Two liquids that will not mix are said to be immiscible. If a chemical that dissolves in two immiscible fluids is added to a container holding both those liquids, a portion of the chemical will dissolve in one of the liquids, and the rest in the other. The partition coefficient is the ratio of the amount of chemical dissolved in each of the two liquids. This concept has important applications in pharmaceutical science and other technologies.
Two immiscible liquids that are combined will quickly separate into separate phases, with a distinct boundary between them. Oil floating on water is a good example. If a chemical is dissolved in this mixture, it will naturally partition itself so that some dissolves in each layer. If we call the liquids A and B, the partition coefficient is just the ratio between the concentration of that chemical in A over its concentration in B. The partition coefficient is a measure of how hydrophilic ("water loving") or hydrophobic ("water fearing") a substance is.
Measurement and Calculation
Water and octanol (a long chain carbon compound) are typically used to find the partition coefficient for a substance. The most reliable means of finding the partition coefficient is to add a known quantity of the substance to a combination of octanol and water and then measure the concentration of the substance in each liquid via ultraviolet-visible spectroscopy. A common way of expressing the partition coefficient -- abbreviated as log P -- is calculated as the base 10 logarithm of the concentration in octanol divided by the concentration in water.
Applications in Science
In environmental science, log P gives an indication of how widely and quickly a chemical pollutant will move through different types of environments, such as rivers or soils. Log P is also used to select the optimal ingredients in a variety of consumer products, such as make-up and dyes, since this can predict how well a chemical ingredient will mix with creams.
Log P can be used to predict how well a chemical will act as a drug. Since most medications have to move through both water and fatty tissues in the body, the perfect drug will be reasonably soluble in both, a property which is reflected in its value of log P. Knowing whether a drug is hydrophilic or hydrophobic can also allow predictions about where it will concentrate in the body, how it will be metabolised and how it will be excreted.
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