Silanization of superficially porous silica particles with p-aminophenyltrimethoxysilane

The surface functionalization of silica particles for chromatographic applications is typically achieved by means of silanization reactions. A silane molecule (e.g., chlorosilanes or alkoxysilanes) with a functional group of interest is attached to the silica surface through a siloxane bond. Even though the surface modification via silanization is widely used, how the reaction parameters affect the degree of surface coverage is rarely discussed, particularly when using the less reactive alkoxysilanes reagents. We studied reaction conditions to graft p-aminophenyltrimethoxysilane (p-APTMS) on superficially porous particles. Theoretical calculations predicted that 3 grafts of p-APTMS per nm ² (~5 μmol/m ² ) can be accommodated on the silica surface. This value is not achieved unless the reaction conditions are optimized. To maximize the surface coverage of the aminophenyl layer at the silica surface, we investigated the influence of temperature, reaction time, and addition of water to the silanization reaction on the surface coverage. After optimization, it was found that using elevated temperatures (130 °C), adding 3.3 equivalents of water per p-APTMS molecule, and reacting for 24 h in decane as the solvent provided a surface coverage as high as 4.5 μmol/m ² (2.7 grafts/nm ² ), approaching the limit suggested via theoretical density functional theory calculations.