Conductometric study of calcium carbonate prenucleation stage: Underlining the role of CaCO₃^o ion pairs

Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO₃^o ion pairs on calcium carbonate prenucleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the formation of CaCO₃^o ion pairs as pre-nuclei as soon as the calcite-equilibrium pH is reached. Additionally CaCO₃^o content increases with pH to form aggregates, which depend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amorphous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nuclei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory effect on the Ca²⁺ and CO₃^2- association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate. During the prenucleation stage, the process begins with the formation of CaCO₃ ion pairs as pre-nuclei. With pH increase and then with the supersaturation coefficient, the number of these entities rises to form aggregates. Aggregation leads to the formation of stable nuclei which will later evolve to crystallize.