Spatial density of water (inner shell left, second shell right) around a central water molecule in pure water (top) and concentrated sodium chloride (bottom).
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Most naturally occurring water has ions (charged atoms) dissolved in it.
Whilst there is plenty of information available about how ions in solution strongly orientate the water molecules that hydrate them, there is surprising paucity of information – and controversy – about how water structure itself (the relative arrangement of one water molecule to another) is affected by the presence of dissolved ions. In pure water this arrangement has a ‘tetrahedral’ structure, giving water the characteristic of a disordered network of hydrogen bonded molecules. Using a series of neutron diffraction experiments on Sandals with hydrogen/deuterium substitution, the structure of water in a number of ionic solutions and over a range of concentrations was investigated. With increased concentration, the first shell of water molecules around a central molecule remains largely intact, but the second shell collapses inwards (see figure), as in pure water under pressure. This behaviour is thought to be due to the ‘electrostriction’ of the ions pulling water molecules closer together on average.
R Mancinelli, A Botti, F Bruni, MA Ricci, (University of Rome III, Italy), AK Soper (ISIS)
Research date: December 2007
Phys. Chem. Chem. Phys. 9(23) (2007) 2959
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