A Study of Molecular Orientation at Neat Liquid-Mica Interface by Sum Frequency Generation (SFG) Spectroscopy

The solid-liquid interface is usually different to bulk liquids since the symmetry break at the interface and provide a nonuniform environment. The rearrangement at solid-liquid interface is central in nanoscale science and technology, due to its influence in the process, such as lubrication, corrosion, heterogeneous reaction, electrochemistry and wetting. The advent of experimental techniques provides a detail information of the solid-liquid interface. Among these different new techniques of studying the interfacial structures, sum frequency generation vibrational spectroscopy shows advantageous to study the solid-liquid interface due to its unique ability in yielding vibrational spectra at such interfaces. Muscovite mica is an important substrate to investigate the ionic liquids structure at the interface. This is largely because it is atomically smooth and flat. A series of studies of ionic liquids at mica interface through surface force apparatus (SFA) and the atomic force microscopy (AFM). In order to compare the orientation of mica-ionic liquids interface, a series of ionic liquids were studied by using sum frequency generation vibrational spectroscopy. Through SFG simulation orientation analysis, it showed that the imidazole ring of ionic liquids, [BMIM][BF4], [BMIM][Tf2N], [BMIM][DCA], and [BMIM][SCN] were almost orientated nearly parallel to the mica surface with their butyl chain toward liquids. The butyl chain is orientated more closed to the normal rather than parallel to the mica surface. To compare the effect of different functional groups of molecules on the muscovite mica, a series of alcohols, nitriles, and alkane were also studied at the muscovite mica interface by using sum frequency generation vibrational spectroscopy. However, the orientation on mica surface still needs to study more in detail since the complexity of orientation of neat liquid on mica surface is introduced by the multiple interactions on mica surface, such as electrostatics, hydrogen bonding, van der Waals and dipole interaction between mica and liquid and liquid itself. A small change of structure and mica surface can make orientation of liquid on mica surface dramatically different.