INFLUENCE OF CHEMICAL MODIFIERS ON THE SOLUBILITY OF O-HYDROXYBENZOIC AND M-HYDROXYBENZOIC ACID IN SUPERCRITICAL CO2

The equilibrium solubilities of o- and m-hydroxybenzoic acid in 3.5 mol % acetone-CO2 and 3.5 mol % methanol-CO2 binary mixtures were measured at temperatures between 318 and 328 K and pressures in the range of 90-200 bar using a continuous flow apparatus. The solubility isotherms in these ternary systems exhibit a retrograde vaporization phenomenon and a common upper crossover pressure for each ternary system. Moreover, the results demonstrate that the addition of small amounts of cosolvents to CO2 increase significantly the solubility of o- and m-hydroxybenzoic acid. A molecular thermodynamic model was developed based on the Scatchard-Hildebrand regular solution and Flory-Huggins theory using the concept of component solubility parameters to obtain the unlike-pair molecular interactions between the solute and cosolvent. An effective volume fraction term that reflects the ''preferential'' solvation of the solute by the cosolvent molecules was included in the model development to account for differences between the bulk solvent environment and that in the immediate vicinity of the solute molecule.