Freezing of glycerol–water mixtures under pressure

We investigated freezing of pure glycerol as well as glycerol–water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0–10 GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5 GPa for pure glycerol to 6.5 GPa for the 3:1 GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2 GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H2O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0–7 GPa range, including at low temperatures.