Molecular Interaction and Polarity Scales

These interactions lower enthalpy and chemical potential just like interactions with liquids. The higher the compression and thus the higher the gas density, the stronger the interactions. Thus gases, adequately compressed and densified, become solvents. The compression must be carried out above the critical temperature Tc of the gas to avoid condensation to a liquid. Generally the pressure p must exceed the critical pressure pc to gain adequate compression. Any gas for which T Tc and p pc is referred to as a supercritical fluid (SF), or more simply as a dense gas [19]. 

SFs are unique—quite unlike liquids—because their solvent power is highly sensitive to pressure changes and can be varied over wide limits by pressure swings. Thus most solutes, once dissolved, can be rapidly and cleanly precipitated from SF solutions by dropping the pressure to one atmosphere, where little solvent power remains. Such flexible control of solubility, combined with improved transport characteristics relative to liquids, has made SFs popular for solvent extraction and chromatography [19-21]. 

In view of the fact that the theoretical basis of A/a° is so complex, many efforts have been made to develop simple scales or series to describe partitioning and solubility properties. By finding the relative positions of solutes and solvents on these scales, solubility can often be roughly predicted. By comparing the scale values of a solute and of two solvents, the partitioning of the solute between the two solvents can be sometimes approximated. 

No single scale can account for the entire spectrum of interactions 

For so-called regular solutions, in which small solute molecules disperse randomly among like-size solvent molecules, AS° can be taken as zero [14]. The enthalpy change AH% for transferring (mixing) pure solute of molar volume Vi into solvent p at high dilution is approximately [14] 

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