Stationary phase effects in reversed-phase chromatography : I. Comparison of energetics of retention on alkyl-silica bonded phases

Abstract

A large number of retention data on various hydrocarbonaceous bonded phases was examined in order to shed light on the energetics of retention in reversedphase chromatography. Plots of logarithmic retention factors measured on column pairs with different eluites but with the same eluent were used in the analysis that was extended also to retention data obtained at different temperatures. Linear correlation with unit slope indicates identical intrinsic thermodynamic behavior for the two columns, i.e., homoenergetic retention, as predicted by the solvophobic theory. Linear correlation with slope different fromunity suggests similar physico-chemical basis for retention on the two columns that is termed homeonergetic. No correlation is taken as proof that, besides solvophobic interactions with the stationary phase ligates, the eluites interact with residual silanol groups. Such heteroenergetic behavior implies the use of experimental conditions outside of solvophobic chromatography. Most bonded phases with medium - or long-chain alkly ligates showed homoenergetic retention when water-rich eluents and relatively non-polar eluites without amino functions were employed, whereas those packed with stationary phases containing short alkyl chain or adamantyl ligates revealed homeonergetic behavior. In contradistinction, only with eluents rich in organic solvent and/or with basic eluites was heteroenergetic behavior observed. It is concluded therefore that under conditions of solvophobic chromatography the main source of the differences between columns packed with hydrocarbonaceous bonded phases is a difference in phase ratio and possibly pore size distribution. In fact, if several columns exhibit homoenergetic retention behaviour it may indicate that the experimental conditions are those for solvophobic chromatography. Nevertheless the analysis clearly demonstrates that when eluites having polar groups are chromatographed in water-lean eluents the interaction with surface silanols can be significant. The approach presented can serve not only as a diagnostic tool for column and chromatographic conditions but also as a means to obtain information on the relative magnitude of column phaase ratios and to predict retention data in solvophobic chromatography.