Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

The molecular-scale structure of the ionic liquid [C(18)mim](+)[FAP](-) near its free surface was studied by complementary methods. X-ray absorption spectroscopy and resonant soft X-ray reflectivity revealed a depth-decaying near-surface layering. Element-specific interfacial profiles were extracted with submolecular resolution from energy-dependent soft X-ray reflectivity data. Temperature-dependent hard X-ray reflectivity, small-and wide-angle Xray scattering, and infrared spectroscopy uncovered an intriguing melting mechanism for the layered region, where alkyl chain melting drove a negative thermal expansion of the surface layer spacing.