Molecular Mechanism Of Water Rotation And Concentration-dependent Properties In Concentrated Electrolyte Aqueous Solutions By Simulations

Water dynamics in the hydration shells of ions and charged molecules play a critical role in a plethora of natural phenomena.Water rotational dynamics in NaSCN and KSCN solutions at a series of concentrations are investigated using femtosecond infrared spectroscopy and theory.Femtosecond infrared measurements,consistent with previous NMR observations,detect that sodium slows down while potassium accelerates the water O-H bond rotation.Results of reported neutron scattering measurements,on the other hand,suggested that these two cations have similar structure-breaking effects on water,and therefore should both acceleratewater rotation through the presumably dominating large-amplitude angular jump component.To explain this discrepancy,theoretical studies with both classical and ab initio models were carried out,which indicate that both ions indeed accelerate the large-amplitude angular jump rotation of the water molecules,while the observed cation specific effect originates from the non-negligible opposite impact of the sodium and potassium cations on the diffusive rotation of water molecules.The dynamics of water molecules surrounding the hydrated ions affects many natural phenomena including protein processes and charge transfer in the aqueous rechargeable ion batteries.Currently,numerous studies have shown that all electrolytes,regardless of their “structure-making/breaking” nature,make water rotate slower at high concentrations,but this phenomenon has not been clearly explained in theory.In this study,we have used the coarse-grained method to theoretically study the reorientation of water molecules in different ionic solutions.Using an extended Ivanov model,water rotation is decomposed into contributions from large-amplitude angular jumps and a slower frame motion.The results show that large hydrated ion clusters are common in concentrated ion solutions,and the coupling of the movement of hydrated ion clusters is the main reason for the slower water rotation speed.