Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li ⁺ revealed by experiments and simulations

Abstract

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K + and Li + ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li + adsorbs to the aqueous solution−vapor interface, while K + does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li + predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K + and Li + ions to a difference in the resilience of their hydration shells.ion adsorption | air−water interface | specific ion effects | Hofmeister series | aqueous ionic solvation M yriad chemical and biochemical processes that occur in aqueous salt solutions exhibit trends that depend systematically on the identities of the salt ions. These trends, which are commonly referred to as specific ion effects, generally follow the Hofmeister series, a ranking of the ability of salt ions to precipitate proteins that was developed by Franz Hofmeister (1) in the late 1800s. The Hofmeister series applies, however, to a wide range of other seemingly unrelated phenomena, such as colloidal stability, critical micelle concentrations, chromatographic selectivity, protein denaturation temperatures, and the interfacial properties of aqueous salt solutions (2, 3). Early attempts to explain the Hofmeister series relied on the notion that salt ions have a long-range effect on the structure of water, with ions on one side of the series acting as "structure makers" and ions on the other side as "structure breakers" (2, 4). However, more recently, several experimental and computational studies have questioned the role of long-range ordering/disordering effects (4-9), and have provided compelling evidence that ion-specific behavior at aqueous interfaces must be taken into consideration when attempting to explain Hofmeister effects (7, 10-13).Specific anion effects on the interfacial properties of aqueous salt solutions, such as surface tensions and surface potentials, closely follow the Hofmeister series for anions (14). For example, surface tension increments (STIs; differences between the surface tension of a salt solution and that of neat water) of sodium salts at the same concentration decrease in the order: SO 4 2− > Cl − > Br − > NO 3 − > I − (15, 16). Molecular dynamics (MD) simulations have predicted that the propensity of anions to adsorb to the solution-vapor interface follows the Hofmeister series in reverse (7,14,17), and this prediction has largely been confirmed experimentally (14,(18)(19)(20)(21)(22). Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2...

Image-charge effects on ion adsorption near aqueous interfaces

“…The ICI depends only on the dielectric constants of the adjacent media and the charge of ions and was invoked in the pioneering work by Wagner (11) and Onsager and Samaras (12) to explain the increase of water surface tension with salt concentration. However, simulation (8, 13-28) and experimen-tal (9,(29)(30)(31)(32)(33)(34)(35)(36)(37)(38) evidence indicates that the ion distribution in the interfacial region is strongly ion-specific, with larger and more polarizable ions tending to accumulate near the air/water interface. The ion-specific adsorption at the air/water interface has been attributed to a number of factors including cavity formation (15,24), ionic polarizability (14,39,40), capillary fluctuations (41)(42)(43), and electrochemical surface potential (44), but the quantitative extent of these effects remains a matter of discussion and disagreement (10,44,45).…”

Electrification of water interface

“…X-ray is a powerful analytical tool to explore the electron structures of materials. X-ray photoelectron spectroscopy (XPS) can measure the surface potential by detecting the shift in the binding energy of the surface's specific electronic orbitals from the isoelectric point [132][133][134][135][136][137]. X-ray absorption fine structure (XAFS) can detect surface-active heavy ions such as bromide ions and transition metal cations [138][139][140].…”

“…11b [113]. The potential of the mean force of Li + , however, shows anomalously weak repulsion [113,134], even though lithium ions are the smallest ions among the alkali metal ions. This fact is related to the large Stokes radius of lithium ions due to the strong bonding of hydration water molecules [113].…”

Uranyl Speciation in the Presence of Specific Ion Gradients at the Electrolyte/Organic Interface

“…These data are consistent with recent X-ray photo-electron spectroscopy interpretations of the prevalence of Li + in lithium iodide solutions at the electrolyte/vapor interface. 78 Prior work has demonstrated that Li + sheds solvating H 2 O within the instantaneous surface, which disfavors residence therein. 61 When combined with observation of relatively consistent NO − 3 concentration in the instantaneous surface and subjacent layers, the negative charge density in the region directly contacting the organic phase is presumed to be an outcome of Li + cation depletion in the instantaneous surface rather than anionic excess.…”