Interfacial Structure of Room-Temperature Ionic Liquids at the Solid-Liquid Interface as Probed by Sum Frequency Generation Spectroscopy

Fundamental and basic information about molecules at the solid-liquid interface is limited primarily because most spectroscopic methods are not able to specifically detect the surface buried between two condensed phases. The second-order nonlinear techniques sum frequency generation (SFG) is able to probe the noncentrosymmetric environment between two bulk phases and has been used in many surface studies. In this Perspective, the use of SFG in studies of ionic liquids at the solid-liquid interface will be discussed. Several interfaces are discussed including dielectrics, semiconductors, and metal. SFG results suggest a specific surface model that the ionic liquid structure persists approximately one layer into the bulk liquid and that this is mostly due to the charge neutralization at the surface.