Direct measurements of the enthalpy of solution of solid solute in supercritical fluids: study on the CO2-naphthalene system

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Enhancing the Rate of the Diels−Alder Reaction Using CO2 + Ethanol and CO2 + n-Hexane Mixed Solvents of Different Phase Regions

The reaction rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthrance in CO2 + ethanol and CO2 + hexane mixed solvents of different compositions were determined by in situ UV/vis spectroscopy at 318.15 K and different pressures. The density of the mixed solvents at different pressures was also determined and the isothermal compressibility was calculated using the density data. The activation volume of the reaction was calculated based on the dependence of rate constant (kc) on pressure. It was demonstrated that the kc was very sensitive to the pressure in the mixed solvents near the critical region and the kc increased dramatically as pressure approached dew points, critical point, and bubble points of the mixed solvents. However, the kc in the mixed solvents outside the critical region or in pure CO2 was not sensitive to pressure. At suitable conditions, kc could be 40 times larger than that in acetonitrile. The activation volume of the reaction was nearly independent of pressure as the pressure was much higher than the phase separation pressure of the mixed solvents, while it increased considerably as pressure approached the bubble points, critical point, and dew points from high pressure. The clustering of the solvent molecules with the reactants and the activated complex in the reaction systems near the phase boundary in the critical region may be the main reason for the interesting phenomena observed. This work also shows that, using pure CO2 as the solvent, the reaction cannot be carried out in the critical region of the solvent due to the limitations of the reactants, while it can be conducted in the critical region of mixed solvents of suitable compositions, where the solvents are highly compressible and the reaction rate can be tuned effectively by pressure.

Chemical reaction in binary mixtures near the critical region: thermal decomposition of 2,2'-azobis(isobutyronitrile) in CO2/ethanol

The effects of pressure and of the composition of the CO2/ethanol mixed solvent in the critical region on the kinetics of the decomposition of 2,2'-azobis(isobutyronitrile) (AIBN) were studied at 333.15 K. The rate constants (kd) in the mixed solvent far from the critical point and in liquid n-hexane and ethanol were also determined for comparison. It was found that kd is very sensitive to pressure in the mixed solvent near the critical point. However, in the mixed solvent outside the critical region kd is nearly independent of pressure. Interestingly, kd in the mixed solvent in the critical region can be higher than that in ethanol at the same temperature, suggesting that no significant enhancement in the reaction rate by a small pressure change in the critical region of the mixed solvent can be achieved by changing the composition of the liquid solvent in the traditional way. Transition-state theory can predict kd in the mixed solvent far from the critical point and in the liquid solvents well. However, it cannot predict kd in the mixed solvent in the critical region. The special intermolecular interaction between the solvent and the reaction species may contribute to this interesting phenomenon. This work also shows that if pure CO2 or ethanol are used as solvents, the reaction cannot be carried out in the critical region of the solvents at the desired temperature, while it can be conducted in the critical region of the mixed solvent of suitable composition, where the solvent is highly compressible.