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

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Materials Science

Background:

• Limited understanding of multivalent cation and water diffusion in hydration shells.
• Contrast with well-studied univalent cation diffusion behavior.

Purpose of the Study:

• Investigate the concentration dependence of multivalent cation and water diffusion.
• Evaluate the applicability of the iceberg model for cation diffusion.
• Clarify the role of hydration water in solute diffusion.

Main Methods:

• Analysis of published translational diffusion coefficients.
• ¹H-pulsed-field-gradient nuclear magnetic resonance (PFG NMR) measurements.
• Temperature-dependent diffusion studies.

Main Results:

• Multivalent cations and water exhibit similar concentration dependence until only hydration water remains.
• Cation charge number influences diffusion dependence in confined water.
• Iceberg model describes cation diffusion until hydration water is present.
• ¹H in hydration shells diffuses faster than Al³⁺ at room temperature, with equal diffusion at 243 K.

Conclusions:

• The iceberg model equivalently describes strong ion-water interactions, not necessarily the physical diffusion pathway.
• Hydration water's role in solute diffusion, especially with weak solute-water interactions, needs re-evaluation.
• Findings offer insights into solute-water interactions in confined environments.