Ion association in AlCl3 aqueous solutions from constrained first-principles molecular dynamics

Area of Science:

• Computational chemistry
• Solution chemistry
• Materials science

Background:

• Understanding ion-pairing in aqueous solutions is crucial for various chemical and biological processes.
• Aluminum chloride (AlCl3) solutions are relevant in industrial applications and environmental science.
• The behavior of ions in solution influences macroscopic properties.

Purpose of the Study:

• To investigate the ion-pairing behavior between Al(3+) and Cl(-) ions in aqueous AlCl3 solutions.
• To determine the preferred configurations and interactions of Al(3+)-Cl(-) ion pairs.
• To analyze the influence of ion pairing on the surrounding water structure and electronic properties.

Main Methods:

• Car-Parrinello-based molecular dynamics (CPMD) simulations.
• Constrained simulations with fixed Al-Cl internuclear separation.
• Calculation of potential of mean force (PMF) to identify stable ion pair configurations.
• Analysis of radial distribution functions and solvent structure.
• Calculation of water molecule dipole moments.

Main Results:

• A global minimum for contact ion pairs (CIP) was found at r(Al-Cl) = 2.3 Å.
• Two local minima for solvent-separated ion pairs (SSIPs) were identified at r(Al-Cl) = 4.4 and 6.0 Å.
• Ion pair positions correlate with Al(3+) hydration shell intervals, indicating a preference for water-deficient regions.
• Significant changes in electronic structure occur when Cl(-) is removed from the Al(3+) first hydration shell.
• Unconstrained simulations confirmed the stability of both CIP and SSIP configurations.

Conclusions:

• Al(3+) and Cl(-) ions form stable contact and solvent-separated ion pairs in aqueous solutions.
• The spatial distribution of Cl(-) ions is influenced by the hydration shells of Al(3+).
• Ion-pair formation significantly impacts the local electronic and solvent structure.