Interfacial pH at electrically charged lipid monolayers investigated by the lipid pH-indicator method

Monomolecular films of various charge densities are prepared by mixing methylstearate with long chain sulfate and quaternary ammonium ions. A lipoid fluorescence pH indicator is embedded in the charged interface. Interfacial pH changes are detected as a function of the charge density of the monolayer and the NaCl concentration of the subphase. In the case of the positive films the results are in agreement with the Gouy-Chapman theory of the diffuse double layer. In the case of the negative film significant deviations are found.