Solvents partition between immiscible

O ince partitioning between immiscible solvents is an equilibrium process, it should be possible to calculate partition coefficients for any solute between any two given solvents if we had a measure of the solvation forces involved. At present these forces are not that well characterized, and the reverse approach can be more enlightening—i.e., a study of how the partition coefficient varies between the systems can yield valuable insight into the types and relative magnitude of the forces involved. 

There has been much speculation in the past on the nature of the circulating thyroid hormone whether it was a simple amino acid, a peptide or polypeptide of thyroxine, or thyroglobulin itself. In 1948 Taurog and Chaikoff produced a considerable amount of evidence that the plasma hormone was indeed thyroxine. Labeled plasma iodine behaved chemically in a manner identical with that of thyroxine added to plasma it was nondialyzable it was precipitated with plasma proteins with zinc hydroxide it was extractable with butanol, and it could be fractionated with carrier thyroxine by its partition between immiscible solvents. This work received confirmation from Laidlaw (1949), and it then became generally accepted that thyroxine alone was the circulating hormone. 

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). 

The importance of thermodynamics in the pharmaceutical sciences is apparent when it is realised that such processes as the partitioning of solutes between immiscible solvents, the solubility of dmgs, micellisation and dmg-receptor interaction can all be treated in thermodynamic terms. This brief section merely introduces some of the concepts of thermodynamics which are referred to throughout the book. Readers requiring a greater depth of treatment should consult standard texts on this subject. ... 

These usually are decomposed by light, air and solvents, so that degradation products are probable impurities. Chromatography and adsorption spectra permit the ready detection of coloured impurities, and separations are possible using solvent distribution, chromatography or crystallisation. Thus, in partition between immiscible... 

The equilibrium distribution of a solute between immiscible solvents is expressed by the distribution (or partition) coefficient (K) which is the ratio of... 

Cleanup can be achieved by partitioning of the alkaloids between immiscible solvents, including salting out techniques [13], following which dissolution in a polar solvent mixture and a simple wash with hexane produced a suitable sample for LC-MS/MS. 

The partition of molecules between two phases can be based on different sorts of equilibrium. Meaningful are equilibria concerning the processes of ion exchange, partition of substances between immiscible solvents (solvent extraction), and accumulation of substances at solid surfaces (adsorption), hi some cases, real chemical bonds are formed, but sometimes only weak forces control the process. These equilibria generally are reversible, and they are mobile, i.e. they tend to react fast to concentration changes. This is a valuable property for sensor applications. Furthermore, they contribute to the accumulation of traces at surfaces, and they are important in manufacturing ordered structures at surfaces. The following discussions are dedicated to equihbria of particular interest. 

The theory of the process can best be illustrated by considering the operation, frequently carried out in the laboratory, of extracting an orgaiuc compound from its aqueous solution with an immiscible solvent. We are concerned here with the distribution law or partition law which, states that if to a system of two liquid layers, made up of two immiscible or slightly miscible components, is added a quantity of a third substance soluble in both layers, then the substance distributes itself between the two layers so that the ratio of the concentration in one solvent to the concentration in the second solvent remains constant at constant temperature. It is assumed that the molecular state of the substance is the same in both solvents. If and Cg are the concentrations in the layers A and B, then, at constant temperature ... 

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. 

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. 

In conventional solvent extraction, a solute is partitioned between two immiscible solvents. Here, used as an... 

MEEKC is a CE mode similar to MEKC, based on the partitioning of compounds between an aqueous and a microemulsion phase. The buffer solution consists of an aqueous solution containing nanometer-sized oil droplets as a pseudo-stationary phase. The most widely used microemulsion is made up of heptane as a water-immiscible solvent, SDS as a surfactant and 1-butanol as a cosurfactant. Surfactants and cosurfactants act as stabilizers at the surface of the droplet. 

Lipophilicity is a molecular property expressing the relative affinity of solutes for an aqueous phase and an organic, water-immiscible solvent. As such, lipophilicity encodes most of the intermolecular forces that can take place between a solute and a solvent, and represents the affinity of a molecule for a lipophilic environment. This parameter is commonly measured by its distribution behavior in a biphasic system, described by the partition coefficient of the species X, P. Thermodynamically, is defined as a constant relating the activity of a solute in two immiscible phases at equilibrium [111,112]. By convention, P is given with the organic phase as numerator, so that a positive value for log P reflects a preference for the lipid phase ... 

The partition coefficient of a substance between several Immiscible solvent pairs can be combined with retention time data to confirm the identity of a substance when a pure standard is available [706]. Devised by Bowman and Beroza, the substance specific partition coefficient ("p-value") was defined as the fractional amount of substance partitioning into the less polar phase of an equal-volume, two-phase system. Only nanogram quantities of sample are required for the measurement and p-values are often sufficiently characteristic to distinguish between closely related substances. 

The operation of a separating funnel depends on partition. A solvent contains some solute. A different solvent, which is immiscible with the first, contains no compound. Because the two solvents are immiscible - which means they do not mix - the separating funnel will show two distinct layers (see Figure 5.15). After shaking the funnel vigorously, and allowing its contents to settle, some of the solute will have partitioned between the two solvents, with some sample passing from the solution into the previously pure solvent 1. 

A great number of separation processes are based on solvent extraction, especially since this is also a concentration technique. For these reasons, solvent extraction will be considered, both from the point of view of the sampling process and from that of the general analytical process. Solvent extraction is ultimately a process of partitioning between two immiscible solvents, and for its optimization it is necessary to know first of all the operational parameters of the system. 

If a substance is soluble in both water and the organic solvent, the result of the extraction depends on the ratio of solubilities if the partition coefficient , e.g. the ratio of the solubility in water to that in ether, is large, correspondingly more ether must be used or the number of extractions must be increased. For this coefficient determines how a substance soluble in two immiscible solvents will distribute itself between them. Whether an aqueous solution should be extracted with a certain amount of ether in one portion or whether it is better to extract several times with smaller portions is... 

The partition of different lipids between two immiscible solvents (countercurrent distribution) is useful for crude fractionation of lipid classes with greatly differing polarities. Repeated extractions in a carefully chosen solvent pair increase the effectiveness of the separation but in practice mixtures of lipids are still found in each fraction. A petroleum ether-ethanol-water system can be used to remove polar contaminants (into the alcoholic phase) when interest lies in the subsequent analysis of neutral glycerides, which may be recovered from the ether phase. Carbon... 

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