Characterization of mesoporous silicas by in situ small angle neutron scattering during isothermal gas adsorption

Abstract

The recent application of in situ small angle neutron scattering (SANS) measurements performed during isothermal gas adsorption is a new technique for the characterization of the structure of porous materials. In this technique the coherent scattering length of the condensed gas or vapour is selected to match that of the porous solid. This technique is illustrated by investigations on two different silica gels, having a highly ordered mesoporous structure. Although both of these gels have a similar nominal `pore' size of ∼6 nm, they have a different pore geometry and longer-range texture. In the first, the gel is produced by a sol–gel process to give a porous network formed by the packing of monosized sol particles. The second contains cylindrical pores which are ordered in a hexagonal array (MCM-41 type structure). Details of the longer range texture and continuity of these mesoporous materials have been obtained from SANS measurements made in situ with contrast matched benzene, adsorbed at 310 K. The results have been analyzed in conjunction with the corresponding adsorption/desorption isotherms which have been measured gravimetrically. This in situ SANS technique is suitable for the evaluation of pore texture (network-connectivity, range of structural homogeneity) in other mesoporous materials, such as aerogels, as discussed.