Partition coefficients can be measured experimentally in various ways (by shake-flask, HPLC, etc.) or estimated by calculation based on a variety of methods (fragment-based, atom-based, etc.). The partitioning of a substance into a solid results in a solid solution.
Partition coefficients can also be defined when one of the phases is solid, for instance, when one phase is a molten metal and the second is a solid metal, or when both phases are solids. For example, the blood/gas partition coefficient of a general anesthetic measures how easily the anesthetic passes from gas to blood. If one of the solvents is a gas and the other a liquid, a gas/liquid partition coefficient can be determined. Conversely, hydrophilic drugs (low octanol/water partition coefficients) are found primarily in aqueous regions such as blood serum. Hydrophobic drugs with high octanol-water partition coefficients are mainly distributed to hydrophobic areas such as lipid bilayers of cells. Partition coefficients are useful in estimating the distribution of drugs within the body. Hence the partition coefficient measures how hydrophilic ("water-loving") or hydrophobic ("water-fearing") a chemical substance is. Most commonly, one of the solvents is water, while the second is hydrophobic, such as 1-octanol. In the chemical and pharmaceutical sciences, both phases usually are solvents.
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The partition coefficient generally refers to the concentration ratio of un-ionized species of compound, whereas the distribution coefficient refers to the concentration ratio of all species of the compound (ionized plus un-ionized). This ratio is therefore a comparison of the solubilities of the solute in these two liquids. In the physical sciences, a partition coefficient ( P) or distribution coefficient ( D) is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. Not to be confused with distribution constant.
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