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

Abstract

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. 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 a hydrophilic to a hydrophobic as the TMS functionalization content was increased, leading to contact angles from 0 ° (complete wetting) to 120 ° (non-wetting). 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), the contact angle was determined from the water sorption isotherms by applying the modified Kelvin equation on the desorption branch of the observed hysteresis loop (which reflects here the thermodynamic liquid-vapour transition). However, on 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 by the application of novel, liquid water intrusion/extrusion experiment, i.e. by applying the Washburn equation on the water intrusion branch (which reflects here the thermodynamic equilibrium vapor-liquid transition of a non-wetting fluid). 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).