Then-octanol-water partition coefficient,K_ow is apartition coefficient for the two-phase system consisting ofn-octanol and water.^[1]K_ow is also frequently referred to by the symbol P, especially in the English literature. It is also calledn-octanol-water partition ratio.^[2][3][4]

K_ow serves as a measure of the relationship betweenlipophilicity (fat solubility) andhydrophilicity (water solubility) of a substance. The value is greater than one if a substance is more soluble in fat-like solvents such as n-octanol, and less than one if it is more soluble in water.^{[citation needed]}

If a substance is present as severalchemical species in the octanol-water system due toassociation ordissociation, each species is assigned its ownK_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]}

In 1899,Charles Ernest Overton andHans Horst Meyer independently proposed that the tadpole toxicity of non-ionizable organic compounds depends on their ability to partition into lipophilic compartments of cells. They further proposed the use of the partition coefficient in an olive oil/water mixture as an estimate of this lipophilic associated toxicity.Corwin Hansch later proposed the use of n-octanol as an inexpensive synthetic alcohol that could be obtained in a pure form as an alternative to olive oil.^[5][6]

K_ow values are used, among others, to assess the environmental fate ofpersistent organic pollutants. Chemicals with high partition coefficients, for example, tend to accumulate in the fatty tissue of organisms (bioaccumulation). Under theStockholm Convention, chemicals with a logK_ow greater than 5 are considered to bioaccumulate.^[7]

Furthermore, the parameter plays an important role in drug research (Rule of Five) andtoxicology.Ernst Overton andHans Meyer discovered as early as 1900 that theefficacy of an anaesthetic increased with increasingK_ow value (the so-calledMeyer-Overton rule).^[8]

K_ow values also provide a good estimate of how a substance is distributed within acell between the lipophilicbiomembranes and the aqueouscytosol.^{[citation needed]}

Since it is not possible to measureK_ow for all substances, various models have been developed to allow for their prediction, e.g.Quantitative structure–activity relationships (QSAR) orlinear free energy relationships (LFER)^[9][10] such as theHammett equation.^[9]

A variant of theUNIFAC system can also be used to estimate octanol-water partition coefficients.^[11] Under REACH, estimates are often derived byEPI Suite orCOSMOtherm.^[12]

• Definition of theK_ow or P-value

TheK_ow or P-value always only refers to a singlespecies or substance:

${\displaystyle K_{\mathrm{o}\mathrm{w}}=P=\frac{c_o^{S_i}}{c_w^{S_i}}}$

with:

• ${\displaystyle c_o^{S_i}}$ concentration of speciesi of a substance in the octanol-rich phase
• ${\displaystyle c_w^{S_i}}$ concentration of speciesi of a substance in the water-rich phase

If different species occur in the octanol-water system by dissociation or association, several P-values and one D-value exist for the system. If, on the other hand, the substance is only present in a single species, the P and D values are identical.

P is usually expressed as acommon logarithm, i.e. Log P (also Log P_ow or, less frequently, Log pOW):

${\displaystyle \log P=\log \frac{c_o^{S_i}}{c_w^{S_i}}=\log c_o^{S_i}-\log c_w^{S_i}}$ Log P is positive for lipophilic and negative for hydrophilic substances or species.

• Definition of the D-value

The P-value only correctly refers to the concentration ratio of a single substance distributed between the octanol and water phases. In the case of a substance that occurs as multiple species, it can therefore be calculated by summing the concentrations of alln species in the octanol phase and the concentrations of alln species in the aqueous phase:

${\displaystyle D=\frac{c_o}{c_w}=\frac{c_o^{S_1}+c_o^{S_2}+\cdots +c_o^{S_n}}{c_w^{S_1}+c_w^{S_2}+\cdots +c_w^{S_n}}}$

with:

• ${\displaystyle c_o}$ concentration of the substance in the octanol-rich phase
• ${\displaystyle c_w}$ concentration of the substance in the water-rich phase

D values are also usually given in the form of the common logarithm as Log D:

${\displaystyle \log D=\log \frac{c_o}{c_w}=\log c_o-\log c_w}$

Like Log P, Log D is positive for lipophilic and negative for hydrophilic substances. While P values are largely independent of thepH value of the aqueous phase due to their restriction to only one species, D values are often strongly dependent on the pH value of the aqueous phase.

Values for logK_ow typically range between -3 (very hydrophilic) and +10 (extremely lipophilic/hydrophobic).^[13]

The values listed here^[14] are sorted by the partition coefficient. Acetamide is hydrophilic, and 2,2′,4,4′,5-Pentachlorobiphenyl is lipophilic.

• Hydrophobic effect
• Dortmund Data Bank

• Virtual Computational Chemistry Laboratory interactive calculation and interactive comparison of several methods
• LogP-Berechnungssoftware von ACDArchived 2006-03-15 at theWayback Machine (commercial)
• Directory of reference works and databases with octanol-water partition coefficientsArchived 2014-04-08 at theWayback Machine
• Comprehensive free database of evaluated octanol-water partition coefficients from Sangster Research LaboratoriesArchived 2009-02-09 at theWayback Machine

• Ecotoxicology
• Pharmacology
• Physical chemistry

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