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

The equilibrium solubilities of o- and m-hydroxybenzoic acid in 3.5 mo l % acetone-CO2 and 3.5 mol % methanol-CO2 binary mixtures were measur ed at temperatures between 318 and 328 K and pressures in the range of 90-200 bar using a continuous flow apparatus. The solubility isotherm s in these ternary systems exhibit a retrograde vaporization phenomeno n and a common upper crossover pressure for each ternary system. Moreo ver, the results demonstrate that the addition of small amounts of cos olvents to CO2 increase significantly the solubility of o- and m-hydro xybenzoic acid. A molecular thermodynamic model was developed based on the Scatchard-Hildebrand regular solution and Flory-Huggins theory us ing the concept of component solubility parameters to obtain the unlik e-pair molecular interactions between the solute and cosolvent. An eff ective volume fraction term that reflects the ''preferential'' solvati on of the solute by the cosolvent molecules was included in the model development to account for differences between the bulk solvent enviro nment and that in the immediate vicinity of the solute molecule.