Structural ordering and ice-like glass transition on cooling the nano-channel water formed within a crystalline framework

• Published in: Journal of physics. Condensed matter Vol. 18; no. 41; pp. 9375 - 9384
• Main Authors: Watanabe, Keisuke, Oguni, Masaharu, Tadokoro, Makoto, Nakamura, Ryouhei
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 18.10.2006; Institute of Physics
• Subjects: Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Glass transitions; Inorganic compounds; Physics; Specific phase transitions; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Glass transition; Specific heat; Nickel complexes; Pore size; Cooling; Hydrogen bonds; Phase transformations; Aqua complex; Enthalpy; Long-range order; Ordering
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/18/41/005

Heat capacities and enthalpy relaxation rates were measured for crystalline [Ni(cyclam)(H2O)2]3(TMA)2*24H2O, where cyclam is 1,4,8,11-tetraazacyclotetradecane, TMA is 1,3,5-benzene tricarboxylic acid, and 24H2O represents the water forming a nano-channel. A phase transition was found to occur at 196.9 K, and a glass transition at 87 K. A potential picture is given for the progress of the ordering of water molecules and hydrogen atoms in the channel. At room temperature, interfacial water molecules form hydrogen bonds with the oxygen atoms of the channel wall, and the aggregation of water molecules is recognized, as the average structure, to be in a crystalline state with a long-range order. The bond formation plays important role in the stabilization of the crystalline framework with a channel structure. The aggregate transforms to a more stable crystalline state at 196.9 K. All the water molecules should be fixed completely there. The positions of the hydrogen atoms on the network are, however, essentially in the disordered state while keeping an ice rule, and freeze at 87 K only with a short-range order in the arrangement. The progress of such ordering of channel water reveals a striking contrast to the behaviour of the water within meso-porous silicas, in which the molecules in the interface are always in the non-crystalline state and those in the pore centre tend to crystallize only when the pore diameter is greater than 2 or 3 nm.