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

• Published in: Crystal growth & design 2013-03, Vol.13 (3), p.1170-1179
• Main Authors: Bruno, Marco, Massaro, Francesco Roberto, Pastero, Linda, Costa, Emanuele, Rubbo, Marco, Prencipe, Mauro, Aquilano, Dino
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; General studies of phase transitions; Nucleation; Physics; Solubility, segregation, and mixing; phase separation; Thermal properties of condensed matter; Thermal properties of crystalline solids; Thermodynamic properties
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg3015817

This paper deals with several properties of calcite crystals. First, the solvated surface energies at 0 K of the {101̅4}, {101̅0}, {112̅0}, {011̅8}, {011̅2}, {0001}, and {213̅4} forms were determined by means of the COSMIC method. We find that the presence of water reduces by ∼7–14% 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 {101̅4}, {101̅0}, {011̅2}, and {0001} forms are present. In a second part of the paper, the free energy at 300 K of the most widely studied {101̅4}/water interface has been determined (γ(101̅4)/w 300 K = 0.412 ± 0.020 J/m2) by combining simple thermodynamic relations and contact angle measurements. Some considerations on the free energy of other interfaces:{101̅0}, {112̅0}, {011̅8}, {011̅2}, {0001}, and {213̅4} 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 γ(101̅4)/w 300 K and calculated (101̅4) solvated surface energy, the latter affected by the limits of the COSMIC method, a rough estimate of the vibrational energy and entropy (γ(101̅4)/w 300 K,vib) and configurational entropy (γ(101̅4)/w 300 K,config) of the (101̅4)/water interface is obtained: (γ(101̅4)/w 300 K,vib + γ(101̅4)/w 300 K,config) = −0.066 ± 0.038 J/m2. Finally, by applying the classical nucleation theory and our estimate of γ(101̅4)/w 300 K 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).