An Indexing Scheme for Optimizing the Choice of Biphasic Systems for Ccc†

Homologous series of N-alkylbenzamides, N-acylcytosines and N-acylcytidines are introduced as standards for comparing the relative polarity of solvent systems employed in countercurrent chromatography (CCC). A solvent system equipollent index, E, is defined as the fractional carbon chain length of the standard which would have a partition coefficient of unity in the solvent system. Alkylbenzamide indices, EB, are suitable to characterize nonpolar to moderately polar systems while more polar systems can be characterized by acylcytosine, ECS, and acylcytidine, ECD, indices. Both organic/aqueous and organic/organic bi-phasic systems can be compared. Disparate systems with the same index are defined as synonymous systems with respect to the standard. As a result of different solvent selectivity, synonymous systems are expected to exhibit different solubility and retention behavior toward chromatographically similar solutes, which may enhance solute separation in some of the systems. Correlation of estimated octanol/water partition coefficients with E values aids the choice of solvent systems for compounds of known structure. Solutes with unknown structures can be compared directly with equipollent standards using other chromatographic methods.