Hydrolysis of Al3+ in Aqueous Solutions: Experiments and Ab Initio Simulations

Abstract

An experimental and computational study on the hydrolysis of Al3+ in aqueous solutions is here reported. Speciation model and formation constants were determined by potentiometric titrations at T = 298.15 K, 0.1 ≤ I/mol L−1 ≤ 1 in aqueous NaCl, NaNO3, NaCl/NaNO3 solutions. The dependence of formation constants on ionic strength is reported in all the ionic media over the range of 0.1–1.0 mol L−1. Under the studied experimental conditions, the formation of Al3(OH)45+ and Al13(OH)327+ species is observed in all the investigated ionic media and ionic strengths. The formation constants of the species formed by Al3+ with Cl− were determined together with the dependence on the ionic strength. Moreover, with the aim of unveiling the molecular structure of the formed Al complexes, quantum-mechanical calculations and state-of-the-art ab initio molecular dynamics simulations under explicit solvation were executed. These computations show, for the first time, the highly cooperative role played by the surrounding water molecules in neutralising mononuclear systems–such as AlCl2+ and AlClOH+–and the hydrolytic polynuclear system, Al3(OH)45+.

Ab Initio Molecular Dynamics Study of the Proton Transfer in Hydroxyl Ion-Induced Hydrolysis of Aluminum Monomers

“…The stability of these products in the solution system is determined by observing whether or not a dehydration reaction occurs. Considering the cost and reasonability of the calculation, one product is considered stable in solution if no dehydration reaction occurs within 25 ps AIMD. , In the presence of one OH – ion in solution, only one product, namely, Al­(H 2 O) 5 (OH) 1 2+ , is observed. This species is stable as no changes were observed during the following 25 ps of AIMD analysis.…”

“…Aluminum compounds, such as alumina, aluminum hydroxide, and aluminum sulfate, find diverse applications in electronic devices, heat insulation, and water treatment. − The sol–gel method is widely employed for the preparation of various aluminum products due to its simplicity, economic applicability, and wide range of applications. − Nanoscale aluminum particles gradually form through hydrolysis and polymerization reactions during the sol process. Despite numerous studies conducted on large polymeric species that exist stably in solution, such as AlO 4 Al 12 (OH) 24 (H 2 O) 12 7+ and Al 30 O 8 (OH) 56 (H 2 O) 24 18+ , the mechanisms behind the formation and growth of these aluminum clusters remain unclear. − Since aluminum oligomers are the basis for the formation of polymeric species, the study of aluminum oligomers, particularly the most fundamental aluminum monomers, contributes to revealing the formation process of polynuclear aluminum clusters and provides guidance for the preparation of aluminum-based materials. − …”