Theoretical insights into the dimerization mechanism of aluminum species at two different pH conditions

Dimerization mechanism of aluminum species is investigated in weakly acidic solutions and in strongly alkaline solutions using density functional theory, respectively. The results indicate the dimerization is controlled by dehydration of monomeric aluminum and the dissolution of dimer is controlled by the hydroxyl bridge cleavage in weakly acidic solutions. The two processes are enhanced by the rising pH. In addition, the main pathway generating the fluorine bridge is not the direct polymerization, but the terminal substitution of F- into polymeric aluminum followed by the transition of terminal F to bridging F. The formation of fluorine bridge promotes dissolution and di-fluorine-bridged dimer should be excluded. In strongly alkaline solutions, the formation processes of three dimers were speculated, and the existence of mono-hydroxyl-bridged dimer is denied due to the extraordinary instability and high formation energy barriers. It is possible that mono-oxygen-bridged and dihydroxyl-bridged dimers coexist in highly alkaline solutions. However, the salts of mono-oxygen-bridged anion are much more stable than salts of di-hydroxyl-bridged anion due to the effect of counter cations and the removal of ambient water molecules.