About Partition Coefficients
What they are
The term partition coefficient commonly refers to the equilibrium distribution of a single substance between two solvent phases separated by a boundary. The phases may be pure substances or solutions. One or both solvents are condensed phases. Partition coefficients are sometimes called distribution coefficients.
A few well-known examples may be mentioned. When one phase is a gas above its critical temperature and the other is a liquid, the partition coefficient is known also as Henry's constant or simple solubility. It is tacitly assumed here that solute concentration is low and is effectively at "infinite dilution". In vapour-liquid equilibrium, the corresponding quantity is the activity coefficient at infinite dilution.
The two solvents may be immiscible liquids, an application readily recognizable as liquid-liquid extraction, used widely in organic synthesis. Use of the octanol-water pair arose in a different context.
What they are used for
Over a century ago, it was discovered that the narcotic action of many simple organic solutes was reflected rather closely by their oil-water partition coefficients (1a). Later, "oil" was replaced by octanol, which better represents the physicochemical environment a foreign substance encounters in living tissue (2a).
In the present Databank, the octanol-water partition coefficient is designated by Kow. It is the equilibrium ratio of solute concentrations in the two solvents:Kow = [solute]oct/[solute]w
Kow first found application in medicinal chemistry and drug design, but it has proved useful in other seemingly unrelated areas as well (1b, 3). For example, Kow (or rather log Kow) correlates with toxicity, soil-water partition coefficients, bioaccumulation and sediment-water partition coefficients. It is as important a parameter as water solubility and vapour pressure in the modelling of the fate of organic pollutants in the environment (4). Such diverse applicability is no accident, because Kow is a quantitative thermodynamic measure of the hydrophilic/lipophilic balance of an organic compound (2a).
How they are measured
Kow is measured in the laboratory often by the "shake-flask" method, as in ordinary liquid-liquid extraction. This is a direct method; a number of indirect methods are also used (1b, 2b, 5).
How good are the measurements
As in other areas of science, Kow measurement methods vary greatly in accuracy and difficulty (2c). The uncertainty of the measured datum may be large, and this sometimes compromises its usefulness in application (6). This is why, in LOGKOW ©, experimental details of measurement are given for reported Kow values.
How Kow can be predicted
Kow can be calculated from the molecular structure of the compound (1c, 2d). There are a number of methods extant, a few of which are available for use on a personal computer. For some purposes, calculated Kow data are sufficiently reliable. For accurate work, the limitations of calculation methods must be considered (1d, 2e, 7). Log Kow may be predicted using various algorithms. See http://www.vcclab.org/lab/alogps.