Solubility of Terephthalic Acid in Subcritical Water

Abstract

The solubility of terephthalic acid (TPA) in subcritical water was measured as a function of the temperature from (349 to 547) K at a pressure of 10.0 MPa. A semibatch flow apparatus was developed for the solubility measurement. N-Methylpyrrolidone was used as a good solvent for collection of TPA saturated in subcritical water without deposition upon cooling. The solubility of TPA in water showed an exponential increase with increasing temperature from 1.25·10–5 at 349 K to 2.99·10–2 at 547 K. The aqueous solubility of TPA was found to be lower than those of m- and o-phthalic acids by one and three orders of magnitude, respectively. The Gibbs free energy of solution (ΔGsol) as well as the enthalpic and entropic components (ΔHsol and –TΔSsol) were calculated from the temperature dependence of the solubility. The thermodynamic analysis showed that the ΔGsol gradually decreases with increasing temperature, as a result of the competition between a large increase in ΔHsol and a large decrease in –TΔSsol. The ΔGsol value decreased in the order of (p-phthalic acid) > (m-phthalic acid) > (o-phthalic acid) due to a decrease in –TΔSsol, whereas ΔHsol showed a small variation among the three isomers.