Only compounds with the proper lipophilicity will pass through the plasma membrane.

Air-water partitioning coefficient (Kaw)

Amphiphilic molecules adsorb at the air-water interface (a good model for the lipid bilayer), decreasing the surface tension.

By measuring the air-water partitioning coefficient Kaw (using Gibbs adsorption isotherm), the lipophicility (= hydrophobicity) of a compound can be assessed.

> Too lipophilic compounds will be retained by the cell membrane: