The surface potential of Langmuir monolayers

A brief outline of an improved model for calculating the surface potential, ΔV, of condensed monolayers is given. The model is used to calculate the contribution of both the hydrocarbon tail and the COOH headgroup to ΔV. In a further series of calculations, the difference Δ(ΔV) between the surface potential of octadecanoic acid and ω-monochloro-octadecanoic acid is determined and found to be within a factor 2 of previously reported experimental values. The model successfully predicts the qualitative dependence of the mean molecular moment on composition in a mixed monolayer of the two acids. In principle, high resolution surface potential measurements are possible with the scanning electrostatic force microscope. Calculations are made which show that the technique is unlikely to have sufficient spatial resolution to image single dipoles except perhaps where these are large and associated with macromolecules. The technique, however, should be capable of providing surface potential measurements on mesoscopic structures to a resolution of some 50 nm.