Ion Correlations at an Electrified Liquid/Liquid Interface

Ion correlations have been suggested as the underlying mechanism of a number of counterintuitive phenomena such as like-charge attraction. Here we present the first molecular-level tests of density profiles predicted by an ion correlations model. Synchrotron x-ray reflectivity reveals ion condensation at the liquid/liquid interface, when polarized with an electric field.\footnote{N. Laanait et al. J. Chem. Phys., \textbf{132}, 171101, 2010} Tuning the density of this ionic layer allows for a detailed study of ion correlations as a function of the Coulomb coupling strength in the system. We propose a parameter-free density functional theory that describes ion-ion correlations within a weighted density approximation and explicitly treats ion-solvent effects through a solvent interaction potential simulated by molecular dynamics. Agreement with the x-ray reflectivity and the interfacial excess charge is found over the entire experimental range of ion-ion correlation energies up to nearly 4 $k_{B}T$. These results suggest that ion correlations in the electrical double layer can be accurately described by mapping to a simple Coulomb system, in this case a one-component plasma.