Thermodynamic studies of chemical equilibrium in supercritical carbon dioxide-cosolvent solutions using UV-Vis spectroscopy

The keto-enol tautomeric equilibrium of 5,5-dimethyl-1,3-cyclohexanedione ( dimedone) was investigated using UV-Vis spectroscopy in various solvents, i ncluding liquid acetonitrile (CH3CN), supercritical carbon dioxide (SC CO2) and SC CO2-CH3CN. The content of the enol tautomer in liquid CH3CN is much higher than that in SC CO2. With the addition of a small amount of cosolve nt CH3CN into SC CO2, the enol content increases with respect to that in pu re SC CO2. The comparison of thermodynamic functions for the tautomerism wa s made in these solvents. It was found that the change of standard internal energy (Delta U-0) in SC CO2-CH3CN is close to that in liquid CH3CN and re markably different from that in pure CO2. On the other hand, the maximum de crease in the change of the standard entropy (Delta S-0) was observed in SC CO2-CH3CN. It indicates that the cosolvent preferentially aggregates about the two tautomers. The Delta S-0 becomes the dominant thermodynamic drivin g force of the tautomerization in the cosolvent-modified SC CO2. (C) 1999 E lsevier Science B.V. All rights reserved.