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

Abstract

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 I h 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 I h 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.