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 Al(3+) ions in an aqueous AlCl(3) 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 (r(Al-Cl)) between the Al(3+) ion and one of the Cl(-) ions held constant. The calculated potential of mean force (PMF) of the Al(3+)-Cl(-) ion pair shows a global minimum at r(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 r(Al-Cl) = 4.4 and 6.0 Å. The positions of the free energy minima coincide with the hydration-shell intervals of the Al(3+) 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 Al(3+) and between the second shell and the bulk. A detailed analysis of the solvent structure around the Al(3+) and Cl(-) ions as a function of r(Al-Cl) is presented. The results are compared to structural data from X-ray measurements and unconstrained CPMD simulations of single Al(3+) and Cl(-) ions and AlCl(3) solutions. The dipole moments of the water molecules in the first and second hydration shells of Al(3+) and in the bulk region and those of Cl(-) ions were calculated as a function of r(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 Al(3+) cation. Finally, two unconstrained CPMD simulations of aqueous AlCl(3) 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.