From Ion Ordering to Microphase Separation—How Charge Frustration Organizes Ions at Surfaces

The role of surface charge at solid-liquid interfaces is fundamental to numerous processes, ranging from materials synthesis, including crystallization and assembly, to biological applications, as well as in energy conversion and storage. One aspect of these processes that remains largely unexplored is the dynamics of ion adsorption and cluster formation leading up to the formation of crystalline films or self-assembled structures. While the Gouy–Chapman theory accounts for the distribution of ions based on their spatial average perpendicular to the surface, the discussion regarding the lateral structure at the interface, particularly the local molecular-level details, is relatively limited.

Using molecularly resolved atomic force microscopy combined with complementary streaming potential apparatus measurements, we investigated the adsorption and precipitation of multivalent ions on mica. Our analysis reveals that electrostatic interactions between the ions and the surface give rise to diverse interfacial structures, including those previously unrealized formations. Divalent ions, such as Mg²⁺, Co²⁺, Ni²⁺ and Zn²⁺, possessing weaker charges, were observed to form large continuous hydroxide monolayers at high pH that reverse the surface potential, with the kinetics of film formation consistent with Classical Nucleation Theory. In contrast, trivalent ions like Al³⁺, Cr³⁺, Fe³⁺, propelled by strong electrostatics, undergo continuous transitions with pH through complex states, such as ordered ion arrays and clusters, and nanostructured films exhibiting microphase separation and significant overcharging. The lateral structure at mica-electrolyte interfaces was understood by Monte Carlo simulations using a charge-frustrated model, where the ions experience competing interactions between short-range chemical bonding and long-range electrostatic forces. The frustration drives the self-organization of ions, leading to diverse interfacial structures.

Additionally, we will present recent findings on electrochemical AFM, demonstrating how the interfacial structure on a semiconductor surface can be manipulated using an external bias. The study provides molecular-level understanding of how electric fields control the spontaneous formation of interfacial nanostructures and offers valuable insights into using electric fields to control crystallization processes for the development of functional materials.