Editing Partition coefficient (section)

{{Short description|Ratio of concentrations in a mixture at equilibrium}}
{{Distinguish|distribution constant}}
{{EngvarB|date = February 2019}}
In the [[physical sciences]], a '''partition coefficient''' ('''''P''''') or '''distribution coefficient''' ('''''D''''') is the ratio of [[concentration]]s of a [[chemical compound|compound]] in a mixture of two [[immiscible]] solvents at [[partition equilibrium|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 [[ionization|un-ionized]] species of compound, whereas the distribution coefficient refers to the concentration ratio of all species of the compound (ionized plus un-ionized).<ref>{{cite book | vauthors = Kwon Y | title = Handbook of Essential Pharmacokinetics, Pharmacodynamics and Drug Metabolism for Industrial Scientists | date = 2001 | publisher = Kluwer Academic/Plenum Publishers | isbn = 978-1-4757-8693-4 | page = 44 | chapter = 4.2.4: Partition and Distribution Coefficients | chapter-url = https://books.google.com/books?id=yt7pBwAAQBAJ&pg=PA44 | location = New York }}</ref>

In the [[chemistry|chemical]] and [[pharmaceutical sciences]], both phases usually are [[solvent]]s.<ref name="Leo">{{cite journal | vauthors = Leo A, Hansch C, Elkins D | author-link2 = Corwin Hansch | title = Partition coefficients and their uses | journal = Chem. Rev. | volume = 71 | issue = 6 | pages = 525–616 | year = 1971 | doi = 10.1021/cr60274a001 }}</ref> Most commonly, one of the solvents is water, while the second is [[hydrophobic]], such as [[1-octanol]].<ref name="Sangster">{{cite book | vauthors = Sangster J | title = Octanol–Water Partition Coefficients: Fundamentals and Physical Chemistry | volume = 2 | series = Wiley Series in Solution Chemistry | publisher = John Wiley & Sons Ltd. | year = 1997 | location = Chichester | pages = 178 | isbn=978-0-471-97397-3 }}</ref> 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 (pharmacology)|distribution]] of drugs within the body. Hydrophobic drugs with high [[octanol-water partition coefficient]]s 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 plasma|blood serum]].<ref>{{cite book | vauthors = Shargel L, Susanna WP, Yu AB | title = Applied Biopharmaceutics & Pharmacokinetics | date = 2012 | publisher = McGraw-Hill Medical | location = New York | isbn = 978-0-07-160393-5 | page = 211 | chapter = Chapter 10: Physiological Drug Distribution and Protein Binding | edition = 6th }}</ref>

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.<ref>{{cite book | vauthors = Golan DE, Tashjian AH, Armstrong EJ, Armstrong AP | title = Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy | date = 2008 | publisher = Lippincott Williams & Wilkins | location = Philadelphia, Pa. | isbn = 978-0-7817-8355-2 | page = 243 | edition = 2nd | chapter = Chapter 15: General Anesthetic Pharmacology | chapter-url = https://books.google.com/books?id=az8uSDkB0mgC&q=gas+blood+partition+coefficient&pg=PA243 }}</ref> 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,<ref name = StallmanSolidification>{{cite book | vauthors = Stallman RE, Ngan AH | title = Modern Physical Metallurgy | date = 2014 | publisher = Elsevier/Butterworth-Heinemann | location = Amsterdam | isbn = 978-0-08-098204-5 | edition = 8th | chapter = Chapter 3: Solidification | pages = 93–120, esp. 106ff}}</ref> or when both phases are solids.<ref>{{cite book | vauthors = Machlin ES | title = An Introduction to Aspects of Thermodynamics and Kinetics Relevant to Materials Science | date = 2007 | publisher = Elsevier | location = Amsterdam|isbn=978-0-08-054968-2 | page = 98 | edition = 3rd | chapter = Chapter 3: Free Energy and Phase Diagrams | chapter-url = https://books.google.com/books?id=a9qMi8IMYmQC&pg=PR6 | quote = Solid–solid equilibria equivalent to the solid–liquid case }}</ref> 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 (chemistry)|association]] or [[Dissociation (chemistry)|dissociation]], each species is assigned its own ''K''<sub>ow</sub> 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|date=June 2021}}<!-- Text from directly transferring relevant information from Octanol-water partition coefficient article, add ref if text sourced there -->