A theoretical kinetic model of the temperature and pH dependent dimerization of orthosilicic acid in aqueous solution

The first steps in a pH-and temperature-dependent theoretical kinetic model of silicate polymerization and dissolution are examined in this work with a combined ab initio and transition state theory based study of the dimerization of H4SiO4. The role of solvation has been of primary concern in this work, and its influence on theoretical activation energies and pre-exponential factors has been thoroughly benchmarked. Relatively inexpensive MP2/6-31+G(d)//HF/6-31+G(d) calculations of octahydrate clusters, with conductor-like polarizable continuum model corrections obtained in the MP2-level single-point calculations, have been shown to lead to a good description of the limited experimentally determined energetics of dimerization for most elementary reactions. Pre-exponential factors computed from this level of theory are found to be relatively insensitive to the level of theory utilized for geometry optimizations, the number of explicit waters, hindered rotor corrections, and variational effects arising from the minimization of rate constants. Within this framework, a kinetic model of the chemistry of H4SiO4 and H3SiO4-, forming H6Si2O7 and H5Si2O7-, has been compiled. Numerical simulations over pH = 3-12 show that a number of pH- and temperature dependent trends in reaction rates and positions of equilibrium are well described with this simple dimerization model. More specifically to the dimerization process, we obtain dimerization constants, log K-dim, of 1.85 and -7.15 for the formation of H6Si2O7 and H5Si2O7- respectively, which compare well with experimentally determined values of 1.2 and -8.5, respectively.