Partition coefficient

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. This ratio is therefore a comparison of the solubilities of the solute in these two liquids. 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).^[1]

In the chemical and pharmaceutical sciences, both phases usually are solvents.^[2] Most commonly, one of the solvents is water, while the second is hydrophobic, such as 1-octanol.^[3] Hence the partition coefficient measures how hydrophilic ("water-loving") or hydrophobic ("water-fearing") a chemical substance is. Partition coefficients are useful in estimating the distribution of drugs within the body. Hydrophobic drugs with high octanol-water partition coefficients are mainly distributed to hydrophobic areas such as lipid bilayers of cells. Conversely, hydrophilic drugs (low octanol/water partition coefficients) are found primarily in aqueous regions such as blood serum.^[4]

If one of the solvents is a gas and the other a liquid, a gas/liquid partition coefficient can be determined. For example, the blood/gas partition coefficient of a general anesthetic measures how easily the anesthetic passes from gas to blood.^[5] 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,^[6] or when both phases are solids.^[7] The partitioning of a substance into a solid results in a solid solution.

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.).

If a substance is present as several chemical species in the partition system due to association or dissociation, each species is assigned its own K_ow value. A related value, D, does not distinguish between different species, only indicating the concentration ratio of the substance between the two phases.^{[citation needed]}

^ Kwon Y (2001). "4.2.4: Partition and Distribution Coefficients". Handbook of Essential Pharmacokinetics, Pharmacodynamics and Drug Metabolism for Industrial Scientists. New York: Kluwer Academic/Plenum Publishers. p. 44. ISBN 978-1-4757-8693-4.
^ Leo A, Hansch C, Elkins D (1971). "Partition coefficients and their uses". Chem. Rev. 71 (6): 525–616. doi:10.1021/cr60274a001.
^ Sangster J (1997). Octanol–Water Partition Coefficients: Fundamentals and Physical Chemistry. Wiley Series in Solution Chemistry. Vol. 2. Chichester: John Wiley & Sons Ltd. p. 178. ISBN 978-0-471-97397-3.
^ Shargel L, Susanna WP, Yu AB (2012). "Chapter 10: Physiological Drug Distribution and Protein Binding". Applied Biopharmaceutics & Pharmacokinetics (6th ed.). New York: McGraw-Hill Medical. p. 211. ISBN 978-0-07-160393-5.
^ Golan DE, Tashjian AH, Armstrong EJ, Armstrong AP (2008). "Chapter 15: General Anesthetic Pharmacology". Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy (2nd ed.). Philadelphia, Pa.: Lippincott Williams & Wilkins. p. 243. ISBN 978-0-7817-8355-2.
^ Stallman RE, Ngan AH (2014). "Chapter 3: Solidification". Modern Physical Metallurgy (8th ed.). Amsterdam: Elsevier/Butterworth-Heinemann. pp. 93–120, esp. 106ff. ISBN 978-0-08-098204-5.
^ Machlin ES (2007). "Chapter 3: Free Energy and Phase Diagrams". An Introduction to Aspects of Thermodynamics and Kinetics Relevant to Materials Science (3rd ed.). Amsterdam: Elsevier. p. 98. ISBN 978-0-08-054968-2. Solid–solid equilibria equivalent to the solid–liquid case

[1] Kwon Y (2001). "4.2.4: Partition and Distribution Coefficients". Handbook of Essential Pharmacokinetics, Pharmacodynamics and Drug Metabolism for Industrial Scientists. New York: Kluwer Academic/Plenum Publishers. p. 44. ISBN 978-1-4757-8693-4.

[Leo-2] Leo A, Hansch C, Elkins D (1971). "Partition coefficients and their uses". Chem. Rev. 71 (6): 525–616. doi:10.1021/cr60274a001.

[Sangster-3] Sangster J (1997). Octanol–Water Partition Coefficients: Fundamentals and Physical Chemistry. Wiley Series in Solution Chemistry. Vol. 2. Chichester: John Wiley & Sons Ltd. p. 178. ISBN 978-0-471-97397-3.

[4] Shargel L, Susanna WP, Yu AB (2012). "Chapter 10: Physiological Drug Distribution and Protein Binding". Applied Biopharmaceutics & Pharmacokinetics (6th ed.). New York: McGraw-Hill Medical. p. 211. ISBN 978-0-07-160393-5.

[5] Golan DE, Tashjian AH, Armstrong EJ, Armstrong AP (2008). "Chapter 15: General Anesthetic Pharmacology". Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy (2nd ed.). Philadelphia, Pa.: Lippincott Williams & Wilkins. p. 243. ISBN 978-0-7817-8355-2.

[StallmanSolidification-6] Stallman RE, Ngan AH (2014). "Chapter 3: Solidification". Modern Physical Metallurgy (8th ed.). Amsterdam: Elsevier/Butterworth-Heinemann. pp. 93–120, esp. 106ff. ISBN 978-0-08-098204-5.

[7] Machlin ES (2007). "Chapter 3: Free Energy and Phase Diagrams". An Introduction to Aspects of Thermodynamics and Kinetics Relevant to Materials Science (3rd ed.). Amsterdam: Elsevier. p. 98. ISBN 978-0-08-054968-2. Solid–solid equilibria equivalent to the solid–liquid case

[1]

[2]

[3]

[4]

[5]

[6]

[7]