partition coefficient (log P)

Imagine shaking a drug in a bottle that holds both oil and water, then letting the layers settle. Some drug ends up in the oil and some in the water. The partition coefficient is simply the ratio of those two concentrations once equilibrium is reached. It tells you whether a molecule prefers fatty (lipid) surroundings or watery ones.

Because the numbers span an enormous range, they are usually reported as log P, the logarithm of the oil-to-water ratio, measured with octanol standing in for the oil. A high positive log P means the drug is lipophilic (fat-loving) and partitions into oil; a negative value means it is hydrophilic (water-loving). Log P is one of the most-used single descriptors in drug design because cell membranes are fatty barriers, so a molecule must be lipophilic enough to slip across them yet water-soluble enough to dissolve and travel in body fluids.

Log P describes only the neutral, unionised molecule. For drugs that gain or lose charge with pH (most acids and bases), the more useful measure is log D, the distribution coefficient at a stated pH, which averages the ionised and unionised forms. Quoting log P without noting whether the drug ionises can be misleading.