Partition Coefficient

• Two immiscible liquids that are combined will quickly separate into separate phases, with a distinct boundary between them. Oil floating on water is a good example. If a chemical is dissolved in this mixture, it will naturally partition itself so that some dissolves in each layer. If we call the liquids A and B, the partition coefficient is just the ratio between the concentration of that chemical in A over its concentration in B. The partition coefficient is a measure of how hydrophilic ("water loving") or hydrophobic ("water fearing") a substance is.
  
  

Measurement and Calculation

• Water and octanol (a long chain carbon compound) are typically used to find the partition coefficient for a substance. The most reliable means of finding the partition coefficient is to add a known quantity of the substance to a combination of octanol and water and then measure the concentration of the substance in each liquid via ultraviolet-visible spectroscopy. A common way of expressing the partition coefficient -- abbreviated as log P -- is calculated as the base 10 logarithm of the concentration in octanol divided by the concentration in water.
  
  

Applications in Science

• In environmental science, log P gives an indication of how widely and quickly a chemical pollutant will move through different types of environments, such as rivers or soils. Log P is also used to select the optimal ingredients in a variety of consumer products, such as makeup and dyes, since this can predict how well a chemical ingredient will mix with creams.
  
  

Pharmaceutical Applications

• Log P can be used to predict how well a chemical will act as a drug. Since most medications have to move through both water and fatty tissues in the body, the perfect drug will be reasonably soluble in both, a property which is reflected in its value of log P. Knowing whether a drug is hydrophilic or hydrophobic can also allow predictions about where it will concentrate in the body, how it will be metabolized and how it will be excreted.