New approach to evaluate pore size distributions and surface areas for hydrophobic mesoporous solids

A new approach to pore size and surface area analysis for hydrophobic mesoporous solids is proposed. Well-defined materials with strongly hydrophobic surfaces were prepared via chemical bonding of octyldimethylsilyl (ODMS) ligands to the surface of large-pore MCM-41 samples. Nitrogen statistical film thickness curves (t-curves) were determined for hydrophobic pores. These t-curves were fitted with the nitrogen adsorption isotherm fora macroporous silica modified with ODMS groups in order to derive the reference t-curve valid in the entire range of relative pressures. An empirical expression for the reference t-curve was found. The reference t-curve and the previously derived corrected form of the Kelvin equation were used to calculate pore size distributions for hydrophobic mesoporous materials, allowing to correctly reproduce the total pore volume, average pore size, and surface area determined using independent calculation procedures. The reference nitrogen adsorption isotherm on hydrophobic materials is reported in a tabular form. Moreover, the accuracy of the BET specific surface area was discussed. Thus, this study allowed us to develop the methodology for accurate and consistent characterization of hydrophobic mesoporous materials using nitrogen adsorption.