Probing Single Ions at Mica-Aqueous Interfaces Using Atomic Force Microscopy

Abstract Over the last decades, atomic force microscopy (AFM) has developed into a tool capable of mapping the equilibrium 3-dimensional organization of liquid molecules at the interface with solids using a vibrating tip. While very successful for pure liquids, measurements in solutions containing dissolved ions or molecules can be more challenging due to the fact that the AFM tip can unwittingly remove single adsorbates during the measurement. This issue is particularly important for high-resolution measurements in liquid, potentially leading to adsorbed species becoming effectively invisible to the AFM despite it resolving details of the smaller liquid molecules. Here, we explore this issue systematically on a known system: the mica-water interface in aqueous solutions of 5 mM RbCl, KCl, NaCl, LiCl and 2 mM CaCl2, and pure water as a reference. The different metal cations all adsorb to the interface, but with different strengths and hydration states. We show that although atomic-level resolution of the hydrated interface can always be achieved, resolving the adsorbed ions depends on the tip vibration amplitude and imaging conditions. Measurements conducted at smaller amplitude are dominated by the equilibrium water molecular arrangement, with ionic effects becoming more prominent at larger amplitudes. This is particularly clear for ions such as Rb+ which adsorb further from the interface compared to ions with higher charge density. The results are discussed for each ion in terms of the hydration state and the measured conservative and dissipative interactions of the measuring tip.