Revealing the fine details of functionalized silica surfaces by solid-state NMR and adsorption isotherm measurements: The case of fluorinated stationary phases for liquid chromatography

The structural and chromatographic characterization of two novel fluorinated mesoporous materials prepared by covalent reaction of 3-(pentafluorophenyl)propyldimethylchlorosilane and perfluorohexylethyltrichlorosilane with 2.5 μm fully porous silica particles is reported. The adsorbents were characterized by solid state 29Si, 13C, and 19F NMR spectroscopy, low-temperature nitrogen adsorption, elemental analysis (C and F), and various chromatographic measurements, including the determination of adsorption isotherms. The structure and abundance of the different organic surface species, as well as the different silanol types, were determined. In particular, the degree of so-called horizontal polymerization, that is, Si-O-Si bridging parallel to the silica surface due to the reaction, under "quasi-dry" conditions, of trifunctional silanizing agents with the silica surface was quantified. Significant agreement was found between the information provided by solid-state NMR, elemental analysis, and excess isotherms regarding the amount of surface residual silanol groups, on the one hand, and the degree of surface functionalization, on the other. Finally, the kinetic performance of the fluorinated materials as separation media for applications in near-ultrahigh-performance liquid chromatography was evaluated. At reduced velocities of about 5.5 (ca. 600 bar backpressure at room temperature) with 3 mm diameter columns and toluene as test compound, reduced plate heights on the order of 2 were obtained on columns of both adsorbents. A modern magnifying glass: What is really on the surface of functionalized silica gels? Is surface functionalization "horizontally" or "vertically" oriented? To what extent did it happen? How many silanol groups reacted, and are residual silanol groups accessible? Solid-state NMR spectroscopy and adsorption isotherms come together to answer some very difficult questions (see figure). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.