Polar attributes of supercritical carbon dioxide

Supercritical carbon dioxide (scCO(2)) is increasingly promoted as an environmentally benign alternative to conventional organic solvents. The supercritical state bridges the gap between liquid and gaseous states by offering gaslike diffusion rates and liquidlike solvent densities, thereby enabling potential opportunities as a reaction and separation medium in chemical industry. Understanding the solvent behavior of liquid and scCO(2) is of critical importance to enable the design Of CO2-philic molecular systems and to expand the use of these solvent systems to a wider range of chemical processes. Historically CO2 was treated as a nonpolar solvent, primarily because of its low dielectric constant and zero molecular dipole moment. CO2 has also been described as a quadrupolar solvent because of its significant quadrupole moment. Recent studies suggest that, as far as the microscopic solvent behavior Of CO2 is concerned, CO2 has the potential to act as both a weak Lewis acid and Lewis base. Also, strong theoretical and experimental evidence indicates that CO2 can participate in conventional or nonconventional hydrogen-bonding interactions. All of these site-specific solute-solvent interactions are important to understand the fundamental nature Of CO2 as a solvent. In this Account, we discuss these polar attributes Of CO2 and their relation to solvation.