Solvation of benzene derivatives in SC-CO2: a molecular dynamics study of fluorination effects

Abstract

According to a molecular dynamics study of benzene (C₆H₆) and its derivatives C₆F₆, CH₃–C₆H₅ and CF₃–C₆H₅ in SC-CO₂ solution, the fluoro compounds are better solvated than the H analogs. This is supported by the analysis of average solute-solvent interaction energies and by free energy perturbation simulations at 305 and 350 K. Stepwise model building of C₆X₆(CO₂)_n aggregates (X = H and F) in the gas phase, however, indicates that up to n = 6, interactions with CO₂ are stronger with benzene, while perfluorobenzene becomes better solvated at higher coordination numbers. Thus, the enhancement of the CO₂-philicity upon fluorination does not stem from enhanced individual interactions with the CO₂ molecule, but from the higher coordination number, due to the increase of solvent accessible surface. Finally, simulations of C₆H₆ and C₆F₆ at the CO₂–water interface reveal that the former is somewhat surface active due to specific O–H⋯π facial interactions between interfacial water molecules, while C₆F₆ mostly sits in the CO₂ phase, in keeping with its larger CO₂-philicity.