Terahertz Driven Ultrafast Energy Dissipation in Aqueous Ionic Solutions

Abstract

Solvation of ions changes the physical, chemical and thermodynamic properties of water. The microscopic origin of this process is believed to be the ion-induced perturbation in the structure and dynamics of the hydrogen (H)-bonding network of water. Here, we provide microscopic insight on the local structural deformation of the H-bonding network of water by ions, via investigating the dissipation of external energy in salt solutions by a novel time-resolved terahertz (THz)-Raman spectroscopy. We resonantly drive the low-frequency rotational dynamics of water molecules by intense THz pulses and probe the Raman response of their intermolecular translational motions. We find that the intermolecular rotational-to-translational energy transfer is enhanced by highly-charged cations and it is drastically reduced by highly-charged anions, scaling with the ion surface charge density and concentration. Our molecular dynamics simulations further reveal that the water-water H-bond strength between the first and the second solvation shells of cations (anions) increases (decreases), signifying the opposite effects of cations and anions on the local structure of water. The impact of ion polarity on the ultrafast energy dissipation in water, resembles the effect of ions on stabilization and denaturation of proteins.

Diffusions in Aqueous Solutions with Multivalent Cations and Especially in Cationic First Hydration Shell

“…In most cases, the diffusion of biomolecules and univalent ions can be represented by this iceberg model while modulated by factors reflecting the strength of the solute–water interaction. These factors include the local viscosity, − hydrogen-bond lifetime, residence time (τ RES ) and rotational correlation time of water in H 2 O first , the rate of water exchange between H 2 O first and the second hydration shell (hereinafter named H 2 O second ), , and ultrafast vibrational properties of H-bonding in hydration shells and the THz-region ion action within the cage that is formed by hydration water. , …”