Nucleation and Polymorphism of Calcium Carbonate by a Vapor Diffusion Sitting Drop Crystallization Technique

Abstract

The nucleation and polymorphism of calcium carbonate have been studied using a microdevice named the crystallization mushroom. This setup allows carrying out precipitation experiments reproducibly by the vapor diffusion sitting drop technique. Within the range of concentrations investigated (from 10 to 500 mmol/L CaCl2 and from 1 to 25 mmol/L NH4HCO3), the dominant polymorph to appear first in the drops was calcite, or mixtures of calcite and vaterite followed by aragonite. Additionally, amorphous calcium carbonate (ACC) was not observed. The order of appearance of the polymorphs in the droplets is explained by intrinsic features of the crystallization mushroom, that is, the slow increase in the ionic activity product caused by slow diffusion of NH3 and CO2 gases, which favors the least soluble phase calcite to crystallize before other more soluble polymorphs. The appearance of calcite as the first nucleating dominant polymorph in the drops allowed us to calculate its surface free energy from induction time measurements assuming the mononuclear nucleation model. The experimentally calculated result of 35 mJ/m2 is lower than the value predicted for homogeneous nucleation. The cause is the existence of heterogeneous nucleation taking place at the air−solution and solution−support interfaces.