Dissociation of protons from fatty acid Langmuir monolayers causes competitive adsorption of other counterions and partial charge neutralization. A new equilibrium model incorporating surface electrochemical and thermodynamic features is presented. The Stern-Gouy-Chapman equations are used, along with a modified Langmuir isotherm in which ion activities in the mixed surface monolayer instead of concentrations are employed. A two-dimensional lattice model with Flory-Huggins interaction parameters χij is invoked to obtain the activities of the ions in the monolayer. The competitive adsorption of ions to fatty acid monolayers is either "ideal" (χij = 0), or nonideal, depending on the ions mixing behavior with protons. Previous electrochemical models apply only for ideal adsorption, which appears to occur for Cd2+ and Ca2+. The nonideal adsorption is characterized successfully by the use of the above interaction parameters. Ba2+ ions show negative deviations from ideal adsorption, i.e., they adsorb less than expected for ideal adsorption in the presence of H+ ions. In contrast, Pb2+ ions show positive deviations from ideal adsorption. The influence of ions on surface electrochemical properties of the monolayer is also calculated from the theory.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry