Surface Layering in Ionic Liquids:
An X-ray Reflectivity Study
Solutskin E, Ocko BM, Taman L, Kuzmenko I, Gog T, Deutsch
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
127 (21): 7796-7804 JUN 1 2005
The surface structure and thermodynamics of two ionic liquids, based on the
1-alkyl-3-methylimiclazolium cations, were studied by X-ray reflectivity
and surface tensiometry. A molecular layer of a density similar to 18 % higher
than that of the bulk is found to form at the free surface of these liquids.
In common with surface layering in liquid metals and surface freezing in
melts of organic chain molecules, this effect is induced by the lower dimensionality
of the surface. The concentrations of the oppositely charged ions within
the surface layer are determined by chemical substitution of the anion. The
temperature-dependent surface tension measurements reveal a normal, negative-slope
temperature dependence. The different possible molecular arrangements within
the enhanced-density surface layer are discussed.