Spectroscopic Analysis of the Local Polarity in Gas Expanded Liquids

Gas Expanded Liquids (GXLs) are ambient temperature organic solvents that undergo volumetric expansion via pressurization with CO₂. Much like supercritical fluid solvents, GXLs provide an advantageous reaction media because of the pressure-tunability of the solvent in order to control phase behavior, transport properties, solvent properties, reaction rates, and reaction selectivity. In addition, the environmental benefits of CO₂ make GXLs an attractive medium especially for the food and pharmaceutical industries. This investigation examines the effects of CO₂ concentration on the Gibb's energy of activation for the isomerization of a probe molecule, 4-(diethylamino) 4'-nitroazobenzene (DENAB). The functionality of DENAB permits the investigation of the cybotactic region, the region immediately surrounding the probe molecule, and enables the distinction between macroscopic and cybotactic region properties. With the addition of CO₂, deviation from pure solvent activation energy signifies a change in the polarity of the cybotactic region. The deviation in activation energy also emphasizes the user control over solvent thermodynamic and kinetic properties.