Interactions of charged Langmuir monolayers with dissolved ions

Abstract

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.