Fig. 6: Water–water hydrogen-bond strength in aqueous salt solutions.

Hydrogen-bond strength from equilibrium trajectories of AIMD simulations are obtained for every water molecule in the system, classified into four disjoint categories: from left to right, a water molecule in the solvation shell of a cation (orange), an anion (green), a water molecule shared between a cation and an anion (violet) and the remaining bulk-type molecules (yellow). For water molecules in the first solvation shell of an ion, the hydrogen bonds correspond to those between water molecules in the first and second solvation shells of that ion. Notably, while the water–water hydrogen-bond strength between the first and second solvation shells of cations increases, it decreases around anions. We have used a simple geometric definition of a hydrogen bond, with an O–O distance of <3.5 Å and an angle of <30°. Error bars indicate standard errors.