Mixing in Langmuir Monolayers: Perfluorotetradecanoic Acid and a Gemini Surfactant without a Linker

A recently reported anionic gemini surfactant, a member of the so-called "gemini without a linker" family, has recently been reported to form closely packed crystalline monolayers at the air-water interface. In this work, the impact on monolayer properties of the compound, C-18-0-C-18, that result from its mixing with a benchmark perfluorinated surfactant, perfluorotetradecanoic acid (PF), is explored. The films exhibit nonideal mixing, as determined by surface pressure-area (pi-A) isotherms and surface potential measurements, and phase-separation between the two components was observed by the direct visualization of the monolayers, and grazing-incident X-ray diffraction at the air-water interface. The pure and mixed films follow similar trends in the order of C-18-0-C-18 < PF < chi(PF) = 0.50 mixed films for both their extent of hysteresis and their stability at the air-water interface. Further, crystallographic data for the mixed film emerge as a simple combination of distinct diffraction patterns characteristic of both the individual components, consistent with the other findings reported here and thus clarify the intermolecular behavior of the binary mixture at the surface.