Yizhak Marcus

Supercritical fluids are used for the extraction of desired ingredients from natural materials, but also for the removal of undesired and harmful ingredients. In this paper, the pertinent physical and chemical properties of supercritical water, methanol, ethanol, carbon dioxide, and their mixtures are provided. The methodologies used with supercritical fluid extraction are briefly dealt with. Advances in the application of supercritical extraction to fuels, the gaining of antioxidants and other useful items from biomass, the removal of undesired ingredients or contaminants, and the preparation of nanosized particles of drugs are described.

The standard entropies S_298°E of deep eutectic solvents (DESs), which are liquid binary mixtures of a hydrogen bond acceptor component and a hydrogen bod donor one, are calculated from their molecular volumes, derived from their densities or crystal structures. These values are compared with those of the components—pro-rated according to the DES composition—to obtain the standard entropies of DES formation Δ_fS. These quantities are positive, due to the increased number and kinds of hydrogen bonds present in the DESs relative to those in the components. The Δ_fS values are also compared with the freezing point depressions of the DESs Δ_fusT/K, but no general conclusions on their mutual relationship could be drawn.

The effect of ions on the structure of water in dilute solutions, whether they are structure-makers or structure-breakers, is manifested also in the volatility of the water. For more than 40 uni-univalent electrolytes, there is a linearly increasing relationship between 2φ(m = 0.4) − φ(m = 0.2), where φ is the osmotic coefficient and m is the molality, and the difference between the viscosity B-coefficients of the cation and anion. Exceptions to this relationship are electrolytes with highly structure-making anions and with hydrophobic cations.

The solubility of gaseous carbon dioxide in a variety of solvents has been extensively studied, the solute interacting with most solvents via dispersion forces. Hence, its Hildebrand solubility parameter, δ_H, may be used to predict its dissolution in liquids. The usual definition of δ_H involves Δ_vH, the molar enthalpy of vaporization, strictly applicable to liquids. This expression is inapplicable for carbon dioxide, being a gas at the temperatures of technical interest (298 ≤ T/K ≤ 333), and various indirect methods have been employed for the determination of δ_H(CO₂,T). The appreciable polarizability of CO₂ and its ability to accept hydrogen bonds from suitable donor solvents prompted the determination of its Hansen solubility parameters. Finally, supercritical carbon dioxide has been extensively used as a solvent so that its δ_H(scCO₂,T,P) is a useful quantity to know. The large discrepancies between the reported quantities for δ_H(CO₂,T) are analyzed and discussed.

The critical temperatures of two dozen deep eutectic solvents, for only some of which these have been estimated previously, were estimated from the temperature dependences of their surface tensions and densities available in the literature according to the Eötvös and the Guggenheim expressions.

Hot, subcritical and supercritical water, methanol, ethanol and their binary mixtures have been employed to treat fuels (desulfurize coal and recover liquid fuels from coal and oil shales) and to extract valuable solutes from biomass. The properties of these solvents that are relevant to their extraction abilities are presented. Various extraction methods: accelerated solvent extraction (ASE), pressurized liquid extraction (PLE), supercritical fluid extraction (SFE, but excluding supercritical carbon dioxide) with these solvents, including microwave- and ultrasound-assisted extraction, are dealt with. The extraction systems are extensively illustrated and discussed.