PORE-STRUCTURE DETERMINATIONS OF SILICA AEROGELS BY XE-129 NMR-SPECTROSCOPY AND IMAGING

Silica aerogels represent a new class of open-pore materials with pore dimensions on a scale of tens of nanometers, and are thus classified as mesoporous materials. In this work, we show that the combination of NMR spectroscopy and chemical-shift selective magnetic resonance imag ing (MRI) can resolve some of the important aspects of the structure o f silica aerogels. The use of xenon as a gaseous probe in combination with spatially resolved NMR techniques is demonstrated to be a powerfu l, new approach for characterizing the average pore structure and stea dy-state spatial distributions of xenon atoms in different physicochem ical environments. Furthermore, dynamic NMR magnetization transfer exp eriments and pulsed-field gradient (PFG) measurements have been used t o characterize exchange processes and diffusive motion of xenon in sam ples at equilibrium. In particular, this new NMR approach offers uniqu e information and insights into the nanoscopic pore structure and micr oscopic morphology of aerogels and the dynamical behavior of occluded adsorbates. MRI provides spatially resolved information on the nature of the flaw regions found in these materials. Pseudo-first-order rate constants for magnetization transfer among the bulk and occluded xenon phases indicate xenon-exchange rate constants on the order of 1 s(-1) for specimens having volumes of 0.03 cm(3). PFG diffusion measurement s show evidence of anisotropic diffusion for xenon occluded within aer ogels, with nominal self-diffusivity coefficients on the order of D = 10(-3) cm(2)/s. (C) 1998 Academic Press.