Mean field theories of ion distributions, such as the Gouy-Chapman theory that describes the distribution near a charged planar surface, ignore the molecular-scale structure in the liquid solution. The predictions of the Gouy-Chapman theory vary substantially from our x-ray reflectivity measurements of the interface between two electrolyte solutions. Molecular dynamics simulations, which include the liquid structure, were used to calculate the potential of mean force on a single ion. We used this potential of mean force in a generalized Poisson-Boltzmann equation to predict the full ion distributions. These distributions agree with our measurements without any adjustable parameters.

Ion distributions play a central role in various settings—from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength of ion–ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameter-free density functional theory that includes ion–ion correlations and ion–solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.

An overview on electrochemistry on the interfaces between two immiscible electrolyte solutions is given.

The properties of polyaniline (PANI) films, grown electrochemically on Au using the potential sweep method to two different upper potential limits (type I and II) to various thicknesses in sulfuric acid solution, were studied systematically using cyclic voltammetry, ac impedance, and for the first time, by tracking the rate of the hydrogen evolution reaction (HER). The HER results show that both films follow the nucleation and growth mechanism initially and that continuing growth occurs primarily at the outer PANI surface. Because type II PANI films are formed at a much more rapid rate than type I films, a greater amount of anodic degradation products are incorporated into type II films. Impedance and HER measurements show that type II films, compared for the first time in this work with type I films, have a more open PANI film/Au interfacial structure. A new observation is that the cyclic voltammograms for type I films show a prepeak which is absent for type II films. Furthermore, constant PANI redox peak potentials are observed for type I films, whereas these shift with film thickness for type II films. This unique observation implies that type II films undergo structural changes during their growth, while type I films remain more uniform. © 1999 The Electrochemical Society. All rights reserved.