Synthesis and morphological characterization of phenyl-modified macroporous–mesoporous hybrid silica monoliths

Abstract

Compared with pure silica-based or organic-polymer monoliths, hybrid organic-silica monoliths offer the combined advantages of mechanically strong stationary phases, simpler preparation protocols, resistance to swelling and shrinking in many solvents and better pH stability. Comprehensive data on the systematic characterization of the pore space morphology of hybrid organic-silica monoliths and their connection to pure silica-based monoliths are still scarce in the literature. In this work, we adapted the general sol–gel procedure with phenyltrimethoxysilane and tetramethoxysilane as precursors to prepare phenyl-modified macroporous–mesoporous silica monoliths via spinodal decomposition involving poly(ethylene glycol). Effects of polycondensation temperature and poly(ethylene glycol) amount were investigated with respect to the corresponding macropore space morphology. We characterized the monoliths by thermogravimetric analysis and infrared spectroscopy (phenyl-modification), nitrogen physisorption and scanning electron microscopy (meso- and macropores) as well as confocal laser scanning microscopy for three-dimensional reconstruction of the macropore space morphology. The statistical analysis of a reconstruction by chord length distributions allowed us to assess the monoliths macropore space heterogeneity through a quantitative approach. Relying exclusively on image analysis, we provide an accurate and model-free description of the void space distribution. Complementary macroporosity profiles were recorded to identify macroscopic heterogeneities inside a monolith. Analyzed structural features are connected to key transport properties of the macropore space. Phenyl-modified monoliths from this work were compared with previous pure silica-based and hybrid organic-silica monoliths regarding the bulk homogeneity of the monoliths and the critical wall region in capillary column format. The comparison with a conventional C18-silica monolith demonstrated a selectivity tuning with the phenyl-modified silica monoliths by π–π-interactions between the stationary phase and aromatic analytes. Application of the phenyl-modified monoliths in capillary liquid chromatography reflected the selectivity behaviour of commercial phenyl-modified silica particles, but with the advantage of a higher separation efficiency for the monolithic stationary phase.