Identification of Single Adsorbed Cations on Mica-Liquid Interfaces by 3D Force Microscopy.

Force microscope provides atomically resolved images of surfaces immersed in a liquid. The presence of different chemical species in the interface (cations, anions, water, neutral atoms) complicates the adscription of the observed features to a given species. We develop a 3D atomic force microscopy method to identify the cations adsorbed on a mica surface from a potassium chloride solution. The method is based on measuring the peak value of the attractive force within the Stern layer. The maximum of the attractive force shows site-specific variations. The positions with the highest attractive force values are associated with the presence of adsorbed potassium ions, while the other positions are associated with a local depletion of the hydration layer. This criterion provides a surface coverage of K cations that is consistent with the one reported by other techniques.