Effects of tert-Butyl Alcohol on Water at the Liquid–Vapor Interface: Structurally Bulk-like but Dynamically Slow Interfacial Water

Abstract

We have investigated the effects of tert-butyl alcohol (TBA) on the structure and dynamics of water at the liquid–vapor interface. The structure of interfacial water has been studied by calculating the structural correlations and vibrational sum frequency generation (VSFG) spectrum from molecular dynamics simulations. It is found that the dangling peak of the VSFG spectrum of the air–water interface near ∼3700 cm–1 almost disappears in presence of TBA at the chosen concentration which means that the interfacial region is covered by the solute molecules. The hydrogen-bonded peak in the VSFG spectrum is found to be red-shifted by ∼100 cm–1 as compared to that of pure the air–water interface despite the fact that the strength of hydrogen bonds in the interfacial region is found to be similar to that of the bulk. This red shift in the VSFG spectrum is found to be a consequence of the cancellation of the nonlinear responses from “up-” and “down”-oriented O–H modes of water in that region. The local structure around the interfacial water is found to be similar to that of bulk water where the oxygen of TBA provides an environment similar to the oxygen of water in the bulk. However, the dynamical properties like the orientational relaxation and vibrational spectral diffusion are found to slow down when one moves from bulk toward the TBA layer at the surface. The effects of intra- and inter-molecular coupling and third-order susceptibility on the VSFG spectrum are also discussed.