Glass Transitions of Ordinary and Heavy Water within Silica‐Gel Nanopores

Abstract

The dynamic properties of water confined within nanospaces are of interest given that such water plays important roles in geological and biological systems. The enthalpy-relaxation properties of ordinary and heavy water confined within silica-gel voids of 1.1, 6, 12, and 52 nm in average diameter were examined by adiabatic calorimetry. Most of the water was found to crystallize within the pores above about 2 nm in diameter but to remain in the liquid state down to 80 K within the pores less than about 1.6 nm in diameter. Only one glass transition was observed, at T(g) = 119, 124, and 132 K for ordinary water and T(g) = 125, 130, and 139 K for heavy water, in the 6-, 12-, and 52-nm diameter pores, respectively. On the other hand, two glass transitions were observed at T(g) = 115 and 160 K for ordinary water and T(g) = 118 and 165 K for heavy water in the 1.1-nm pores. Interfacial water molecules on the pore wall, which remain in the noncrystalline state in each case, were interpreted to be responsible for the glass transitions in the region 115-139 K, and internal water molecules, surrounded only by water molecules in the liquid state, are responsible for those at 160 or 165 K in the case of the 1.1-nm pores. It is suggested that the glass transition of bulk supercooled water takes place potentially at 160 K or above due to the development of an energetically more stable hydrogen-bonding network of water molecules at low temperatures.