Simultaneous study of mechanical and electrical properties of a self assembled insoluble monolayer by axisymmetric drop shape analysis (ADSA)

A novel technique has been devised to measure mechanical and electrical properties of a lipid monolayer in contact with an electrolytic solution and supported by a metal electrode. The technique is based on a sessile drop apparatus in which the surface of a mercury drop, covered with the lipid film, can be alternately increased or decreased, while optically measuring the drop shape, hence the interfacial tension, by the ADSA technique. At the same time the film polarization state is controlled by a voltage clamp circuit and differential capacitance is measured by a lock-in amplifier. By this technique it is possible to obtain, e.g. the relation between surface pressure and differential capacitance, or surface pressure and polarization, that cannot be obtained with any other technique in a single experiment and on a single film. Films formed on the drop surface have been shown to be stable for many hours.