Characterisation and Applications of CO2-Expanded Solvents.

The use of CO2 as an alternative to traditional organic solvents has been an extensive area of research over the last several decades with research focusing mainly on supercritical applications. Gas eXpanded Liquids (GXLs) combine the advantages of liquid CO2 and co-solvents. Much like its supercritical counterpart, the solvent power of GXLs can be tuned by varying the liquid phase concentration as a function of pressure. Determination of solvent-solute interactions is key to the understanding of solvent properties in liquids and expanded solvents. Spectroscopic measurements of a range of binary mixtures of organic solvent with carbon dioxide have been recorded to calculate solvatochromic parameters for gas expanded liquids. Data obtained for gas expanded solvents showed a significant change in local polarity upon addition of CO2, modifying the properties of traditional organic solvents. Protic solvents were found to behave anomalously to conventional aprotic solvents. Density, relative permittivity, and CO2 solubility at 25 °C and 50 bar pressure for a range of CO2-expanded solvents are reported for the first time. The dissolution of CO2 into liquid organic solvents to generate expanded liquids has resulted in significant changes in bulk solvent properties. Collation of relative permittivity data and solvatochromism data of the expanded liquids has given an insight into the structural changes occurring in the local and bulk regions of the solvent, resulting in the occurrence of preferential solvation. Variation in these solvent properties are understood by the determination of molar free volumes which was correlated with the Hildebrand solubility parameter showing that the expansion of molecular solvents is controlled by the thermodynamics of cavity formation. A range of applications have been probed using gas expanded liquids as replacement solvents. One of the most prominent advantages of GXLs for chemical synthesis is their adjustable solvating power. Areas such as biphasic chemistry, selective reactions, solubility, and phase behaviour studies have been explored. It was found that in each application the CO2 expanded solvent had a varied ‘role’.