Unraveling the Molecular Pathways for Structure “Making” and “Breaking” by Ions in Water

Aqueous anions play a crucial role in chemical and biological processes. They are traditionally classified as "structure makers" or "structure breakers" based on their impact on the viscosity of electrolyte solutions. Until now, this behavior has been assumed to stem from a single restructuring mechanism of the hydrogen (H) bonding network of water, that could align with macroscopic properties. Correlated Vibrational Spectroscopy (CVS) measurements reveal that this is not the case. Rather, anions modify water-water H-bonds through multiple distinct pathways, with frequency shifts correlating with charge transfer, and intensity changes quantifying variations in the number of interacting/orientationally cross-correlated H-bonds. The different ways through which anions impact water structure can be explained in terms of Hard-Soft-Acid-Base theory. Hard anions only affect water H-bonds through electrostatics. By contrast, soft anions weaken the H-bonds via charge transfer but simultaneously increase their concentration. The two effects for soft anions nearly cancel each other out in terms of structure breaking/making, resulting in macroscopic behavior that is similar to hard anions in spite of dramatically different molecular-level effects.