On the fluctuations that drive small ions toward, and away from, interfaces between polar liquids and their vapors

Abstract

Contrary to the expectations from classic theories of ion solvation, spectroscopy and computer simulations of the liquid-vapor interface of aqueous electrolyte solutions suggest that ions little larger than a water molecule can prefer to reside near the liquid's surface. Here we advance the view that such affinity originates in a competition between strong opposing forces, primarily due to volume exclusion and dielectric polarization, that are common to all dense polar liquids. We present evidence for this generic mechanism from computer simulations of (i) water and (ii) a Stockmayer fluid near its triple point. In both cases, we show that strong surface enhancement of small ions, obtained by tuning solutes' size and charge, can be accentuated or suppressed by modest changes in either of those parameters. Statistics of solvent polarization, when the ion is held at and above the Gibbs dividing surface, highlight a basic deficiency in conventional models of dielectric response, namely, the neglect of interfacial flexibility. By distorting the solution's boundary, an ion experiences fluctuations in electrostatic potential and in electric field whose magnitudes attenuate much more gradually (as the ion is removed from the liquid phase) than for a quiescent planar interface. As one consequence, the collective responses that determine free energies of solvation can resolve very differently in nonuniform environments than in bulk. We show that this persistence of electric-field fluctuations additionally shapes the sensitivity of solute distributions to ion polarizability. dielectric continuum ͉ ion solvation ͉ liquid-vapor interface ͉ polarizability ͉ water S imple salts dissolve in water because the electrostatic reaction fields induced by separated ions in solution provide greatly favorable energies, more than sufficient to offset intrinsic costs of dissociation. These substantial solvation energies are captured with remarkable accuracy by simple linear response models for solvent polarization, for instance dielectric continuum theory (DCT) (1). Molecular simulations of bulk solutions further indicate that the presumption of Gaussian-distributed polarization fluctuations underlying these theories is appropriate even on microscopic scales (2). Based on these facts, one might expect small ions (little larger than a water molecule) to avoid interfaces with lower dielectric media, such as vapor, as predicted by straightforward DCT calculations. Surprisingly, a large body of computational work (pioneered by Jungwirth and Tobias) (refs. 3 and 4 and references therein, 5-9) and many experiments (10, ref. 11 and references therein, 12-14) suggest that certain small ions are in fact more concentrated near the liquid-vapor interface than in bulk aqueous solution. This counterintuitive phenomenon not only challenges the textbook understanding of ion solvation in polar liquids but also bears importantly on a wide range of important molecular processes, e.g., formation of sea salt aerosols and subsequent halogen-release mechani...

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…Recent simulations have shown that anions at the air-water interface specifically bias the height fluctuations of thermal capillary waves over extended domains. [12][13][14] Given the importance of LR-SIE across many fields, [15][16][17] we deemed pertinent to inquire whether such mechanism is universal and applies to other liquids, 1,18,19 or it is inherent to water. Herein, we report experiments addressing this fundamental question.…”

Atomistic simulation of ion solvation in water explains surface preference of halides

“…Because Br − is more reactive than Cl − , the surface preference has an impact on the chemistry in the atmosphere, despite the fact that the content of Cl − in bulk sea water is about three orders of magnitude higher than that of Br − (15). The energetic and structural mechanisms underlying the surface preference of large halide anions have been studied in quite some detail (16)(17)(18), but the phenomenon is still not understood quantitatively (10). Only very recently has the surface absorption free energy of one halide anion, Br − , been determined experimentally (19).…”

Reactivity and Dynamics at Liquid Interfaces

“…281, 282, 302, 306-308, 314, 316-318 Closely related to this concept is the role played by the fluctuations in the solvent-ion interactions in driving small ions towards and away from the interface. 294 …”

“…Note that the total surface excess, which includes the depleted sub-layer, still gives rise to an overall negative surface excess in agreement with the Gibbs adsorption equation. 268,290 In addition to the mutual induced polarizabilities of the water and the ions, other entropic and energetic 293 considerations and surface water fluctuations 294 have been implicated in the local surface enhancement of ions.…”