The hydration repulsion between charged surfaces as an interplay of volume exclusion and dielectric saturation effects

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 experi-charged 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 ®nite size of the counterions and the variable 1004 M (KNO3 or KCl) they observed the typical DLVOdielectric permittivity across the electric double layers. We involve maximum. However, at electrolyte concentrations higher thanthese two effects in the theory by coupling the Poisson±Boltzmann 1003 M they did not observe the expected DLVO maximumequation with the Bikerman and Booth equations. The resulting and primary minimum; instead a strong short-range repulsionnonlinear ordinary differential equation of second order is solved was detected, see also Ref. (6). Empirically, this force callednumerically. The theory is applied to interpret available experi-mental data for the dependence of the surface force on the distance the ‘‘hydration repulsion’ ’ appears to follow an exponential measured by means of the surface force apparatus. Excellent law (6) agreement between theory and experiment is obtained with reason-able values of the two adjustable parameters: the area per surface fhydr (h) f0e0h /l0, [1.1]charge and the counterion adsorption energy. It turns out that the contribution of the ionic-excluded volume to the hydration force where the decay length l 0 0.6–1.1 nm for 1:1 electrolytesis several times greater than the contribution due to the dielectri