Charged solid-solution surfaces are ubiquitous throughout nature, where they regulate surface potential, control ion adsorption, and influence colloidal stability. This has important consequences in diverse geochemical systems, including the adsorption of transition metals, contaminants, and rare earth elements onto mineral surfaces.

The properties of charged interfaces are commonly understood using the modified Gouy Chapman theory and related models, which describe how screening ions accumulate at the interface to generate an electrical double layer (EDL) that regulates the interfacial potential. However, most models focus on simple high-symmetry systems, such as homogenously charged planes, and neglected lateral variations in surface charge density. This is often a significant oversight, since the charge density of real surfaces can vary widely due to the presence of various crystal terminations, surface defects, surface reconstructions, or adsorbates. Even canonically neutral surfaces like NaCl (001) present an array of positively and negatively charged ions at the atomic scale. Emerging work even suggests that competitions between ion-clustering and electrostatic forces can drive the spontaneous formation of nanostructured interfaces. Models that account for surface charge heterogeneity are needed to understand such systems.

Here we present new simulations on the energetics of heterogeneously charged interfaces, conducted using a finite-volume Poisson Boltzmann solver. We simulate charged interfaces that display stripes of positive and negative surface charge, and then systematically study how variations in ionic strength, surface charge density, and stripe width influence the interfacial energy. We show that stripe-interactions have significant influence over the interfacial energy, and we identify crossover lengths at which the system transitions from being governed by geometric screening to electrolyte screening. Our work also reveals deficiencies in standard paradigms. Whereas traditional models focus almost exclusively on the screening behavior of the electrolyte solution, our simulations show that the dielectric properties of the substrate have significant influence on the energetics of heterogeneously charged substrates. These results provide new insights into mechanisms of ion adsorption, colloidal stability, and the formation of interfacial nanostructures.

Benjamin Legg

Pacific Northwest National Laboratory

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