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Octanol-water partition coefficient

## Summary

The n-octanol-water partition coefficient, Kow is a partition coefficient for the two-phase system consisting of n-octanol and water.[1] Kow is also frequently referred to by the symbol P, especially in the English literature. It is also called n-octanol-water partition ratio.[2][3][4]

Kow serves as a measure of the relationship between lipophilicity (fat solubility) and hydrophilicity (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 several chemical species in the octanol-water system due to association or dissociation, each species is assigned its own Kow 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]

## History

In 1899, Charles Ernest Overton and Hans 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]

## Applications

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

Furthermore, the parameter plays an important role in drug research (Rule of Five) and toxicology. Ernst Overton and Hans Meyer discovered as early as 1900 that the efficacy of an anaesthetic increased with increasing Kow value (the so-called Meyer-Overton rule).[8]

Kow values also provide a good estimate of how a substance is distributed within a cell between the lipophilic biomembranes and the aqueous cytosol.

## Estimation

Since it is not possible to measure Kow for all substances, various models have been developed to allow for their prediction, e.g. Quantitative structure–activity relationships (QSAR) or linear free energy relationships (LFER)[9][10] such as the Hammett equation.[9]

A variant of the UNIFAC system can also be used to estimate octanol-water partition coefficients.[11]

## Equations

• Definition of the Kow or P-value
The Kow or P-value always only refers to a single species or substance:
${\displaystyle K_{\mathrm {ow} }=P={\frac {c_{o}^{S_{i}}}{c_{w}^{S_{i}}}}}$
with:
• ${\displaystyle c_{o}^{S_{i}}}$  concentration of species i of a substance in the octanol-rich phase
• ${\displaystyle c_{w}^{S_{i}}}$  concentration of species i 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 a common logarithm, i.e. Log P (also Log Pow 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 D-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 all n species in the octanol phase and the concentrations of all n species in the aqueous phase:
${\displaystyle D={\frac {c_{o}}{c_{w}}}={\frac {c_{o}^{S_{1}}+c_{o}^{S_{2}}+\dots +c_{o}^{S_{n}}}{c_{w}^{S_{1}}+c_{w}^{S_{2}}+\dots +c_{w}^{S_{n}}}}}$
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 the pH 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.

## Example values

Values for log Kow typically range between -3 (very hydrophilic) and +10 (extremely lipophilic/hydrophobic).[12]

The values listed here[13] are sorted by the partition coefficient. Acetamide is hydrophilic, and 2,2′,4,4′,5-Pentachlorobiphenyl is lipophilic.

Substance log KOW T Reference
Acetamide −1.155 25 °C
Methanol −0.824 19 °C
Formic acid −0.413 25 °C
Diethyl ether 0.833 20 °C
p-Dichlorobenzene 3.370 25 °C
Hexamethylbenzene 4.610 25 °C
2,2′,4,4′,5-Pentachlorobiphenyl 6.410 Ambient

## References

1. ^ Sangster J (1997). Octanol-water partition coefficients : fundamentals and physical chemistry. Chichester: Wiley. ISBN 0-471-97397-1. OCLC 36430034.
2. ^ Mackay D (2021). Multimedia environmental models : the fugacity approach. J. Mark Parnis (Third ed.). Boca Raton, FL. ISBN 978-1-000-09499-2. OCLC 1182869019.
3. ^ Hodges G, Eadsforth C, Bossuyt B, Bouvy A, Enrici MH, Geurts M, et al. (2019). "A comparison of log Kow (n-octanol–water partition coefficient) values for non-ionic, anionic, cationic and amphoteric surfactants determined using predictions and experimental methods". Environmental Sciences Europe. 31 (1). doi:10.1186/s12302-018-0176-7.
4. ^ Hendriks AJ, van der Linde A, Cornelissen G, Sijm DT (July 2001). "The power of size. 1. Rate constants and equilibrium ratios for accumulation of organic substances related to octanol-water partition ratio and species weight". Environmental Toxicology and Chemistry. 20 (7): 1399–420. PMID 11434281.
5. ^ Lipnick RL (1989). "Narcosis, electrophile and proelectrophile toxicity mechanisms: Application of SAR and QSAR". Environmental Toxicology and Chemistry. 8 (1): 1–2. doi:10.1002/etc.5620080101.
6. ^ Hansch C (June 2011). "The advent and evolution of QSAR at Pomona College". Journal of Computer-aided Molecular Design. 25 (6): 495–507. doi:10.1007/s10822-011-9444-y. PMID 21678028.
7. ^ Stockholm Convention on Persistent Organic Pollutents (POPs) (PDF). Secretariat of the Stockholm Convention. Geneva: United Nations Environment Programme. 2018. pp. Annex D.
8. ^ Evers AS, Crowder M (2009). "Mechanisms of Anesthesia and Consciousness". In Barash PG, Cullen BF, Stoelting RK, Catalan MK, Stock MC (eds.). Clinical Anesthesia. Lippincott Williams & Wilkins. p. 106. ISBN 978-0-7817-8763-5.
9. ^ a b Dearden JC (September 1985). "Partitioning and lipophilicity in quantitative structure-activity relationships". Environmental Health Perspectives. 61: 203–28. doi:10.1289/ehp.8561203. PMC 1568760. PMID 3905374.
10. ^ Kellogg GE, Abraham DJ (July 2000). "Hydrophobicity: is LogP(o/w) more than the sum of its parts?". European Journal of Medicinal Chemistry. 35 (7–8): 651–61. doi:10.1016/s0223-5234(00)00167-7. PMID 10960181.
11. ^ Gani R, Abildskov J, Kontogeorgis G (2004-06-30). "Application of property models in chemical product design". In Kontogeorgis GM, Gani R (eds.). Computer Aided Property Estimation for Process and Product Design: Computers Aided Chemical Engineering. Elsevier. ISBN 978-0-08-047228-7.
12. ^ Cumming H, Rücker C (September 2017). "Octanol-Water Partition Coefficient Measurement by a Simple 1H NMR Method". ACS Omega. 2 (9): 6244–6249. doi:10.1021/acsomega.7b01102. PMC 6644330. PMID 31457869.
13. ^ "Dortmund Data Bank (DDB)". Dortmund Data Bank Software & Separation Technology (DDBST) GmbH. Retrieved 2020-05-20.