Dynamics of liquids in porous media: Investigating competitive adsorption using NMR relaxometry
The work presented in this thesis has been focused towards the development of NMR relaxometry methods to investigate competitive adsorption and microphase separation of liquid mixtures imbibed within porous media. The work was done on three different mesoporous catalysts and catalyst supports, namely anatase titania, γ-alumina and silica. The main NMR relaxometry method that was applied was Fast Field Cycling (FFC) NMR, which has been shown to quantitatively probe surface-adsorbate interactions of liquids in porous media. First, a model study of binary liquid mixtures imbibed within anatase titania and γ-alumina was performed to assess the applicability of FFC-NMR to multi-component systems, and to compare the FFC-NMR data with a high field NMR relaxometry method. In a semi-quantitative analysis, FFC-NMR was shown to provide clear, unambiguous results relating to the surface-adsorbate interactions of both the stronger and weaker interacting species within the liquid mixtures. The high field NMR relaxometry results were more ambiguous, and therefore more open to misinterpretation. In the following chapters, a theoretical modelling approach was developed to quantitatively describe the interactions of short, linear alcohols with an anatase titania support. The optimized modelling approach was applied to alcohol-water mixtures within the same porous medium, and showed an increased level of microphase separation for aqueous solutions with longer alcohols as well as tetrahydrofuran. This effect was shown not to depend on the bulk miscibility of the mixture, but rather on the surface-adsorbate interaction strength of the weaker interacting species. The competitive adsorption of ethanol-water mixtures imbibed within silica supports was also studied, initially through fixed field T1-T2 correlation experiments, and subsequently through FFC-NMR methods. Due to overlapping relaxation environments, it was necessary to implement an advanced FFC-NMR pulse sequence with T2-encoding. The low field NMR relaxometry experiments and FFC-NMR dispersion profiles were in agreement with one another and found that the form of microphase separation of ethanol-water mixtures depends on the properties of the silica support, thereby explaining a discrepancy in the literature and providing a quantitative analysis of the dynamics of these systems.