Excess protons in water-acetone mixtures. II. A conductivity study

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Computational Chemistry

Background:

• Proton transfer in water is crucial for many chemical and biological processes.
• Previous simulations suggested disruption of proton transfer by aprotic solvents like acetone.
• Understanding solvent effects on ion mobility is key to many electrochemical applications.

Purpose of the Study:

• To experimentally measure proton mobility in aqueous-acetone mixtures across a wide composition range.
• To investigate the underlying proton transport mechanisms using molecular dynamics simulations.
• To identify composition-dependent changes in proton transfer and diffusion.

Main Methods:

• Experimental measurement of proton and Li+ mobility in aqueous-acetone mixtures (0.05 ≤ xw ≤ 1.00).
• Molecular dynamics simulations for acetone-rich mixtures.
• Analysis of infinite dilution molar conductivities (Λ(0)) and comparison with simulation data.

Main Results:

• Two critical composition points (xw ≈ 0.25 and xw ≈ 0.8) show qualitative changes in proton transport.
• Below xw ≈ 0.25, proton diffusion is vehicular and matches Li+ diffusion.
• Above xw ≈ 0.25, proton mobility diverges from Li+, indicating the onset of Grotthuss mechanism; Eigen structure interconversion becomes significant.
• At xw ≈ 0.8, HCl conductivity shows a qualitative change attributed to solvent molecule exchange in the hydronium ion's solvation shell.

Conclusions:

• Proton transport in water-acetone mixtures exhibits distinct regimes governed by composition.
• The Grotthuss mechanism becomes feasible above a specific acetone concentration (xw ≈ 0.25).
• Changes in solvation shell structure significantly influence proton mobility at high acetone concentrations (xw ≈ 0.8).