THERMODYNAMIC PROPERTIES AND PHASE-TRANSFORMATIONS IN METHYLHYDRAZINEMONOHYDRATE

Liquid methylhydrazine monohydrate (CH3NHNH2 . H2O; molar mass = 0.064 09 kg mol(-1) ) can be easily quenched into a glassy state. On reheati ng, the glass transition occurs near 160 K (T-g) with an important hea t capacity variation (Delta C-p=96.8 J mol(-1)K(-1)). This glass is ex tremely ''fragile'' according to Angell's classification. Depending on the thermal treatment, this glass transition is followed by successiv e phase transformations with slow and complex kinetics which are descr ibed here in detail. Thermodynamic properties were investigated by mea ns of adiabatic calorimetry. Enthalpies and heat capacities were deter mined or extrapolated from 100 K to 265 K for the glassy state and all condensed phases: the liquid, the undercooled liquid, the metastable and the stable solid phases. The melting temperature T-fus,T-s and the melting enthalpy Delta H-fus,H-s for the stable solid phase were esti mated to be 234 K and 15.4 k J mol(-1), respectively. The temperature evolution of the enthalpy for the metastable solid phase was investiga ted using previous measurements for the metastable solid phase: T-fus, T-m 219 K and Delta H-fus,H-m = 9.8 k J mol(-1). The spontaneous evolu tion of the temperature of the undercooled liquid was followed from 18 8 K to 222.4 K under adiabatic conditions. This is interpreted in term s of phase transformations and kinetics. Three different processes can be considered. The first process leads to the quasi-complete transfor mation into the metastable solid phase from the undercooled liquid. Th en the crystallization of the stable solid phase starts from the remai ning undercooled liquid and further from the metastable solid phase. T he third process corresponds to the melting of the metastable solid ph ase at 219 K and the rapid and simultaneous crystallization into the s table solid phase. At the end of the process, the complete transformat ion of the stable solid phase is observed.