Neutron diffraction, isotopic substitution and the structure of aqueous solutions

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Abstract
The structural properties of liquids that contain more than one atomic species are difficult to unravel. The essential reason for this is that conventional diffraction experiments measure a rather coarse average of all the ½ v ( v +l) partial structure factors that characterize a liquid containing v species. This paper describes the way that neutron diffraction experiments carried out on samples that are identical in all respects, except that the isotope of one or more of the species has been changed, can overcome this problem. It is shown that the systematic use of isotopes, by virtue of the dependence of the neutron scattering amplitude on isotope, enables partial structure factors to be extracted directly from diffraction data. A detailed account of the method applied to a range of aqueous solutions of electrolytes under various experimental conditions is given. In particular, first-order dfference experiments yield information on ion-water conformations. Data are now available for the cations Ni 2+ , Ca 2+ and Li + and these results will be discussed in detail. The data for Ni 2+ are of particular interest because they show that the substantial angle of tilt between the plane of the water molecule and the Ni-O axis gradually disappears as the concentration is reduced. The only anion studied so far is Cl - but the experiments have been carried out for a wide range of counter ions. We have shown that for such different electrolytes as CaCl 2 , NaCl and LiCl, the nature of the hydration around the Cl - ion is essentially the same. Finally, the method of second-order difference yields directly ion-ion correlations. The experiments described include Cl-Cl structure factors in NaCl and NiCl 2 solutions, and the Ni-Ni structure factor in NiCl 2 solutions. Comparisons made with theoretical predictions based on the primitive model of electrolytes show that in certain cases, the molecular nature of the water is a crucial factor in determining ion-ion correlations. In other cases, the primitive model contains most of the essential physics.