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...
A theory is presented which allows us to accurately calculate the surface tensions and the surface potentials of electrolyte solutions. Both the ionic hydration and the polarizability are taken into account. We find a good correlation between the Jones-Dole viscosity B coefficient and the ionic hydration near the air-water interface. The kosmotropic anions such as fluoride, iodate, sulfate, and carbonate are found to be strongly hydrated and are repelled from the interface. The chaotropic anions such as perchlorate, iodide, chlorate, and bromide are found to be significantly adsorbed to the interface. Chloride and bromate anions become weakly hydrated in the interfacial region. The sequence of surface tensions and surface potentials is found to follow the Hofmeister ordering. The theory quantitatively accounts for the surface tensions of 10 sodium salts for which there is experimental data.
We explore the effects of counterion condensation on fluid-fluid phase separation in charged colloidal suspensions. It is found that formation of double layers around the colloidal particles stabilizes suspensions against phase separation. Addition of salt, however, produces an instability which, in principle, can lead to a fluid-fluid separation. The instability, however, is so weak that it should be impossible to observe a fully equilibrated coexistence experimentally.
The relationship between enhancement flow and structure of core-softened fluids confined inside nanotubes has been studied using nonequilibrium molecular dynamics simulation. The fluid was modeled with different types of attractive and purely repulsive two length scale potentials. Such potentials reproduce in bulk the anomalous behavior observed for liquid water. The dual control volume grand canonical molecular dynamics method was employed to create a pressure gradient between two reservoirs connected by a nanotube. We show how the nanotube radius affects the flow enhancement factor for each one of the interaction potentials. The connection between structural and dynamical properties of the confined fluid is discussed, and we show how attractive and purely repulsive fluids exhibit distinct behaviors. A continuum to subcontinuum flow transition was found for small nanotube radius. The behavior obtained for the core-softened fluids is similar to what was recently observed in all-atom molecular dynamics simulations for classical models of water and also in experimental studies. Our results are explained in the framework of the two length scale potentials.
We present a theory of dilute aqueous suspensions of microgel particles. It is found that as the number of charged monomers in the polymer network composing mesoscopic gel increases, the particles undergo a swelling transition. Depending on the hydrophobicity of the polymer, this transition can be either continuous or discontinuous. Furthermore, similar to charge stabilized colloidal particles, we find that the electrophoretic mobility of the microgel is controlled by an effective charge. Unlike the colloids, however, for which the effective charge grows asymptotically with the logarithm of the bare charge, the effective charge of an ionic microgel scales as Z eff ϳZ 0.5 . The findings are in good agreement with the experimental measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.