Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties

Area of Science:

• Physical Chemistry
• Solution Dynamics
• Spectroscopy

Background:

• Understanding the behavior of concentrated electrolyte solutions is crucial for various chemical and physical processes.
• Previous studies have investigated the concentration-dependent dynamics of lithium chloride (LiCl) solutions.
• The role of ion-water clusters in solution dynamics remains an area of active research.

Purpose of the Study:

• To investigate the concentration-dependent dynamics of highly concentrated lithium bromide (LiBr) solutions.
• To examine the temperature-dependent dynamics of two high concentrations of LiCl solutions.
• To compare these dynamics with existing LiCl concentration-dependent data and elucidate the underlying mechanisms.

Main Methods:

• Ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy was employed to obtain dynamical data.
• Analysis of OHD-OKE decays revealed multiple exponential components, indicating complex dynamics.
• Transition state theory was applied to temperature-dependent data to determine thermodynamic parameters.

Main Results:

• Fastest decay components (t1, t2) showed minimal dependence on concentration or temperature, similar to pure water.
• Slower decay components (t3, t4) significantly increased in contribution and slowed down with increasing concentration and decreasing temperature.
• Bulk viscosity was found to be linearly dependent on the correlation time of the slow dynamics (τc^slow) for both LiBr and LiCl solutions across different conditions.

Conclusions:

• The slow dynamics in concentrated LiBr and LiCl solutions are attributed to large ion/water clusters.
• Decreasing temperature in LiCl solutions is dynamically equivalent to increasing concentration at room temperature.
• A universal relationship exists between bulk viscosity and the slow dynamics, governed by the behavior of ion-water clusters.