Ion Association in AlCl₃ Aqueous Solutions from Constrained First-Principles Molecular Dynamics

The Car–Parrinello-based molecular dynamics (CPMD) method was used to investigate the ion-pairing behavior between Cl^– and Al³⁺ ions in an aqueous AlCl₃ solution containing 63 water molecules. A series of constrained simulations was carried out at 300 K for up to 16 ps each, with the internuclear separation (<i>r</i>_Al–Cl) between the Al³⁺ ion and one of the Cl^– ions held constant. The calculated potential of mean force (PMF) of the Al³⁺–Cl^– ion pair shows a global minimum at <i>r</i>_Al–Cl = 2.3 Å corresponding to a contact ion pair (CIP). Two local minima assigned to solvent-separated ion pairs (SSIPs) are identified at <i>r</i>_Al–Cl = 4.4 and 6.0 Å. The positions of the free energy minima coincide with the hydration-shell intervals of the Al³⁺ cation, suggesting that the Cl^– ion is inclined to reside in regions with low concentrations of water molecules, that is, between the first and second hydration shells of Al³⁺ and between the second shell and the bulk. A detailed analysis of the solvent structure around the Al³⁺ and Cl^– ions as a function of <i>r</i>_Al–Cl is presented. The results are compared to structural data from X-ray measurements and unconstrained CPMD simulations of single Al³⁺ and Cl^– ions and AlCl₃ solutions. The dipole moments of the water molecules in the first and second hydration shells of Al³⁺ and in the bulk region and those of Cl^– ions were calculated as a function of <i>r</i>_Al–Cl. Major changes in the electronic structure of the system were found to result from the removal of Cl^– from the first hydration shell of the Al³⁺ cation. Finally, two unconstrained CPMD simulations of aqueous AlCl₃ solutions corresponding to CIP and SSIP configurations were performed (17 ps, 300 K). Only minor structural changes were observed in these systems, confirming their stability