Charge Frustration Drives Ion Ordering and Microphase Separation at Mica-Electrolyte Interfaces

The significance of surface charge at solid-liquid interfaces extends to crucial roles in diverse chemical processes that include crystallization and self-assembly. 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. Building upon the classical mean field description derived from the Gouy–Chapman model, the electrostatic attraction between a charged interface and counterions in solutions prompts oppositely charged ions from the solution to accumulate at the interface, forming an electric double layer (EDL), with distinct properties from those observed in bulk solutions. While the model 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.<br/>In this study, we explored the interfacial structure of mica, a mineral with an atomically flat surface and intrinsic negative charge, in electrolytes containing different multi-valent cations, utilizing <i>in-situ</i> liquid phase atomic force microscopy (LP-AFM) prior to and during the formation of crystalline films. We find that, as the solution pH is gradually raised, cations adsorb and form a monolayer hydroxide film on mica surface. Divalent ions, such as Mg²⁺, Ni²⁺, Zn²⁺, and Co²⁺, form large continuous monolayers in a manner that aligns with expectations based on classical nucleation theory. For trivalent ions like Al³⁺, Fe³⁺, and Cr³⁺, hydrolyzed species adsorb in increasing amounts as pH is increased. However, instead of exhibiting a random distribution, these ions reveal intricate lateral ordering and eventually evolve into an ordered ion network. Upon further pH increase, the ion network undergoes a transition to a discontinuous film with a persistent network of gaps. The links between the surface nanostructure and local surface charge were investigated using three-dimensional fast force mapping (3D FFM) and complementary streaming potential apparatus (SPA) measurements. As films form, an inversion from negative to positive zeta potential on mica is observed through SPA and is accompanied by the appearance of a long-range tip-sample forces through 3D FFM. These findings suggest that electrostatic interactions between positively charged ions/films and a negatively charged substrate stabilize the surface nanostructure. Furthermore, films created by trivalent ions are more strongly charged compared to those formed by divalent ions.<br/>The lateral structure at mica-electrolyte interfaces was simulated using a charge-frustrated lattice gas model, where the ions experience competing interactions between short-range chemical bonding and long-range electrostatic forces. For weak charge effects, the film undergoes a first-order transition from sparse adsorbates to large continuous sheets, aligning with the behavior observed with divalent ions. When the charge effect is sufficiently strong, the surface forms various states, including ordered patterns of ions, ion clusters, and microphase-separated partial films, corresponding to the behavior observed with trivalent ions. This 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.