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

Abstract

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, and24H2O represents the water forming a nano-channel. A phase transition was found to occur at196.9 K, and a glass transition at 87 K. A potential picture is given for the progress of theordering of water molecules and hydrogen atoms in the channel. At room temperature,interfacial water molecules form hydrogen bonds with the oxygen atoms of the channelwall, 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 playsimportant role in the stabilization of the crystalline framework with a channelstructure. 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 thehydrogen atoms on the network are, however, essentially in the disordered statewhile keeping an ice rule, and freeze at 87 K only with a short-range order in thearrangement. The progress of such ordering of channel water reveals a strikingcontrast to the behaviour of the water within meso-porous silicas, in which themolecules in the interface are always in the non-crystalline state and those in the porecentre tend to crystallize only when the pore diameter is greater than 2 or 3 nm.