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

Area of Science:

• Surface Science
• Electrochemistry
• Materials Science

Background:

• The electrical double layer (EDL) governs interfacial phenomena.
• Understanding ion organization in the Stern layer is crucial for controlling interfacial properties.
• Previous studies have explored ion adsorption but lacked high-resolution structural details.

Purpose of the Study:

• To investigate the influence of anion size on cation Wigner crystal-like structure (WCLS) formation at the silica-electrolyte interface.
• To elucidate the role of ionic charge density and hydration in dictating ion arrangement.
• To provide atomic-level insights into the structure of the Stern layer.

Main Methods:

• High-resolution atomic force microscopy (AFM) imaging.
• Systematic variation of calcium halide electrolytes (CaCl2, CaBr2, CaI2) at a controlled pH (10.5).
• Analysis of cation WCLS dimensions and inter-ionic spacings.

Main Results:

• Anion size systematically controlled Ca2+ WCLS dimensions.
• CaCl2 formed Ca2+ structures with spacing matching Cl- diameter (3.6-3.8 Å), indicating direct contact.
• CaBr2 and CaI2 showed Ca2+ structures with spacings larger than the respective anion diameters (4.8 Å for Br-, 5.0-5.1 Å for I-), suggesting partial hydration.
• Observed spacing differences correlate with anion charge density and hydration energy.

Conclusions:

• Anion size and hydration are key factors controlling cation organization in the Stern layer.
• High charge density anions (Cl-) can displace hydration water, enabling direct cation-anion contact.
• Lower charge density anions (Br-, I-) retain partial hydration shells, influencing cation spacing.
• Findings advance the understanding of EDL structure and ion-adsorption mechanisms at charged interfaces.