Solvent Extraction for Bioseparations

The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of liquid-liquid distribution over the traditional term solvent extraction.10 However, solvent extraction is still used prevalently in the literature. Solvent extraction utilizes the partition of a solute between two practically immiscible liquid phases one solvent phase and the other aqueous phase.11 A separatory funnel can be used in a lab to carry out solvent extraction. Of course, a simple test tube can also be used in conjunction with a glass pipette. The organic phase (solvent phase) is usually the top phase and the aqueous phase bottom phase. However, some organic solvents are heavier than water (for example, methylene chloride s specific gravity is 1.33 at 20°C) and in such cases the organic phase becomes the bottom phase. 

TABLE I Partition Coefficients of Some Biomolecules in Solvent Extraction 

Source Reprinted with permission from Belter et al.11 Copyright 1988 John Wiley Sons, 

R is the universal gas constant and T is the absolute temperature in degrees Kelvin. The concept of solubility parameter can be used to predict the partition coefficient qualitatively in the selection of solvents.22 We can write K as 

Obviously, some solvents are better than others for the extraction of a particular compound. A solvent system should provide a suitable partition coefficient, preferably much greater than unity or much less than unity, depending on whether you want the solute to migrate to the solvent phase or to stay in the aqueous phase. The following factors must be considered during the selection of a suitable solvent system23,24  

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Supercritical fluid extraction processes are particularly appropriate for the separation and isolation of biochemicals where thermal decomposition, chemical modification, and physiologically-active solvents are undesirable. Examples of these bioseparations include the extraction of oils from seeds using carbon dioxide (1), of nicotine from tobacco using carbon dioxide-water mixtures (2), and of caffeine from coffee beans again using carbon dioxide-water mixtures (3). 

Very often, particularly in bioseparation, extraction is used to separate solutes from each other fBelter et al.. 1988 Frank etal.. 2008 Schiebel, 1978). In this situation we can use fractional extraction with two solvents as illustrated in Figure 13-5. In fractional extraction the two solvents are chosen so that solute A prefers solvent 1 and concentrates at the top of the column, while solute B prefers solvent 2 and concentrates at the bottom of the column. In Figure 13-5 solvent 2 is labeled as diluent so that we can use the nomenclature of Table 13-2. The column sections in Figure 13-5 are often separate so that each section can be at a different pH or temperature. This will make the equilibrium curve different for the two sections. It is also common to have reflux at both ends (Schiebel, 1978). 

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