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 (CaCl 2, CaBr 2, CaI 2 ) at pH 10.5 form hexagonally close-packed Ca 2+ structures with spacings of 3.6–3.8 Å (CaCl 2 ), 4.8 Å (CaBr 2 ), and 5.0–5.1 Å (CaI 2 ). The CaCl 2 spacing matches the Cl – diameter, suggesting direct Ca 2+ –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 Ca 2+ interactions sufficient to displace hydration waters, while the lower charge densities of Br – and I – lead to less strong attractions with Ca 2+ 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.