Supercritical fluid extraction 25 (3) NCFs can be highly discriminating and frequently offer a greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or injecting entrainers(see Section 2.3. 2) usually serves to reduce selectivity Although some selectivity is sacrificed it is often preferable to operate at high pressures (and temperatures) to obtain sufficient solubility to make a process viable. The conditions cited for a specific process are often arrived at from an optimisation of these opposing effects of selectivity and solubility. However, the electivity that is exploited in extraction processes is sometimes an intrinsic roperty of the NCF solvent and is not always dramatically changed by the onditions (e.g. in the selective extraction of triglycerides from phospholipids General princip Effect of molecular structure Any pragmatic assessment of a solvent extraction process must examine what type of molecules are soluble and to what extent. With nCFs the molecular structure of the solute is of major importance as small changes in molecular weight and functional groups can affect solubility to a greater extent than with liquid solvents. In fact the viability of many simple separation processes using NCFs can be realised without recourse to extensive solubility data covering a wide range of conditions. Francis (1954)has pains takingly measured the solubilities of 261 substances in liquid CO2, and this pioneering work still acts as a useful guide to the relative solubilities of different classes of com- pounds in NCF CO. Dandge et al. (1985) have used this and other data to correlate the solubilities of different classes of chemical compounds with molecular structure. Some of the broad principles emerging from this work are given below (1)Solubility is reduced by increasing polarity. A good illustration of this is to be found in the relative solubilities of ethanol and ethylene glycol in liquid CO2. Whereas the former is completely miscible(M), increasing the overall polarity by introducing a second hydroxyl group reduces the solubility to 0. 2%. Miscibility with liquid CO2 can recovered by methy lation of the Oh groups, which reduces the polarity of the molecule ethanol dimethyl ether (02%) (2) Solubility declines with increasing molecular weight and for any homologous series the solubility decreases rapidly beyond a given carbon number. This effect is illustrated in Fig. 2.5, which also serves to demonstrate the effect of polarity on solubility since the more polar alcohol has a lower carbon number'cut off than the parent alkaneSupercritical fluid extraction 25 NCFs can be highly discriminating and frequently offer a greater selectivity than liquid solvents. Any attempt at increasing solubilities by changing conditions or injecting entrainers (see Section 2.3.2) usually serves to reduce selectivity. Although some selectivity is sacrificed it is often preferable to operate at high pressures (and temperatures) to obtain sufficient solubility to make a process viable. The conditions cited for a specific process are often arrived at from an optimisation of these opposing effects of selectivity and solubility. However, the selectivity that is exploited in extraction processes is sometimes an intrinsic property of the NCF solvent and is not always dramatically changed by the conditions (e.g. in the selective extraction of triglycerides from phospholipids; Section 2.6.2). (3) General principles Effect of molecular structure Any pragmatic assessment of a solvent extraction process must examine what type of molecules are soluble and to what extent. With NCFs the molecular structure of the solute is of major importance as small changes in molecular weight and functional groups can affect solubility to a greater extent than with liquid solvents. In fact the viability of many simple separation processes using NCFs can be realised without recourse to extensive solubility data covering a wide range of conditions. Francis (1954) has pains￾takingly measured the solubilities of 261 substances in liquid C02, and this pioneering work still acts as a useful guide to the relative solubilities of different classes of com￾pounds in NCF C02. Dandge et al. (1985) have used this and other data to correlate the solubilities of different classes of chemical compounds with molecular structure. Some of the broad principles emerging from this work are given below: (1) Solubility is reduced by increasing polarity. A good illustration of this is to be found in the relative solubilities of ethanol and ethylene glycol in liquid C02. Whereas the former is completely miscible (M), increasing the overall polarity by introducing a second hydroxyl group reduces the solubility to 0.2%. Miscibility with liquid C02 can be recovered by methylation of the OH groups, which reduces the polarity of the molecule. HO 4 Ho # OH CH30 - OCH3 ethylene glycol ethylene glycol dimethyl ether ethanol (M) (0.2%) (M) (2) Solubility declines with increasing molecular weight and for any homologous series the solubility decreases rapidly beyond a given carbon number. This effect is illustrated in Fig. 2.5, which also serves to demonstrate the effect of polarity on solubility since the more polar alcohol has a lower carbon number 'cut off' than the parent alkane