In-situ small-angle neutron scattering study of pore filling and pore emptying in ordered mesoporous silica

The capillary condensation and capillary emptying of water and perfluoropentane in ordered mesoporous SBA-15 silica is studied by in-situ small-angle neutron scattering (SANS). The SANS data can be perfectly described by a simple analytical model for spatially random pore filling (Laue scattering) for the entire range of pore-filling fractions. From this it is concluded that recently proposed pore correlations due to elastic interactions between neighbouring pores upon capillary condensation do not play a role in this system. The pores fill randomly according to their size distribution, in perfect agreement with the classical Kelvin equation. The relation between the overall pore-filling fraction as determined from the volumetric sorption isotherm, and the fraction of completely filled pores as obtained from the fit of the SANS data, allows conclusions to be drawn about the thermodynamic metastability of the adsorption process.