Surface Layering in Ionic Liquids:  An X-ray Reflectivity Study

Abstract

The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 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.

Checkerboard Self-Patterning of an Ionic Liquid Film on Mercury

“…Theoretical progress in understanding the interplay of interactions in, and of the strong-correlation aspects of, RTILs is hampered by the scarcity of molecular-resolution structural data, particularly at interfaces, where such measurements are highly challenging. While a few such studies of liquid/vapour (l/v ) RTIL interfaces are available [8,9], only two address liquid/solid (l/s) interfaces [10,11]. No bulk liquid/liquid (l/l ) interface studies were published.…”

Small-Angle X-Ray Scattering of Ionic Liquids

“…This also extends to the liquid/air interface of ILs, where a surface layer with a higher mass density than that of the bulk has been identified due to the low dimensionality at the free surface [131]. A subsequent question is on how this unusual packing affects the dynamics at the interface.…”

Simultaneous measurement of X-ray specular reflection and off-specular diffuse scattering from liquid surfaces using a two-dimensional pixel array detector: the liquid-interface reflectometer of BL37XU at SPring-8

“…for the Cl À anion (Solutskin et al, 2005;Larriba et al, 2009)], the [C 12 mim]Cl molecules are considered to form a monolayer in which the dodecyl chain faces upwards and the Cl À anion locates next to the imidazolium ring at a depth of 10 Å .…”