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

Abstract

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.

Capillary condensation in deformable mesopores: wetting versus nanomechanics

“…These authors base their proposition on detailed analyses of sorption isotherms where the surmised correlation effects can be ascertained only rather indirectly. However, very recent neutron scattering experiments on SBA-15 silica contradict the clustering phenomenon [27]. The neutron scattering data are fully consistent with the assumption that the spatial distribution of pores of different sizes remains random despite the sorption-induced deformation of individual pores in the sample.…”

Improvement in the Pore Size Distribution for Ordered Mesoporous Materials with Cylindrical and Spherical Pores Using the Kelvin Equation

“…According to the capillary condensation and capillary evaporation theory, the most accepted mechanisms for the filling or emptying of the mesopores are: In addition, it is known that MCM-41 and SBA-15 materials present cylindrical pores open at both ends [34][35][36] and the SBA-16 materials show cage-like structures with spherical pores [12]. Therefore, all the considerations stated above should be taken into account in those traditional methods.…”

“…It is noticeable that the additional term is obtained without considering the accepted filling mechanism for this isotherm branch. For these materials that exhibit cylindrical pores open at both ends [34][35][36], they assume the formation of hemispherical menisci in the capillary condensation, which has not physical meaning.…”

Neutron and X-Ray Porosimetry

“…The hypothesis of sorption strain-induced long-range correlations during pore filling was tested using in situ SANS by studying capillary condensation and emptying of water and perfluoropentane (C 5 F 12 ) in SBA-15 [27]. The BET specific surface area of the granulated SBA-15 sample was 826 m 2 /g and the total specific pore volume 1.134 cm 3 /g.…”

“…The obtained SANS data were fitted to analytical model for spatially random (not correlated) pore filling described by equation that takes into account contribution from all three types of scattering [27]:…”