Stationary phase effects on the dynamic affinity of low-molecular-mass displacers

In this paper, the selectivity of a variety of cation-exchange stationary phases was investigated using a homologous series of displacer molecules based on pentaerythritol. These displacers were derived from pentaerythritol and contained either four trimethyl ammonium groups [pentaerythrityl-(trimethylammonium chloride)4, PE(TMA)4], benzene rings [pentaerythrityl-(benzyl dimethylammonium chloride)4, PE(DMABzCl)4], heptyl groups [pentaerythrityl-(heptyl dimethylammonium iodide)4, PE(DMAHepI)4] or cyclohexyl groups [pentaerythrityl-(cyclohexyl dimethylammonium iodide)4, PE(DMACyI)4]. This series enabled us to probe the secondary interactions that can play a role in the affinity of low-molecular-mass displacers for different stationary phases. The relative affinities of these displacers were examined using a displacer ranking plot based on the steric mass action (SMA) isotherm model. While hydrophobicity and aromaticity played important roles in generating the affinity to the hydrophilized polystyrene-divinylbenzene (Source 15S) and polymethacrylate-based (Toyopearl 650M) resins, these secondary interactions had a minimal impact on the selectivity in agarose resins coated with dextran (SP Sepharose XL), "gel in a shell" (S Ceramic HyperD F), and monolithic (Bio-Rad Uno S6) cation-exchange materials. Further, the results with a tentacular stationary phase (Fractogel EMD) suggest that the alkyl chains on PE(DMAHepI)4 play an important role in increasing the affinity, possibly because of strong interactions between the alkyl moiety and the polymer matrix as well as between the charged groups and the polyelectrolyte tentacles. The results of this study provide insight into the design of high affinity, low-molecular-mass displacers for different cation-exchange stationary phase materials.