Abstract
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.
Highlights
Salt water is ubiquitous, playing crucial roles in geological and physiological processes
Compared to the dynamical information measured by vibrational spectroscopy[7], the liquid structure can be directly probed by diffraction experiments[4,17,18]
In ab initio molecular dynamics[33] (AIMD), the electronic structure of the ground state is generated on the fly from density functional theory[34] (DFT)
Summary
Salt water is ubiquitous, playing crucial roles in geological and physiological processes. Because of the similar pressure-like effect, it has been widely assumed that the water structure in NaCl solutions is distorted in the same way since SXX(Q) is mostly determined by the partial structure factors SOOðQÞ17,18.
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