EFFECT OF HIGH-SALT CONCENTRATIONS ON WATER-STRUCTURE

THE characteristic tetrahedral structure of water is known to be disru pted by changes in pressure and temperature(1-3). It has been suggeste d that ions in solution may have a similar perturbing effect(4,5). Her e we use neutron diffraction to compare the effects of applied pressur e and high salt concentrations on the hydrogen-bonded network of water . We find that the ions induce a change in structure equivalent to the application of high pressures, and that the size of the effect is ion -specific. Ionic concentrations of a few moles per litre have equivale nt pressures that can exceed a thousand atmospheres. We propose that t hese changes may be understood in terms of the partial molar volume of the ions, relative to those of water molecules. The equivalent induce d pressure of a particular ion species is correlated,vith its efficacy in precipitating, or salting-out, proteins from solution(6).