Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

Abstract

Understanding the complex thermodynamic behavior of confined amphiphilic molecules in biological or mesoporous hosts requires detailed knowledge of the stacking structures. Here, we present detailed solid-state NMR spectroscopic investigations on 1-butanol molecules confined in the hydrophilic mesoporous SBA-15 host. A range of NMR spectroscopic measurements comprising of (1)H spin-lattice (T(1)), spin-spin (T(2)) relaxation, (13)C cross-polarization (CP), and (1)H,(1)H two-dimensional nuclear Overhauser enhancement spectroscopy ((1)H,(1)H 2D NOESY) with the magic angle spinning (MAS) technique as well as static wide-line (2)H NMR spectra have been used to investigate the dynamics and to observe the stacking structure of confined 1-butanol in SBA-15. The results suggest that not only the molecular reorientation but also the exchange motions of confined molecules of 1-butanol are extremely restricted in the confined space of the SBA-15 pores. The dynamics of the confined molecules of 1-butanol imply that the (1)H,(1)H 2D NOESY should be an appropriate technique to observe the stacking structure of confined amphiphilc molecules. This study is the first to observe that a significant part of confined 1-butanol molecules are orientated as tilted bilayered structures on the surface of the host SBA-15 pores in a time-average state by solid-state NMR spectroscopy with the (1)H,(1)H 2D NOESY technique.

Axial Diffusion in Liquid-Saturated Cylindrical Silica Pore Models

“…By accounting for the loading dependence of the diffusion coefficient, the agreement between different data sets could be improved . In mesoporous materials such as silica, completely saturated with fluids, the static and dynamic fluid properties may differ from their corresponding bulk values and are affected by the pore diameter and the chemical nature of the inner surface of the pore. − A structural parameter characterizing the pore topology is the tortuosity, often defined by the ratio of the actual path and the shortest path from A to B or by the ratio of the two paths squared . The latter definition can be approximated by the ratio of the bulk diffusion coefficient of a chemically inactive species and its diffusion coefficient measured under confinement. , The notion of chemical inactivity is important to avoid any superimposed effect resulting from interactions with the pore walls that may lead to rather complex diffusion behavior such as a strong reduction or even an enhanced self-diffusivity. , However, also structural properties such as constrictions, reduced connectivity, or roughness affect the diffusion coefficient ratio .…”