Entropy dominated subambient temperature separations in reversed-phase liquid chromatography

Abstract

Subambient temperature selectivity in reversed-phase liquid chromatography (RP-LC) is an entropy dominated process. Since reduced temperature results in a more ordered stationary phase as well as more effective intramolecular interactions, it is useful for resolving solutes which exhibit similar retentions at ambient and higher temperatures, such as isomeric compounds. We have been investigating the effects of temperature on RP-LC selectivity for polycyclic aromatic hydrocarbon (PAH) and polyphenyl isomers over the temperature range of −20 to 70°C using methanol-water mobile phases and monomeric octadecyl stationary phases. We have observed that a high bonding density monomeric reversed-phase column (4.4 μmol m −2) exhibits improved selectivity over a low density column (1.5 μmol m −2). Shape selectivity for both columns is enhanced at temperatures of 25° C or less and this is especially demonstrated by the high density phase. Van 't Hoff plots (ln k′ vs. 1/ T) have been constructed to observe whether solute retention behavior is linear over the temperature range studied. This is observed over the entire temperature range for the low bonding density stationary phase, yet distinct curvature is noted at ca. 25°C for the high density phase. One explanation of this observation is a “phase transition” of the bonded octadecyl chains from a liquid-like state at higher temperatures to a more ordered and extended crystalline-like state at lower temperatures.