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.