Studies on the electrical double layer structure at the water/air interface

Effective dipole moments (calculated from experimental data of surface tension and electric surface potential) of some homologous normal alcohols and carboxylic acid were found to vary linearly with the number of carbon atoms in the hydrocarbon chain. Values of effective dipole moments were used for the determination of the effective dipole moments of water molecules , and the dielectric permittivity of the water subphase (₁), as well as in the vicinity of the hydrophobic part of adsorbed molecule (₂). The latter was found to decrease with the increase of the hydrocarbon chain length. Knowing the effective dipole moment of surface water dipoles, the average orientation angle () of water molecules at the inteface was estimated. The calculated potential drop of water varies within the range –0.038 to –2.38 V for two extreme orientations of water dipoles at the surface.     

Investigation of electrical properties of the electrical double layer on porous glasses

A series of Controlled Porosity Glasses was employed to investigate their electrical surface properties. Surface charge density and adsorption of electrolyte ions were examined, in order to link initial glass composition and physicochemical properties with its surface electrical double layer characteristics. Results prove the increase of surface boron concentration and consequently growing of negative charge and adsorption properties. Authors find it more complicated to explain characteristics of positive charge changes, which require further investigation.     

Surface-bound molecular rulers for probing the electrical double layer

Herein, we report the first experimental investigation on the effect of varying the position of redox-active moieties, within the electrical double layer, on the apparent formal potential and on the electron transfer rate constant. This was achieved using a rigid class of molecules, norbornylogous bridges, to place redox species (ferrocene) at a fixed position above the surface of the electrode. Cyclic voltammetry and alternating current voltammetry were used to calculate the apparent formal potential and the electron transfer rate constant for the electron transfer between the ferrocene and the gold electrode. We use the effect of electric field on the apparent formal potential measurement of the surface-bound redox species to calculate the potential drop from the initiation of the electrical double layer to different distances above it. It was found that self-assembled monolayers formed from ω-hydroxyalkanethiol have a potential profile very similar to that described by classical theories for bare metal electrodes. A steep drop in potential in the Stern layer was observed followed by a smaller potential drop in the Gouy-Chapman layer. The electron transfer rate constant was found to decrease as the distance between the ferrocene moiety and the initiation of the double layer is increased. Thus, the electron transfer rate constant appears to be dependent on ion concentration.     

Self-assembly in the electrical double layer of ionic liquids

We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation-anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.     

Geometrical factors in the capacity of the electrical double layer

A method for determination of double layer capacities at silver iodide-solution interfaces is presented. The influence of counterion on capacity is subjected to special study in the present work. Capacities at the silver iodide interface are compared with those at the mercury interface (Grahame) and interpreted in terms of theGouy-Stern double layer model. Some characteristic differences between capacities at mercury and silver iodide interfaces are explained in terms of different structural factors.     

Temperature effects on electrical double layer at solid-aqueous solution interface

Despite the significant influence of solution temperature on the structure of electrical double layer, the lack of theoretical model intercepts us to explain and predict the interesting experimental observations. In this work, we study the structure of electrical double layer as a function of thermochemical properties of the solution by proposing a phenomenological temperature dependent surface complexation model. We found that by introducing a buffer layer between the diffuse layer and stern layer, one can explain the sensitivity of zeta potential to temperature for different bulk ion concentrations. Calculation of the electrical conductance as function of thermochemical properties of solution reveals the electrical conductance not only is a function of bulk ion concentration and channel height but also the solution temperature. The present work model can provide deep understanding of micro- and nanofluidic devices functionality at different temperatures.     

Structure and dynamics of nanoscale electrical double layer

Electrical double layer (EDL) at substrate–solution interface plays essential roles in basic electrochemistry, energy conversion, desalination and separation, stochastic single-entity sensing, and other applications. The EDL structure generally refers to the inhomogeneous distribution of solution ions at the interfacial region. Dynamic changes in the EDL structure due to the transport of charges at the nanometer scale are the physicochemical origin of recently resolved novel nanotransport phenomena. High surface area materials and devices are potentially advantageous for better applications by providing more accessible interfaces. It is of high importance to emphasize that interfacial structures are indications of capacity, while the efficiency is often related to dynamics. This review discusses emerging transport phenomena under steady-state conditions and the transient deviations in prototype channel-type nanopores as unit elements for porous electrodes/membranes. The fundamental governing mechanism and structure–function correlations will be discussed in the context of energy harvesting and storage, desalination and phase transition, and resistive pulse sensing at the nanometer scale toward single-event/entity resolutions.     

