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).
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).
Silica particles are widely used as a support material for chemically-bound stationary phases in chromatographic separation processes. The tuning of textural properties and surface chemistry of stationary phase materials (SPMs) is crucial to enhance their selectivity to certain compounds and the efficiency of the separation process. Silica supports have the advantage that their surface can be modified with a large variety of hydrophilic and hydrophobic functional groups, but their influence on the silica surface properties has not been evaluated in detail. In this sense, the contact angle is a key parameter for the assessment of surface chemistry but its quantification in the pore walls is particularly challenging and requires a combination of various tools and experimental techniques. In this work we demonstrate that by combining water adsorption and intrusion measurements is possible to derive reliable information of the effective contact angle θ of adsorbed water for wetting (θ = 0°), partial wetting (θ < 90°), and non-wetting situations (θ > 90°) observed on the pore walls of the SPMs under study. Furthermore, NMR relaxometry experiments reveal that the T1,ads.film/T2,ads.film-ratio can be correlated with the effective adsorption strength of water on the surface. Indeed, we find a linear correlation between the negative inverse of the T1,ads.film/T2,ads.film -ratio (-T2,ads.film/T1,ads.film) with the contact angle determined from water vapor adsorption and intrusion experiments for the investigated SPMs. Our work clearly demonstrates for the first time that water vapor adsorption experiments and novel water intrusion technique coupled with NMR relaxometry can be used as complementary techniques to quantitatively analyze the wettability behavior and surface chemistry of nanoporous materials.
Silica particles are widely used as a support material for chemically-bound stationary phases in chromatographic separation processes. The tuning of textural properties and surface chemistry of stationary phase materials (SPMs) is crucial to enhance their selectivity to certain compounds and the efficiency of the separation process. Silica supports have the advantage that their surface can be modified with a large variety of hydrophilic and hydrophobic functional groups, but their influence on the silica surface properties has not been evaluated in detail. In this sense, the contact angle is a key parameter for the assessment of surface chemistry but its quantification in the pore walls is particularly challenging and requires a combination of various tools and experimental techniques. In this work we demonstrate that by combining water adsorption and intrusion measurements is possible to derive reliable information of the effective contact angle θ of adsorbed water for wetting (θ = 0°), partial wetting (θ < 90°), and non-wetting situations (θ > 90°) observed on the pore walls of the SPMs under study. Furthermore, NMR relaxometry experiments reveal that the T1,ads.film/T2,ads.film-ratio can be correlated with the effective adsorption strength of water on the surface. Indeed, we find a linear correlation between the negative inverse of the T1,ads.film/T2,ads.film -ratio (-T2,ads.film/T1,ads.film) with the contact angle determined from water vapor adsorption and intrusion experiments for the investigated SPMs. Our work clearly demonstrates for the first time that water vapor adsorption experiments and novel water intrusion technique coupled with NMR relaxometry can be used as complementary techniques to quantitatively analyze the wettability behavior and surface chemistry of nanoporous materials.
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