Observation of ionic layering at liquid-solid interfaces by cryo-electron microscopy

Electrically charged liquid-solid interfaces play an important role in many electrochemical phenomena encountered in biology, energy, and the environment. The ability to probe such systems with single-ion resolution is important to a basic understanding of their behaviors and the development of new ionic technologies. Using cryo-electron microscopy, we directly visualize individual counterions and reveal their discrete interfacial layering. Comparison with simulations suggests the strong effects of finite ionic size and electrostatic interactions. Besides, we reveal correlated ionic structures under extreme confinement, with the channel widths approaching the ion diameter (~1 nm). Our experiment opens up a new arena to study liquid-solid interfaces at the single-ion level.