High-pressure phase behavior of the carbon dioxide-trifluoropropylisobutyl polyhedral oligomeric silsesquioxane binary system

The solubility of a bifunctional polyhedral oligomeric silsesquioxane, trifluoropropylisobutyl (TFIB) POSS, in supercritical carbon dioxide (scCO2) was investigated. In the temperature and pressure ranges of 308 K to 323 K and 8.8 MPa to 14.5 MPa, respectively, TFIB POSS is soluble in scCO2 up to 2.0 x 10- 2 by weight fraction corresponding to 1.0 x 10- 3 by mole fraction, which is between the solubility of its monofunctional counterparts, octaisobutyl POSS and octatrifluoropropyl POSS. The study includes the modeling of the TFIB POSS-CO2 binary system phase equilibrium with the density-based semi-empirical equations and Peng Robinson+COSMO segment activity coefficient (PR+COSMOSAC) equation of state (EOS). The prediction of the equation of state has been improved for the cage-structured molecule by introducing new electrostatic and dispersion contributions. While the density-based relations provide better fits to the solubility isotherms, the main advantage of the EOS is its applicability for a priori phase equilibrium predictions for the CO2-POSS systems in the absence of the solute critical properties.