Mechanisms of the viscosity decrease and increase of aqueous CsCl

Aqueous salt solutions occur in many aspects of chemistry, biology, and geology. Increasing the concentration of most aqueous salt solutions increases the viscosities. In contrast, adding CsCl to water initially decreases the viscosity, but at moderate concentration further addition increases it. While this phenomenon is well known, the molecular mechanisms for the reduction and increase have not been elucidated. We used ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE) and IR pump-probe experiments, as well as density functional theory to investigate the impact of Cs⁺ ions on water dynamics, interactions, and structure. OHD-OKE experiments demonstrated that the dynamics of the water hydrogen bond (H-bond) network underpin the viscosity of CsCl solutions. Transient IR spectra of HOD in H₂O interacting with Cs⁺ showed a significant blue shift, a hallmark of hydrogen bonds weaker than those of pure water. Due to its low charge density, Cs⁺ is distinct from high charge density cations, e.g., Na⁺ and Li⁺, which have been observed to strengthen water hydrogen bonds and drive a large, monotonic increase in viscosity with concentration. The results showed that water hydrogen bonds in the Cs⁺ second solvation shell are weaker than typical water-water hydrogen bonds, and these weak hydrogen bonds give rise to faster collective structural dynamics, leading to reduced viscosity. However, at sufficiently high salt concentrations, the low number of water molecules per ion pair leads to water clusters. Water confined in small clusters slows H-bond rearrangement, leading to an increase in viscosity.