Near-infrared spectroscopic solubility measurement for thermodynamic analysis of melting point depressions of biphenyl and naphthalene under high-pressure CO2

Abstract

Melting temperatures of organic solids are often depressed by high-pressure CO2 due to a dissolution of CO2 in the molten organic compounds. For thermodynamic analysis of the melting point depression, solubilities of CO2 in molten biphenyl and naphthalene were measured by near-infrared spectroscopy at various temperatures and pressures up to 20MPa. Molarity of the organic component was determined from the 3νCH absorption band, and that of CO2 from the 2ν1+ν3 band. Mole fraction of CO2 in the liquid phase was found to be an increasing function of the pressure up to 0.6 at 20MPa and a weakly decreasing function of the temperature. The solubility data were used for modeling of the mixtures by the Peng–Robinson equation of state with a binary interaction parameter k12. Calculation of the solid–liquid–gas phase equilibrium for the model fluid qualitatively described a large decrease in the melting temperature with increasing pressure up to 10MPa followed by a small change at higher pressures. The melting point change was interpreted by the two competing effects: hydrostatic pressure effect increases the melting point by ca. 8°C at 20MPa, whereas CO2 solubility effect reduces it by ca. 30°C. Decomposition of the solubility effect into ideal and non-ideal mixing parts revealed that the non-ideality increases the melting point by more than 10°C.