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 angstrom at 0.2 M to 5.5 angstrom at 1.0 M and the damped, oscillatory, cation concentrations indicative of solute multilayering in the subsurface region. This nonmonotonic profile is consistent with an analysis of surface tension measurements by the Gibbs adsorption equation that predicts negative adsorption. Molecular dynamics simulations provide physical insight into the observed behavior, implicating the high charge on erbium for its nonmonotonic variation with depth. This work suggests that future studies employing higher-valent cations will enhance the understanding of liquid/vapor interfaces and their widespread importance in areas ranging from atmospheric chemistry to metal-ion separations.