SOLID-STATE C-13 NMR-STUDIES OF IONIC SURFACTANTS ADSORBED ON C-18 AND C-8 SILICAS - IMPLICATIONS FOR MICELLAR LIQUID-CHROMATOGRAPHY

Solid-state C-13 NMR spectroscopy techniques, including cross polariza tion (CP), magic angle spinning (MAS), and high-power proton decouplin g, have been used to study the interactions of two ionic surfactants w ith octadecylsilica (C-18) and octylsilica (C-8) HPLC stationary phase s. The two surfactants, cetyltrimethylammonium bromide (CTAB) and sodi um dodecyl sulfate (SDS), are commonly used in micellar reversed-phase liquid chromatography (RPLC). Variable contact time CP/MAS C-13 NMR d ata suggest that differences in selectivity between SDS and CTAB micel lar RPLC are due to the differing nature of the SDS- and CTAB-bonded p hase association. For CTAB the association leads to a more hydrophobic bulk stationary phase, whereas SDS adsorption results in the formatio n of an anionic, hydrophilic surface layer. These results suggest that proper matching of surfactant monomer and bonded stationary phase is critical to ensure selective separations in micellar RPLC. (C) 1994 Ac ademic Press, Inc.