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.

A theory is presented which allow us to accurately calculate the critical coagulation concentration (CCC) of hydrophobic colloidal suspensions. For positively charged particles the CCC's follow the Hofmeister (lyotropic) series. For negatively charged particles the series is reversed. We find that strongly polarizable chaotropic anions are driven towards the colloidal surface by electrostatic and hydrophobic forces. Within approximately one ionic radius from the surface, the chaotropic anions loose part of their hydration sheath and become strongly adsorbed. The kosmotropic anions, on the other hand, are repelled from the hydrophobic surface. The theory is quantitatively accurate without any adjustable parameters. We speculate that the same mechanism is responsible for the Hofmeister series that governs stability of protein solutions.PACS numbers: 61.20. Qg, 64.75.Xc, 64.70.pv, 82.45.Gj All of biology is specific. The ion channels which control the electrolyte concentration inside living cells are specific to the ions which they allow to pass. The tertiary structure of proteins is sensitive to both the pH and to specific ions inside the solution. It has been known for over a hundred years that while some ions stabilize protein solutions, often denaturing them in the process, others lead to protein precipitation. In the fields as diverse as biophysics, biochemistry, electrochemistry, and colloidal science, ionic specificity has been known -and puzzled over -for a very long time. It has become known as the "Hofmeister effect" or the "lyotropic" series of electrolytes, depending on the field of science. The traditional physical theories of electrolytes completely fail to account for the ion specificity. The Debye-Hückel (DH) theory of electrolytes and the Onsager-Samaras theory of surface tensions treat ions as hard spheres with a point charge located at the center [1]. The cornerstone of colloidal science, the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory of stability of lyophobic colloidal suspensions is based on an even simpler picture of hard sphere-like colloidal particles interacting with the pointlike ions through Coulomb potential. The DLVO theory showed that the primary minimum of colloid-colloid interaction potential -arising from the mutual van der Waals (dispersion) attraction -is not accessible at low electrolyte concentrations because of a large energy barrier. When the electrolyte concentration is raised above the critical coagulation concentration (CCC), the barrier height drops down to zero leading to colloidal flocculation and precipitation. The DLVO theory, however, predicts that the CCC should be the same for all monovalent electrolytes, which is clearly not the case [2][3][4][5]. In fact it has been known for a long time that the effectiveness of electrolyte at precipitating hydrophobic colloids follows the lyotropic series. For positively charged particles the CCC concentration of sodium thiocyanide is an order of magnitude lower than the CCC of sodium fluoride. Even more dramatic is...

A theory, based on a modified Poisson-Boltzmann equation, is presented that allows us to calculate the excess interfacial tension of an electrolyte-oil interface accurately. The chaotropic (structure-breaking) ions are found to adsorb to the water-oil interface as the result of large polarizability, weak hydration, and hydrophobic and dispersion interactions. However, kosmotropic (structure-making) anions as well as potassium and sodium ions are found to be repelled from the interface. The adsorption of I(-) and ClO(4)(-) is found to be so strong as to lower the interfacial tension of the water-oil interface, in agreement with the experimental data. The agreement between the calculated interfacial tensions and the available experimental data is very good. The theory is used to predict the interfacial tensions of six other potassium salts, for which no experimental data is available at the moment.

IranWe introduce a new method for simulating colloidal suspensions with spherical colloidal particles of dielectric constant different from the surrounding medium. The method uses exact calculation of the Green function to obtain the ion-ion interaction potential in the presence of a dielectric discontinuity at the surface of the colloidal particle. The new method is orders of magnitude faster than the traditional approaches based on series expansions of the interaction potential.

We present a new approach to efficiently simulate electrolytes confined between infinite charged walls using a 3d Ewald summation method. The optimal performance is achieved by separating the electrostatic potential produced by the charged walls from the electrostatic potential of electrolyte. The electric field produced by the 3d periodic images of the walls is constant inside the simulation cell, with the field produced by the transverse images of the charged plates canceling out. The non-neutral confined electrolyte in an external potential can be simulated using 3d Ewald summation with a suitable renormalization of the electrostatic energy, to remove a divergence, and a correction that accounts for the conditional convergence of the resulting lattice sum. The new algorithm is at least an order of magnitude more rapid than the usual simulation methods for the slab geometry and can be further sped up by adopting a particle-particle particle-mesh approach.

We review the present understanding of the behavior of ions at the air-water and oil-water interfaces. We argue that while the alkali metal cations remain strongly hydrated and are repelled from the hydrophobic surfaces, the anions must be classified into kosmotropes and chaotropes. The kosmotropes remain strongly hydrated in the vicinity of a hydrophobic surface, while the chaotropes loose their hydration shell and can become adsorbed to the interface. The mechanism of adsorption is still a subject of debate. Here, we argue that there are two driving forces for anionic adsorption: the hydrophobic cavitational energy and the interfacial electrostatic surface potential of water. While the cavitational contribution to ionic adsorption is now well accepted, the role of the electrostatic surface potential is much less clear. The difficulty is that even the sign of this potential is a subject of debate, with the ab initio and the classical force fields simulations predicting electrostatic surface potentials of opposite sign. In this paper, we will argue that the strong anionic adsorption found in the polarizable force field simulations is the result of the artificial electrostatic surface potential present in the classical water models. We will show that if the adsorption of anions would be as large as predicted by the polarizable force field simulations, the excess surface tension of NaI solution would be strongly negative, contrary to the experimental measurements. While the large polarizability of heavy halides is a fundamental property and must be included in realistic modeling of the electrolytes solutions, we argue that the point charge water models, studied so far, are incompatible with the polarizable ionic force fields when the translational symmetry is broken. The goal for the future should be the development of water models with very low electrostatic surface potential. We believe that such water models will be compatible with the polarizable force fields and can then be used to study the interaction of ions with hydrophobic surfaces and proteins.

We present a theory that is able to account quantitatively for the surface and interfacial tensions of different electrolyte solutions. It is found that near the interface, ions can be separated into two classes: the kosmotropes and the chaotropes. While the kosmotropes remain hydrated near the interface and are repelled from it, the chaotropes loose their hydration sheath and become adsorbed to the surface. The anionic adsorption is strongly correlated with the Jones-Dole viscosity B-coefficient. Both hydration and polarizability must be taken into account to obtain a quantitative agreement with the experiments. To calculate the excess interfacial tension of the oil-electrolyte interface, the dispersion interactions must also be included. The theory can also be used to calculate the surface and the interfacial tensions of acid solutions, predicting a strong surface adsorption of hydronium ion.