Temperature and solvation effects on homologous series selectivity in reversed phase liquid chromatography

Although the theoretical basis for temperature effects in reversed phase liquid chromatography (RPLC) has been well established, temperature is an under-utilized parameter for enhancing resolution in RPLC systems. However, the use of subambient operating temperatures can bring about significant increases in selectivity which are useful in separating solutes with subtle differences in their morphology. We have been examining the effects of temperature, particularly in the subambient range, and mobile phase solvation on RPLC solute selectivity for a homologous series of non-polar compounds on monomeric C18 stationary phases. By using solute sets consisting of structural homologs, we have examined whether linear free energy relationships are operant, as well as measured enthalpic and entropic contributions to retention and selectivity over a wide temperature range. The solute homologous series sets include flexible chain solutes, such as the methylene series consisting of the C1–C6 n-alkylbenzenes rod-like solutes, such as the phenylene series consisting of benzene, biphenyl, p-terphenyl and p-quaterphenyl and plate-like solutes, such as the fused-ring series consisting of benzene, naphthalene, anthracene and 2,3-benzanthracene. This work is relevant in better understanding RPLC retention mechanisms as well as solute partitioning in other organized assemblies, and also is resulting in improved separation methods for many structurally similar compounds of environmental and pharmaceutical importance.