Imaging Arrangements of Discrete Ions at Liquid–Solid Interfaces

Abstract

The individual and collective behavior of ions near electrically charged interfaces is foundational to a variety of electrochemical phenomena encountered in biology, energy, and the environment. While many theories have been developed to predict the interfacial arrangements of counterions, direct experimental observations and validations have remained elusive. Utilizing cryo-electron microscopy, here 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. We also uncover correlated ionic structures under extreme confinement, with the channel widths approaching the ionic diameter (∼1 nm). Our work reveals the roles of ionic size, valency, and confinement in determining the structures of liquid−solid interfaces and opens up new opportunities to study such systems at the single-ion level.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

“…These challenges can be overcome with cryogenic TEM (cryo-TEM) microscopy. − By vitrifying the liquid through rapid cooling, the native state of the sample is preserved, and the sample is protected from the vacuum and the electron beam. Li et al recently used cryo-TEM to image the arrangements of discrete counterions at electrically charged liquid–solid interfaces . By adding positively charged, amine-functionalized gold nanorods to aqueous solutions of phosphotungstic acid (H 3 PW 12 O 40 ), they captured the crowding and layering behavior of Keggin anions [PW 12 O 40 ] 3– at the nanorod surfaces (Figure A–C).…”

“…(D,E) Cryo-TEM images of the ionic solution confined between aggregated parallel nanorods, which show monolayer or bilayer ionic structures, depending on the confinement width. Adapted from ref . Copyright 2020 American Chemical Society.…”

“…Defining and determining the pore size relevant for transport in these systems is another central, yet often nontrivial, task. As a few examples, size-exclusion assays have been used for CNT membranes, , vibrational spectroscopy for arrays of ultralong CNTs, purified and precharacterized CNTs as starting materials for CNT porins, ,,, and direct electron microscopy for CNT nanocapillaries . Given the difficulty in preventing leaks, the CNT diameter should not be inferred from transport characteristics if the goal is to study transport.…”

Gating with Charge Inversion to Control Ionic Transport in Nanopores

“…The accumulation of counterions on the surface is possible through the spatial correlation of counterions that lowers the system’s potential energy and overcomes electrostatic repulsion . The adsorbed counterions have been postulated to form a strongly correlated liquid (SCL), like the structure of a Wigner crystal, , as recently revealed under electron microscopy . Thus far, the experimental efforts have been focused primarily on using nanopores and nanochannels as a model system to identify conditions where overcharging occurs.…”

“… 1 The adsorbed counterions have been postulated to form a strongly correlated liquid (SCL), like the structure of a Wigner crystal, 1 , 11 as recently revealed under electron microscopy. 12 Thus far, the experimental efforts have been focused primarily on using nanopores and nanochannels as a model system to identify conditions where overcharging occurs. Measurements of electrokinetic phenomena such as streaming current, 7 open circuit potential, 10 and current–voltage ( i – V ) curves 6 , 9 , 13 − 15 have indeed identified the critical concentration needed to switch the sign of the effective surface charge.…”