Partitioning, between organic solvents

Partitioning between organic solvents is used in the extraction of analytes from oily excipients such as in the extraction of steroid creams prior to HPLC analysis. The... 

Y. Kubota, Ion-Transfer Voltammetry of Organic Compounds at Organic Solvent/Water Interface. Study on Partition of Organic Compounds between Organic Solvent and Water, MS thesis, Fukui Prefectural University, Fukui 1998. 

Give several reasons why it is important to know something about the partitioning behavior of a given compound between organic solvents and water. 

The rationale for this approach was based on the similarity between the dissolution of an organic solute in water, and its partitioning between two solvents. Thus, the equilibrium between an organic solute and its saturated aqueous solution was thought to be similar to that of the partitioning of the solute between itself and water, and a linear relationship between log S (S = solubility in moles/lit) and log p was sought. This study by Hansch et al (I) reported the following QSAR model ... 

In MLC, solutes are separated on the basis of their differential partitioning between bulk solvent and micelles in the mobile phase or surfactant-coated stationary phase. For water-insoluble species or for species strongly bound to micelles, partitioning can also occur via direct transfer between the micellar pseudophase and the modified stationary phase. Partition equilibria are affected by a variety of factors, such as the nature and concentration of surfactant and organic modifiers, temperature, ionic strength, and pH. 

We already know that organic compounds tend to partition between immiscible solvents so that the ratio of concentrations in the two solvents is constant (the pcirtition coefScient). We also know that organic acids and bases tend to associate with or dissociate from hydrogen ions according to pH, as described by the Henderson-Hasselbalch (H-H) equation. We can therefore picture a situation in which the two solvents may each contain both ionised and un-ionised material (Fig. 8.6). 

In a simple liquid-liquid extraction the solute is partitioned between two immiscible phases. In most cases one of the phases is aqueous, and the other phase is an organic solvent such as diethyl ether or chloroform. Because the phases are immiscible, they form two layers, with the denser phase on the bottom. The solute is initially present in one phase, but after extraction it is present in both phases. The efficiency of a liquid-liquid extraction is determined by the equilibrium constant for the solute s partitioning between the two phases. Extraction efficiency is also influenced by any secondary reactions involving the solute. Examples of secondary reactions include acid-base and complexation equilibria. 

A weak acid, HA, with a fQ of I.O X 10, has a partition coefficient, Rp, between water and an organic solvent of I20( What restrictions on the sample s pH are necessary to ensure that a minimum of 99.9% of the weak acid is extracted in a single step from 50.0 mF of water using 50.0 mF of the organic solvent ... 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

Closely related to water solubility as a polarity measure is the partition coefficient of a substance between water and an immiscible organic solvent. Most commonly the organic solvent is selected to be n-octanol, and the symbol P is given to the octanol/water partition coefficient. Then log P is a quantitative measure of hydro-phobicity and, therefore, of nonpolarity. Table 8-3 gives log P values for many of... 

Filtration of the catalyst over a Hyflo pad and removal of the solvent left a yellow crystalline residue. The crude mixture of ketone and potassium acetate was partitioned between methylene chloride (300 cc) and water (1 liter), The layers were separated and the water layer washed with methylene chloride (3 x 50 cc). The organic layers were combined, washed with 3N sodium hydroxide solution (2 x 50 cc), water (3 x 100 cc), dried over anhydrous sodium sulfate and filtered. The solvent was removed and the product recrystallized from ethanol to give 2-amino-2 -fluorobenzophenone as yellow prisms melting at 126° to 128°C,... 

The reaction systems used for modification of triglycerides usually consist of a lipase catalyst and a small amount of water dispersed in a bulk organic phase containing the reactants and, if required, a water immiscible solvent. The small amount of water in the reaction system partitions between the catalyst and the bulk organic phase. 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

Predictions of bioaccumulation assume a standard model of dissolution in fat and are based on partition between water and organic solvent. The better studied tributyltin has been shown to partition based on binding to protein rather than dissolution in fat this might account for discrepancies between observed and predicted BCFs. 

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