Characterization of Functionalized Chromatographic Nanoporous Silica Materials by Coupling Water Adsorption and Intrusion with Nuclear Magnetic Resonance Relaxometry

Nanoporous silica is widely used as a support material for chemically bound stationary phases in chromatographic separations as well as for catalytic applications. Tuning of textural properties and surface chemistry of stationary phase materials (SPMs) is crucial to enhance their selectivity to certain compounds and the resulting process efficiency. Nanoporous silica supports are beneficial, as surface modifications are possible with a large variety of hydrophilic and hydrophobic functional groups, but their influence on the 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 pores is challenging and requires a combination of various experimental techniques. This work demonstrates that combining water adsorption and intrusion measurements allows for the determination of the effective contact angle of adsorbed water on the pore walls for wetting, partial wetting, and nonwetting situations using functionalized hydrophilic and hydrophobic silica SPMs as model materials. Furthermore, NMR relaxometry experiments reveal that the ratio of the spin-lattice (T₁) to spin-spin (T₂) relaxation time of the adsorbed water film (T_1,ads.film/T_2,ads.film ratio) can be correlated with the effective adsorption strength of water on the surface. Indeed, a linear correlation between the negative inverse relaxation time ratio (−T_2,ads.film/T_1,ads.film) and the contact angle is observed. Our work demonstrates that water vapor adsorption and water intrusion experiments coupled with NMR relaxometry can be used as complementary tools to quantify the wettability and surface chemistry of nanoporous materials. This approach allows for addressing important aspects of nanoscale wettability in the context of material synthesis as well as for their application in chromatographic separation processes and catalysis.