Ions at the Air-Water Interface: An End to a Hundred-Year-Old Mystery?

Abstract

Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atmospheric chemistry. Yet neither simulations, experiments, nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this paper a new theory is proposed which allows us to explicitly calculate the ionic density profiles, the surface tension, and the electrostatic potential difference across the solution-air interface. Predictions of the theory are compared to experiments and are found to be in excellent agreement. The theory also sheds new light on one of the oldest puzzles of physical chemistry -the Hofmeister effect.PACS numbers: 61.20. Qg, 82.45.Gj Since van't Hoff's experimental measurements of osmotic pressure more than 120 years ago, electrolyte solutions have fascinated physicists, chemists, and biologists alike [1]. The theory of Debye and Hückel (DH) [2] was able to address almost all of the properties of bulk electrolytes. On the other hand, electrolyte-air interface remains a puzzle up to now. The mystery appeared when Heydweiller [3] measured the surface tension of various electrolyte solutions and observed that it was larger than the interfacial tension of pure water. While the dependence on the type of cation was weak, a strong variation of the excess surface tension was found with the type of anion. The sequence was reverse of the famous Hofmeister series [4], which was known to govern stability of protein solutions against salting-out. An explanation for this behavior was advanced by Wagner [5] and Onsager and Samaras [6] (WOS), who argued that when ions approach the dielectric air-water interface, they see their image charge and are repelled from it. This produces a depletion zone which, with the help of thermodynamics, can be related to the excess surface tension. The theory and its future modifications [7], however, were unable to account for the Hofmeister series and showed strong deviations from the experimental measurements above 100mM concentrations. The fact that something was seriously wrong with the WOS approach was already clear in 1924, when Frumkin measured the potential difference across the airwater interface and found that for all halogen salts -except for fluoride -the electrostatic potential difference (air − water) was more negative for solution than for pure water [8]. This suggested that anions were able to approach the interface closer than the cations, or even be adsorbed to it! This contradicted the very foundation of the WOS theory. The confused state of affairs continued for the next 70 years, until the photoelectron emission experiments [9, 10, 11] and the polarizable force fields simulations [12] showed that Frumkin was right, and ions might be present at the interface. The situation, however, remains far from resolved. Simulations predict so much adsorption that the excess surface tension of NaI solution becomes negative, contrary to experiments [13]. Furthermore, while the electron spectroscopy was findin...

On the theoretical description of weakly charged surfaces

“…To avoid the complexity of solving the equation for the Green function (2.12), previous work usually invoked approximate schemes, e.g., by replacing the spatially varying screening length by the bulk Debye length 14,26,27,37,38 or using a WKB-like approximation. [34][35][36] However, the screening on the image force at the dielectric interface is inhomogeneous, long-ranged, and accumulative, which cannot be captured fully by these approximate methods.…”

“…The self-energy due to image charge repulsion appears in the Boltzmann factor and is responsible for the depletion layer in the ion distribution near the surface, as shown in Figure 5. Note, however, in the original WOS theory as well as in subsequent treatments, 14,26,27,37,38 the image charge term was added to the Boltzmann factor ad hoc based on physical intuition, whereas in our theory, its appearance is the result of systematic derivation. Therefore, our theory not only recovers the WOS theory (upon making additional approximations, e.g., by using the constant bulk screening length for the image force potential) but also provides the means for systematically improving the WOS theory.…”

Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

“…The neutrality of the total charge is an important condition in deriving the ion distribution near the interface in the electric double layer theory. For an uncharged hydrophobic surface such as the water/air interface, positive and negative ions can still be separated in the interfacial region (B10 Å) due to different propensities towards the interface between the cations and anions [5][6][7][8] ; such effect is called specific ion effect [9][10][11][12][13] since it is driven by non-electrostatic interactions that vary significantly between different ions even for ions with the same electrovalency (for example, F À and I À ). In the scenario of aqueous electrolytes confined by hydrophobic surfaces where the pore size is comparable in size to the interfacial region determined by the specific ion effect, a natural question raised is how the tendency of charge separation near the interface reconciles with electroneutrality inside nanoconfined regions.…”

Ion fluxes through nanopores and transmembrane channels

“…Such artifacts of classical water and ion models have been recently observed in the studies of ionic solvation near interfaces, when compared to the full ab initio simulations [23]. It has been shown that for ionic solvation in an interfacial geometry properly constructed dielectric continuum models agree better with results of full ab initio simulations than the classical explicit water models [23][24][25].…”