EFFECTS OF THE INTERFACIAL STRUCTURE ON THE HYDRATION FORCES BETWEEN LATERALLY UNIFORM SURFACES

In the hydration force theories it is customary to assume that the hyd rophilic surface is an infinitely thin plane. In many cases, however, the polar surface of a molecule or the surface of the corresponding ag gregate is corrugated and soft, the surface/solution interface then be ing really a deformable interphase. Our theoretical analysis shows tha t the interfacial ''smearing'' may strongly influence the apparent dec ay length of the hydration force between two laterally homogeneous sur faces, representative of polar phospholipid membranes. This length is approximately given by Lambda(eff) similar or equal to d(p) Lambda/[2s Lambda + (1 - 2s)d(p)], where d(p) is the decay length of the interfa cial polarity profile, s less than or equal to 0.5 a measure of the in terfacial softness, and Lambda the solvent correlations decay length. When the interfacial width is much larger than the intrinsic decay len gth of the correlations in pure water, and the interface is sufficient ly soft, the former becomes the chief determinant of the range of hydr ation-dependent intersurface force. This introduces new possibilities for the probing of interfacial structure and rigidity in the atomic fo rce or osmotic-stress measurements and provides new mechanisms for the information exchange between the opposing hydrophilic regions.