How acidic is water on calcite

We have used density functional theory in combination with the COSMO-RS implicit solvent model to calculate the pKa for water adsorbed on various sites on a calcite surface. The mineral surfaces were modeled using 80 atom clusters. We found that, on kink sites, water is acidic with pKa as low as 3.5, implying that, at equilibrium, the kink sites are associated with one hydroxyl ion in addition to water. We also calculated pKa for ethanol and phenol adsorbed on calcite and compared the free energies of adsorption for ethanol and water. Ethanol was found to bind more strongly on the {10.4} terrace and on the obtuse step, whereas on kink sites, water (in the form of hydroxyl ions) binds more strongly than ethanol (in the form of ethoxide). Our results are relevant for modeling the dissolution and precipitation of minerals and show how the OH functional group behaves in the interaction between organic molecules and mineral surfaces.

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