Interfacial Depth Profiling of the Orientation and Bonding of Water Molecules across Liquid−Liquid Interfaces

Abstract

Molecular interactions that create the interfacial properties present at a junction between water and several hydrophobic liquids are the focus of this paper. This study employs molecular dynamics simulations to generate vibrational sum frequency (VSF) spectral profiles of water across the interfacial depth of three systems: the carbon tetrachloride−water, chloroform−water, and dichloromethane−water interfaces. These spectral profiles are calculated as functions of both frequency and interfacial depth, providing a visual description of interfacial water structure that can be difficult to elucidate from density profiles and sum frequency spectral intensities, or spectra that vary only as functions of frequency. VSF spectral intensities calculated for the OH stretch region that are integrated over the entire interfacial region are shown to compare well with the experimental VSF data for these systems. VSF spectral depth profiles show how the widths of the interfaces vary with the density and polarity of the organic phase and where different types of water species reside in the interfacial region. Furthermore they highlight the major molecular level differences found in these three interfacial regions and, in particular, identify oriented water monomers deeply immersed within the dichloromethane phase far removed from the Gibbs dividing surface. The penetration of water has important implications for how we view transport across water−hydrophobic liquid interfaces.