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The nanoscale organization of electrolyte solutions at interfaces is often described well by the electrical double-layer model. However, a recent study has shown that this model breaks down in solutions of LiClO4 in acetonitrile at a silica interface, because the interface imposes a strong structuring in the solvent that in turn determines the preferred locations of cations and anions. As a surprising consequence of this organisation, the effective surface potential changes from negative at low electrolyte concentration to positive at high electrolyte concentration. Here we combine previous ion-current measurements with vibrational sum-frequency-generation spectroscopy experiments and molecular dynamics simulations to explore how the localization of ions at the acetonitrile–silica interface depends on the sizes of the anions and cations. We observe a strong, synergistic effect of the cation and anion identities that can prompt a large difference in the ability of ions to partition to the silica surface, and thereby influence the effective surface potential. Our results have implications for a wide range of applications that involve electrolyte solutions in polar aprotic solvents at nanoscale interfaces. [ABSTRACT FROM AUTHOR]

The development of modern membranes for ionic separations and energy-storage devices such as supercapacitors depends on the description of ions at solid interfaces, as is often provided by the electrical double layer (EDL) model. The classical EDL model ignores, however, important factors such as possible spatial organization of solvent at the interface and the influence of the solvent on the spatial dependence of the electrochemical potential; these effects in turn govern electrokinetic phenomena. Here we provide a molecular-level understanding of how solvent structure can dictate ionic distributions at interfaces using a model system of a polar, aprotic solvent, propylene carbonate, in its enantiomerically pure and racemic forms, at a silica interface. We link the interfacial structure to the tuning of ionic and fluid transport by the chirality of the solvent and the salt concentration. The results of nonlinear spectroscopic experiments and electrochemical measurements suggest that the solvent exhibits lipid-bilayer-like interfacial organization, with a structure that is dependent on the solvent chirality. The racemic form creates highly ordered layered structure that dictates local ionic concentrations, such that the effective surface potential becomes positive in a wide range of electrolyte concentrations. The enantiomerically pure form exhibits weaker ordering at the silica surface, which leads to a lower effective surface charge induced by ions partitioning into the layered structure. The surface charge in silicon nitride and polymer pores is probed through the direction of electroosmosis that the surface charges induce. Our findings add a new dimension to the nascent field of chiral electrochemistry, and emphasize the importance of including solvent molecules in descriptions of solid–liquid interfaces. [ABSTRACT FROM AUTHOR]

Nanopores in thin membranes play important roles in science and industry. Single nanopores have provided a step-change in portable DNA sequencing and understanding nanoscale transport while multipore membranes facilitate food processing and purification of water and medicine. Despite the unifying use of nanopores, the fields of single nanopores and multipore membranes differ – to varying degrees – in terms of materials, fabrication, analysis, and applications. Such a partial disconnect hinders scientific progress as important challenges are best resolved together. This Viewpoint suggests how synergistic crosstalk between the two fields can provide considerable mutual benefits in fundamental understanding and the development of advanced membranes. We first describe the main differences including the atomistic definition of single pores compared to the less defined conduits in multipore membranes. We then outline steps to improve communication between the two fields such as harmonizing measurements and modelling of transport and selectivity. The resulting insight is expected to improve the rational design of porous membranes. The Viewpoint concludes with an outlook of other developments that can be best achieved by collaboration across the two fields to advance the understanding of transport in nanopores and create next-generation porous membranes tailored for sensing, filtration, and other applications. [ABSTRACT FROM AUTHOR]

We present experimental approaches to probe the ionic conductivity of solid electrolytes at the meso- and nanoscales. Silica ionogel based electrolytes have emerged as an important class of solid electrolytes because they maintain both fluidic and high-conductivity states at the nanoscale, but at the macroscale they are basically solid. Single mesopores in polymer films are shown to serve as templates for cast ionogels. The ionic conductivity of the ionogels was probed by two experimental approaches. In the first approach, the single-pore/ionogel membranes were placed between two chambers of a conductivity cell, in a set-up similar to that used for investigating liquid electrolytes. The second approach involved depositing contacts directly onto the membrane and measuring conductivity without the bulk solution present. Ionic conductivity determined by the two methods was in excellent agreement with macroscopic measurements, which suggested that the electrochemical properties of ionogel based electrolytes are preserved at the mesoscale, and ionogels can be useful in designing meso-scaled energy-storage devices. [ABSTRACT FROM AUTHOR]

Hydrophobic nanopores provide a model system to study hydrophobic interactions at the nanoscale. Such nanopores could also function as a valve since they halt the transport of water and all dissolved species. It has recently been found that a hydrophobic pore can become wetted i.e. filled with condensed water or an aqueous solution of salt when a sufficiently high electric field is applied across the membrane. The wetting process is reversible thus when the voltage is lowered or switched off, the pore comes back to a closed state due to water evaporation in the pore. In this manuscript we present experimental studies on how the switching between conducting and non-conducting states can be regulated by the electrolyte concentration. Transport properties of single nanopores modified with alkyl chains of different lengths were recorded in salt concentrations between 10 mM and 1 M KCl. Nanopores modified with propyl chains exhibited gating in 10 mM KCl and were open for ionic transport for all voltages at higher salt concentrations. Nanopores modified with decyl chains did not conduct current in 10 mM and exhibited repeatable hydrophobic gating in 100 mM and 1 M KCl. The results are explained in the context of Maxwell stress in confined geometry with local surface charges, which change the shape of the water–vapor interface and promote wetting. [ABSTRACT FROM AUTHOR]

The ionic conductivity of porous MnO2 at the nanoscale is not well understood, despite possible importance in battery charging/discharging processes. It is demonstrated here that MnO2 in different oxidation states exhibits different ionic conductivities and surface charge characteristics, which are probed by reversal potential measurements. [ABSTRACT FROM AUTHOR]

Polymeric nanopores show unique transport properties and have attracted a great deal of scientific interest as a test system to study ionic and molecular transport at the nanoscale. By means of all-atom molecular dynamics, we simulated the ion dynamics inside polymeric polyethylene terephthalate nanopores. For this purpose, we established a protocol to assemble atomic models of polymeric material into which we sculpted a nanopore model with the key features of experimental devices, namely a conical geometry and a negative surface charge density. Molecular dynamics simulations of ion currents through the pore show that the protonation state of the carboxyl group of exposed residues have a considerable effect on ion selectivity, by affecting ionic densities and electrostatic potentials inside the nanopores. The role of high concentrations of Ca2+ions was investigated in detail. [ABSTRACT FROM AUTHOR]

Hydrophobic interactions and local dewetting of hydrophobic cavities have been identified as a key mechanism for ionic gating in biological voltage-gated channels in a cell membrane. Hydrophobic interactions are responsible for rectification of the channels, i.e. the ability to transport ions more efficiently in one direction compared to the other. We designed single polymer nanopores with a hydrophobic gate on one side in the form of a single layer of C10 or C18 thiols. This nanoporous system behaves like an ionic diode whose direction of rectification is regulated by the pH of the electrolyte. In addition, reversible dewetting of the hydrophobic region of the pore was observed as voltage-dependent ion current fluctuations in time between conducting and non-conducting states. The observations are in accordance with earlier molecular dynamics simulations, which predicted the possibility of spontaneous and reversible dewetting of hydrophobic pores.