Editing Partition coefficient (section)

== Partition coefficient and log ''P'' ==
[[Image:Separatory funnel with oil and colored water.jpg|thumb|upright|An equilibrium of dissolved substance distributed between a hydrophobic phase and a hydrophilic phase is established in special glassware such as this [[separatory funnel]] that allows shaking and sampling, from which the log ''P'' is determined. Here, the green substance has a greater solubility in the lower layer than in the upper layer.]]
The '''partition coefficient''', abbreviated '''''P''''', is defined as a particular ratio of the [[concentration]]s of a [[solute]] between the two solvents (a biphase of liquid phases), specifically for un-[[Dissociation (chemistry)|ionized]] solutes, and the [[logarithm]] of the ratio is thus '''log ''P'''''.<ref name = "Comer_2001">{{cite book | veditors = Testa B, van de Waterbed H, Folkers G, Guy R, Comer J, Tam K | title = Pharmacokinetic Optimization in Drug Research: Biological, Physicochemical, and Computational Strategies | chapter = Lipophilicity Profiles: Theory and Measurement | date = 2001 | publisher = Wiley-VCH | location = Weinheim | isbn = 978-3-906390-22-2 | pages = 275–304 | doi = 10.1002/9783906390437.ch17 | vauthors = Comer J, Tam K }}</ref>{{rp|275ff}} When one of the solvents is water and the other is a [[non-polar solvent]], then the log ''P'' value is a measure of [[lipophilicity]] or [[hydrophobicity]].<ref name = "Comer_2001"/>{{rp|275ff}}<ref name = Martin2ndCh4/>{{rp|6}} The defined precedent is for the lipophilic and [[hydrophilic]] phase types to always be in the [[numerator]] and [[denominator]] respectively; for example, in a biphasic system of ''n''-[[octanol]] (hereafter simply "octanol") and water:

: <math>\log P_\text{oct/wat} = \log_{10}\left(\frac{\big[\text{solute}\big]_\text{octanol}^\text{un-ionized}}{\big[\text{solute}\big]_\text{water}^\text{un-ionized}}\right).</math>

To a first approximation, the non-polar phase in such experiments is usually dominated by the un-ionized form of the solute, which is electrically neutral, though this may not be true for the aqueous phase. To measure the ''partition coefficient of ionizable solutes'', the [[pH]] of the aqueous phase is adjusted such that the predominant form of the compound in solution is the un-ionized, or its measurement at another pH of interest requires consideration of all species, un-ionized and ionized (see following).

A corresponding '''partition coefficient''' for ionizable compounds, abbreviated '''log ''P'' <sup>''I''</sup>''', is derived for cases where there are dominant [[Dissociation (chemistry)|ionized]] forms of the molecule, such that one must consider partition of all forms, ionized and un-ionized, between the two phases (as well as the interaction of the two equilibria, partition and ionization).<ref name = Martin2ndCh4/>{{rp|57ff,69f}}<ref name="logp_review">{{Cite journal | title = Lipophilicity Profiles of Ampholytes| vauthors = Pagliara A, Carrupt PA, Caron G, Gaillard P, Testa B | doi = 10.1021/cr9601019 | pmid = 11851494 | year = 1997 | journal = Chemical Reviews | pages = 3385–3400| volume = 97 | issue = 8 }}</ref> ''M'' is used to indicate the number of ionized forms; for the {{mvar|I}}-th form ({{math|1=''I'' = 1, 2, ... , ''M''}}) the logarithm of the corresponding partition coefficient, <math>\log P_\text{oct/wat}^I</math>, is defined in the same manner as for the un-ionized form. For instance, for an octanol–water partition, it is

: <math>\log\ P_\text{oct/wat}^\mathrm{I} = \log_{10}\left(\frac{\big[\text{solute}\big]_\text{octanol}^I}{\big[\text{solute}\big]_\text{water}^I}\right).</math>

To distinguish between this and the standard, un-ionized, partition coefficient, the un-ionized is often assigned the symbol '''log ''P''<sup>0</sup>''', such that the indexed <math>\log P_\text{oct/wat}^I</math> expression for ionized solutes becomes simply an extension of this, into the range of values {{math|''I'' > 0}}.{{citation needed|date=March 2016}}