Deuterium NMR Spectroscopy of Solvents Adsorbed onto the Surface of Mesoporous Silica

A series of simple deuterium NMR experiments are used to investigate the surfaces of differently structured silicas by studying the sorption of deuteriated solvents. Two mesoporous silicas (MCM-41 and H1-silica), which display hexagonally arranged channels of uniform diameter are studied, as well as amorphous column silica. The deuterium NMR lineshape strongly depends on the anisotropic environment of the adsorbed deuteriated species (D2O, benzene-d6, pyridine-d5). This allows differentiation between probe molecules adsorbed to the internal surface (anisotropic) and to the external surface (isotropic, non-ordered surface of particles). Solvents adsorbed within the channels of the silica nanostructure retain a degree of orientational order while moving on the surface, thus producing a Pake pattern. In contrast, solvents adsorbed on the outside of the ordered channels and on domain boundaries retain no orientational order while moving on the surface, therefore giving rise to a singlet in the deuterium NMR spectrum. This phenomenon allows to judge and compare the qualities of hexagonal ceramic nanostructures. The results show that H1-silica exhibits a significantly higher degree of order, due to a larger domain size, than MCM-41.