Small-angle X-ray scattering studies of adsorption in Vycor glass

Porous Vycor is examined by measuring CH2Br2 adsorption in situ with small-angle X-ray scattering. When a class of pores fills with condensed vapors of this particular adsorbate it ceases to act as scatterer and only the remaining empty pores produce a measurable intensity. By determining a number of scattering curves at various relative pressure loadings details on the structure of the glass as well as on the adsorption/desorption mechanism are obtained. Pore chord length and specific surface area are estimated from Porod tangent analysis to 78 Å and 108 m2/g. Comparison of the results with those reported for N2 and Ar adsorption is also given. The role of network effects on the shape of the hysteresis loop is considered and the pore-blocking hypothesis is verified from the scattering spectra. The pore connectivity is calculated to 5.6. The X-ray data are further treated with the inverse Fourier transformation technique. Pore-size distributions are extracted and weighed against the prediction of the Kelvin equation.

Although narrower pores have drained than those which had been filled with condensate, the scattering curve is shorter in the former than in the latter case, indicating that the number of pores that should have drained is much less than the number of pores that had been filled despite their larger size. This is a direct evidence of network effects.Figure optionsDownload as PowerPoint slide