Ion Distributions at the Nitrobenzene-Water Interface Electrified by a Common Ion

TL;DR

This study uses synchrotron x-ray reflectivity to investigate ion distributions at a nitrobenzene-water interface, revealing discrepancies with traditional theories and supporting models that include detailed free energy profiles.

Contribution

It demonstrates that ion distributions at the liquid-liquid interface are better described by Poisson-Boltzmann models incorporating explicit free energy profiles, challenging existing theories.

Findings

Standard models like Gouy-Chapman do not match measurements.

Poisson-Boltzmann with free energy profiles aligns well with data.

Interfacial liquid structure significantly influences ion distributions.

Abstract

Synchrotron x-ray reflectivity is used to study ion distributions at the liquid-liquid interface between a nitrobenzene solution of tetrabutylammonium tetraphenylborate (TBATPB) and a water solution of tetrabutylammonium bromide (TBABr). The concentration of TBABr is varied to alter the ion distribution. Our x-ray measurements are inconsistent with several commonly used theories of ion distributions, including Gouy-Chapman, modified Verwey-Niessen, and the MPB5 version of the Poisson-Boltzmann equation. These structural measurements are described well by ion distributions predicted by a version of the Poisson-Boltzmann equation that explicitly includes a free energy profile for ion transfer across the interface when this profile is described by a simple analytic form or by a potential of mean force from molecular dynamics simulations. These x-ray measurements from the liquid-liquid interface provide evidence for the importance of interfacial liquid structure in determining interfacial ion distributions.