Octanol-water partition coefficient

Key takeaways

Octanol-water partition coefficient belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Octanol-water partition coefficient to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Octanol-water partition coefficient from memory before moving on to harder problems.

Reference excerpt

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

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.

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. 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). 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) such as the Hammett equation. A variant of the UNIFAC system can also be used to estimate octanol-water partition coefficients. Under REACH, estimates are often derived by EPI Suite or COSMOtherm.

Equations

Definition of the Kow or P-value The Kow or P-value always only refers to a single species or substance:

K o w = P = c o S i c w S i {\displaystyle K_{\mathrm {ow} }=P={\frac {c_{o}^{S_{i}}}{c_{w}^{S_{i}}}}}

with:

c o S i {\displaystyle c_{o}^{S_{i}}} concentration of species i of a substance in the octanol-rich phase

c w S i {\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 {\displaystyle \log P} ⁠ (also log Pow or, less frequently, Log pOW):

log ⁡ P = log ⁡ c o S i c w S i = log ⁡ c o S i − log ⁡ c w S i {\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 {\displaystyle \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 all n species in the octanol phase and the concentrations of all n species in the aqueous phase:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Octanol-water partition coefficient

Start with the simplest possible case. Write down what Octanol-water partition coefficient claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Octanol-water partition coefficient before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Octanol-water partition coefficient ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Octanol-water partition coefficient

In research: Octanol-water partition coefficient appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Octanol-water partition coefficient in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Octanol-water partition coefficient is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ecotoxicology, Pharmacology, Physical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Octanol-water partition coefficient outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Octanol-water partition coefficient in 20 minutes

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Frequently asked questions

What is Octanol-water partition coefficient in simple terms?

The n-octanol-water partition coefficient, Kow is a partition coefficient for the two-phase system consisting of n-octanol and water. Kow is also frequently referred to by the symbol P, especially in the English literature.

Why does Octanol-water partition coefficient matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Octanol-water partition coefficient?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Octanol-water partition coefficient.