Magnitude of the solvation pressure depends on dipole potential

As polar surfaces in solvent are brought together, they experience a large repulsive interaction, termed the solvation pressure. The solvation pressure between rough surfaces, such as lipid bilayers, has been shown previously to decay exponentially with distance between surfaces. In this paper, we compare measured values of the solvation pressure between bilayers and the dipole potential for monolayers in equilibrium with bilayers. For a variety of polar solvents and lipid phases, we find a correlation between the measured solvation pressures and dipole potentials. Analysis of the data indicates that the magnitude of the solvation pressure is proportional to the square of the dipole potential. Our experiments also show that the oriented dipoles in the lipid head-group region, including those of both the lipid and solvent molecules, contribute to the dipole potential. We argue that (i) the field produced by these interfacial dipoles polarizes the interbilayer solvent molecules giving rise to the solvation pressure and (ii) both the solvation pressure and the dipole potential decay exponentially with distance from the bilayer surface, with a decay constant that depends on the packing density of the interbilayer solvent molecules (1-2 A in water). These results may have importance in cell adhesion, adsorption of proteins to membranes, characteristics of channel permeability, and the interpretation of electrokinetic experiments.