Theoretical Studies of the Formation Mechanisms, Thermodynamic Stabilities, and Water-Exchange Reactivities of Aluminum-Salicylate Complexes in Aqueous Solution

Abstract

The formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of 1:1 monomer aluminum–salicylate (Al–salicylate) complexes in acidic aqueous solution are investigated using the density functional theory-quantum chemical cluster model (DFT-CM) method. (1) The formation pathways for possible monodentate and bidentate Al–salicylate configurations are modeled with the gas phase-supermolecule-polarizable continuum model (GP-SM-PCM). It shows that the formation pathways for the Al–salicylate complexes follow the Eigen-Wilkins mechanism, where the dissociation of an inner-shell coordinated water of Al3+ is the rate-determining step. (2) The formation constants K aq for different Al–salicylate configurations are estimated based on the total Gibbs free energy changes ΔG° for their overall formation pathways. It is indicated that in the acidic aqueous solution at pH ∼ 3, the main existence form of the 1:1 monomer Al–salicylate complex is the phenol-deprotonated bidentate Al­(Sal)­(H2O)4 + with six-membered ring. Its log K aq is calculated as 13.8, in good agreement with the literature values of 12.9–14.5. (3) The water-exchange reactions are modeled for different Al–salicylate configurations. The water-exchange rate constant for Al­(Sal)­(H2O)4 + is estimated as log k H2O = 3.9 s–1, close to the experimental value of 3.7 s–1. It proves again that this configuration is the dominant form under experimental conditions.

Density Functional Theory Studies on the Real and Apparent Water-Exchange Reaction Kinetics of Al3+ in Aqueous Solution

“…The water-exchange reaction of Al 3+ is the first step in many biological, material, and geochemical processes related to Al speciation . In natural waters and at mineral–solution interfaces, the interaction of Al 3+ with inorganic or organic ligand X to form the Al­(III)–X complex follows the Eigen–Wilkins mechanism, which involves the rapid formation of an outer-sphere ion pair and a slower ligand-exchange reaction, where the inner-shell coordinated water of Al 3+ is substituted by X. ,− In the outer-sphere ion pair Al­(H 2 O) 6 3+ ·X, ligand X is located in the second hydration shell of Al 3+ . When Al­(H 2 O) 6 3+ dehydrates to form Al­(H 2 O) 5 3+ , the competition between X and the surrounding solvent water molecules to attack pentacoordinated Al­(H 2 O) 5 3+ will be a critical step in the speciation change of Al 3+ .…”