New Estimates of the Free Energy of Calcite/Water Interfaces for Evaluating the Equilibrium Shape and Nucleation Mechanisms

This paper deals with several properties of calcite crystals. First, the solvated surface energies at 0 K of the {10 (1) over bar4}, {10 (1) over bar0}, {11 (2) over bar0}, {01 (1) over bar8}, {01 (1) over bar2}, {0001}, and {21 (3) over bar4} forms were determined by means of the COSMIC method. We find that the presence of water reduces by similar to 7-1496 the dry surface energy at 0 K, in a homogeneous way. The solvated and dry equilibrium shapes at 0 K result to be nearly homothetic: in both cases the {10 (1) over bar4}, {10 (1) over bar0}, {01 (1) over bar2}, and {0001} forms are present. In a second part of the paper, the free energy at 300 K of the most widely studied {10 (1) over bar4}/water interface has been determined (gamma(300 K)((10 (1) over bar4)/w) = 0.412 +/- 0.020 J/m(2)) by combining simple thermodynamic relations and contact angle measurements. Some considerations on the free energy of other interfaces:{10 (1) over bar0}, {11 (2) over bar0}, {01 (1) over bar8}, {01 (1) over bar2}, {0001}, and {21 (3) over bar4} are also proposed. We show that our results are consistent with thermodynamic constraints and fit quite well the known dependence of the solubility of calcite on grain size experimentally determined. Furthermore, as a side result and for the first time, on the basis of our estimate of gamma(300 K)((10 (1) over bar4)/w) and calculated (10 (1) over bar4) solvated surface energy, the latter affected by the limits of the COSMIC method, a rough estimate of the vibrational energy and entropy (gamma(300 K,)((10 (1) over bar4)/w) (vib)) and configurational entropy (gamma(300 K,)((10 (1) over bar4)/w) (config)) of the (10 (1) over bar4)/water interface is obtained: (gamma(300 K,)((10 (1) over bar4)/w) (vib) + gamma(300 K,)((10 (1) over bar4)/w) (config)) = -0.066 +/- 0.038 J/m(2). Finally, by applying the classical nucleation theory and our estimate of gamma(300 K)((10 (1) over bar4)/w) we calculated the activation energy for homogeneous and heterogeneous nucleation at room temperature. We found extremely high values for such thermodynamical quantity, suggesting that the direct formation of calcite should be prevented. This strongly supports the view that calcite formation should proceed throughout the early formation of an amorphous calcium carbonate phase (ACC).