Surface modification and ligand loading effects in imidazolium-based zwitterionic stationary phases for hydrophilic interaction chromatography.

Imidazolium-based zwitterionic stationary phases represent a promising class of materials for hydrophilic interaction chromatography (HILIC). However, systematic studies examining the influence of silica surface modification strategies on ligand loading, material morphology, and chromatographic mass transfer behavior remain limited. In this work, porous silica was modified using 3-mercaptopropylsilatrane (MPS) and the conventional 3-mercaptopropyltrimethoxysilane (MPTMS) approach, followed by functionalization with vinylimidazolium and octenylimidazolium zwitterionic ligands via thiol-ene click chemistry. MPS modification enabled higher sulfur surface coverage resulting in increased ligand loading compared to silica treated with the MPTMS approach. The type of ligand appears to govern material morphology and packability with vinylimidazolium-based materials exhibiting pronounced agglomeration, whereas octenylimidazolium-based materials retained a free-flowing morphology and could be readily packed into columns. In contrast, mass transfer behavior was observed to be strongly influenced by ligand loading. The octenylimidazolium zwitterionic ligand on MPTMS modified silica with approximately 1.25-fold lower ligand loading than the corresponding MPS modified ligand, exhibited a 1.90-fold lower resistance to mass transfer despite comparable immobilized water layer volumes. Nitrogen adsorption-desorption measurements showed that higher ligand loading resulted in greater reductions of surface area, pore width and volume of silica, suggesting partial occupation of the pore network by the bonded ligands and more restricted diffusion. Evaluation of acids, bases, and water-soluble vitamins further indicated that retention involves contributions from adsorption-related interactions in addition to hydrophilic partitioning. Together, these results suggest that the increased resistance to mass transfer could arise from reduced intraparticle diffusion and greater adsorption-related interactions associated with higher ligand loading.