Anion Size Controls Cation Wigner Crystal-Like Structures at Silica Interfaces

High-resolution atomic force microscopy (AFM) images reveal that anion size systematically controls the dimensions of cation Wigner crystal-like structures (WCLS) at silica–electrolyte interfaces. Calcium halide solutions (CaCl₂, CaBr₂, CaI₂) at pH 10.5 form hexagonally close-packed Ca²⁺ structures with spacings of 3.6–3.8 Å (CaCl₂), 4.8 Å (CaBr₂), and 5.0–5.1 Å (CaI₂). The CaCl₂ spacing matches the Cl^– diameter, suggesting direct Ca²⁺–Cl^– contact, whereas Br^– and I^– systems show consistent 0.7–0.9 Å offsets above their ionic diameters, indicating partially hydrated states. This behavior reflects the balance between ionic charge density and hydration effects. The high charge density of Cl^– enables strong Ca²⁺ interactions sufficient to displace hydration waters, while the lower charge densities of Br^– and I^– lead to less strong attractions with Ca²⁺ and partial hydration layers are preserved. These findings demonstrate how ion size and hydration control Stern layer ion organization, providing new insights into the electrical double layer structure.