Octanol-water partition coefficient

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.

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.

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.

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).

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).[2]

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)[3][4] such as the Hammett equation.[5]

A variant of the UNIFAC system can also be used to estimate octanol-water partition coefficients.[6]

Equations
  • Definition of the Kow or P-value
The Kow or P-value always only refers to a single species or substance:
with:
  • concentration of species i of a substance in the octanol-rich phase
  • 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):
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:
with:
  • concentration of the substance in the octanol-rich phase
  • 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:
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

The values listed here[7] 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

See also

Sources
  • Kai-Uwe Goss: Der Oktanol/Wasser Verteilungskoeffizient – Das Allheilmittel der Umweltchemie? UWSF-ESPR-Beitragsserie: Persistente Organische Schadstoffe (POPs), UWSF 15(4), S. 273–279; doi:10.1065/uwsf2003.01.050.
  • Burkhard Heuel-Fabianek: Partition Coefficients (Kd) for the Modelling of Transport Processes of Radionuclides in Groundwater (PDF; 9,4 MB), JÜL-Berichte, Forschungszentrum Jülich, Nr. 4375, 2014, ISSN 0944-2952.

References
  1. Sangster, J. (1997). Octanol-water partition coefficients : fundamentals and physical chemistry. Chichester: Wiley. ISBN 0-471-97397-1. OCLC 36430034.
  2. Barash, Paul G. (2009). Clinical Anesthesia. Lippincott Williams & Wilkins. ISBN 978-0-7817-8763-5.
  3. Dearden, J C (September 1985). "Partitioning and lipophilicity in quantitative structure-activity relationships". Environmental Health Perspectives. 61: 203–228. doi:10.1289/ehp.8561203. ISSN 0091-6765. PMC 1568760. PMID 3905374.
  4. G, Eugene Kellogg; Dj, Abraham (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.
  5. John C. Dearden: Partitioning and Lipophilicity in Quantitative Structure-Activity Relationships. Environ. Health Perspect. 1985 September; 61: 203–228; PMC 1568760.
  6. I, Georgios M. Kontogeorgis; Gani, Rafiqul (2004-06-30). Computer Aided Property Estimation for Process and Product Design: Computers Aided Chemical Engineering. Elsevier. ISBN 978-0-08-047228-7.
  7. "DDBST - DDBST GmbH". www.ddbst.com. Retrieved 2020-05-20.
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