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 (LiCI, NaCl, and NaBr) and structure-breaking salts (KCI 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 ROE patterns (Cl-O/NaCl and Br-O/NaBr) while two structure-breaking salts show a normal minimum point (Cl-O/KCI and Br-O/KBr). Additionally, we observed that the ratio (R-12) 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-12 increased with increasing salt concentrations while R-12 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. (C) 2018 Elsevier B.V. All rights reserved.