Surface Phases and Surface Freezing in an Ionic Liquid

Room temperature ionic liquids (RTILs), a novel class of liquid salts, are intensively studied for their basic science and numerous emerging applications. When undercooled, RTILs comprising long alkyl chains often exhibit liquid crystal (LC) bulk phases. However, only one molecular-resolution experimental structure study was published for their LC surface phases. We measured the temperature evolution of another LC surface phase, using surface specific A-resolution X-ray methods. This phase’s existence range, 90 °C, much exceeds the corresponding bulk phase’s 3 °C. Its thickness, L, confirms the theory-predicted logarithmic temperature dependence, with an amplitude equaling the bulk correlation length. Surprisingly, at L’s divergence temperature, a ∼20 A thick, hexagonally packed, crystalline monolayer forms at, and fully covers, the sample’s surface. It is identified as a surface-frozen Langmuir–Gibbs film and fundamentally differs from the only reported RTIL surface crystal, a Coulomb-dominated, four-lay...