted explanation for strong exponential repulsion. In their recent Review Article Israelachvili and Wennerstrom suggest that it is caused by the finite size of adsorbed hydrated counterions. Here I report calculations of the interaction between surfaces in an aqueous solvent showing that the repulsion is related to the hydration of ions in the double layer, although the full picture is more complex than originally envisaged.
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