Solute retention in reversed-phase chromatography as a function of stationary phase properties: Effect of n-alkyl chain length and ligand density

Summary

Two series of bonded phases were synthesized employing LiChrospher Si 100, 10 μm and n-alkyldimethylmonochlorosilanes as silanizing reagents. In series A the n-alkyl chain length, n, of the bonded phase was varied between 1 and 20 at a constant ligand density of 3.5±0.2μmol·m⁻². In series B the ligand density, d, was gradually changed from 0 to 4.1μmol·m⁻² on the C₁, C₄, C₆, C₈ and C₁₈ bonded phases, respectively.

The capacity factors of benzoic acid esters and anilines as solutes were found to increase linearly with the n-alkyl chain length of packings at constant eluent composition (series A) up to a so-called critical chain length, n_crit, where the capacity factor remained constant. n_crit was in the range from 11 to 14. The same pattern was observed when plotting the solute capacity factor against the ligand density of the packing at constant n and constant eluent composition (series B). The critical ligand density, α_crit, varied between 2.3 and 3.2 μmol·m⁻² depending on n and the solute. Furthermore, solute retention was slightly higher on RP packings with an even number of carbon atoms in the alkyl chain than on those with an odd number.

These findings are consistent with the results of Berendsen and de Galan (J. Chromatogr., 196, 21 (1980)), Dill (J. Phys. Chem., 91, 1980 (1987)) and Simpson and Lau (to be published). The observed phenomena reflect the dynamic structure of RP silicas which are related to the mobility of solvated n-alkyl chains. Due to the lack of a precise conformational analysis of the surface of RP silicas, a semiquantitative model was applied to interprete the described dependencies.