THE INFLUENCE OF SILICA STRUCTURE ON REVERSED-PHASE RETENTION

The literature concerning the deleterious effects of silanol groups on reversed-phase separations is reviewed. Methods for their estimation on the surface of silica range from chemical, by reaction of surface silanols with a suitable reagent to physical procedures such as infrared and NMR spectroscopy. Most procedures are seen to be dependent upon the equilibrium between silanol groups and surface adsorbed water. The possible interactions between solutes and silanol groups, ion exchange, hydrophobic and silanophilic, all contribute to the overall retention mechanism; the performance problems with basic molecules in reversed-phase chromatography are mainly a consequence of ion exchange on active silanol groups. The acidity of silanol groups has been ascribed to incomplete rehydroxylation of silicas and to the presence of metal atoms within the silica matrix. The available evidence currently points to the latter as being the main source of the deleterious effects of silica structure on reversed-phase separations.