Nanoscale insights on the freezing front propagation and ion behaviors during seawater freezing

Seawater freezing holds significant importance in scientific and industrial applications. However, there is a lack of in-depth understanding of the nanoscale mechanism of seawater freezing. In this study, the nanoscale characteristics and mechanisms of seawater freezing are investigated by the molecular dynamics method. Since the solid-liquid interfacial free energy decreases with increasing subcooling or decreasing NaCl solution concentration, the speed of freezing front propagation increases. The formation of nano brine pockets is observed, which is analyzed based on the solid-liquid interfacial free energy. The energy barriers of Na⁺ and Cl⁻ diffusing from the freezing front into the NaCl solution are smaller than these of diffusing into the hexagonal ice. As a result, most Na⁺ and Cl⁻ are rejected into the solution, whereas only a few dope into the hexagonal ice. Furthermore, the energy barrier of Cl⁻ diffusion into hexagonal ice is smaller than that of Na⁺, which makes the amount of Cl⁻ doping into hexagonal ice greater than that of Na⁺. This study contributes to a deeper understanding of nanoscale characteristics and mechanisms of seawater freezing, which shall promote related technologies such as seawater freeze desalination.