The heat capacity and glass transition of hyperquenched glassy water

Abstract

The glass transition and heat capacity of hyperquenched glassy water have been studied by differential scanning calorimetry and by isothermal measurements from 103 K to a temperature where its crystallization to cubic ice is complete. Glassy water shows a thermally reversible glass-liquid transition and has a T_g, of 136 ± 1 K. The activation energy of structural relaxation in the transition range is ∼55kjmol⁻¹ and is a reflection of the energy required to break two hydrogen bonds before a rotational-translational diffusion of a water molecule in the H-bonded network can occur. The temperature width of the transition is ∼12°, and the increase in the heat capacity is 1.6±0.1JK⁻¹ mol⁻¹. Liquid water formed on heating the glassy water to 146 K is more stable against crystallization than that which exists near 232 K. The hyperquenched glassy form of water can be thermodynamically continuous with liquid water, but whether or not it has the same structure as water above 273 K or supercooled water near the postulated γ-type transition is not known.