Molecular dynamics simulation of impurity effects near the NaCl interface during the initial rapid stages of growth

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For salt processing, it is necessary to understand impurity incorporation into NaCl crystals at the molecular level. In this study, molecular dynamics (MD) simulations of NaCl crystal growth were performed to analyze the solution structure near the interface between a NaCl crystal and NaCl–NaBr or NaCl–NaI solutions and to calculate the dehydration energy in the presence of impurities (Br− and I−). Furthermore, the adsorption mechanism of solute ions onto the NaCl surface during the initial growth stage of batch crystallization was investigated. Both Br− and I− were found to decrease the adsorption rate, but the impurity effects of these ions were different. Moreover, the adsorption rate in the presence of Br− was faster than that in the presence of I−. In the presence of Br−, the adsorption rate was decreased by the rate-limiting effect of the growth rate of Br−. By contrast, in the presence of I−, the adsorption rate was reduced by the abundance of I− near the interface, as the supply of Cl− near the interface was limited by electrostatic effects. In addition, the local concentration of impurities (Br− and I−) near the crystal–solution interface was higher than the saturated concentration. This difference from the bulk composition suggests that impurities may even crystallize at unsaturated concentrations.