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

Abstract

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 CaCO 3 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 CaCO 3 o ion pairs as pre-nuclei as soon as the calcite-equilibrium pH is reached. Additionally CaCO 3 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 Ca 2+ and CO 3 2− association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate.

Designing Solid Materials from Their Solute State: A Shift in Paradigms toward a Holistic Approach in Functional Materials Chemistry

“…While calcium carbonate remains an essential model system for the further development and refinement of the PNC pathway, , it is also the system where the pivotal role of the solventwaterduring the early stages of crystallization is now crystal clear. The release of hydration water is the entropic key contribution which drives pre-nucleation ion association, beyond the ion pair, allowing for the formation of even larger PNCs . The solvent plays a key role when Mg is introduced, by inhibiting calcite growth and directly promoting aragonite nucleation in the early stages of CaCO 3 formation .…”

“…The release of hydration water is the entropic key contribution which drives pre-nucleation ion association, 58 beyond the ion pair, 59 allowing for the formation of even larger PNCs. 60 The solvent plays a key role when Mg is introduced, by inhibiting calcite growth and directly promoting aragonite nucleation in the early stages of CaCO 3 formation. 61 The assessment of water dynamics allowed the localization of the liquid−liquid binodal limit, at which PNCs become nanodroplets of a new phase, 36 while water can control the prestructuring in the as-formed amorphous intermediates.…”

Investigation of scale inhibitors effect on calcium carbonate nucleation process

“…Among these models, it has been shown that the one proposed by McCleskey et al is the most suitable for the conductivity calculation. The methodology and basic steps of the curves modelling have been described in detail elsewhere [17].…”

“…So, the present work focuses on the effect of chemical substances on the CaCO 3 ion pair formation and aggregation at the earlier nucleation stage. For this, our recently developed method [17] for highlighting the role of CaCO 3° ion pairs in the nucleation-growth of calcareous scale was exploited. As inhibitors, polyacrylate and polyphosphate substances were tested.…”

Influence of foreign salts on the CaCO3 pre-nucleation stage: application of the conductometric method

“…In our previous work, 22 a modelling method of the resistivity vs. pH precipitation curves was developed using thermodynamic calculations of all equilibrium between various species of the CaCO 3 –CO 2 –H 2 O system. The method consisted of the comparison of four models based on equations.…”