A MOLECULAR-DYNAMICS STUDY OF SOLVATED ORTHOSILICIC ACID AND ORTHOSILICATE ANION USING PARAMETERIZED POTENTIALS

Ab initio calculations have been used extensively in the last decade t o model silicate mineral surfaces. A major problem with both the use a nd validation of ab initio calculations is that including more than a few solvent molecules requires enormous amounts of computer time. In t his work, we propose a means of alleviating this problem by introducin g a parameterized force field for Si-O-H systems. The parameterized mo del is constructed by appending an Si-O interaction to a previously pu blished molecular dynamics model for water. By fitting the parameters of the Si-O interaction to the structure and vibrational spectrum of H 4SiO4, and retaining the O-H interactions in the water model, we obtai n a representation of the H4SiO4 which quite accurately reproduces the gas-phase deprotonation energy of H4SiO4 computed from quantum mechan ical calculations. Molecular dynamics calculations are then used to mo del the free energy change for the deprotonation of H4SiO4 in aqueous solution. While the predicted value of the free energy change (16 kcal /mol) is 2.5 kcal/mol larger than the accepted experimental value of 1 3.5 kcal/mol, the difference between the gas-phase deprotonation energ y computed from quantum mechanics and the solution phase deprotonation free energy measured experimentally is predicted to within 1%.