Surface charge, surface dipoles and membrane conductance

The conductance of lipid membranes in the presence of nonactin is changed by the adsorption of small amounts of ionic and zwitterionic surfactants. The conductance changes are, in many instances, not accounted for by the variation in surface charge or diffuse double layer potential as calculated from Gouy-Chapman theory. The changes are, however, all accurately accounted for by the variation in total potential across the membrane interface. This potential includes contributions from surface dipoles and specific adsorption, as well as any diffuse double layer effects not included in the Gouy-Chapman theory.The total potential changes were inferred from Volta or compensational potential changes at bulk oil (and monolayer)/aqueous solution interfaces. Surface charge densities were found by standard thermodynamic methods involving the use of the Gibbs equation. Electrokinetic potentials for the appropriate surfaces were also measured and, in general, agreed well with the diffuse double layer potentials calculated from the Gouy-Chapman theory.