Thermodynamic insight into the mechanism of NaCl nucleation from solution

Understanding nucleation of crystals from solution is crucial in calculating nucleation rates and designing crystallization processes to control polymorphism. In this work, we examine the non-classical nucleation mechanism that has been reported in nucleation of NaCl from highly concentrated aqueous solutions [1]. By obtaining the free energy change of nucleation as a function of two structure specific nucleus size coordinates, we demonstrate the thermodynamic preference for nucleation through a composite cluster, where the crystalline nucleus is surrounded by an amorphous layer. The composite cluster model of Iwamatsu [2] describes the free energy change of cluster formation very well. The thermodynamic properties extracted from the fit show that the amorphous phase is less stable than the metastable solution phase, whereas the crystalline phase is the most stable. Finally, we calculate the nucleation rate of the crystalline cluster using our 2D system and compare it with nucleation rates obtained using a 1D free energy profile. This study is preparatory to an investigation of polymorphism in glycine crystallization.

[1] Jiang et al., J. Chem. Phys. 150, no. 12 (2019): 124502.
[2] Iwamatsu, M. J. Chem. Phys., 134 no. 16 (2011): 164508.