Octanol Water Partition Coefficient – What does it Mean for Exposure Assessment?

The octanol/water partition coefficient is determined by a relatively simple test.   It is done by taking roughly equal volumes of octanol (an “oily” alcohol) and water, mixing them together and throwing the chemical of interest into the mix and mixing some more.   We all know that oil and water do not mix and as a result when everything settles down we get an oil layer of octanol on top of the water layer with the chemical of interest dissolved in each.   The ratio of the amount in the oil to the amount that got dissolved in the water is the octanol/water partition coefficient or Kow.   This number indicates whether a chemical is a lipophilic (a “fat lover”) or hydrophilic (a “water lover”).   For example, methanol would go mostly into the water while benzene goes mostly into the octanol.   Most organics are highly lippophillic such that the scale is made to be logarithmic.   Thus, we take the log of the ratio which is called log Kow or pKow.   If the Kow is 1000 the the log Kow or pKow would be 3.    Since only 17% of methanol goes into the octanol the Kow ratio is 0.17 and the pKow is -0.77.   See the previous blog on logarithms (A Simple Refresher of Logarithms, July 2013) if this is at all unclear.

The pKow of benzene is 2.13 which mean it really likes fats and generally dislikes water.  What does this all mean for those of us in the exposure assessment business.   Well first of all it tells us roughly where benzene or any other fat loving chemical will wind up in the environment.   It will tend to wind up in environmental compartments that are rich in organic constituents like some soils and most sediment.   If inhaled or ingested, it will tend to accumulate in areas of the body that have a lot of blood flow and a lot of fat. 

In general, chemicals with a high pKow tend to bioaccumulate or biocentrate in organisms within the environment.   That is, they concentrate in the tissues of animals -as you go up the food chain.  DDT was a prime example of this in the case of the Bald Eagle.  Since we are at the top of the food chain this could also be problematic for us.

Finally, chemicals with high pKow tend to be absorbed by the skin much more readily that those with low pKow.   Look at the following equation for dermal adsorption from Dr. Wil tenBerge’s web site:

From this it appears that pKow and molecular weight drive the dermal absorption of chemicals.

Of course, the higher the pKow, the lower the water solubility and this is an important considerations in some exposure assessments.   One can get faked out with this fact which will be the subject of my next blog and how I received what was perhaps my greatest surprise ever to date in doing an exposure assessment.