Molecular dynamics study on the anomalous behavior of phase transition heats of alcohols at vapor–liquid interface

Abstract

The surface adsorptions of methanol, ethanol and 1-propanol liquids at vapor–liquid interface are investigated with the surface adsorption theory and molecular dynamics simulations (MD). The surface layer thicknesses of these three liquids calculated with MD increase with temperature. It is found that the thickness of the surface layer becomes smaller as the intermolecular interaction becomes stronger. The heat of phase transition from the bulk to surface determined with the experimental surface tension data decreases from methanol to ethanol and increases from ethanol to propanol. This anomalous behavior is different from the usual one of the phase transition heats which changes monotonously with the alkyl chain length in literature. Since the reversible phase transition heat is equal to T(Ss-Sα), the entropy changes of these three liquids are calculated with MD simulations and are in agreement with the variation trend of phase transition heats determined from the experimental data. This is a qualitative confirmation of our surface adsorption theory from the viewpoint of the molecular dynamic simulations.