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

Abstract

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 CO32– 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 CO32– 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.