Wall thickness determination of hydrophobically functionalized MCM-41 materials

Abstract

We have developed a new protocol for simple and convenient synthesis of MCM-41 porous material. Post-synthetic grafting of –Si(CH₃)₂R groups, where R stands for methyl, phenyl, or hexyl, respectively, yielded hydrophobic porous material by addition of an organic layer to the surface silanols. Modified materials and the hydrophilic precursor were characterized by a combination of nitrogen adsorption and small angle neutron scattering. We found that pore sizes determined by applying BET and KJS models to nitrogen adsorption data depend strongly on the chosen model as expected. Pore sizes from neutron scattering data do not demand simplifying model assumptions and were therefore used in order to calculate the thickness of the grafted organic layer. Comparison of the experimentally measured thickness with the theoretical lengths of the grafted groups allowed us to deduce the conformation of the organic moiety. A detailed analysis of the neutron scattering profile yielded a fractal surface dimensionality of 2.2 for all samples in agreement with literature data for hydrophilic MCM-41 solids and confirms that post-synthetic grafting does not impair the precursor material. This is a crucial point as the precursor is absolutely necessary to evaluate the thickness of the grafted layer.