Neutron Scattering and Dielectric Studies on Dynamics of Methanol and Ethanol Confined in MCM-41

Abstract

The dynamics of methanol and ethanol molecules confined in MCM-41 with pore diameters d = 2.1, 2.8, and 3.7 nm has been investigated by quasi-elastic neutron scattering (QENS) and dielectric measurements. Two kinds of adsorbed amounts of liquids were studied: surface-adsorbed and capillary-condensed. The QENS measurements were performed only for methanol at room temperature and revealed that the molecular motions became slower in the following order: bulk, capillary-condensed, and surface-adsorbed liquid. The effect of confinement on the dynamics of capillary-condensed methanol molecules is stronger than in the case of acetonitrile, indicating higher cooperativity of molecular motions in liquid methanol. The dielectric measurements for methanol and ethanol in the frequency range 102−107 Hz and temperature range 120−300 K showed that the molecular motions became slower in the same order as for the QENS measurements. The slowest molecular motions of surface-adsorbed liquid must result from strong interaction between liquid molecules and the pore surface, which was demonstrated by adsorption isotherm and Fourier transform infrared spectra measurements. The slow molecular motions of capillary-condensed liquid can be explained by the influence of slow molecules near the pore surface through the cooperativity of molecular motions and/or formation of an ordered structure possibly due to van der Waals interactions between hydrogen-bonded chains.