Multiionic and Permittivity-Related Effects on the Diffuse Electric Double Layer Structure at Solid-Electrolyte Solution Interfaces

J J López-García,J Horno,C Grosse

doi:10.1155/2018/4316894

Abstract

The structure and differential capacitance of the diffuse part of the electric double layer at solid-electrolyte solution interfaces are examined using a theoretical model that takes into account the finite ion size by modeling the solution as a suspension of polarizable insulating spheres in water. This formalism is applied to binary and mixed electrolyte solutions using the “Boublik–Mansoori–Carnahan–Starling–Leland” (BMCSL) theory for the steric interactions among ions. It is shown that the ionic size differences have a strong bearing on the diffuse part of the electric double layer structure, as well as on the differential capacitance dependence on the surface potential for mixed electrolytes.

Highlights

In order to compare the Carnahan– Starling and BMCSL theory predictions, we first consider the simplest case of binary electrolyte solutions
Even when steric interactions are independent of the counterion size according to CS, the repelling dielectrophoretic force is stronger for Na+ than for K+, leading to a concentration behavior that is qualitatively similar to that predicted by BMCSL
We present a theoretical account based on the Poisson-Boltzmann equation of the influence of the finite ionic size on the diffuse double layer properties of binary and mixed electrolyte solutions. is is done combining for the first time the “Boublik–Mansoori–Carnahan–Starling–Leland” (BMCSL) theory for the steric interactions among ions with all the effects that result from the representation of ions as dielectric spheres. ese include the dependence of the solution permittivity on the local ionic concentration, calculated here by means of the Maxwell mixture formula, and the appearance of two additional forces acting on the ions, namely, the Born and the dielectrophoretic forces that depend on the permittivity and the electric field gradients, respectively

Summary

Introduction

It is well-known that when solid objects are placed in contact with an aqueous electrolyte solution, they usually acquire a surface charge and spatial distributions of charge and electric potential, known as the electric double layer (EDL), appear close to the interfaces. is structure plays a crucial role in materials science, electrochemistry, colloid and polymer science, biophysics, and the study of nanofluidic devices [1,2,3]. Most of the EDL interpretations that followed continued using the original Stern idea, i.e., to divide the EDL into two parts: an inner part in contact with the solid surface and a diffuse electric double layer (DEDL) In most of these studies, it was further assumed that the remaining discrepancies with experimental results originated from an overly simplified model of the inner layer, which has been corrected including specific ionic adsorption, solvent reduction, strong metal-solvent interactions, etc. Most studies only consider binary electrolyte solutions, in which the DEDL is basically composed of a single ionic species (the counterion) [26] so that the Bikerman and Carnahan–Starling expressions can be used to calculate ionion interactions. Carnahan–Starling, bringing the theoretical results closer to those experimentally observed

Theory

Results

Conclusions