Electrokinetic Phenomena in Organic Solvents

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.

Gating ion and fluid transport with chiral solvent

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.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Enhanced Ionic Current Rectification through Innovative Integration of Polyelectrolyte Bilayers and Charged-Wall Smart Nanochannels

The tools utilized by humans continue to shrink and speed up. Lab-on-a-chip (LOC) is one of the most recent techniques for decreasing the size of chemical systems. Today, LOCs have made substantial strides in developing nanomaterial fabrication techniques. Controlling and regulating the fluid and ion mobility in these systems is crucial. Layer-by-layer (LBL) soft layers are one of the most effective strategies for controlling fluid flow in channels. In light of the present constraints for developing these systems and the high expense of experimental investigations, it is vital to employ modeling to minimize costs and comprehend their underlying ideas and operations. In this study, we examined the influence of the LBL soft layer's presence in the charged nanochannels on the ion transport parameters. To examine the effect of the coating length of the LBL soft layer, we first examined three lengths of coating: one with a length greater than half (type (I)), one with a length equal to half (type (II)), and one with a length less than half (type (III)) of the nanochannel length. Then, by solving Poisson-Nernst-Planck and Navier-Stokes equations, we determined the influences of pH, soft layer charge density (N_PEL/N_A), bulk concentration (C₀), and hard surface charge density (σ) on the ionic current rectification (R_f) and selectivity (S) of the nanochannel. The maximum rectification of 30.65 was achieved using a nanochannel of type (III) and σ = +10 mC/m². The current results demonstrate a promising hybrid architecture consisting of an LBL soft layer and a smart charged nanochannel for enhanced rectification.

Electrodiffusioosmosis induced negative differential resistance in micro-to-millimeter size pores through a graphene/copper membrane

Negative differential resistance (NDR) is one of the nonlinear transport phenomena in which ionic current decreases with the increase in electromotive potential. Electro-osmosis, diffusio-osmosis, and surface charge density of pores are the driving forces for observing NDR in nanoscale ion transport. Here, we report electrodiffusioosmosis induced NDR using micro to millimeter size pores in a two-dimensional (2D) graphene-coated copper (Gr/Cu) membrane. Along with NDR, we also observed ion current rectification (ICR), in which there is preferential one-directional ion flow for equal and opposite potentials. The experimentally observed NDR effect has been validated by performing ion transport simulations using Poisson-Nernst-Planck (PNP) equations and Navier-Stokes equations with the help of COMSOL Multiphysics considering salinity gradient across the membrane. Charge polarization induced electro-osmotic flow (EOF) dominates over diffusio-osmosis, causing the backflow of low concentration/conductivity solution into the pore, thereby causing NDR. This finding paves the way toward potential applications in ionic tunnel diodes as rectifiers, switches, amplifiers, and biosensors.

Design and development of an automated experimental setup for ion transport measurements

The ion transport measurements using various ion-exchange membranes (IEMs) face several challenges, including controllability, reproducibility, reliability, and accuracy. This is due to the manual filling of the solutions in two different reservoirs in a typical diffusion cell experiment with a random flow rate, which results in the diffusion through the IEM even before turning on the data acquisition system as reported so far. Here, we report the design and development of an automated experimental setup for ion transport measurements using IEMs. The experimental setup has been calibrated and validated by performing ion transport measurements using a standard nanoporous polycarbonate membrane. We hope that the present work will provide a standard tool for realizing reliable ion transport measurements using ion-exchange membranes and can be extended to study other membranes of various pore densities, shapes, and sizes.

Enhanced electro-osmosis in propylene carbonate salt solutions

Properties of solid-liquid interfaces and surface charge characteristics mediate ionic and molecular transport through porous systems, affecting many processes such as separations. Herein, we report experiments designed to probe the electrochemical properties of solid-liquid interfaces using a model system of a single polyethylene terephthalate (PET) pore in contact with aqueous and propylene carbonate solutions of LiClO₄. First, the existence and polarity of surface charges were inferred from current-voltage curves recorded when a pore was placed in contact with a LiClO₄ concentration gradient. Second, the electro-osmotic transport of uncharged polystyrene particles through the PET pore provided information on the polarity and the magnitude of the pore walls' zeta potential. Our experiments show that the PET pores become effectively positively charged when in contact with LiClO₄ solutions in propylene carbonate, even though in aqueous LiClO₄, the same pores are negatively charged. Additionally, the electro-osmotic velocity of the particles revealed a significantly higher magnitude of the positive zeta potential of the pores in propylene carbonate compared to the magnitude of the negative zeta potential in water. The presented methods of probing the properties of solid-liquid interfaces are expected to be applicable to a wide variety of solid and liquid systems.

Surface Potential/Charge Sensing Techniques and Applications

Surface potential and surface charge sensing techniques have attracted a wide range of research interest in recent decades. With the development and optimization of detection technologies, especially nanosensors, new mechanisms and techniques are emerging. This review discusses various surface potential sensing techniques, including Kelvin probe force microscopy and chemical field-effect transistor sensors for surface potential sensing, nanopore sensors for surface charge sensing, zeta potentiometer and optical detection technologies for zeta potential detection, for applications in material property, metal ion and molecule studies. The mechanisms and optimization methods for each method are discussed and summarized, with the aim of providing a comprehensive overview of different techniques and experimental guidance for applications in surface potential-based detection.

Electrodiffusioosmosis-Induced Negative Differential Resistance in pH-Regulated Mesopores Containing Purely Monovalent Solutions

Negative differential resistance (NDR) refers to a unique electrical property where current decreases with increasing voltage. Herein, we report experimental evidence showing that the NDR effect can be observed in mesopores that feature charged pore walls and are subjected to a KCl concentration gradient. NDR in our system originates from the solution and ion flows driven by the synergistic effects of electroosmosis [electroosmotic flow (EOF)] and diffusioosmosis, the so-called electrodiffusioosmosis. Experiments reveal that in addition to the ion current rectification, the mesopores considered here exhibit the NDR phenomenon that is dependent on the magnitude and direction of the salinity gradient and on pH. The NDR behavior can be observed only at conditions at which the EOF and diffusioosmosis occur in the opposite directions: diffusioosmosis fills the tip opening with a high concentration solution, while EOF brings a low concentration solution to the pore. All experimental findings are supported by our numerical model, which takes into account the interfacial site reactions of acidic and basic functional groups on the entire pore membrane surfaces. Our results provide an important insight into how liquid pH, salinity gradients, interfacial site reactions, and pore geometries can influence the current-voltage characteristics of mesopores, enriching transport modes that can be induced by voltage.