X-ray Reflectivity Reveals a Nonmonotonic Ion-Density Profile Perpendicular to the Surface of ErCl3 Aqueous Solutions

Complex interactions that determine ionic ordering in the bulk of electrolyte solutions are modified by surface-region inhomogeneities. We present results from an investigation of surface-ionic profiles that provide insights into the underlying physical chemistry in this region. X-ray reflectivity measurements from the liquid surfaces of aqueous ErCl3 solutions reveal in unprecedented detail a nonmonotonic electron density profile, which is interpreted in terms of a nonmonotonic surface distribution of cations (Er3+) and their relationship to the bulk. The combination of a heavy, multivalent Er3+ and a lighter, monovalent anion (Cl–) results in a significant cation depletion layer at the surface followed by a subsurface region of notably enhanced Er3+. Studying a series of solutions as a function of solute concentration reveals marked changes in Er3+ distribution, the most notable of which are the depletion layer thickness variation from 7.8 A at 0.2 M to 5.5 A at 1.0 M and the damped, oscillatory, cation...