Calorimetric Finding of Phase and Glass Transitions Concerning the Ordering of the Water Located between the Silicate Layers of the Clay Na-RUB-18

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  • Watanabe Keisuke
    Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology
  • Oguni Masaharu
    Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology

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The behavior of the water accommodated between the silicate layers of a clay Na8[Si32O64(OH)8]·32H2O was investigated by using adiabatic calorimetry. A phase transition was observed showing a heat-capacity peak at 194 K and its entropy was roughly estimated to be 5.2 J K−1 (H2O-mol)−1. The transition was interpreted to be essentially due to an order–disorder process involving two orientations of the water molecule which can form hydrogen bonds with siloxane-oxygen atoms. It is thought that the water molecules move with a large amplitude of vibration as well as disorder between the two permissible orientations at room temperature and become gradually ordered to take a more stable orientation of the two below 194 K. A glass transition was found at a temperature Tg=106 K, due to freezing of the water molecule in the orientations, which causes a little disorder to remain. The potential barrier (Δεa) of the reorientation was assessed from the Tg to be about 34 kJ mol−1, which corresponds to the energy to break two hydrogen-bonds. There remains a possibility that the silanol-hydrogen atom takes a tunneling state involving two accessible sites between two silanol-oxygen atoms; however, it was not confirmed in this work.

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