Quantitative Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by combining Adsorption, Liquid Intrusion and solid-state NMR spectroscopy

We have developed a comprehensive strategy for assessing the surface chemistry of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques and solid-state NMR spectroscopy. For this we have chosen a well-defined system of model materials, i.e., the highly ordered mesoporous silica molecular sieve SBA-15 in its pristine state and functionalized with different amounts of trimethylsilyl groups. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under Magic Angle Spinning was employed. 1H two-dimensional single quantum double quantum MAS NMR spectra reveal an intimate mixture of TMS and residual OH groups on the surface. A full textural characterization of the materials was obtained by high-resolution argon at 87 K adsorption, coupled with the application of dedicated methods based on non-local-density functional theory (NLDFT). Based on the known texture of the model materials, we developed a method allowing one to determine the effective contact angle of water adsorbed on the pore surfaces, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to a hydrophobic as the TMS functionalization content was increased. For wetting and partial wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure P0 and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, on non- wetting surfaces, pore condensation occurs at pressures above the saturation pressure P0. In this case we investigated the pore filling of water by the application of a novel, liquid water intrusion/extrusion methodology, allowing one to derive the effective contact angel of water on the pore walls even in case of non-wetting. Complementary molecular simulations provide density profiles of water on pristine and TMS-grafted silica surfaces, which agree with the obtained experimental data. Summarizing, we present a comprehensive and reliable methodology for assessing the hydrophilicity/hydrophobicity of siliceous nanoporous materials, which has the potential to optimize applications in heterogeneous catalysis and separation (e.g,.chromatography).