Abstract:
Quantum chemical calculations at the density functional level (B3LYP functional) with full geometry optimisation have been performed on the effect of protonation and of adsorption of alkali cations (Li+, Na+, K+, Rb+, and Cs+) on the siloxane bond strength in silicate minerals. The influence of pH was modelled by assuming a fully protonated surface model, (OH)3Si-O-Si(OH)3, at pH lower than the point of zero charge (pzc), while for pH = pzc and pH > pzc, the cation was assumed to interact with a deprotonated surface -O- site. At low pH, addition of cations is found to strengthen the siloxane bond in agreement with experiment for the alkali metals, but not for the interaction with H3O+. At high pH, the siloxane bond is weakened by the addition of alkali, in agreement with experiment for feldspar dissolution. Inclusion of the surface hydroxyl groups is found to be important particularly when solvation of the ions at the surface is considered; up to three water molecules have been included in the geometry optimisation. Solvation of the ions interacting with the surface is found to give very important contributions to the computed reaction energies.