Measurement of partition coefficients in waterless biphasic liquid systems by countercurrent chromatography

Countercurrent chromatography (CCC) is a chromatographic separation technique that uses a liquid as a stationary phase. Centrifugal forces are used to immobilize the liquid stationary phase when the liquid mobile phase is pushed through it. In CCC, the solutes are separated according to their liquid-liquid partition coefficients. The solutes studied were the alkylbenzene homologues from benzene to hexylbenzene and some polyaromatic hydrocarbons (PAHs) from naphthalene to coronene. Their liquid-liquid partition coefficients were measured in the five waterless biphasic systems formed by heptane, as the apolar liquid phase of the five biphasic systems, and four dipolar aprotic solvents, dimethyl sulfoxide, dimethylformamide, furfural, and N-methylpyrrolidone, and the polar proton-donor solvent methanol. The coefficients were compared to the corresponding capacity factors obtained by classical liquid chromatography on octadecyl-bonded silica. For the five biphasic solvent systems studied, linear relationships were found between the partition coefficients and the sp(3) and sp(2) hybridized carbon atom number for the alkylbenzene and PAH series, respectively. The sp(2) and sp(3) transfer energies were estimated, and their ratio was used to quantify the solvent selectivity toward aromatic extraction.