Structural and dynamical properties of guest molecules confined in mesoporous silica materials revealed by NMR

Abstract

In the last fifteen years several novel porous silica materials, which are periodically structured on the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental studies of surface–substrate interactions, for studies of the dynamics of guest molecules in confinement and for studies of the effect of confinement on the structural and thermophysical properties of fluids. Examples of such confinement effects include the change of the freezing and melting points or glass transitions of the confined liquids. These effects are studied by combinations of several NMR techniques, such as ¹⁵N- and ²H-solid-state NMR line shape analysis, MAS NMR and NMR diffusometry with physico-chemical characterization techniques such as nitrogen adsorption and small angle diffraction of neutrons or X-rays. This combination does not require crystalline samples or special clean and well defined surfaces such as conventional surface science techniques, but can work with typical ill-defined real world systems. The review discusses, after a short introduction, the salient features of these materials and the applied NMR experiments to give the reader a basic knowledge of the systems and the experiments. The rest of the review then focuses on the structural and dynamical properties of guest molecules confined in the mesoporous silica. It is shown that the confinement into the pores leads to fascinating new features of the guests, which are often not known for their bulk phases. These features depend strongly on the interplay of the their interactions with the silica surface and their mutual interactions.