An X-ray scattering and quasielastic neutron scattering study on the structure and dynamic properties of low-temperature methanol confined in ordered microporous carbon and mesoporous organosilica pores

Abstract

We utilized large-angle X-ray scattering (LAXS) and quasielastic neutron scattering (QENS) to measure the structure and dynamic properties of methanol confined in ordered microporous carbon (OMC) and periodic mesoporous organosilica (PMO) materials with phenyl groups embedded in a silica matrix (Ph-PMO). The pore diameters of the OMC and Ph-PMO materials obtained by the nitrogen adsorption and desorption isotherm analysis are 18.7 Å and 30.0 Å, respectively. The LAXS data of the methanol molecules confined in the Ph-PMO pores at a low temperature (230 to 298 K) reveal that the structure of the capillary-condensed methanol molecules in the Ph-PMO pores consists of the monolayer of methanol on the surface and the hydrogen-bonded chains of the methanol molecules at the central part of the pores. As the temperature decreases, the methanol chains in the Ph-PMO pores gradually become ordered; however, the degree of this enhancement with temperature is not as significant as that for water in the Ph-PMO pores. The QENS spectra were analyzed using a jump-diffusion model to describe the motion of the methanol molecules. For the methanol molecules confined in the OMC pores, as the temperature decreases, the hydrogen-bonded axial chain structure of methanol in the pores is enhanced and the movement of the confined methanol molecules slows down. A comparison of the dynamic properties of the methanol and water molecules under the same conditions shows that the confinement effect has a greater impact on the dynamic characteristics of the methanol molecules than on the water molecules. For the methanol molecules confined in the amphiphilic Ph-PMO with a relatively large capillary pore, a kind of ‘bulk-like’ behavior was observed, especially at 315 K, which is in accord with the behavior of the water molecules in the Ph-PMO pores. In addition, similar to the case for the water molecules, we found evidence of the immobile and mobile fractions of the confined methanol molecules.