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

•In situ solubility measurements of CO2 in molten biphenyl and naphthalene.•The solubility data at various temperatures and pressures up to 20 MPa.•Thermodynamic analysis of the melting point depression based on the solubility data.•Decomposition of the melting point change into solubility and pressure effects.•Separation of the solubility effect into ideal and non-ideal mixing parts.

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 20 MPa. 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 20 MPa 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 10 MPa 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 20 MPa, 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide