Dielectric study on two dynamic phases of 1-propanol confined in mesopores of MCM-41

Abstract

Two dynamic phases were recognized on the 1-propanol molecules confined in MCM-41 with pore diameters d = 2.1, 2.4, 2.7 and 3.6 nm by dielectric measurements, in which two types of confined states of liquid were investigated: surface-adsorbed (sa) and pore-filled (pf) liquid. The dielectric measurements in the frequency range 103–107 Hz and temperature range 120–300 K showed that the molecular motions became slower in the following order: bulk, pf and sa liquid, which is the same order as for methanol and ethanol confined in MCM-41 reported previously. For pf samples, two relaxation components, which correspond to molecules near the pore surface and at the center of the pores, were observed separately. This is somewhat different from the behavior of methanol and ethanol confined as pf state in which two relaxation components were also detected but a clear separation between them was not observed. This implies that 1-propanol molecules near the pore wall interact weakly with those at the central part of the pores. For the MCM-41 sample with the smallest pore diameter (d = 2.1 nm), however, the dielectric spectra of the pf sample were very similar to those of the sa sample. That is, the dynamic motion of molecules in the pf sample was inhibited by narrow space surrounded by monolayer molecules similarly to that in the sa sample

Thermal characterization of static and dynamical properties of the confined molecular systems interacting through dispersion force

“…These results have suggested an inhomogeneous molecular picture for nanopore-confined liquids composed of inner pore molecules and interfacial molecules on the pore wall. More recent results from dielectric relaxation experiments on methanol, ethanol [11] and 1-propanol [12] confined in MCM-41 with pore diameters 2.1-3.7 nm were interpreted in a similar manner.…”

“…Preparation of nano-confined liquids is generally accomplished by introducing a molecular liquid into a solid material with pores of a nanometer scale. To this end, silica-based materials, such as silica gel, controlled pore glasses (CPGs) [2,3,5,[7][8][9], Vycor glass [6], MCM-41 [10][11][12][13][14] and SBA-15 [13,14], have been widely employed as the porous materials. Initially, Jackson and McKenna reported glass transition characterization for ortho-terphenyl (o-TP) and benzyl alcohol liquids confined into surfacetreated CPG pores with diameters down to 4 nm by using differential scanning calorimetry (DSC) [7].…”

“…Takahara et al [12] have concluded that the rather clear separation between the interfacial and inner-pore related dielectric-relaxation peaks for MCM-41-confined 1-propanol, as compared to the poor separation between the dielectricrelaxation peaks for methanol and ethanol, is in virtue of a weaker interaction of interfacial molecules with inner-pore molecules in comparison with the interaction with the pore wall. Indeed, the molecular interaction between pore wall and interfacial molecules has been cogitated to be a key fact in conditioning the dynamical properties of a confined system [1].…”