Beyond the electrical double layer model: ion-dependent effects in nanoscale solvent organization.

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.

Rectified and Salt Concentration Dependent Wetting of Hydrophobic Nanopores.

Nanopores lined with hydrophobic groups function as switches for water and all dissolved species, such that transport is allowed only when applying a sufficiently high transmembrane pressure difference or voltage. Here we show a hydrophobic nanopore system whose wetting and ability to transport water and ions is rectified and can be controlled with salt concentration. The nanopore we study contains a junction between a hydrophobic zone and a positively charged hydrophilic zone. The nanopore is closed for transport at low salt concentrations and exhibits finite current only when the concentration reaches a threshold value that is dependent on the pore opening diameter, voltage polarity and magnitude, and type of electrolyte. The smallest nanopore studied here had a 4 nm diameter and did not open for transport in any concentration of KCl or KI examined. A 12 nm nanopore was closed for all KCl solutions but conducted current in KI at concentrations above 100 mM for negative voltages and opened for both voltage polarities at 500 mM KI. Nanopores with a hydrophobic/hydrophilic junction can thus function as diodes, such that one can identify a range of salt concentrations where the pores transport water and ions for only one voltage polarity. Molecular dynamics simulations together with continuum models provided a multiscale explanation of the observed phenomena and linked the salt concentration dependence of wetting with an electrowetting model. Results presented are crucial for designing next-generation chemical and ionic separation devices as well as understanding fundamental properties of hydrophobic interfaces under nanoconfinement.

Charge Inversion and Calcium Gating in Mixtures of Ions in Nanopores.

Calcium ions play important roles in many physiological processes, yet their concentration is much lower than the concentrations of potassium and sodium ions. The selectivity of calcium channels is often probed in mixtures of calcium and a monovalent salt, e.g., KCl or NaCl, prepared such that the concentration of cations is kept constant with the mole fraction of calcium varying from 0 and 1. In biological channels, even sub-mM concentration of calcium can modulate the channels' transport characteristics; this effect is often explained via the existence of high affinity Ca2+ binding sites on the channel walls. Inspired by properties of biological calcium-selective channels, we prepared a set of nanopores with tunable opening diameters that exhibited a similar response to the presence of calcium ions as biochannels. Nanopores in 15 nm thick silicon nitride films were drilled using focused ion beam and e-beam in a transmission electron microscope and subsequently rendered negatively charged through silanization. We found that nanopores with diameters smaller than 20 nm were blocked by calcium ions such that the ion currents in mixtures of KCl and CaCl2 and in CaCl2 were even ten times smaller than the ion currents in KCl solution. The ion current blockage was explained by the effect of local charge inversion where accumulated calcium ions switch the effective surface charge from negative to positive. The modulation of surface charge with calcium leads to concentration and voltage dependent local charge density and ion current. The combined experimental and modeling results provide a link between calcium ion-induced changes in surface charge properties and resulting ionic transport.

Gating of Hydrophobic Nanopores with Large Anions.

Understanding ion transport in nanoporous materials is critical to a wide variety of energy and environmental technologies, ranging from ion-selective membranes, drug delivery, and biosensing, to ion batteries and supercapacitors. While nanoscale transport is often described by continuum models that rely on a point charge description for ions and a homogeneous dielectric medium for the solvent, here, we show that transport of aqueous solutions at a hydrophobic interface can be highly dependent on the size and hydration strength of the solvated ions. Specifically, measurements of ion current through single silicon nitride nanopores that contain a hydrophobic-hydrophilic junction show that transport properties are dependent not only on applied voltage but also on the type of anion. We find that in Cl--containing solutions the nanopores only conducted ionic current above a negative voltage threshold. On the other hand, introduction of large polarizable anions, such as Br- and I-, facilitated the pore wetting, making the pore conductive at all examined voltages. Molecular dynamics simulations revealed that the large anions, Br- and I-, have a weaker solvation shell compared to that of Cl- and consequently were prone to migrate from the aqueous solution to the hydrophobic surface, leading to the anion accumulation responsible for pore wetting. The results are essential for designing nanoporous systems that are selective to ions of the same charge, for realization of ion-induced wetting in hydrophobic pores, as well as for a fundamental understanding on the role of ion hydration shell on the properties of solid/liquid interfaces.

Modulation of Charge Density and Charge Polarity of Nanopore Wall by Salt Gradient and Voltage.

Surface charge plays a very important role in biological processes including ionic and molecular transport across a cell membrane. Placement of charges and charge patterns on walls of polymer and solid-state nanopores allowed preparation of ion-selective systems as well as ionic diodes and transistors to be applied in building biological sensors and ionic circuits. In this article, we show that the surface charge of a 10 nm diameter silicon nitride nanopore placed in contact with a salt gradient is not a constant value, but rather it depends on applied voltage and magnitude of the salt gradient. We found that even when a nanopore was in contact with solutions of pH equivalent to the isoelectric point of the pore surface, the pore walls became charged with voltage-dependent charge density. Implications of the charge gating for detection of proteins passing through a nanopore were considered, as well. Experiments performed with single 30 nm long silicon nitride nanopores were described by continuum modeling, which took into account the surface reactions on the nanopore walls and local modulation of the solution pH in the pore and at the pore entrances. The results revealed that manipulation of surface charge can occur without changing pH of the background electrolyte, which is especially important for applications where maintaining pH at a constant and physiological level is necessary. The system presented also offers a possibility to modulate polarity and magnitude of surface charges in a two-electrode setup, which previously was accomplished in more complex multielectrode systems.