Characterization of functionalized chromatographic silica materials : Coupling water adsorption and intrusion with NMR-relaxometry

Silica is widely used as a support material for chemically-bound/bonded stationary phases in chromatographic separations. Tuning of textural properties and surface chemistry of stationary phase materials (SPMs) is crucial to enhance their selectivity to certain compounds. 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 non-wetting situations. 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, a linear correlation between the negative inverse ratio with 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.