Sum frequency generation spectroscopy of tetraalkylphosphonium ionic liquids at the air–liquid interface

Abstract

Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C-H stretching region (2750-3100 cm^-1) for [P_{8 8 8 <i>n</i>} ][Cl], where <i>n</i> = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for <i>n</i> = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of <i>n</i> = 12 or <i>n</i> = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with <i>n</i> = 8, 10, or 12) confirmed <i>gauche</i> defects, with enhancement for the long alkyl chain and an abrupt increase of <i>gauche</i> occurrence from <i>n</i> = 8 to <i>n</i> = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented.