Ion-specific Effects on Freezing and Salt Rejection of Aqueous Solutions

Investigating how dissolved impurities influence the freezing of aqueous solutions is crucial across cryosphere to atmosphere. Among these impurities, ions are common, and their behavior at the ice-solution interface has been a subject of research for the last few decades (Rasmussen et al, Desalination 573:117216, 2024). In this study, we explore the specific effects of various ions on ice crystal growth and salt rejection using molecular dynamics techniques via atomistic simulations. In pure water and electrolytes of halide anions with Na⁺ countercations, we observed that ice growth rates follow the order F^- < Cl^- < Br^- < H₂O, in accordance with the Hofmeister series, whereas ion rejection to the solution side follows the order F^- > Cl^- ≈ Br^-. In freeze desalination, the process cost is dictated by the balance between the ice growth rate and ion rejection efficiency. Moreover, the type of anion present in the salt solution does not significantly affect the rejection rate of the cation. This knowledge of ion-specific behavior in freezing processes opens up a range of possibilities for engineering the related fields, e.g., freeze desalination, water treatment, and cryopreservation.