CHEMICAL COMPLEXING AGENTS FOR ENHANCED SOLUBILITIES IN SUPERCRITICAL FLUID CARBON-DIOXIDE

The effect of the strong Lewis base tri-n-butyl phosphate (TBP) on the solubility of benzoic acid and hydroquinone (HQ) in supercritical fluid carbon dioxide is reported. TBP is shown to be a much stronger cosolvent for these solutes than methanol. For example, 2% TBP increases hydroquinone's solubility by a factor of 250. The principles of chemical reaction equilibria are combined with the Peng-Robinson equation of state in order to model these results. The behavior of the hydroquinone-carbon dioxide-TBP system is shown to be attributable to the formation of an HQ-TBP2 complex having an enthalpy of formation of -18.9 kcal/mol. The performance of this chemical model is compared to that of a recently developed density-dependent local composition (DDLC) model.