Characterization of porous glass by adsorption of dibromomethane in conjunction with small-angle x-ray scattering

Abstract

Two porous glass samples of the Vycor type, one in a monolithic form and the other as a fine powder, were examined by the method of small-angle x-ray scattering (SAXS). Adsorption isotherms of dibromomethane (${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$) were measured on the same samples, and analyzed according to the Kelvin equation. The SAXS measurements were repeated on samples that had been brought to equilibrium with known relative pressures (p/${\mathit{p}}_0$) of ${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$, which has a similar electron density as Vycor. Analysis of the data, in terms of fractal geometry, was also accomplished. The fractal dimension for the dry samples was estimated to be 2.5±0.1. Furthermore, as the adsorbate loading was increased defractalization was observed. Subsequently, the fractality in Vycor was estimated to have an upper cutoff limit equal to 15 Å. A correlation between the two methods was demonstrated, by reconstructing the adsorption isotherm from the SAXS data, in two ways: first, by plotting the scattering intensities against p/${\mathit{p}}_0$ and second by plotting the Porod slopes against p/${\mathit{p}}_0$.