Interfacial layers between ion and water detected by terahertz spectroscopy

Dynamic fluctuations in the hydrogen-bond network of water occur from femto- to nanosecond timescales and provide insight into the structural/dynamical aspects of water at ion-water interfaces. Employing terahertz spectroscopy assisted with molecular dynamics simulations, we study aqueous chloride solutions of five monovalent cations, namely, Li, Na, K, Rb, and Cs. We show that ions modify the behavior of the surrounding water molecules and form interfacial layers of water around them with physical properties distinct from those of bulk water. Small cations with high charge densities influence the kinetics of water well beyond the first solvation shell. At terahertz frequencies, we observe an emergence of fast relaxation processes of water with their magnitude following the ionic order Cs > Rb > K > Na > Li, revealing an enhanced population density of weakly coordinated water at the ion-water interface. The results shed light on the structure breaking tendency of monovalent cations and provide insight into the properties of ionic solutions at the molecular level.