The modelling of solvent structure in the electrical double layer

A Civilized Model electrolyte is one in which the ions and solvent molecules are regarded as distinct molecular species and treated on an equal basis. Recent efforts to use a Civilized Model to study the effects of solvent structure in the properties of the electrical double layer are discussed. By modelling the electrolyte as a simple ion-dipole mixture, it is possible to gain valuable insight in areas such as: 1) the successes and limitations of the Gouy-Chapman-Stern picture; 2) the derivation as opposed to a postulation of the Stern layer; 3) the influence of the charged surface on the magnitudes of the apparent Stern capacities (e.g. the “low” capacitances of the mercury/solution interface vs. the “high” capacitance of inorganic oxides); 4) the effect of solvent structure on the potential profile in the diffuse layer; 5) the interpretation of the electrokinetic potential; and 6) the role of solvent orientation on the x potential.     

Influence of electrolyte characteristics on the electrochemical parameters of electrical double layer capacitors

Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in propylene carbonate with the addition of different 1 M Me₃EtNBF₄, Me₂Et₂NBF₄, MeEt₃NBF₄, Et₄NBF₄, Et₃PrNBF₄ and Et₃BuNBF₄ electrolytes have been tested by cyclic voltammetry, chronoamperometry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, Ragone plots (energy density vs. power density dependencies) and other characteristics have been discussed. The influence of the electrolyte molar mass on the electrochemical characteristics of the nanoporous carbon electrode cells has been established. The applicability limits of the Srinivasan and Weidner model have been tested.     

Poisson-Boltzmann description of the electrical double layer including ion size effects

The electrical double layer is examined using a generalized Poisson-Boltzmann equation that takes into account the finite ion size by modeling the aqueous electrolyte solution as a suspension of polarizable insulating spheres in water. We find that this model greatly amplifies the steric effects predicted by the usual modified Poisson-Boltzmann equation, which imposes only a restriction on the ability of ions to approach one another. This amplification should allow for an interpretation of the experimental results using reasonable effective ionic radii (close to their well-known hydrated values).     

The determination of the free energy of an electrical double layer

A well-known charging process is used to obtain the free energy of an electrical double layer, in which the double layer is built up by a transfer of ions from one phase to another. The present study formulates proofs that this charging process cannot determine the free energy of an electrical double layer.     

Improved understanding of the effect of electrical double layer on pressure-driven flow in microchannels

Traditionally, the effects of electrical double layer on pressure-driven flow in microchannels were modeled by using the Poisson-Boltzmann equation and the fluid momentum equation with a flow-induced body force term. Such a model, however, usually underestimate the electrical double layer effects on the flow. In this study, a theoretical model of the electrical double layer field is developed to provide a better understanding of the electrical double layer effects. The electrical potential and ionic concentration distribution in dilute solutions in small microchannels are investigated by numerically solving this new model. This newly developed model predicted the deficit of counter-ions in the bulk liquid region due to the accumulation of counter-ions in the EDL region, and the surplus of co-ions in the bulk liquid region due to rejection of the co-ions in the EDL region. The presence of the net charges in the bulk liquid region is responsible for the strong electroviscous effects in dilute solutions in small microchannels.     

Contact conditions for the charge in the theory of the electrical double layer

In this paper, from the Born-Green-Yvon equations of the liquid-state theory, we derive a general expression for the charge-density contact value at charged interfaces. This relation is discussed, in particular, for symmetrical electrolytes. We emphasize an essential coupling between the electric properties and the density profile. Limiting behavior at small and large charges at the interface is discussed.     

The electrical double layer on gold probed by electrokinetic and surface force measurements

Gold surfaces, obtained by vacuum deposition of 15-nm gold films on glass and silica wafers, were studied in aqueous solutions by streaming potential measurements and colloidal-probe AFM force measurements. In the force measurements both a bare and a gold-coated silica particle (6 microm in diameter) have been used as colloidal probes. From the streaming potential measurements we determined the zeta-potential of the gold surface, while from the force measurements the diffuse double-layer potential psi(d) was obtained by fitting the data to the DLVO theory or to the nonlinear Poisson-Boltzmann equation. Measured interactions were found to be entirely due to overlap of electric double layers with no indication of attractive Van der Waals forces. Results of both types of measurements are in good agreement. The double layer potential strongly depends on the pH, probably as a result of the presence of oxide species on the gold surface. Insight in the double layer potential of polarizable interfaces such as the gold/electrolyte solution interface is the first step for understanding the effect of externally applied potentials on the adsorption behavior of charged species.