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Pure Appl. Chem., 2013, Vol. 85, No. 1, pp. 289-305

Published online 2012-10-29

Thermal behavior, structure, and dynamics of low-temperature water confined in mesoporous organosilica by differential scanning calorimetry, X-ray diffraction, and quasi-elastic neutron scattering

Mai Aso1, Kanae Ito1, Hiroaki Sugino1, Koji Yoshida1, Takeshi Yamada2, Osamu Yamamuro2, Shinji Inagaki3 and Toshio Yamaguchi4,1*

1 Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
2 Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-0882, Japan
3 Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan
4 Advanced Materials Institute, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Abstract: Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and quasi-elastic neutron scattering (QENS) measurements have been made at 200~330 K for capillary-condensed water confined in periodic mesoporous organosilica (PMO) materials with the phenyl groups embedded in silica matrix (Ph-PMO; pore diameter 30 Å). The DSC data showed that the capillary-condensed water in Ph-PMO freezes at 228 K. X-ray radial distribution functions (RDFs) showed that the tetrahedral-like hydrogen-bonded structure of water is distorted in Ph-PMO pores, compared with bulk water; however, with lowering temperature the tetrahedral moiety of water is gradually recovered in the pores. Below the freezing point, cubic ice Ic was formed in the Ph-PMO pores. The QENS data showed that the translational diffusion constant and the residence time and the rotational relaxation time of water molecule in Ph-PMO are comparable with those in bulk. The corresponding activation energies suggested that the more hydrophobic the nature of the wall is, the smaller the activation energy of diffusion and rotation of a water molecule; this implies that water molecules confined in the hydrophobic pores are present in the core of the pores, whereas those in the hydrophilic pores strongly interact with the silanol groups.