Unique and unexpected behaviour of water molecules that is unmatched by any known gas, liquid or solid states has been discovered by scientists, including one of Indian-origin. A new tunnelling state of water molecules confined in hexagonal ultra-small channels - 5 angstrom across - of the mineral beryl has been described by the researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL). An angstrom is 1/10-billionth of a metre, and individual atoms are typically about one angstrom in diameter.
The discovery, made possible with experiments at ORNL’s Spallation Neutron Source and the Rutherford Appleton Laboratory in the UK, demonstrates features of water under ultra confinement in rocks, soil and cell walls, which scientists predict will be of interest across many disciplines.
“At low temperatures, this tunnelling water exhibits quantum motion through the separating potential walls, which is forbidden in the classical world,” said lead author Alexander Kolesnikov of ORNL’s Chemical and Engineering Materials Division.
“This means that the oxygen and hydrogen atoms of the water molecule are ‘delocalised’ and therefore simultaneously present in all six symmetrically equivalent positions in the channel at the same time.
“It’s one of those phenomena that only occur in quantum mechanics and has no parallel in our everyday experience,” said Kolesnikov.
The existence of the tunnelling state of water shown in the study should help scientists better describe the thermodynamic properties and behaviour of water in highly confined environments such as water diffusion and transport in the channels of cell membranes, in carbon nanotubes and along grain boundaries and at mineral interfaces in a host of geological environments.
While previous studies have observed tunnelling of atomic hydrogen in other systems, the discovery that water exhibits such tunnelling behaviour is unprecedented.
The neutron scattering and computational chemistry experiments showed that, in the tunnelling state, the water molecules are delocalised around a ring so the water molecule assumes an unusual double top-like shape.
First principle simulations made by Narayani Choudhury of Lake Washington Institute of Technology and University of Washington-Bothell showed that the tunnelling behaviour is coupled to the vibrational dynamics of the beryl structure.
“The average kinetic energy of the water protons directly obtained from the neutron experiment is a measure of their motion at almost absolute zero temperature and is about 30 per cent less than it is in bulk liquid or solid water,” Kolesnikov said.
“This is in complete disagreement with accepted models based on the energies of its vibrational modes,” said Kolesnikov. The research was published in the journal Physical Review Letters.