LIQUID-LIQUID SEPARATION EXPLAINS “NON-CLASSICAL” BEHAVIOR DURING CALCIUM CARBONATE CRYSTALLIZATION

Recent experimental characterizations of the early stages of calcium carbonate crystallization reveal an abundance of nanometer-sized so-called “pre-nucleation” clusters that appear prior to the formation of an amorphous intermediate phase. The prevailing interpretation of the clusters as thermodynamically stable species is incompatible with classical nucleation theories that predict formation of unstable clusters within supersaturated solutions, as well as long-standing speciation models with established predictive capabilities. This study demonstrates that a liquid-liquid phase separation occurs within the range of concentrations that pre-nucleation clusters are observed. Amorphous calcium carbonate forms by coalescence and solidification of nanoscale droplets of an ion-rich dense liquid phase, which suggests experimental observations can be reconciled within the context of classical phase separation models without invoking non-classical constructs.