  Dynamic fluctuations in hydrogen-bond network of water occur from femto- to
nano-second timescale and provides insights into 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 surrounding water molecules and form interfacial layers of
water around them with physical properties distinct from that 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 ion-water interface. The results shed light on the
structure breaking tendency of monovalent cations and provide insights into the
properties of ionic solutions at the molecular level.
