Water distribution at the electrified interface of deep eutectic solvents.

Deep eutectic solvents (DESs) are a new class of solvents with wider potential window than that of water and high electrochemical stability, making them potential candidates for a wide range of electrochemical systems. However, due to the hygroscopic nature of DESs, the presence of latent water is unavoidable. Therefore, understanding the interfacial structure and the electrosorption and distribution of residual water at the electrified interface is of great importance for the use of these solvents in electrochemical systems. Using atomistic molecular dynamics, we explore the electrosorption and distribution of different amounts of water in 1 : 2 choline chloride-urea DES (Reline) at the electrified graphene interface. We found that both the water distribution and the interfacial structure are sensitive to the electrification of the graphene electrode. As a result, it is found that for moderately charged electrodes, water shows a preferential asymmetric adsorption in the vicinity of the positively charged electrode, partly due to strong intermolecular interactions with anions through hydrogen bonds. In contrast, for highly charged electrodes, water adsorbs at both electrodes due to a strongly enhanced external electrostatic interaction between the electrodes and the water dipoles.

Correction: Water distribution at the electrified interface of deep eutectic solvents.

[This corrects the article DOI: 10.1039/C9NA00331B.].

Atomistic Insight into the Electrochemical Double Layer of Choline Chloride-Urea Deep Eutectic Solvents: Clustered Interfacial Structuring.

Green, stable, and wide electrochemical window deep eutectic solvents (DESs) are ideal candidates for electrochemical systems. However, despite several studies of their bulk properties, their structure and properties under electrified confinement have barely been investigated, which has hindered widespread use of these solvents in electrochemical applications. In this Letter, we explore the electrical double layer structure of 1:2 choline chloride-urea (Reline), with a particular focus on the electrosorption of the hydrogen bond donor on a graphene electrode using atomistic molecular dynamics simulations. We discovered that the interface is composed of a mixed layer of urea and counterions followed by a mixed charged clustered structure of all of the Reline components. This interfacial structuring is strongly dependent on the balance between intermolecular interactions and surface polarization. These results provide new insights into the electrical double layer structure of a new generation of electrolytes whose interfacial structure can be tuned at the molecular level.

Numerical insights into the early stages of nanoscale electrodeposition: nanocluster surface diffusion and aggregative growth.

Fundamental understanding of the early stages of electrodeposition at the nanoscale is key to address the challenges in a wide range of applications. Despite having been studied for decades, a comprehensive understanding of the whole process is still out of reach. In this work, we introduce a novel modelling approach that couples a finite element method (FEM) with a random walk algorithm, to study the early stages of nanocluster formation, aggregation and growth, during electrochemical deposition. This approach takes into account not only electrochemical kinetics and transport of active species, but also the surface diffusion and aggregation of adatoms and small nanoclusters. The simulation results reveal that the relative surface mobility of the nanoclusters compared to that of the adatoms plays a crucial role in the early growth stages. The number of clusters, their size and their size dispersion are influenced more significantly by nanocluster mobility than by the applied overpotential itself. Increasing the overpotential results in shorter induction times and leads to aggregation prevalence at shorter times. A higher mobility results in longer induction times, a delayed transition from nucleation to aggregation prevalence, and as a consequence, a larger surface coverage of smaller clusters with a smaller size dispersion. As a consequence, it is shown that a classical first-order nucleation kinetics equation cannot describe the evolution of the number of clusters with time, N(t), in potentiostatic electrodeposition. Instead, a more accurate representation of N(t) is provided. We show that an evaluation of N(t), which neglects the effect of nanocluster mobility and aggregation, can induce errors of several orders of magnitude in the determination of nucleation rate constants. These findings are extremely important towards evaluating the elementary electrodeposition processes, considering not only adatoms, but also nanoclusters as building blocks.

A numerical study of the assumptions underlying the calculation of the stationary zone mass transfer coefficient in the general plate height model of chromatography in two-dimensional pillar arrays.

