The hydrolysis and carbonate complexation of the alkaline earth cations Ca 2q and Sr 2q were investigated by using a combined experimental and modeling approach. The modeling approach uses both macroscopic thermodynamic models and molecular modeling at the Ε½ . density functional theory DFT level. The molecular modeling calculations identify possible speciation schemes in the thermodynamic modeling and provide molecular level insight into the macroscopically observed thermodynamic measurements. In order to develop accurate thermodynamic models valid to high electrolyte concentration and to test for the possible existence of species suggested by the molecular models, experimental measurements were . Ε½ . Ε½ . made on the solubility of Ca OH , Sr OH 8H O, and carbonate compounds extending to 2 2 2 . y 1 . y 1 Ε½ . Ε½ . high base molality f 5 mol kg , or carbonate molality f 2 mol kg . A thermodynamic model is developed that satisfactorily explains the macroscopic aqueous thermodynamic data and correlates with the molecular modeling results. The first published values of the Ε½ . 2 y Ε½ . Ε½ . 2y Ε½ . equilibrium constants for the formation of Sr CO aq and Ca CO aq , and of the 3 2 3 2 . Ε½ . solubility product of Sr OH 8H O are also provided. In certain cases, specifically when the 2 2
DFT calculations suggest that the hydroxyl groups are more closely associated with the first hydration layer of water molecules than directly with the central alkaline earth cation, the thermodynamic relations for these cationαhydroxyl interactions are described by means of Pitzer ion-interaction parameters, rather than by the explicit introduction of an aqueous hydrolysis species.