A link between viscosity and cation-anion contact pairs: adventure on the concept of structure-making/breaking for concentrated salt solutions

We investigated structure properties of structure-making salts (LiCl, NaCl, and NaBr) and structure-breaking salts (KCl and KBr) solutions of 0.5 M to 4 M using TIP4P/2005 water model and OPLS ionic force fields. The viscosity results show a good trend with experimental values for all salts. For the water structure, the O–O number decreased with increasing salt concentrations but is independent of the salt types. There is an extra shoulder (Cl-O/LiCl) or no obvious low-point at the first minimum point of anion‑oxygen RDF patterns (Cl-O/NaCl and Br-O/NaBr) while two structure-breaking salts show a normal minimum point (Cl-O/KCl and Br-O/KBr). Additionally, we observed that the ratio (R) of primary and second shell cation-anion numbers showed a similar trend as the viscosity behaves with increasing salt concentration. For structure-making salts, values of R increased with increasing salt concentrations while R values of structure-breaking salts slightly decreased with concentration. These observations may indicate that the change in viscosity is due to the disturbance of cation-anion contact hydration pairs, not the free water hydrogen network