Thermal behavior, structure, and dynamics of low temperature water confined in mesoporous materials MCM-41

Abstract

Thermal behavior, structure and dynamics of water confined in mesoporous materials MCM-41 with various pore sizes have been investigated over a temperature range of 298 to 170 K by differential scanning calorimetry (DSC), FTIR, X-ray diffraction (XRD), quasielastic neutron scattering (QENS), and neutron spin echo (NSE) measurements. The DSC data showed that the smaller the pore size the lower the freezing and melting points. It has been found that water confined in pores of 20 Å diameter keeps the liquid state at temperatures down to 170 K. The X-ray radial distribution functions showed that the capillary-condensed water in the pores has more distorted tetrahedral structure than in bulk water with decreasing pore size. The QENS data showed that when the pore size decreases the translational diffusion of confined water is retarded to almost half compared with bulk water. The temperature dependence of the relaxation time of rotational diffusion and the residence time of translational diffusion of confined water showed the Arrhenius type behavior. The NSE data showed that the relaxation time of water confined in pores whose diameter is 20 Å follows the VTF equation above 220 K, whereas below 220 K they shows the Arrhenius type behavior, i.e. the dynamical crossover takes place in the confined water. © 2011 The Japan Society for Analytical Chemistry.