Bridging Structure, Dynamics, and Thermodynamics: An Example Study on Aqueous Potassium Halides.

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Computational Chemistry

Background:

• Aqueous salt systems are fundamental to many natural and industrial processes.
• Ions significantly perturb the structure and dynamics of surrounding water molecules.
• Understanding these ion-specific effects is crucial for explaining bulk solution properties.

Purpose of the Study:

• To deconstruct ion-specific perturbations on water structure and dynamics at an atomistic level.
• To link local water environments around ions to bulk thermodynamic properties of aqueous salt solutions.
• To elucidate the molecular origins of hydration enthalpy and entropy.

Main Methods:

• Combined experimental techniques: neutron scattering and nuclear magnetic resonance (NMR).
• Computational modeling for atomistic scale resolution.
• Analysis of water molecule environments within the first hydration shell.

Main Results:

• Enthalpy of hydration can be determined by considering only water molecules in the first hydration shell.
• Ion-specific enthalpic interactions with water extend only up to approximately 4 Å.
• Ions induce structure in the first hydration shell, but this perturbation does not extend into bulk water; dynamics increase down the halide series.

Conclusions:

• Monovalent potassium halide ions primarily induce local perturbations to water structure and dynamics.
• Decreasing ionic charge density down the halide series leads to less negative hydration enthalpy and increased water dynamics.
• Local structural and dynamic changes in hydration shells explain bulk thermodynamic properties like hydration entropy.