DESCRIPTION OF RETENTION MECHANISM BY SOLVOPHOBIC THEORY - INFLUENCE OF ORGANIC MODIFIERS ON THE RETENTION BEHAVIOR OF HOMOLOGOUS SERIES IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY

The retention behaviour of different homologous series (2 less-than-or-equal-to n(c) less-than-or-equal-to 22) was studied on three different bonded chain lengths (C1, C-14 and C18) and in aqueous-organic solvent mixtures commonly used in reversed-phase liquid chromatography [methanol, acetonitrile and tetrahydrofuran (THF)]. For all solutes (ligands) without pi-electrons, and regardless of bonded chain length, the plots log k' vs. phi (organic modifier ratio) in methanol-water or acetonitrile-water are correlated with the evolution of surface tension of these mixtures. On the other hand, for ligand which possess pi-electrons, there is a modification in these curves which is due to specific pi-pi-interactions added to the pure solvophobic effect. In this respect, pure methanol was found to be a better eluent than pure acetonitrile excepted for the ligands with pi-electrons, for which the opposite was observed. For the THF-water mixtures, the plots of log k' vs. phi were always below those for methanol-water or acetonitrile-water. Moreover, the plots of log k' vs. n(c) did not increase linearly. In this pure solvent, retention is due to an exclusion mechanism. These results were in agreement with conformational infrared studies of alkyl chains in THF, methanol and acetonitrile. The overall results could rationalize the fact that retention prediction based on methylene selectivity was often erroneous when THF was used. Nevertheless, they show the possibility of optimizing separations of homologous series from the results obtained from a single gradient experiment with any commonly used organic modifier.