Solid-state nuclear magnetic resonance investigations of surface layers in oxide glass systems

A solid-state NMR study of leached surface layers of sodium-aluminosilicate glasses and characterization of multicomponent silicate glass surfaces is presented. The structure and composition of newly formed surface layers and mechanisms of their formation have been investigated to elucidate the surface and sub-surface chemistry. In addition, results of testing the applicability and sensitivity of solid-state NMR for characterization of commercial fiberglass surfaces are described.; Cross-polarization from protons is used as a surface-selective NMR technique, since in these leached glasses all protons are present near the surface. CPMAS, 1H → 29Si CPMAS, 1H → 27Al CPMAS NMR and 1H/29Si/ 27Al cross-polarization transfer of populations via double-resonance (CP-TRAPDOR) experiments were employed to provide information about silicon and aluminum environments in the altered layers. Transformation of Al from four- to six-fold coordination is documented after leaching and alteration of the surface layers. The fraction of AlVI in the altered layer increases with the increase of duration of dissolution. The observed AlVI/(AlVI+AlIV) ratio correlates with the observed dissolution rate of the glass samples and the Al/Si ratio in the bulk. Such a correlation supports the hypothesis that AlVI forms on surfaces during dissolution. 29Si MAS, 27Al MAS and 27Al → 29Si CPMAS NMR experiments additionally describe the bulk network structures in aluminosilicates. In certain instances, repolymerization of the silicon network via formation of additional Q4(0Al) silicon sites was observed in samples of leached glasses.; NMR techniques were developed to identify surface degradation products, monitor chemical and structural changes on silicate glass surfaces that result from atmospheric exposure or from aqueous attack, and more importantly, allow a direct spectroscopic probe of surface silanol groups. Since NMR spectroscopy is too insensitive a technique to probe low surface area samples quantitatively, model and commercial silicate fiber surfaces are modified with a silylation agent ((3,3,3-trifluoropropyl) dimethylchlorosilane) containing –CF ₃ end-groups. This allows the first 19F single-resonance NMR spectrum detection of a commercial fiberglass surface. In addition, a quantification analysis is presented, and the number of 19F atoms associated with this probe molecule attached to a fiber system are estimated.