The electrical‐double layer revisited

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.

Nanopores: synergy from DNA sequencing to industrial filtration – small holes with big impact

“…There are already reports on unusual transport properties of nanopores in contact with aprotic organic solvents such as acetonitrile and propylene carbonate. 108,109 When in contact with silica pores, these solvents form a highly organized structure that persists over a few nanometers. A multipronged experimental and modeling approach revealed that the organized solvent structure, not the solid properties, determines the distribution of ions and electric potential at the interface.…”

“…A multipronged experimental and modeling approach revealed that the organized solvent structure, not the solid properties, determines the distribution of ions and electric potential at the interface. 108 There has also been interest in understanding solid/liquid interfaces in ionic liquids together with ion transport through these liquids in nanoconfinement, 110 and using them as media that facilitate sensing. 111 Introducing complex solutions to single nanopores can, however, be difficult because of material instabilities and nonspecific adsorption, or fouling.…”

Aprotic Solvent Accumulation Amplifies Ion Current Rectification in Conical Nanopores

“…It is also important to consider a recent study by Polster et al 36 and an earlier study by Berne et al 37 where MeCN molecules have been shown to form lipid-like bilayers on silica surfaces, meaning that, in neat MeCN, the surface exhibits an effective negative charge. Polster et al 36 hypothesized that, at certain concentrations of lithium perchlorate (LiOCl 4 ), the effective surface charge becomes positive due to the interaction of the supporting electrolyte ions with the MeCN bilayer.…”

“…It is also important to consider a recent study by Polster et al 36 and an earlier study by Berne et al 37 where MeCN molecules have been shown to form lipid-like bilayers on silica surfaces, meaning that, in neat MeCN, the surface exhibits an effective negative charge. Polster et al 36 hypothesized that, at certain concentrations of lithium perchlorate (LiOCl 4 ), the effective surface charge becomes positive due to the interaction of the supporting electrolyte ions with the MeCN bilayer. The outcome of the theoretical calculations assuming this hypothesis would remain the same however as an effective positive surface charge in the presence of a supporting electrolyte is assumed, the magnitude of which is affected by the concentration of ions in given regions at the silica surface.…”

“…It was also shown that the surface charge in the double-junction diode region has a significant effect on the direction and degree of rectification. Thus, the smaller inversion in the DCM system can be explained by a smaller surface charge on the nanopore wall due to its smaller dipole density (ν) in accordance with eq 6, as described by Plett et al 24 It is also important to consider a recent study by Polster et al 36 and an earlier study by Berne et al 37 where MeCN molecules have been shown to form lipid-like bilayers on silica surfaces, meaning that, in neat MeCN, the surface exhibits an effective negative charge. Polster et al 36 hypothesized that, at certain concentrations of lithium perchlorate (LiOCl 4 ), the effective surface charge becomes positive due to the interaction of the supporting electrolyte ions with the MeCN bilayer.…”