Hydrogen-bond mediated and concentrate-dependent NaHCO₃ crystal morphology in NaHCO₃–Na₂CO₃ aqueous solution: Experiments and computer simulations

Adding Na₂CO₃ to the NaHCO₃ cooling crystallizer, using the common ion effect to promote crystallization and improve product morphology, is a new process recently proposed in the literature. However, the mechanism of the impact of Na₂CO₃ on the crystal morphology is still indeterminate. In this work, the crystallization of NaHCO₃ in water and Na₂CO₃–NaHCO₃ aqueous solution was investigated by experiments and molecular dynamics simulations (MD). The crystallization results demonstrate that the morphology of NaHCO₃ crystal changed gradually from needle-like to flake structure with the addition of Na₂CO₃. The simulation results indicate that the layer docking model and the modified attachment energy formula without considering the roughness of crystal surface can obtain the crystal morphology in agreement with the experimental results, but the lower molecules of the crystal layer have to be fixed during MD. Thermodynamic calculation of the NaHCO₃ crystallization process verifies that the common ion effect from Na⁺ and the ionization equilibrium transformation from CO₃^2– jointly promote the precipitation of NaHCO₃ crystal. The radial distribution function analysis indicates that the oxygen atoms of Na₂CO₃ formed strong hydrogen bonds with the hydrogen atoms of the (0 1 1) face, which weakened the hydration of water molecules at the crystal surface, resulting in a significant change in the attachment energy of this crystal surface. In addition, Na⁺ and CO₃^2– are more likely to accumulate on the (0 1 1) face, resulting in the fastest growth rate on this crystal surface, which eventually leads to a change in crystal morphology from needle-like to flake-like.