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

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.