Alexandre P. dos Santos

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.

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.

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

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.

Effects of the dielectric discontinuity on the counterion distribution in a colloidal suspension.

We introduce a new method for simulating colloidal suspensions with spherical colloidal particles of dielectric constant different from the surrounding medium. The method uses an approximate 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 method is very accurate and is orders of magnitude faster than the traditional approaches based on series expansions of the interaction potential.

Surface and interfacial tensions of Hofmeister electrolytes

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.

Colloidal charge renormalization in suspensions containing multivalent electrolyte

A theory is proposed which allows us to self-consistently calculate the effective colloidal charge and the counterion and coion density profiles in suspensions containing both multivalent and monovalent electrolytes. The formation of counterion-coion clusters is explicitly taken into account. The theory predicts that sufficiently strongly charged colloidal particles will become overcharged. The addition of monovalent electrolyte decreases the counterion condensation and diminishes the amount of charge reversal. Predictions of the theory are compared with the Monte Carlo simulations and are found to be in excellent agreement without any adjustable parameters.

Surface tensions and surface potentials of acid solutions

A theory is presented which allows us to quantitatively calculate the excess surface tension of acid solutions. The H(+), in the form of hydronium ion, is found to be strongly adsorbed to the solution-air interface. To account for the electrostatic potential difference measured experimentally, it is necessary to assume that the hydronium ion is oriented with its hydrogens pointing into the bulk water. The theory is quantitatively accurate for surface tensions and is qualitative for electrostatic potential difference across the air-water interface.

Electrostatic correlations in colloidal suspensions: density profiles and effective charges beyond the Poisson-Boltzmann theory.

A theory is proposed which allows us to calculate the distribution of the multivalent counterions around a colloidal particle using the cell model. The results are compared with the Monte Carlo simulations and are found to be very accurate in the two asymptotic regimes, close to the colloidal particle and far from it. The theory allows to accurately calculate the osmotic pressure and the effective charge of colloidal particles with multivalent counterions.

Electrolytes between dielectric charged surfaces: Simulations and theory

We present a simulation method to study electrolyte solutions in a dielectric slab geometry using a modified 3D Ewald summation. The method is fast and easy to implement, allowing us to rapidly resum an infinite series of image charges. In the weak coupling limit, we also develop a mean-field theory which allows us to predict the ionic distribution between the dielectric charged plates. The agreement between both approaches, theoretical and simulational, is very good, validating both methods. Examples of ionic density profiles in the strong electrostatic coupling limit are also presented. Finally, we explore the confinement of charge asymmetric electrolytes between neutral surfaces.