SURFACE DIPOLE POTENTIAL AT THE INTERFACE BETWEEN WATER AND SELF-ASSEMBLED MONOLAYERS OF PHOSPHATIDYLSERINE AND PHOSPHATIDIC-ACID

The nature and magnitude of the surface dipole potential chi at a memb rane/water interface still remain open to discussion. By combining mea surements of differential capacity C and charge density sigma at the i nterface between self-assembled monolayers of phosphatidylserine and p hosphatidic acid supported by mercury and aqueous electrolytes of diff erent concentration and pH, a sigmoidal dependence of chi upon sigma i s revealed, with the inflection at sigma = 0. This behavior is strongl y reminiscent of the surface dipole potential due to reorientation of adsorbed water molecules at electrified interfaces. The small increase in C with a decrease in the frequency of the AC signal below similar to 80 Hz, as observed with phospholipid monolayers with partially prot onated polar groups, is explained either by a sluggish collective reor ientation of some polar groups of the lipid or by a sluggish movement of protons across the polar head region.