When two liquids can be brought into contact and later separated, there is the opportunity to find materials that distribute unevenly between them. For example, non-polar compounds added to water and an organic solvent that is not miscible with water will shun the water and dissolve mostly in the organic solvent. This is the basis for many industrial isolation and purification schemes. The surface tension between water and organic solvents is so high, however, that solvent extraction procedures will destroy the configurations of some important biochemicals such as proteins. Although there are ways to make the protein distribute unevenly between two liquids, its structure is usually ruined.

When each of the solvents is mostly water, the interfacial tension is barely measurable and induces no damage. Proteins, other macromolecules, and cell components such as mitochondria distribute in the phases or collect at the interface. Two-phase aqueous systems provide a mild method for purification of proteins, and scale up to large volumes presents no engineering problems. The polymers can have functional groups that improve distribution coefficients of the biochemical products, but the costs for these polymers are high. Although highly promising, two-phase aqueous methods are used only for valuable products because the cost of the polymers is too high and they are not easily recovered for reuse. Another drawback is distribution coefficients not far from 1 for most proteins. This contrasts with distribution coefficients of more than 100 for non-polar compounds distributed between aqueous and organic phases. Several extraction stages are needed to get acceptable yields when the distribution coefficients are unfavorable.

The distribution coefficient is :