Chemically modified silica gel is typically used as a stationary phase in high pressure liquid chromatography (HPLC).  As the mobile phase containing the sample flows over the surface,  interactions with the surface cause different compounds to be eluted at different times.  The structure and density of chains on the surface strongly impact the ability to separate similar compounds.  A series of bonded phases were prepared using surface polymerization of trifunctional and difunctional ligands, and the structural morphology was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy using cross-polarization and magic angle spinning (CP/MAS). Mixed-phase surfaces were prepared using mixtures of trifunctional long chain (C18) ligands with a) trifunctional and b) difunctional short chain (C1) ligands, and these surfaces were compared to the corresponding single-phase surfaces consisting of only long or short chain ligands.  Incorporation of short chains increases the ligand density and the degree of cross-linking between ligands compared to the surface consisting exclusively of long chains.  When the percentage of long chain ligands in the mixture is high, a horizontally polymerized monolayer of chains is formed on the silica surface, for both trifunctional and difunctional short chains.  When the percentage of trifunctional short chains is increased, some vertical polymerization occurs, caused by molecular stacking of the highly reactive short chains near the surface.  However, the degree of cross-linking is quite high, suggesting the short chains cross-link both vertically, away from the surface, and horizontally across the surface.  No such vertical polymerization is observed for the bulkier difunctional short chains.  These different bonded phases were packed into HPLC columns, and the retention times of selected compounds were compared for the different surfaces.  The effect of overall chain density as well as the ratio of long to short chains on the surface on the HPLC retention times was explored.

Characterization of chemically modified silica prepared by surface polymerization of a mixture of long and short alkyl chains using NMR spectroscopy and HPLC

247th National Meeting (2014)

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