The Role of Water Dimers in the Initial Stage of Salt Crystallization

Probing early-stage crystallization in hydrated environments is crucial for elucidating the microscopic mechanism of crystal growth. However, capturing these processes remains challenging because of the nanometric dimensions of nanocrystals and the dynamic role of water in solvation and ion-ion interactions. Here, we employ a cryogenic scanning probe microscopy platform, which integrates qPlus-type atomic force microscopy with a frozen-solution preparation technique, to directly visualize hydrated sodium chloride (NaCl) nanocrystals at atomic resolution. We observe double- to 5-stranded ionic chain structures, which are hydrated by water dimers. Those structures promote the anisotropic growth of NaCl nanocrystals. Density functional theory calculations reveal that the water dimer can substantially stabilize the chain-shaped configurations by optimizing the water-water and water-ion interactions. In contrast, larger crystals favor isotropic crystalline lattices due to dominant bulk ionic interactions. These findings highlight the unique role of water dimers in the initial crystallization process of salts. Furthermore, this work demonstrates the potential of cryogenic scanning probe microscopy as a powerful tool for probing the crystallization process at atomic resolution, particularly in hydrated environments.