The present study investigates the validity of one of the key assumptions underlying the general plate height model of chromatography, i.e., the presumed independency of the individual band broadening contributions. More precisely, it is investigated under which conditions the mass transfer inside the stationary zone (e.g., porous pillars) is independent from the axial transport of species outside this zone, and how strongly any such dependency would affect the validity of the general plate height model of chromatography. For this purpose, detailed calculations of the species concentration distribution inside and outside the porous pillars of a computer-mimic of a porous pillar array column have been made. These simulations revealed a clear interplay between the mass transfer inside and outside the pillars, manifesting itself as an asymmetry of the species concentration distribution inside the pillars. The latter is in disagreement with the basic assumption used to calculate the value of the C(s)-term of the general plate height model. The asymmetry-effect is largest at low reduced velocities, high retention factors and high intra-pillar diffusion coefficients. Fortunately, these are conditions where the C(s)-term is relatively small, which might explain why the general plate height model of chromatography (and based on the symmetry assumption) can represent the band broadening in a porous pillar array within an accuracy on the order of some 1-2%.

Modelling the relation between the species retention factor and the C-term band broadening in pressure-driven and electrically driven flows through perfectly ordered 2-D chromatographic media.

The band broadening that can be expected in perfectly ordered cylindrical pillar arrays has been calculated for a wide range of intra-particle diffusion coefficients (D(sz)) and zone retention factors (0

Relaxation effect on the Onsager coefficients of mixed strong electrolytes in the mean spherical approximation.

The Fuoss-Onsager continuity equations are solved by using the equilibrium pair distribution functions of the mean spherical approximation in the case of equal diameters. An analytical expression is obtained for the relaxation effect on the Onsager coefficients of mixed strong electrolytes. This work also extends the existing expressions for the conductivity of binary and ternary electrolytes to any number of ions.

Influence of ion properties on the equilibrium and transport properties of electrolyte solutions.

Strong electrolytes are described in the framework of the primitive model in which the solvent is regarded as a dielectric continuum, using the mean spherical approximation. The analytical solution of the equilibrium and transport properties is dependent on the ions' diameters and valencies. For hydrated or nonspherical ions, an effective diameter must be fitted. A sensitivity study of the osmotic coefficient and the transport coefficients is performed on theoretical 1-1, 2-1, and 3-1 electrolytes, up to a total ion concentration of 2 mol/L.

Numerical investigation of transient current density distributions for multi-ion electrolytes at a rotating disk electrode.

A versatile model for the simulation of transient multiion transport and reaction processes is applied to investigate current density distributions over a rotating disk electrode for linear voltammetric sweep experiments. The model accounts for ion transport by convection, diffusion, and migration, in combination with Butler-Volmer type electrode reactions. For several process conditions (reversible and irreversible reactions, excess or lack of supporting electrolyte), the current density distribution over the disk surface is examined and the transient current response is compared to results from the more commonly used one-dimensional axial approach. The impact of migrational effects on the nonuniform local process conditions over the disk surface is illustrated, and the resulting effect on the current peak height, width, and position is investigated. A mathematical correlation for the current peak height as a function of the reacting ion transference number is established.

Theoretical comparison of the band broadening in nonretained electrically and pressure-driven flows through an ordered chromatographic pillar packing.

Using a well-validated computational fluid dynamics simulation method, based on a multi-ion transport model, a detailed analysis of the differences in band broadening between pressure-driven (PD) and electrically driven (ED) flows through perfectly ordered, identical chromatographic pillar packings has been made. It was found that, although the eddy-diffusion band-broadening contributions were nearly completely absent in the considered structure, the ED flow still yields much smaller plate heights than the PD flow. This difference could be fully attributed to the different ways in which the ED and PD velocity profiles reshape when passing through a tortuous pore structure with undulating cross section. Whereas in the PD case the parabolic tip of the band front is continually squeezed and extended each time it passes a pore constriction, the ED flow displays some kind of band front restoring mechanism, with which the fluid elements of the band front are (at least partly) laterally re-aligned after each pore constriction passage. This could be clearly visualized from a series of step-by-step images of the progression of a sharply "injected" species band moving through the packing under ED and PD conditions.

Numerical solution of a multi-ion one-potential model for electroosmotic flow in two-dimensional rectangular microchannels.

A new more general numerical model for the simulation of electrokinetic flow in rectangular microchannels is presented. The model is based on the dilute solution model and the Navier-Stokes equations and has been implemented in a finite-element-based C++ code. The model includes the ion distribution in the Helmholtz double layer and considers only one single electrical' potential field variable throughout the domain. On a charged surface(s) the surface charge density, which is proportional to the local electrical field, is imposed. The zeta potential results, then, from this boundary condition and depends on concentrations, temperature, ion valence, molecular diffusion coefficients, and geometric conditions. Validation cases show that the model predicts accurately known analytical results, also for geometries having dimensions comparable to the Debye length. As a final study, the electro-osmotic flow in a controlled cross channel is investigated.