Understanding the anatomy of capacitance at interfaces between two immiscible electrolytic solutions

How to polarise an interface with ions: the discrete Helmholtz model

The distribution of electrolytes in an electric field usually relies on theories based on the Poisson-Boltzmann formalism. These models predict that, in the case of a metallic electrode, ionic charges screen the electrode potential, leading to concentration-dependent ion distributions. This theoretical framework was first applied at solid-liquid interfaces and then transposed to soft interfaces. However, in this latter case, the potential in which the electrolytes evolve is not homogeneous, which is less amenable to a mean-field description. In this report, we show that at polarised soft interfaces the potential difference takes place between two closely interacting ionic monolayers. In this configuration, ions of opposite charges directly neutralise each other leading to an absence of diffuse layers and charge screening by surrounding ions. Thus, independently of the electrolyte concentrations, the surface charge density is a linear function of the potential difference, which results in a constant capacitance.

Electric-double-layer potential distribution in multiple-layer immiscible electrolytes

In this Brief Report, we calculate the electric-double-layer (EDL) electrostatic potential in a system of several layers of immiscible electrolytes. Verwey-Niessen theory predicts that at the interface between two immiscible electrolytes back-to-back EDLs are formed. The present analysis extends this idea to the case where the immiscible liquids are contained inside a domain with given electrostatic potentials at its boundaries, where the thickness of the individual liquid layer can be comparable to the EDL thickness. Such a system gives rise to a situation where the overall EDL electrostatic potential in the system is dictated by the competitive influences of the boundary-induced effects and the effects induced by the jump in the ion-solvent interaction potential at the liquid-liquid interfaces. Invoking Debye-Hückel linearization, we derive an analytical result for the EDL electrostatic potential for two immiscible electrolyte layers, and extend it for a general system of N such immiscible electrolyte layers. We demonstrate that, depending upon the nature of the interfacial ion-solvent interaction potential jump, the overall EDL potential may manifest a strong influence of the boundary conditions or may invert the influence of the boundary conditions. Effects such as a variation of the ratio of the permittivity or the thickness of the liquids also dictate the overall potential profiles.

Giant Stark effect in quantum dots at liquid/liquid interfaces: a new option for tunable optical filters

Control of the fundamental absorption edge of a quantum dot with an applied electric field has been limited by the breakdown fields of the solid-state material surrounding the dot. However, much larger fields can be applied at the interface of two immiscible electrolytic solutions (ITIES) in an electrochemical cell. These electric fields also localize the quantum dots at the ITIES. Our analysis shows that semiconductor nanocrystals localized at the ITIES should have electric-field-tunable optical properties across much of the visible spectrum. The transparency of the liquids in such cells indicates that this configuration would be well suited for electrically tunable optical filters with wide-angle acceptance.

Electrowetting with electrolytes

A theory of electrowetting is developed for systems containing an interface between two immiscible electrolytic solutions. Laws for the dependence of contact angle on electrode potential are presented. Ionic impermeability of the liquid-liquid interface and nonlinear double-layer responses rationalize observed phenomena such as contact-angle saturation and droplet contraction or detachment. The theoretical results can be applied to design new, precisely controllable microfluidic devices.