Small-angle x-ray diffraction of Kr in mesoporous silica: Effects of microporosity and surface roughness

Kr has been adsorbed in SBA-15, a template-grown mesoporous silica substrate. A volumetric adsorption isotherm is presented. The Bragg peaks of the pore array have been measured by small angle x-ray diffraction and analyzed as function of the filling fraction in terms of the electron density contrast, the radial position of the liquid-vapor interface, and film roughness, both for the regime of film growth and capillary condensation. The results are compared with the theory of Saam and Cole. It is shown that the microporosity of the matrix leads to a delay of capillary condensation. The peculiar dependence of the film thickness on the filling fraction points to a high fractal dimension of the pore walls.