Water is one of the most basic molecule we can find in the Universe but this tiny molecule is still keeping some interesting secrets. Its physic could be very surprising.  Among its various properties, water can exist in three different states, either as solid ice, liquid water, or vapour gas. But this common knowledge could change in a near future. A team of researchers from Stockholm University in Sweden has found a new liquid form for water with a different density.

Water molecules are polarized and it exists some sort of dynamic network between these molecules. It has been postulated that water’s hydrogen-bonding network can exist in two liquid forms of different densities, namely high-and low-density liquid water. During recent work, these forms were recently simulated but direct experimental evidence was still missing. Furthermore, it was also hypothesized that the observed high- and low-density amorphous ice forms are the glassy counterparts of these two liquid. Similar observations of transitions have been reported by studying water confined in protein crystals and aqueous solutions, which indicate the occurrence of the transition between two viscous liquid states of water. However, there is contradicting evidence that the transition involves several intermediate forms.

Coherent X-ray diffraction setup. (A) The experimental scattering geometry that was used employs two detectors. (B) The scattering pattern allows distinguishing high- and low-density forms. (C) The scattering pattern recorded in another geometry is used to extract the dynamics.

In a recent paper, researchers from Stockholm University made interesting progress in the study of liquid water. They worked on the debated question of the transition between the two liquid states and brought more evidence of the phenomenon. In their last experiments, they combined wide-angle X-ray scattering with X-ray photon-correlation spectroscopy to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar.

“The new remarkable property is that we find that water can exist as two different liquids at low temperatures where ice crystallization is slow.”

Anders Nilson, Stockholm University

By analyzing the structure factor and the radial distribution function, they showed the coexistence of two structurally distinct domains is observed at T = 125 K. They studied both the dynamics in momentum space, and the structural relaxation on the nanometer length scale. The team observed that above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion.

Consequences of these new results could be very important. A better understanding of water properties will help researchers in many fields, like developing better purification and desalination processes or giving a better understanding of how life evolves in water.

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