Motivation (slide 1)
The aqueous chemistry of aluminum, the most abundant metal in the earth's crust, remains the subject of fundamental importance for geochemistry after many years of intensive research.
The environmental and health issues due to the increased acidity of many natural waters have also increased attention to the aluminum hydrolysis.
Since aluminum exhibits amphoteric properties and distinctly pH-dependent behavior in aqueous solutions, its hydrolysis products have also many practical applications, ranging from the pharmaceutical design to purification of water.
The aluminum ion, Al3+ , exists as an octahedral (6-fold coordinated) hexahydrate ion, Al(H2O)63+ in acidic solutions. Over a very narrow pH range, 5.5 < pH < 6.2, the hexahydrate undergoes hydrolysis by successive four deprotonations, producing species with uniformly decreasing coordination numbers.
The final product of the hydrolysis process, the aluminate ion, Al(OH)4- , is tetrahedraly (4-fold) coordinated. This change in coordination of Al3+ from octahedral to tetrahedral form occurs to accomodate changes in ligands charges and polarizations, i.e., hydrolisys products exhibit successive strengthening of coordination bonds with the hydroxyl groups most tightly bound to the aluminate ion, Al(OH)4. However, the microscopic description of described above processes, especially hydrolysis mechanisms, remains incomplete.
Ab initio molecular dynamics of aluminum solvation
by Mark I. Lubin and John H. Weare, UCSD