Structure of the ethylammonium nitrate surface: an X-ray reflectivity and vibrational sum frequency spectroscopy study

X-ray reflectivity and vibrational sum frequency spectroscopy are used to probe the structure of the ethylammonium nitrate (EAN)−air interface. X-ray reflectivity reveals that the EAN−air interface is structured and consists of alternating nonpolar and charged layers that extend 31 Å into the bulk. Vibrational sum frequency spectroscopy reveals interfacial cations have their ethyl moieties oriented toward air, with the CH₃ C₃ axis positioned 36.5° from interface normal. This structure is invariant between 15 and 51 °C. On account of its molecular symmetry, the orientation of the nitrate anion cannot be determined with certainty.