Yan Levin

Electrostatic correlations: from plasma to biology

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 come to be fully appreciated only quite recently. In this paper, we will review the thermodynamic consequences of electrostatic correlations in a variety of systems ranging from classical plasmas to molecular biology.

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

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 Letter 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.

Criticality in the hard-sphere ionic fluid

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-Huckel (DH) approach. Simple DH theory yields a critical point at T∗ ≡ kBTa/q2 = 116, which is only about 15% above the best recent simulation estimates (Tc,sim∗ = 0.052–0.056) but a critical density (ϱc∗ ≡ ϱca3 = 164π ⋍ 0.005 that is much too small (ϱc,sim∗ = 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 pairing of + and − charges improves ϱc∗ significantly. Bjerrums theory of the (required) association constant is revisited critically; Ebelings 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 neural dipolar pairs correlates fairly well with Gillans 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 Tc.

Surface tensions, surface potentials, and the Hofmeister series of electrolyte solutions.

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.

Ion specificity and the theory of stability of colloidal suspensions

A theory is presented which allows us to accurately calculate the critical coagulation concentration of hydrophobic colloidal suspensions. For positively charged particles, the critical coagulation concentrations 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 lose 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.

Simple model for attraction between like-charged polyions

Abstract:We present a simple model for the possible mechanism of appearance of attraction between like charged polyions inside a polyelectrolyte solution. The attraction is found to be short ranged, and exists only in the presence of multivalent counterions. It is produced by the correlations in layers of condensed counterions surrounding each polyion and is only weakly temperature dependent. We find the attraction to be maximum at zero temperature and dimish as the temperature is raised. The attraction is only possible if the number of condensed counterions exceeds the threshold,

Ions at the water-oil interface: interfacial tension of electrolyte solutions.

n > Z/2\alpha

Charge inversion in DNA-amphiphile complexes: Possible application to gene therapy

, where α is the valence of counterions and Z is the polyion charge.