title: Why do co-solvents enhance the solubility of solutes in supercritical fluids? New evidence and opinion.
abstract: The effects of two polar co-solvents, chlorodifluoromethane and acetone, on the solubility and enthalpy of a solution of 1,4-naphthoquinone in supercritical (SC) CO2 were studied. We found that the dissolution process becomes less exothermic in the presence of the co-solvents relative to that in pure CO2, although the solubility is enhanced significantly by the co-solvents. This indicates that the increase in the solubility by adding co-solvents results from the increase of the entropy of solution. On the basis of the unexpected results we propose a new mechanism for the solubility enhancement of the solute by the co-solvents in supercritical fluids (SCF); this should be applicable to cases in which the local density of the SC solvent around the solute and the co-solvent is larger, and the co-solvent associates preferentially with the solute. The results are also very important for the understanding of other fundamental questions of SCF science, such as the effect of co-solvents on the thermodynamic and kinetic properties of the reactions in SCFs.
authors.affiliation: Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
authors.forename: Xiaogang
authors.initials: X
authors.lastname: Zhang
authors.forename: Buxing
authors.initials: B
authors.lastname: Han
authors.forename: Zhenshan
authors.initials: Z
authors.lastname: Hou
authors.forename: Jianling
authors.initials: J
authors.lastname: Zhang
authors.forename: Zhimin
authors.initials: Z
authors.lastname: Liu
authors.forename: Tao
authors.initials: T
authors.lastname: Jiang
authors.forename: Jun
authors.initials: J
authors.lastname: He
authors.forename: Hongping
authors.initials: H
authors.lastname: Li