Theoretical curves obtained from the Demchak and Fort (DF) model are compared with the experimental surface potential curves published by Vogel for mixed Langmuir films made from palmitic acid and tri-fluoropalmitic acid. In disagreement with the results expected from the DF model, the surface potential did not decrease linearly with the concentration of tri-fluoropalmitic acid. We show that a possible explanation for this unexpected finding encompasses two important changes in the DF model which are consistent with the formation and growth of microdomains in Langmuir monolayers. First, account must be taken of the dipole-dipole interaction in the calculation of the contribution of each layer. This results in an effective dielectric constant which depends on the polarizability and on the domain structure. Second, for agreement between experiment and theory to be reached, we still had to assume, together 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. This occurs at the critical area where the growth of the domains causes major structuring at the monolayer-water interface.