29-SILICON NMR EVIDENCE FOR THE IMPROVED CHROMATOGRAPHIC SILOXANE BOND STABILITY OF BULKY ALKYLSILANE LIGANDS ON A SILICA SURFACE

A stable bond stationary phase for reversed-phase high-performance liq uid chromatography, with a diisobutyl-n-octadecylsilane derivatized su rface, was studied using Si-29 cross-polarization magic angle spinning (CP MAS) NMR. Fumed silica surfaces (Aerosil), trimethylsilylated to different extents, were used to illustrate the effect of ligand surfac e loading on the hydrogen bonding contribution to the ligand silane CP MAS NMR signal. Spectral comparison of the diisobutyl-n-octadecylsila ne derivatized silica with the conventional dimethyl-n-octadecylsilane derivatized silica revealed significantly decreased hydrogen bonding of residual silanols to the ligand siloxane bond in the diisobutyl-n-o ctadecyl phase. This illustrates the increased steric protection of th e ligand siloxane bond by the bulky alkyl substituents, which is assum ed to be the reason for the improved hydrolytic stability at low pH of this phase.