Differentiating and characterizing geminal silanols in silicas by 29Si NMR spectroscopy

Single and geminal hydroxyl species in silicas have been characterized using solid-state 29Si NMR spectroscopy. Differentiating hydroxyl types is important in understanding their roles in chemical toxicity mechanisms for inhaled crystalline silicas responsible for silicosis. 1H–29Si cross polarization NMR spectroscopy has been employed to obtain 29Si NMR chemical shift data and signal accrual and relaxation characteristics. Spectral deconvolution is used to examine relative single and geminal hydroxyl resonance areas for a series of representative silicas and silica gels. Silicon-containing materials examined include 1878a quartz, and 1879a cristobalite from the National Institute for Science and Technology, kaolin, and several widely used respirable silicas and silica gels. Geminal hydroxyls were observed in every case, with relative resonance areas accounting for 21–65% of total hydroxyl signals. Factors affecting relative areas measured as a function of contact time, relaxation, and surface area are discussed. Subsequent 29Si and 31P NMR studies of a silica coated with various sodium hydrogen phosphates show preferential single silanol–phosphate interaction for basic phosphates, and oligomerization products for acidic phosphates. Geminal hydroxyl resonance areas displayed significant error (4–17%) for low surface area silicas, limiting this method to studies exhibiting major changes in chemical or spectroscopic properties.

Geminal hydroxyls are abundant and chemically distinct from single hydroxyls on silicas, complicating measurements and mechanisms in studies of surface reactivity..Figure optionsDownload high-quality image (43 K)Download as PowerPoint slideResearch highlights
► Silicon 29 NMR differentiates and characterizes geminal and single silanols.
► Geminal hydroxyl concentrations are much higher than expected.
► Geminal silanols do not interact with phosphate coating species.
► Differences in silanol reactivity may control silica surface toxicity.