Molecular orientation near liquid–vapor interface of methanol: Simulational study

Abstract

Computer simulations with molecular dynamics technique were carried out to investigate properties of liquid–vapor interface of methanol for a wide temperature range of 160–350 K. The estimated surface excess thermodynamic quantities, especially surface entropy, are characteristic of strongly hydrogen-bonding liquids, but the density profile resembles that of simple fluids. Orientational structuring near the surface was also studied, and one apparent tendency, much stronger than that of water, was observed; the methanol molecule projects its methyl group toward the vapor phase. This orientational ordering can be understood as a result of putting a hydrophobic methyl group outside of liquid phase to maximize the number of hydrogen bonds near the surface for energetical stabilization. A considerable part of the thermodynamic anomaly is due to this orientational ordering. Since the electric dipole is almost parallel to the surface in this orientation, the calculated surface potential is quite small, about −30 mV at room temperature.