Thermodynamics of Dissolution of Calcium Hydroxycarboxylates in Water

Aqueous solubility of calcium L-lactate, calcium D-gluconate, and calcium D-lactobionate increases with temperature (10-30 degrees C investigated), most significantly for the least soluble D-gluconate, while the calcium ion activity of the saturated solutions decreases with temperature, as measured electrochemically, most significantly for the most soluble D-lactobionate. This unusual behavior is discussed in relation to dairy processing and explained by endothermic binding of calcium to hydroxycarboxylate anions determined to have Delta H-ass degrees = (31 +/- 3) kJ.mol(-1) for L-lactate, (34 +/- 2) kJ.mol(-1) for D-gluconate, and (29 +/- 3) kJ.mol(-1) for D-lactobionate in 1:1 complexes with thermodynamic binding constants at 25 degrees C of K-ass = 49 (L-lactate), 88 (D-gluconate), and 140 (D-lactobionate). Quantum mechanical calculations within density functional theory (DFT) confirm the ordering of strength of binding. The complex formation is entropy driven with Delta S-ass degrees > 0, resulting in decreasing calcium ion activity in aqueous solutions for increasing temperature, even for the saturated solutions despite increasing solubility.