Controlling Solvation and Mass Transport Properties of Biobased Solvents through CO2 Expansion: A Physicochemical and Molecular Modeling Study

Abstract

Gas-expanded liquids have been studied during past years; however, the physicochemical properties of some of these fluids still need to be characterized and understood. In particular, the study of properties concerning solvation and mass transport is key for industrial applications. This work presents the characterization of eight CO2-expanded biosourced solvents: organic carbonates (dimethyl, diethyl, ethylene, and propylene carbonates), anisole, veratrole, γ-valerolactone, and 2-methyltetrahydrofuran. Two approaches have been used: spectroscopic measurements and molecular modeling. Phase equilibrium was determined for each CO2/biosourced solvent system, and then the solvatochromic probe Nile Red was used to determine changes in dipolarity/polarizability (π* Kamlet–Taft parameter) by CO2 pressure. Molecular dynamics calculations were performed to determine the density and viscosity changes with CO2 pressure. It is shown in this study that the degree of modulation of dipolarity/polarizability parameter can go from that of pure solvent (around 0.4 for linear organic carbonates) to negative values, close to that of pure CO2 at the T and P used in this study. Concerning transport properties, such as density and viscosity, a great decrease in both these properties’ values was observed after swelling of the solvent by CO2, for instance, in linear organic carbonates where density can decrease to 50% the density of pure solvent; concerning viscosity a decrease of up to 90% was measured for these compounds. It was observed that the solubility of CO2 and then modulation of properties were higher in linear organic carbonates than in the cyclic ones. This study shows once more that CO2 has a great capacity to be used as a knob for triggering changes in the physicochemical properties of green biosourced solvents that can help to implement these solvents in industrial applications.