X-ray diffraction studies on methanol-water, ethanol-water, and 2-propanol-water mixtures at low temperatures

X-ray diffraction measurements over a range from ambient to freezing point temperatures have been made on methanol-water, ethanol-water, and 2-propanol-water mixtures, whose compositions are around mole fractions of the structural transition of solvent clusters at 25 °C, i.e. ∼0.3, ∼0.2, and ∼0.1, respectively. The present results showed that the structure of dominant clusters formed in the mixtures at 25 °C is still kept at low temperatures, except that O⋯O hydrogen bonds formed in the mixtures are gradually ordered with lowering temperature. At the freezing point temperatures, ice Ih is crystallized in methanol-water and ethanol-water mixtures below the mole fraction of structural transition. Above the mole fraction of structural transition, methanol-water mixtures are kept in the metastable liquid state, but ethanol hydrate is crystallized in ethanol-water mixtures. In the case of 2-propanol-water mixtures, ice Ih is crystallized from the mixtures at all the mole fractions (0.05, 0.1, and 0.2) investigated because microheterogeneity in 2-propanol-water mixtures at 25 °C is most enhanced among the three alcohol-water mixtures. From the present results, it was concluded that the structure of dominant clusters formed in the mixtures at the ambient temperature is reflected into that of frozen alcohol-water mixtures.