Ion distributions at the nitrobenzene–water interface electrified by a common ion

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.