Surface Dipole Potential at the Interface between Water and Self-Assembled Monolayers of Phosphatidylserine and Phosphatidic Acid

AbstractThe nature and magnitude of the surface dipole potential χ at a membrane/water interface still remain open to discussion. By combining measurements of differential capacity C and charge density σ at the interface between self-assembled monolayers of phosphatidylserine and phosphatidic acid supported by mercury and aqueous electrolytes of different concentration and pH, a sigmoidal dependence of χ upon σ is revealed, with the inflection at σ=0. This behavior is strongly 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 ∼80Hz, as observed with phospholipid monolayers with partially protonated polar groups, is explained either by a sluggish collective reorientation of some polar groups of the lipid or by a sluggish movement of protons across the polar head region