Temperature-Jump Investigation of Alkyl Chain Length Effects on Sorption/Desorption Kinetics at Reversed-Phase Chromatographic Interfaces

The influence of the alkyl chain length on the kinetics of solute retention at reversed-phase chromatographic surfaces is examined. A Joule-discharge temperature-jump relaxation experiment was used to monitor reversible sorption/desorption kinetics at C4- and C8-modified silica/solution interfaces. Biexponential sorption/desorption relaxation kinetics were observed for a charged fluorescent probe, 1-anilino-8-naphthalenesulfonate (ANS), on both C4- and C8-silica surfaces. Both relaxation rates on C4 surfaces were sufficiently slow to be measured and increased linearly with solute concentration. One of the relaxations on a C8 surface is too fast to be resolved from the heating rate, similar to behavior of the solute on a longer chain C18-silica. These observations suggest that sorption kinetics on the intermediate chain length surfaace, C4-silica, are different from kinetics on longer chain length surfaces, C8- and C18-silica. From a fit of the data to a two-step kinetic model, the rates of both adsorption and partition of the ionic probe on the C4 chain are estimated; both rates exhibit significant influence over the equilibrium constant. The relaxation rate of a neutral probe, N-phenyl-1-naphthylamine, is also measured; the results indicate a fast (diffusion-controlled) adsorption step, followed by a detectable barrier to partition that is similar to the partition barrier for ANS on the C4-silica surface. These results show that the alkyl chain length of modified silica strongly influences retention kinetics.