Absence of the Density Minimum of Supercooled Water in Hydrophobic Confinement

The surface effect on the peculiar dynamic and thermodynamic properties of supercooled water, such as the density, has been puzzling the scientific community for years. Recently, using the small angle neutron scattering method, we were able to measure the density of H2O confined in the hydrophobic mesoporous material CMK1-14 from room temperature down to the deeply supercooled temperature 130 K at ambient pressure. We found that the well-known density maximum of water is shifted 17 K lower and, more interestingly, that the previously observed density minimum in hydrophilic confinement disappears. Furthermore, the deduced thermal expansion coefficient shows a much broader peak spanning from 240 to 180 K in comparison with the sharp peak at 230 K in hydrophilic confinement. These present results may help in the understanding of the effect of hydrophobic/hydrophilic interfaces on the properties of supercooled confined water.