Quantitative treatment of surface potentials in Langmuir films from aromatic amphiphiles

It is shown that the surface potentials of Langmuir monolayers from aromatic compounds can be interpreted using the three-layer capacitor model of Demchak and Fort, with the same local dielectric constants employed for aliphatic compounds. Based on new data for monolayers from an acid, amine and ester compounds with an aromatic hydrophobic part, we show that the dielectric constant for the monolayer/water interface region is 6±1. Good agreement between experiment and theory was obtained by taking the dielectric constant of the monolayer/interface as 3.0±0.6, which are essentially the same as those obtained for long tail aliphatic compounds.