Structure and single particle dynamics of the vapour-liquid interface of acetone-CO2 mixtures

Abstract

Molecular dynamics computer simulations of the liquid-vapour interface of acetone-CO2 mixtures are performed in the canonical (N,V,T) ensemble at 30 thermodynamic state points, ranging from 280 to 460 K and from about 10 to 116 bar, covering the entire composition range from neat CO2 to neat acetone. The molecules forming the first layer at the molecularly rough liquid surface as well as those of the next three subsurface molecular layers have been identified by the ITIM method, and the surface properties of the liquid phase are analyzed in a layer-wise manner. The arrangement of the molecules both within the macroscopic plane of the interface and along its normal axis, as well as their surface orientation and single particle dynamics at the liquid surface are analyzed in detail. It is found that, in accordance with their higher affinity to the vapour phase, CO2 molecules are enriched at the liquid surface, moreover, even within the surface layer they prefer to occupy positions that are more exposed to the bulk vapour phase than those preferred by acetone. In other words, within the molecularly wavy surface layer, CO2 molecules prefer to stay at the crests, while acetone molecules prefer to stay in the troughs. On the other hand, the lateral arrangement of the surface molecules is found to be more or less random. Both molecules prefer to stay perpendicular to the liquid surface, but this preference only involves the first molecular layer, and this preference is governed by the electrostatic interaction of the surface molecules. Both molecules perform considerable lateral diffusion at the liquid surface during their stay there, this diffusion being faster for the CO2 than for the acetone molecules, but not as much faster than in the bulk liquid phase.