Controversial Evidence on the Point of Minimum Density in Deeply Supercooled Confined Water

The issue of the existence of a minimum of density at 210 K in water confined in MCM41 is tackled by neutron diffraction with H/D isotopic substitution over a wide range of momentum transfer, which allows refining in a single experiment both the meso- and the atomic-scale structure. We observe clear density fluctuations of confined water, with temperature-dependent length scale, due to changes of the balance between cohesive and adhesive interactions. These results cast some doubt about previous interpretation of SANS experiments on water confined in the same substrate as evidence for the existence of a point of minimum density at 210 K. This interpretation is based on the assumption that water homogeneously occupies the available volume within the confining substrate. We demonstrate that this is not the case in water confined in MCM-41-S-15, and provide an alternative explanation of small-angle neutron scattering results.