Characterization and stability of silanized and polymer-coated octadecyl reversed phases

Three different reversed phases for high-performance liquid chromatography obtained with monofunctional octadecylsilane or with cross-linked polymethyloctadecylsiloxane with and without trimethylsilane precapping of the substrate were synthesized on the same batch of Nucleosil silica. After extensive characterization, these phases were subjected to artificial ageing under simulated routine conditions. Changes in properties of the bonded-phase packings were quantified with chromatographic techniques, solid-state 29Si NMR and elemental analysis. These changes are correlated with lipophilic and polar selectivity, accessibility of silanol groups at the surface, loss of silanes/siloxanes, separation performance and silica degradation of the stationary phases. Improved surface shielding properties and stability towards eluents of high pH are considerable advantages of cross-linked polymer-coated and octadecyl stationary phases. Precapping of the substrate with trimethylsilanes prior to polymer coating reduces the concentration of silanol groups to less than 50% and yields stationary phases which exhibit further decreased silanol interactions because the residual silanol groups are shielded in a double manner by the trimethylsilyl groups and by the polymer layer on top of it. This also leads to an increase in stability. The non-precapped polymer-coated stationary phase showed an advanced cross-linking of the polymers after ageing experiments with high pH buffer solutions, resulting in improved silanol shielding and large shifts in selectivity. After precapping with trimethylsilane-enolate the polymeric methyloctadecylsiloxane coating proved to be more stable than the non-precapped phase. Also, the precapped phase showed hardly any chemical bonding with the silica surface; only Van der Waals and other dispersive interactions with the anchored trimethylsilanes at the surface exist. The high mobility of the polymer coating at the pore surface of the substrate is retained for the precapped stationary phase, also after simulated ageing experiments. This precapped polymer coated stationary phase showed an almost consistent separation performance after ageing experiments with high pH buffer solutions, but a small shift in selectivity was observed.