High resolution AFM imaging of ion adsorption and charge distribution at heterogeneous solid-liquid interfaces

The interfacial liquid does not behave like a bulk liquid, and often undergoes particular structural arrangements, depending on the local solid-liquid and liquid-liquid molecular interactions. However, experimental research remains challenging due to the lack of techniques offering sufficient resolution over inhomogeneous surfaces. Here we present a novel approach based on ultrahigh resolution Atomic Force Microscopy and spectroscopy in the dynamic mode. We explore in unprecedented detail the adsorption of ions at heterogeneous gibbsite-silica surfaces in contact with aqueous electrolytes of variable concentration. Dynamic force spectroscopy with a lateral resolution of ~10nm and atomic resolution imaging consistently reveal a preferential adsorption of divalent Mg2+ and a2+ as compared to monovalent Na+ and K+ cations including adsorption sites on the atomically flat mineral surfaces. We present an atomic model for the adsorption sites of the ions and compare the local surface charge to a surface speciation model based on the Poisson Boltzmann equation coupled to a representation of the surface in a basic Stern model.