MEAN-FIELD THEORY OF DEHYDRATION TRANSITIONS

It has been shown in our previous paper [Phys. Rev. A 40, 6431 (1989)] that repulsive hydration forces between parallel hydrophilic surfaces separated by a thin water film are determined by the lateral distribution of solvated surface groups. This theory is now extended to include correlations in fluctuations of the solvated group distribution between the opposing surfaces. The solution is obtained in a treatment formally analogous to the "spherical model" for Ising systems. It is shown that the correlations between the opposing surfaces give an attractive contribution to the hydration forces. Conditions governing the crossover from net repulsion to net attraction are studied. Hydration attraction appears and increases gradually with a decrease in temperature below a critical value or with variation of other system parameters. Full transition to the net attractive force leads to formation of a dehydrated contact between the surfaces. The results are applied to describe dehydration transitions in multilayer lipid systems.