Structure and Dynamics of Acetonitrile at the Air/Liquid Interface of Binary Solutions Studied by Infrared−Visible Sum Frequency Generation

Infrared−visible sum frequency generation (SFG) vibrational spectroscopy was employed to investigate the molecular level details of acetonitrile (CH3CN) structure at the air/liquid interface of binary solutions with per-deuterated acetonitrile (CD3CN), water, and carbon tetrachloride (CCl4). In the case of the CH3CN−CD3CN system, the SFG signal intensity for the symmetric CH stretch (CH(s)) mode increases proportionally with the bulk mole fraction of CH3CN (xCH3CN). However, the SFG signal intensity of the CH(s) mode from the CH3CN−water system is rapidly enhanced, reaches a maximum, and gradually attenuates as xCH3CN is varied from 0 to 1. Mixtures of CH3CN and CCl4 generally exhibit less SFG intensity for the CH(s) mode compared to the CH3CN−CD3CN and CH3CN−water systems for a corresponding xCH3CN. For quantitative analysis, the contributions to the SFG signal intensity were separated into the orientation factor and surface number density. As a consequence, it was found that CH3CN molecules in the CH3CN−CD3CN mixtures exhibit a concentration-independent orientation similar to those in neat CH3CN. For the CH3CN−water system, CH3CN molecules become less tilted with respect to the surface normal and more ordered than those in neat CH3CN by lowering xCH3CN. In contrast, CH3CN molecules in the CH3CN−CCl4 system are generally less ordered and more tilted with respect to the surface normal compared to the case of neat CH3CN.