Surface Potential of a Planar Charged Lipid-Water Interface. What Do Vibrating Plate Methods, Second Harmonic and Sum Frequency Measure?

The interfacial electrical potential is an important parameter influencing, for instance, electrochemical reactions and biomolecular interactions at membranes. A deeper understanding of different methods that measure quantities related to the surface potential is thus of great scientific and technological relevance. We use lipid monolayers with varying charge density and thoroughly compare the results of surface potential measurements performed with the vibrating plate capacitor method and second harmonic generation spectroscopy. The two techniques provide very different results as a function of surface charge. Using the molecular information on lipid alkyl chain, lipid headgroup, and interfacial water provided by sum frequency generation spectroscopy, we disentangle the different contributions to the surface potential measured by the different techniques. Our results show that the two distinct approaches are dominated by different molecular moieties and effects. While the shape of the SPOT method response as a function of charge density is dominated by the lipid carbonyl groups, the SHG results contain contributions from the interfacial water molecules, the lipids and hyper-Rayleigh scattering.