Role of solvent density

The solubility of a liquid component in a near-critical solvent depends on a number of factors. These include solute volatility and the number and strengths of the solute/solvent and solvent/solvent molecular interactions in the solvent-rich phase and of the solute/solvent and solute/solute interactions in the phase being extracted. The occurrence of these interactions is density-dependent. The situation for a solid solute is conceptually rather simpler since the solvent will not normally dissolve in the solid phase. 

The exact interpretation of this behaviour is complex. In essence, however, it may be regarded as due to the competing effects of the reduction in solvent density and the increase in solute volatility which accompany the temperature rise. At pressures below about 300 bar, density decreases comparatively rapidly with isobaric increase in temperature, in the temperature range of interest, and it is the effect of this factor on solubility which predominates. At higher pressures than this the influence of temperature on density becomes less marked and an increase in solubility with temperature occurs. This behaviour is probably typical of solutes of very low solubility. 

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