Structure and Kinetics of Fatty Acid Langmuir Monolayers on Zinc Salt Solutions

The adsorption of zinc cations under behenic acid Langmuir monolayers was investigated by means of isotherm measurements, grazing incidence X-ray diffraction and Brewster angle microscopy. The structure of the films was characterized as a function of Zn2+ concentration, for three different counterions (chloride, iodide, bromide) and at two subphase pHs (5.5 and 7.5). At pH 5.5 and in the studied concentration range, Zn2+ adsorption leads to a condensation of the fatty acid monolayer with the same phase transitions as over pure water. In contrast, at higher pH the organic X-phase is evidenced immediately above a concentration threshold without any ion organization. Even though Cu2+ and Zn2+cations induce both the fatty acid X-phase, the kinetics of its formation appears strongly different. Indeed, as for Mg2+ and Cd2+, the intermediate new I-structure is evidenced in the course of Zn2+ adsorption although superstructures are observed only for Mg2+ and Cd2+. However, for Zn2+, the I-phase evolves to the final state through a new structure called X′ and a continuous X′–X transition. Finally, any effect of the counterion is evidenced neither during the kinetic process nor in the final state.