The electrical‐double layer revisited

Abstract

Abstract The electrical‐double layer (EDL) model is fundamental to our understanding of interactions in ionic solutions, and is widely used in chemical, biological, and technological contexts, particularly in the description of aqueous electrolyte solutions. However, recent experiments have raised questions regarding the validity of this model in polar, aprotic solvents; some observations, such as a surface potential that changes sign with increasing salt concentration, are not consistent with the EDL picture. We demonstrate in a model system, acetonitrile at a silica interface, that solvent organization dictates the ionic distributions. Ion‐transport measurements in nanopores, surface‐selective spectroscopy, and molecular dynamics simulations reveal that the distribution of ions in acetonitrile at a silica interface is determined by the lipid‐bilayer‐like organization that the interface imposes upon the liquid, which accounts for the change in sign of the potential. Our findings emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces. Key Points In a range of acetonitrile‐based electrolyte solutions in contact with silica, the effective surface potential is negative at low salt concentrations and positive at high salt concentrations. The long‐range, lipid‐bilayer‐like organization of acetonitrile at this interface dictates the preferred locations for cations and anions. Due to this organizational effect, the classical electrical double‐layer modal is not applicable to the acetonitrile/silica system, as likely also does not hold for other systems involving polar, aprotic liquids.

Keywords

• aprotic solvents
• electrochemistry
• interfaces
• ion current
• ionic solutions
• molecular dynamics