Solid–liquid phase equilibria and enthalpy changes in the water + 2,5-dimethylhexane-2,5-diol system

Abstract

The solid–liquid phase diagram has been determined for the water + 2,5-dimethylhexane-2,5-diol system. A solid hydrate containing four moles of water for each mole of diol forms in the system and melts incongruently at 311.56 K. Differential scanning calorimetry was used to determine the enthalpy of fusion of the pure diol (27.9 kJ mol^–1) and of the tetrahydrate (52.6 kJ mol^–1 of hydrate). The peritectic reaction accounts for 46 kJ mol^–1 of the enthalpy of fusion of the hydrate.

Titration calorimetry was used to determine the ΔH at the peritectic temperature for the reaction of the solid diol with water to form the saturated solution (16.0 kJ mol^–1). The above data were combined with enthalpy-of-fusion and heat-capacity data from the literature for water to obtain the ΔH at 311.56 K for the formation of the solid hydrate from the solid diol and solid water (– 8 ± 2 kJ mol^–1).

The hydrate system shows excellent promise for thermal-energy storage.