Abstract.Electrostatic correlations play an important role in physics, chemistry and biology. In plasmas they result in thermodynamic instability similar to the liquid-gas phase transition of simple molecular fluids. For charged colloidal suspensions the electrostatic correlations are responsible for screening and colloidal charge renormalization. In aqueous solutions containing multivalent counterions they can lead to charge inversion and flocculation. In biological systems the correlations account for the organization of cytoskeleton and the compaction of genetic material. In spite of their ubiquity, the true importance of electrostatic correlations has become fully appreciated only quite recently. In this paper, I will review the thermodynamic consequences of electrostatic correlations in a variety of systems ranging from classical plasmas to molecular biology.

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...

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A non-perturbative theory is presented which allows to calculate the solvation free energy of polarizable ions near a water-vapor and water-oil interfaces. The theory predicts that larger halogen anions are adsorbed at the interface, while the alkali metal cations are repelled from it. The density profiles calculated theoretically are similar to the ones obtained using the molecular dynamics simulations with polarizable force fields.PACS numbers: 61.20. Qg, 82.45.Gj, There are a number of long standing mysteries in the fields of physical chemistry and biophysics. The Hofmeister effect [1], which has now been known for over 120 years is, perhaps, one of the oldest and most puzzling ones. Hofmeister observed that different ions have very different effect on stability of protein solutions. While some electrolytes are very efficient at salting-out proteins, others lead to protein precipitation only at much larger concentrations. A related mystery, which is also very old, has to do with the surface tensions. Some hundred years ago Heydweiller [2] noted that adding a strong electrolyte to water leads to increase in the surface tension of the water-air interface. While the dependence on the type of cation is weak, there is a strong variation of the excess surface tension with the type of anion -the lighter halides lead to larger excess surface tension than the heavier ones. The sequence is precisely the reverse of the Hofmeister one. Both effects are completely unaccounted for by the current theories of electrolytes, which go back to the pioneering work of Debye and Hückel (DH) [3,4].Application of the DH theory to the study of interfacial properties of electrolyte solutions was initiated by Wagner [5] and continued by Onsager and Samaras (OS) [6]. These approaches were based on the observation that the image charge induced at the air-water interface repels ions from the surface, creating a depletion layer. The Gibbs adsorption isotherm then leads to the conclusion that the surface tension of aqueous electrolytes must be higher than that of pure water. Unfortunately, there is no way to account for ionic specificity within these theories. Since the hydrated size of all halide ions is nearly the same -and this is the only parameter that enters into DH theory -the OS approach predicts that the surface excess should be independent of the type of ion. Recently there have been proposed some other approaches, but none have proven completely satisfactory [7].Some clues to the failure of the DH and the OS theories started to appeared in the 1990s when the photoelectron emission experiments [8] and molecular dynamics simulations with polarizable force fields showed that contrary to the common wisdom, there were ions present at the airwater interface [9,10]. The simulations found that while hard alkali metal ions such as Potassium and Sodium and small halides such as Fluoride [11] are repelled from the interface, the softer more polarizable anions such as Bromide and Iodine are actually attracted to it [12]. Presence of highly re...

A physically based mean-field theory of criticality and phase separation in the restricted primitive model of an electrolyte (hard spheres of diameter a carrying charges + q) is developed on the basis of the Debye-Hiickel (DH) approach. Simple DH theory yields a critical point at T* -kBTa/q 2 = 1/16, which is only about 15% above the best recent simulation estimates (Tc,si m = 0.052-0.056) but a critical density p* =-pea 3 = 1/64~z -0.005 that is much too small (Pc.sam -0.023 0.035). Allowing for hard-core exclusion effects reduces these values slightly. However, correction of the DH linearization of the Poisson-Boltzmann equation by including pairin9 of + and -charges improves p* significantly. Bjerrum's theory of the (required) association constant is revisited critically; Ebeling's reformulation is strongly endorsed but makes negligible numerical difference at criticality and below. The nature and size of the associated, dipolar ion pairs is examined quantitatively and their solvation free-energy in the residual fluid of free ions is calculated on the basis of DH theory. This contribution to the total free energy proves crucial and leads to a rather satisfactory description of the critical region. The temperature variation of the vapor pressure and of the density of neutral dipolar pairs correlates fairly well with Gillan's numerical cluster analysis. Possible improvements to allow for larger ion clusters and to better represent the denser ionic liquid below criticality are discussed. Finally, the replacement of the DH approximation for the ionic free energy by the mean spherical approximation is studied. Reasonable critical densities are generated but the MSA critical temperatures are all 40-50% too high; in addition, the predicted density of neutral clusters seems much too low near criticality and, along with the vapor pressure, appears to decrease too rapidly by an exponential factor below T c.