Effect of excess electrical charge density at the interface in electrolyte solution

Abstract

The phenomena happening at the electrolyte solution interface is very important because it is common in chemistry and biological processes. Among them, the ion specificity called Hofmeister effect in general and the surface tension increase at the air-electrolyte solution interface have not been understood for a long time. The macromolecular interaction(Hofmeister effect) and the surface tension increase depend on the specific ion used and there is a concrete order in power to change the bulk properties among the ions. It is so general that there will be a simple explanation. Because cation and anion have different affinity to the interface, the cation and anion density at the interface are different so that it makes the intrinsically neutral interface charged. Therefore, we adopted the concept of excess interfacial charge density. We calculated the adsorption Gibbs free energy in electrolyte solution and measured the excess charge density indirectly from the probe cation density using resonant second harmonic generation. The proposed adsorption Gibbs free energy predicted that the excess charge density grows with the square root of the bulk electrolyte concentration and it agrees well with the experimental result. The excess interfacial charge density follows the Hofmeister series and depends on ion species only through the difference between ion`s surface and bulk energy. The surface tension change is calculated by modifying the Gibbs adsorption equation. The bulk osmotic pressure increases surface tension and the electrostatic repulsion between excess interfacial charges act in opposite way. The calculation predicts that surface tension increases linearly with the bulk electrolyte concentration and follows the Hofmeister series in reverse order, which is well-known. The Hofmeister effect, ion-specific effect on the macromolecular interaction in electrolyte solution, is explained quantitatively with the binary mixture model. It is calculated the relation b...