data suggest. In Ref.
(2) the predicted pH pzc is 01.8, about Molecular statics calculations on gas-phase and solvated clusters 4 log units lower than the experimental value. References and on gas-phase and solvated slabs representing aqueous species (7) and ( 5) presented a model including the static dielectric and surfaces were applied to investigate acid/base reactions on constant of the mineral as an additional parameter. Paramesilica surfaces. Our gas-phase approach, which was previously ters fitted to equations describing this physical model were applied to goethite, predicts a surface p K a of 8.5 for the reaction capable of correlating the pH pzc for a wide variety of miner-ΓΊSiOH r ΓΊSiO 0 / H / which is in good agreement with estimates als, including silica. The MUSIC model was reformulated based on potentiometric titration. However, the model gives an in Ref. ( 6). This work focussed on taking into account hyunrealistically large pK a for the reaction ΓΊSiOH / 2 r ΓΊSiOH / H / . The model dependence of this result was checked by using drogen bonding interactions (8), also giving significantly two different types of interaction potentials, one based on quantum improved results for silica. These types of studies (1-3, 7) mechanical calculations on H 4 SiO 4 clusters, and another empirical all begin with a physical model of proton adsorption and end model fitted to the structure and elastic properties of a-quartz. by fitting model parameters to experimental data essentially Because these models gave similar results, we hypothesize that the using linear free energy relationships (LFER). Hereafter we failure of the gas-phase models is due to intrinsic solvation effects refer to this approach as the empirical LFER approach.
not accounted for by our previously developed correlations. We
We have recently applied molecular modeling methods to tested this idea by carrying out energy minimization calculations calculate the pH at the pristine point of zero charge (pH pzc )
on gas-phase clusters with one hydration shell as well as molecular (9) and the surface charge vs pH curves (10) for goethite.
dynamics simulations on fully-solvated H 5 SiO / 4 and a fully sol-
Our approach was to calculate gas-phase deprotonation enervated (0001) surface of b-quartz. Though we are unable to establish a quantitative measure of the pK a of SiOH 2 groups, the sol-gies DE for the successive removal of protons from an vated systems do indicate that SiOH groups do not protonate in Fe(III)(H 2 O 6 ) molecule using the model described in (11).
any of our solvated models.