SURFACE-POTENTIALS OF MIXED LANGMUIR FILMS - A MODEL CONSISTENT WITH A DOMAIN-STRUCTURED MONOLAYER

Theoretical curves obtained from the Demchak and Fort (DF) model are c ompared with the experimental surface potential curves published by Vo gel for mixed Langmuir films made from palmitic acid and tri-fluoropal mitic acid. In disagreement with the results expected from the DF mode l, the surface potential did not decrease linearly with the concentrat ion of tri-fluoropalmitic acid. We show that a possible explanation fo r this unexpected finding encompasses two important changes in the DF model which are consistent with the formation and growth of microdomai ns in Langmuir monolayers. First, account must be taken of the dipole- dipole interaction in the calculation of the contribution of each laye r. This results in an effective dielectric constant which depends on t he polarizability and on the domain structure. Second, for agreement b etween experiment and theory to be reached, we still had to assume, to gether with the already known change in the dielectric constant at the water interface, that some of the CH3 dipoles in the mixed film give a negative contribution in the presence of neighboring CF3 dipoles. Th is occurs at the critical area where the growth of the domains causes major structuring at the monolayer-water interface.