X-ray reflectivity from liquid-liquid interface reveals the limitations of classical Gouy-Chapman theory

The distribution of ions in solution at an interface is key to the fundamental understanding of electrochemistry as well as to the design of materials and devices such as biomembranes. So far, classical descriptions of ion distributions, such as the Guoy-Chapman theory (see, G. Gouy, C. R. Acad. Sci. 149, 654 (1910) and D. L. Chapman, Phil. Mag. Ser. 6 25, 475 (1913)), which ignores the details of molecular structure, have been widely used. Professor M. Schlossman (University of Illinois at Chicago) and his colleagues recently performed very precise X-ray reflectivity measurements to obtain experimentally ion distributions at the interface between solutions (0.01 ~0.08M) of tetrabuytlammonium (TBA) tetraphenylborate (TPB) in nitrobenzene and aqueous TBA bromide. They found significant deviations from the Guoy-Chapman theory in describing their data. However, on the other hand, molecular dynamics calculations produced potentials that could be used to predict distributions with the Poisson-Boltzmann equation without adjustable parameters. The experiments were done at the Chemistry and Materials section of the Consortium for Advanced Radiation Sources (ChemMatCARS) beamline 15-ID at the Advanced Photon Source (APS, at Argonne National Laboratory). For more information, see the paper, "Ion Distributions near a Liquid-Liquid Interface", L. Guangming et al., Science, 311, 216-218 (2006).