Supercritical fluid extraction 19 SOLID LIQUID P Tc 50 Fig. 2.2. Pressure-temperature phase diagram for COz showing isochores(g cm") though the latter condition is often relaxed in the technical literature. A substance above its critical temperature therefore behaves like a gas and always occupies all available volume as a single phase. However, unlike a gas, a Scf cannot be condensed to a d-gas state by application of pressure. Similarly when the critical pressure is exceeded it is possible to go from a SCF state to a compressed liquid condition by cooling, but a single-phase filling all available volume is always maintained It should be appreciated that there are no phase boundaries delineating the SCF state and therefore no sharp changes in physical properties occur on entering this region Transition to the SCf state from a gas or liquid is thus an 'invisible process. However, it a coexisting liquid-gas mixture is heated at constant volume along the vapour pressure curve, the density of the liquid phase decreases while that of the gas phase increases,ul at the critical point they become equal and the meniscus between them disappears. As this point is approached density fluctuations of microscopic dimensions give rise to a distinctive light-scattering phenomenon known as 'critical opal Although the supercritical state offers a greater range of density, which in turn provides greater control over solubilities, the liquid state of compressed gases is often mployed in extraction processes, particularly for separation of thermolabile components at low temperatures. In order to avoid restrictive and confusing nomenclature, it is convenient to use the term near-critical liquid(NCL)to distinguish the state of compressed gas just below T from a'normal'liquid at NTP, for which T< Te. The term near critical fluid(NCF) will be used in this chapter to represent both SCF and NCL states of compressed-gas solvents Many liquids commonly employed as solvents enter an SCF state on heating, but for most purposes the critical temperatures are too high to permit their use as SCF solventsSupercritical fluid extraction 19 I 600 - 1.1 11.0 - 400 - - - k3 e a -50 0 Tc 50 100 T (“C) Fig. 2.2. Pressure-temperature phase diagram for CO, showing isochores (g ~rn-~). though the latter condition is often relaxed in the technical literature. A substance above its critical temperature therefore behaves like a gas and always occupies all available volume as a single phase. However, unlike a gas, a SCF cannot be condensed to a coexisting liquid-gas state by application of pressure. Similarly when the critical pressure is exceeded it is possible to go from a SCF state to a compressed liquid condition by cooling, but a single-phase filling all available volume is always maintained. It should be appreciated that there are no phase boundaries delineating the SCF state and therefore no sharp changes in physical properties occur on entering this region. Transition to the SCF state from a gas or liquid is thus an ‘invisible’ process. However, if a coexisting liquid-gas mixture is heated at constant volume along the vapour pressure curve, the density of the liquid phase decreases while that of the gas phase increases, until at the critical point they become equal and the meniscus between them disappears. As this point is approached density fluctuations of microscopic dimensions give rise to a distinctive light-scattering phenomenon known as ‘critical opalescence’. Although the supercritical state offers a greater range of density, which in turn provides greater control over solubilities, the liquid state of compressed gases is often employed in extraction processes, particularly for separation of thermolabile components at low temperatures. In order to avoid restrictive and confusing nomenclature, it is convenient to use the term ‘near-critical liquid’ (NCL) to distinguish the state of a compressed gas just below T, from a ‘normal’ liquid at NTP, for which T < T,. The term ‘near critical fluid’ (NCF) will be used in this chapter to represent both SCF and NCL states of compressed-gas solvents. Many liquids commonly employed as solvents enter an SCF state on heating, but for most purposes the critical temperatures are too high to permit their use as SCF solvents