Mechanisms of the viscosity decrease and increase of aqueous CsCl

Area of Science:

• Physical Chemistry
• Chemical Physics
• Geochemistry

Background:

• Aqueous salt solutions are ubiquitous in science.
• Most salts increase solution viscosity with concentration.
• Cesium chloride (CsCl) exhibits unusual viscosity behavior, decreasing then increasing with concentration, with underlying mechanisms unclear.

Purpose of the Study:

• To elucidate the molecular mechanisms behind CsCl's unique viscosity effects in water.
• To investigate the impact of Cs+ ions on water dynamics, interactions, and structure.

Main Methods:

• Ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE).
• Infrared (IR) pump-probe spectroscopy using HOD in H2O.
• Density functional theory (DFT) calculations.

Main Results:

• OHD-OKE revealed water hydrogen bond (H-bond) network dynamics govern CsCl solution viscosity.
• IR spectra showed Cs+ weakens water H-bonds, unlike high-density cations (e.g., Na+, Li+).
• Weakened H-bonds in the Cs+ second solvation shell accelerate dynamics, reducing viscosity at low concentrations. At high concentrations, water clustering slows dynamics, increasing viscosity.

Conclusions:

• Cs+ ions uniquely interact with water, weakening H-bonds due to low charge density.
• The concentration-dependent viscosity of CsCl solutions arises from competing effects on water H-bond dynamics and structure.
• Understanding these ion-specific effects is crucial for aqueous solution chemistry, biology, and geology.