Direct Octanol Water Partition Coefficient Determination Using Co-Current Chromatography

Abstract

Octanol-water partition coefficients. K_oct, can be measured directly using countercurrent chromatography (CCC) with an octanol-water biphasic solvent system. The mobile phase is water saturated with octanol, and the stationary phase is octanol saturated with water. The measurable K_oct range was 0.003 to 300. A liquid stationary octanol phase permits the development of a dual-mode elution method using CCC which extends the measurable K_oct range to 5,000 (log K_oct = 3.7). The co-current CCC method was developed to extend the range to K_oct = 20,000 (log K_oct = 4.3). In co-current CCC both the water and the octanol phase move in the same direction at different rates. Using the complete theory that was developed recently (Berthod, A. Analusis, 18, 352, 1990), it was possible to measure with a hydrostatic CCC apparatus loaded with 2 cartridges (800 channels, 49 mL. internal volume) the K_oct values 10 solutes. The K_oct values ranged from 11 (Phthalimide) to 20,000 (Phenanthrene). The continuous detection problem was solved by the post-column addition of 2-propanol as a solubilizing agent with mild heating. As predicted by the theory, the maximum K_oct selectivity depends on the octanol to water flow rate ratio. It was found that the efficiency increases with octanol flow rate and. at a given octanol flow rate, the efficiency increases with the solute retention volume (or solute hydrophobicity). The continuous input of octanol at the head of the CCC apparatus produced an autofocusing effect which limits band broadening and peak tailing of the injected band of solute located in the octanol phase.