The influence of surfactant head group on miscibility in mixed hydrocarbon-perfluorocarbon monolayers

Abstract

Simple binary mixtures of hydrogenated and perfluorinated surfactants are typically immiscible in monolayers, with mixtures forming highly-structured films at the air-water and solid-air interfaces. While dispersion interactions between tail groups of the surfactants play a key role in dictating the micron-scale morphology of such mixed films, the influence of the head group on film morphology has been minimally explored. In this work, surfactant head group effects are investigated by comparing thermodynamic miscibility in combination with film morphology in binary mixed films comprised of nonadecanoic acid (ND; CH3(CH2)17COOH) with perfluorotetradecanoic acid (PF; CF3(CF2)12COOH), as well as the corresponding fatty alcohol nonadecanol (NDOH; CH3(CH2)18OH) with PF. Both the fatty acid and fatty alcohol were immiscible with PF in monolayers, with the systems exhibiting small, negative deviations from ideal mixing and forming phase-separated domains under a variety of preparation conditions. Micron and nanometer-scale film morphologies for the two sets of films were significantly different, however, with fatty acid mixtures tending to form polygonal, faceted domains enriched in hydrocarbon whereas the fatty alcohol mixtures formed significantly less structured domains, both at the solid- and liquid-air interfaces. Differences in film morphologies are discussed in terms of head and tail group interactions, and placed in context of existing literature on closely-related monolayer systems.