Heat capacity changes in glass-forming aqueous solutions and the glass transition in vitreous water

The heat capacities of supercooled liquid and glassy states of several aqueous electrolyte systems have been measured by differential scanning calorimetry over a range of compositions and temperatures. The partial molal heat capacity of water in the glass-forming composition range is constant. In the glassy state it is the same as that of ice within experimental uncertainty. The partial molal heat capacity change at the glass transition for water in these solutions falls in the range 19-25 J mol^-1 deg^-1 which is much smaller than that observed for vapor-deposited vitreous water. It is suggested that the latter value is possibly in error due to a relaxational contribution, and that the lower heat capacity indicated for water, under conditions in which anomalous fluctuations are frozen out, permits a thermodynamic connection between supercooled water and vapor-deposited glassy states of water without requiring improbable or counter-intuitive entropy states.