Studies of Solute-Chain Interaction under Reordering/Resolvation Conditions: Correlation between Solute Structure and Changes in Retention for Hydroxylated Aliphatic and Aromatic Compounds

Reversed-phase liquid chromatography is perhaps the most important means of separating thermally labile low molecular weight compounds. In a majority of cases, separations are carried out using linear alkyl phases. Hence developing a better understanding of such bonded materials and the fundamental processes controlling solute retention and selectivity continues to be important. There have been a number of theories suggested to describe the nature of the bonded microlayer. A common view is that the immobilized chains are surrounded by intercollated solvent, the composition of which may or may not be close to that of the bulk mobile phase. Further the microlayer can swell or shrink depending on solvent composition. This latter property has been discussed in terms of a breathing model. The current investigation is an extension of previous studies of conformational changes of n-alkyl bonded phases under totally aqueous mobile phase conditions. Relative changes in retention, delta k sub r before and after recordingresolvation of an octyl phase have been studied using various linear, branched, cyclic and aromatic compounds. The magnitude of delta k sub r can be correlated to structural features in the solute, such as size, shape, rigidity and substitution. The largest values of delta k sub r were obtained from rigid solutes with multiple hydroxylated groups.