UV-Vis spectroscopic studies of solute–solvent and solute–cosolvent interactions in supercritical carbon dioxide

Abstract

The keto–enol tautomeric equilibria of ethyl acetoacetate in supercritical (SC) CO₂ with and without ethanol cosolvent were investigated over the pressure range from 75.9 to 185 bar at 308.2 K using UV-Vis spectroscopy. In order to obtain the local densities of solvent and cosolvent about the solute, we correlated the standard Helmholtz free energy of the tautomerization with a solvent parameter based on the Onsager reaction field theory. In both SC CO₂ and SC CO₂–ethanol, the local density enhancement of CO₂ in the vicinity of the solute is most remarkable at the reduced density close to unity. The highest degree of aggregation of cosolvent about the solute takes place at the lowest pressure investigated, which is slightly above the critical pressure. Moreover, the clustering of ethanol with the solute competes with that of CO₂. In addition, the local concentration enhancement of ethanol was studied in the concentration range of 0 to 6 mol%. It was found that the local concentration of ethanol is enhanced significantly at lower cosolvent concentrations.