Dynamically and structurally heterogeneous 1-propanol/water mixtures

1-propanol/water mixtures over the whole composition range (0 < X-V <= 1; X-V is the 1-propanol volume fraction) are shown to be structurally and dynamically heterogeneous. By combining structural (x-ray diffraction), thermodynamic (differential scanning calorimetry) and dynamical probes (dielectric spectroscopy) we construct the pertinent phase diagram. It consists of liquid 1-propanol, liquid water, hexagonal ice and different hydrates, the latter sharing the same lattice. The phase diagram can be discussed in terms of four regimes, all having in common a droplet arrangement of the minority component. When water droplets are strongly confined by 1-propanol (regime I, 0.92 < X-V <= 1; soft confinement), water is unable to crystallize. It has dynamics reminiscent to the ultra-viscous water phase known as high-density liquid (HDL). When water droplets are moderately confined (regime II, 0.75 < X-V <= 0.92) water can crystallize via homogeneous nucleation. Strikingly, the homogeneous nucleation temperature is at 205 K, well within no-man's land. The result is in line with earlier reports that soft confinement is the key to enter into the no-man's land. When 1-propanol is the minority component (regimes III and IV), the structure and the dynamics are dominated by the 1-propanol/water interface with the formation of hydrates. The corresponding dynamical features suggest a link between hydrate formation and the two metastable phases of ultra-viscous water, HDL and low-density liquid.

Automated summary

1-propanol/water mixtures exhibit structural and dynamical heterogeneity over the entire composition range. The behavior of water molecules is influenced by the degree of confinement by 1-propanol, leading to droplet arrangement or crystallization. When 1-propanol is the minority component, the structure and dynamics are dominated by the 1-propanol/water interface and the formation of hydrates.