Why do co-solvents enhance the solubility of solutes in supercritical fluids? new evidence and opinion

摘要

The efects of two polar cosolvents,chlorodifluoromethane and acetone,on the solubility and enthalpy of a solution of 1,4-naphthoquinone in supercritical (SC) CO_2 were studied.We found that the dissolution process becomes less exothermic in the presence of the co-solvents relative to that in pure CO_2,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 ehancement of the solute by the co-solvents insupercritical fluids (SCF);this should be applicable to cases in which the local density of the SC solvent around the solute and the cosolvent 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.