The use of Flory-Huggins theory in interpreting partitioning of solutes between organic liquids and water

Abstract

Solute partitioning data for dilute solutions have almost invariably been interpreted by equating experimental values of -RT in Kx (wherein Kx is the mole fraction partition coefficient) to delta mu infinity, the standard Gibbs energy change for solute transfer from one solvent to another. Recently, it has been alleged that this relation is insufficiently general. Instead, the statistical mechanical Flory-Huggins (FH) theory has been recommended for use, because it is designed to account for disparities in molecular size between solute and solvent. Our examination of the thermodynamics of partitioning shows that: (1) The customary interpretation is not only entirely correct (providing only that the solute is dilute), but is model-independent. (2) The dilute limit of the FH theory is seen to agree entirely with the usual interpretation of -RT in Kx, once certain misnomers are cleared away. (3) The use of FH theory being urged upon us in fact serves only to extract from delta mu infinity (the latter quite correctly determined as -RT in Kx) the contact part of delta mu infinity in order to obtain information on hydrophobic interactions. Some caveats are cited concerning such use of the FH statistical mechanical model.