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 quantitatively assessing the hydrophilicity/ hydrophobicity 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 allowing one to accurately tailor the surface chemistry while maintaining the well-defined pore structure. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under Magic Angle Spinning was employed. A full textural characterization of the materials was obtained by high-resolution argon 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 novel methodology allowing one to determine the effective contact angle of water adsorbed on the pore surfaces from complete wetting to non-wetting, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to hydrophobic with increasing TMS functionalization. For wetting and partial wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure P0 (i.e., at P/P0 < 1) and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, for non-wetting surfaces, pore condensation occurs at pressures above the saturation pressure (i.e., at P/P0 > 1). In this case we investigated the pore filling of water (i.e. the vapour-liquid phase transition) 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 (mimicking the tailored, functionalized experimental silica surfaces) which allow for a molecular view on the adsorbed water phase. Within this context, the presented methodology can be considered a powerful toolbox for a comprehensive surface chemistry assessment for pristine and functionalized mesoporous silica surfaces, but can also be extended to any other nanoporous material. It has the potential to provide important guidance for the design and selection of porous materials for various applications including separation applications (such as chromatography), heterogeneous catalysis etc.