Salt water is ubiquitous in nature. Understanding the effect of the dissolved salts on the tetrahedral hydrogen-bond network of solvent water is essential to uncover the mechanisms underlying various physical, chemical, biological, and geological processes. Despite centuries of investigations, whether or not water’s structure is drastically changed by dissolved ions is still debated.
Pioneering studies of electrolyte solutions, began as early as the 19th century by Hofmeister, later framed the textbook theory of ions being either structure “breaker” or “maker”. More recently, the structure of salt solutions has been investigated by modern experimental methods. In particular, there are two highly-cited works published in Nature  (https://www.nature.com/articles/378364a0) and Science  (https://www.science.org/doi/full/10.1126/science.1084801), which probed the structure and dynamics of ionic solutions by neutron diffraction and infrared spectroscopy, respectively. However, rather opposite conclusions are drawn from their experimental data. On one hand, the spectroscopy experiment concludes that there is no major difference between the water outside the ions’ first solvation shell and neat water. On the other hand, the neutron diffraction experiments argued that water structure beyond the ions’ first solvation shell is significantly distorted in a similar fashion to neat water under a few thousand atmospheres pressure. These opposite opinions have been under intense debate for over 25 years and are still unresolved. In principle, ab initio molecular dynamics computer simulations can resolve this fundamental question, starting from the principles of quantum mechanics. However, until recently, the needed computational burden simply made the studies impossible.
In this work, we address the above controversy by employing neural network based molecular dynamics computer simulations . The adopted state-of-the-art deep learning techniques allow us to predict the structure of salt solutions from first-principles, but with significantly reduced computational effort.
Our theoretical predictions of structure factors agree quantitatively with that measured in the neutron diffraction experiments [6, 7] as shown in Fig.1. In neat water (Fig. 1a), the first peak of the structure factor becomes higher with elevated pressure, while an attenuation is observed on its second peak. Notably, the structure factor of NaCl solutions in Fig. 1b exhibits rather similar behavior, however as a function of increased salt concentration. For pure water, the prominent changes of the structure factor under high pressures are caused by drastic distortions of the tetrahedral structure throughout the liquid. 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. However, our detailed analyses in real space suggest that the changes in the structure factors of NaCl solutions have a distinct microscopic origin from water under high pressures.
Fig. 2a shows the microscopic structure of sodium chloride solutions. The electronic orbitals in a water molecule adopt the well-known sp3 hybridization, giving rise to the near-tetrahedral structure of water. On the contrary, Na+ and Cl- ions both have a closed-shell electronic configuration. Therefore, structures of ionic first solvation shells are largely different from the tetrahedral structure of pure water. The fraction of first solvation shells increases with increasing salt concentration, which results in the change of structure factors (Fig. 1b) and radial distribution functions (Fig. 2b) of NaCl solutions with increasing salt concentration.
To study ion’s effects beyond ionic first solvation shells, we calculated the oxygen-oxygen radial distribution function in the absence of the ionic first solvation shells, i.e., only the free water (FW) molecules are accounted, which is referred to as gOOFW(r). The resulting gOOFW(r) of NaCl solutions are presented in Fig. 2c together with the gOO(r) of neat water. For all the concentrations under consideration, gOOFW(r) largely recovers the bulk structure of neat water. Rather similar pictures are also seen in the simulated KCl and NaBr solutions. Therefore, water-structure modifications in salt solutions are largely confined to ionic first solvation shells, which is different from pure water undergoes by applying external pressure as large as thousands of atmospheres. Outside ionic first solvation shells, the difference between the FW structures in the NaCl solutions and neat water is small but visible. For more subtle ionic effects outside ionic first solvation shells, more delicate order parameters beyond the radial distribution function, such as the orientational correlation , should be carefully analyzed in future work.
For more information, please read our paper in Nature Communications, “Dissolving salt is not equivalent to applying a pressure on water”, link: https://doi.org/10.1038/s41467-022-28538-8
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