Characterization of silanol reactivity and acidity on octadecyl-bonded chromatographic supports by29Si solidstate nuclear magnetic resonance spectroscopy and surface titration

doi:10.1016/S0021-9673(01)90534-5

Journal of Chromatography A

Volume 319, 1985, Pages 9-21

https://doi.org/10.1016/S0021-9673(01)90534-5 Get rights and content

Abstract

Surface analysis and chromatographic data are used to examine the structure of surface groups in a reversed-phase chromatographic environment involving an octadecyl-derivatized silica substrate. The support is characterized by²⁹Si solid-state nuclear magnetic resonance spectroscopy and surface titration, and results are compared to other spectroscopic methods for the quantitation of surface species. Quantitative nuclear magnetic resonance data permits examination of the number and reactivity of single versus geminal hydroxyl sites as a function of octadecyldimethylsilyl coverage. It is postulated that an acidic, reactive subset of surface silanols exists that includes a large proportion of the geminal silanol sites, and a subset of the single silanol sites including hydrogen-bonded species such as the vicinal silanols. Knowledge of surface structure helps to explain various types of chromatographic behavior including the inaccessibility of some surface region silanols at moderate silane coverages, the partition-like behavior of octadecyl substrates, and the pH-dependent retention of non-polar solutes in reversed-phase chromatography.

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The calorimetric measurements of methanol and hexane heats of immersion were carried out on different silica gels. Based on the difference in immersion heats, a methodology for the determination of the number of silanols on the surface is presented. The calculated concentration of residual silanols on the silica gel surface agreed with data found in the literature. The proposed methodology, based on a calculation of possible hydrogen bond formation, was also tested on the series of bonded stationary phases with different coverage densities. A very good correlation between the calculated number of accessible residual silanols and the coverage density of bonded ligands was observed.
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NMR spectroscopy1 has been used extensively to detect species on solid surfaces possessing NMR-active nuclei. Maciel et al. first reported solid state 29Si NMR studies which detect surface silicon in a variety of silica gels [14–16]. Q3 was observed near −100 ppm (relative to tetramethylsilane) in 29Si NMR spectra, while Q4 was found near −110 ppm and Q2 near −91 ppm in VCT studies.
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Characterization of silanol reactivity and acidity on octadecyl-bonded chromatographic supports by29Si solidstate nuclear magnetic resonance spectroscopy and surface titration

Abstract

J. Chromatogr.

J. Chromatogr.

J. Amer. Chem. Soc.

J. Phys. Chem.

J. Amer. Chem. Soc.

Separ. Sci.

Anal. Chem.

Anal. Chem.

Anal. Chem.

Anal. Chem.

Characterization of silanol reactivity and acidity on octadecyl-bonded chromatographic supports by²⁹Si solidstate nuclear magnetic resonance spectroscopy and surface titration