by Israelachvili et al. (2,3) and Pashley (4,5), who examined We demonstrate that the hydration repulsion between smooth the validity of DLVO theory at small film thicknesses in expericharged surfaces can be attributed to the interplay of two effects ments with films from aqueous electrolyte solutions confined which are not taken into account in the conventional DLVO the-between two mica surfaces. At electrolyte concentrations below ory. These are the finite size of the counterions and the variable 10 04 M (KNO 3 or KCl) they observed the typical DLVO dielectric permittivity across the electric double layers. We involve maximum. However, at electrolyte concentrations higher than The physical importance of the hydration force is that it the presence of electrolyte at ionic strengths above 1 m M. ᭧ 1996 stabilizes some dispersions preventing coagulation in the Academic Press, Inc.primary DLVO minimum. It is believed that the hydration Key Words: counterion adsorption; dielectric saturation; hydra-force is connected with the binding of strongly hydrated tion energy of ions; hydration repulsion; surface force apparatus; ions at the interface. This is probably the explanation of the volume exclusion effect for ions.