Dissolving salt is not equivalent to applying a pressure on water

By advanced machine learning techniques, first-principles simulations find that dissolving salt in water does not change water structure drastically. It is contrary to the notion of pressure effect which has been widely applied over past 25 years. Salt water is ubiquitous, playing crucial roles in geological and physiological processes. Despite centuries of investigations, whether or not water's structure is drastically changed by dissolved ions is still debated. Based on density functional theory, we employ machine learning based molecular dynamics to model sodium chloride, potassium chloride, and sodium bromide solutions at different concentrations. The resulting reciprocal-space structure factors agree quantitatively with neutron diffraction data. Here we provide clear evidence that the ions in salt water do not distort the structure of water in the same way as neat water responds to elevated pressure. Rather, the computed structural changes are restricted to the ionic first solvation shells intruding into the hydrogen bond network, beyond which the oxygen radial-distribution function does not undergo major change relative to neat water. Our findings suggest that the widely cited pressure-like effect on the solvent in Hofmeister series ionic solutions should be carefully revisited.

Automated summary

By using advanced machine learning techniques, researchers have found that dissolving salt in water does not lead to drastic changes in the structure of water. Contrary to previous beliefs about the pressure effect, the study shows that the ions in salt water only disrupt the hydrogen bond network within the first solvation shells, without significantly altering the oxygen radial-distribution function. This finding challenges the widely accepted pressure-like effect in Hofmeister series ionic solutions.