Neutron diffraction studies of electrolytes in null water: a direct determination of the first hydration zone of ions

A method of neutron diffraction is described which enables the first hydration zone of small cations to be investigated at atomic resolution. It is shown that the cation structures of aqueous electrolyte solutions dissolved in a 'null' mixture of water (H2O) and heavy water (D2O), can be calculated directly from the neutron scattering patterns. The hitherto unresolved structure around Na+ is used to illustrate the power of this method, the accuracy of which is discussed formally with reference to standard nickel chloride solutions. Possible applications to a variety of other systems and at different thermodynamic states are proposed.