SOLID-LIQUID EQUILIBRIA AND MODELING OF THE BINARY-SYSTEMS WATER-METHYLHYDRAZINE AND WATER-1,1-DIMETHYLHYDRAZINE

Solid-liquid equilibria of the system water-methylhydrazine are investigated by direct thermal analysis on heating. Three liquidus curves can be observed corresponding to ice, methylhydrazine and a stable congruently melting methylhydrazine monohydrate. Another metastable form of monohydrate can be observed, its transformation into the stable form being monotropic. A model deduced from the study of the water-alkylhydrazines vapor-liquid equilibria and representing strong interactions between molecules as chemical associations, allows, using the van Laar relation, to reproduce correctly all the liquidus curves. This modeling is justified by the evolution of Raman spectra of the liquid phase. The crystal structure of pure methylhydrazine, determined at 179 K by X-ray diffraction on a single crystal, is favorable to the existence of strong interactions between methylhydrazine molecules. The model is also used to reproduce the system water-1,1-dimethylhydrazine.