Solute effects on supercritical fluid

Abstract

Supercritical carbon dioxide has been widely used as solvent, although its solubility is much lower than that of typical organic solvents in liquid state in general. Entrainer effects are known and utilised, in which adding low-weight molecules, such as alcohols (e.g. methanol and ethanol) and hydrocarbons (e.g. pentane and hexane), increases the solubility and the chemical reactivity. In this study, we investigated the change of local fluid structure and thermodynamic properties with molecular dynamics simulations, focusing on the entrainer effect. A monatomic fluid mixture model based on the Lennard-Jones model potential was adopted for simplicity; the parameters of which roughly correspond to those of carbon dioxide (solvent) and ethanol (solute). A series of simulations were performed in the vicinity of the gas–liquid critical point (CP). The radial distribution function and the coordination number reveal the enhancement of the solvent local density. The solvent chemical potential was evaluated with the Widom's particle insertion method. The chemical potential change by adding solutes is smallest in magnitude at the CP, qualitatively agreeing with recent experimental analysis and interpretation with a small-angle neutron scattering method.

Keywords:

• supercritical fluid
• entrainer effect
• chemical potential
• molecular simulation

Acknowledgements

This study was inspired by neutron scattering experiments of Dr T. Sato, Prof. M. Sugiyama and their co-workers. The authors are grateful to Prof. M. Sugiyama at Kyoto University for stimulating discussions over their recent experiments and incessant encouragement.