Modification of surface crystal formation on mother crystal by additive molecules

Additive molecules and excess reactant species (NaOH and NH4F) in NaF crystallization were used to control surface crystal formation on a mother crystal. When the surface integration was inhibited or bulk diffusion facilitated, more surface crystals were found on the mother crystal due to an increase in the interfacial supersaturation on the crystal. The inclusion of KCl as an additive and increases in the reactant concentration and agitation speed resulted in a higher population of surface crystals on the mother crystal, because surface integration was inhibited. However, surface crystal formation was suppressed by the polymer gelatin due to retardation of the bulk diffusion by the gelatin layer formed around the crystal. In addition, surface crystal formation significantly changed according to the feeding mode, and the excess species of NaOH originating from the NH4F feeding mode were more effective in facilitating surface crystal formation than the excess species of NH4F coming from the NaOH feeding mode due to the smaller ionic radii of NaOH than of NH4F.