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

Abstract

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 Å at 0.2 M to 5.5 Å 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.

Erbium(III) Coordination at the Surface of an Aqueous Electrolyte

“…36 h of scan-upon-scan data collection by measurements and observations of the totally reflected beam intensity and position with a phosphor screen with a digital camera. Surface tension measurements made at the start and at the conclusion of the X-ray absorption experiments are identical to those reported beforehand; 35 the results are diagnostic of a pure and clean electrolyte−He interface.…”

“…34 The results showed an upper limit for the Er concentration at the interface that was consistent with a uniform concentration extending from the bulk to the surface. Complementary X-ray reflectivity experiments, which provided subnanometer spatial resolution along the direction perpendicular to the aqueous ErCl 3 electrolyte-vapor surface, showed a nonmonotonic variation of Er concentration in this direction, with a depletion layer at the surface followed by a subsurface enhancement layer, 35 a finding contrary to our initial assumption from a separations perspective but consistent with known negative adsorption behaviors of multivalent cations at an interface. 10,36−42 Both surface depletion and subsurface enrichment of Er 3+ lead to local variations in concentration that can induce different interion orderings in The Journal of Physical Chemistry B Article much the same manner as found in bulk electrolytes of different concentrations.…”

“…This result, while unexpected, fits together well with the nonmonotonic Er 3+ distribution and oscillatory intrinsic electron density profile, ρ int (z), into the same depth of the electrolyte. 35 Although the results presented here are for an aqueous−air interface, the system properties are comparable to aqueous−organic interfaces in that the dielectric constants of paraffinic organic phases (∼2) are essentially equivalent to that for air (∼1), and both are much smaller than those (∼78) for the aqueous phases.…”

Water-mediated ion–ion interactions are enhanced at the water vapor–liquid interface

“…The enhancement of ion density near the interface coupled with ion-induced quenching of capillary waves may lead to layering of ions in the direction perpendicular to the interface. Recent X-ray reflectivity experiments have shown layering of erbium chloride at the air-water interface (26).…”

Fenton chemistry at aqueous interfaces

“…The possibility that doubly charged Fe 2+ cations might be present in shallow interfacial layers is supported by molecular dynamics calculations, which predict that the concentration of doubly charged Mg 2+ peaks at ∼4 Å below the Gibbs dividing surface at values twice as large as its concentration in bulk water (83). Recent X-ray reflectivity studies confirmed the existence of nonmonotonic cation density profiles within ∼1-nm interfacial layers of aqueous electrolyte solutions (84). We cannot rule out the possibility that impinging gases are hydrated before colliding with the liquid surface (85, 86), but we deem it inconsequential because O 3 -(H 2 O) n and 6 2+ has a distorted octahedral geometry, on account of the broken symmetry, that lets O 3 approach the Fe 2+ center via low-energy associative interchange pathways (88).…”