Overcharging and Free Energy Barriers for Equally Charged Surfaces Immersed in Salt Solutions

Role of Counterion in the Adsorption of Ionic Amphiphiles at Fluid Interfaces

This communication represents the chemical alternative to the previous two papers dealing with the influence of positively charged alkali cations on the adsorption properties of the series of the standard surfactant system of alkali-perfluorocarbon octanoates. Now, this contribution describes the adsorption properties of the negatively charged cationic surfactant series of trimethyldodecyl-ammonium halides. In our latest contributions, we have put forward a new model of adsorption of ionic surfactants. It says that the surface excess of the adsorbed anionic surfactant is exclusively determined by the cross-sectional area of the positive counterion. This, however, has been demonstrated by applying relevant, positively charged (alkali) counterions only, i.e., by anionic surfactants. In this article, we extend the new model to negatively charged counterions (halides) applying the cationic standard surfactant series of the trimethyldodecylammonium-halides. A big difference between the hydration behavior of the positively charged alkali and the negatively charged counterions has become striking. Thus, for example, whereas the ratio between the naked ion radius of the cesium and of the lithium cation is almost 2-fold, it is practically equal for the chloride and the iodide anion. Surprisingly, however, the relevant adsorption data are practically identical. This means that the bigger, negatively charged halide counterions interact considerably more strongly with their residual ionic surfactant group than the positively charged alkali cations with theirs. Due to this, the size of the hydrated negative halide ions is considerably greater than that of the relevant positive alkali ions. These specialties can well be explained by the Stern model of charge distribution across a naked ion's surface. It shows that for the electrostatic interaction between counterion and ionic surfactant headgroup, the peculiarities of the polar solvent of water will play a crucial role, too. By these investigations our new model of adsorption of ionic amphiphiles is further extended and gives finally evidence that it is of general validity.

Interactions between conducting surfaces in salt solutions

In this work, we simulate interactions between two perfectly conducting surfaces, immersed in a salt solution. We demonstrate that these forces are quantitatively different from those between (equally charged) non-conducting surfaces. There is, for instance, a significant repulsion between net neutral surfaces. On the other hand, there are also qualitative similarities, with behaviours found with non-conducting surfaces. For instance, there is a non-monotonic dependence of the free energy barrier height, on the salt concentration, and the minimum essentially coincides with a flat profile of the apparent surface charge density (i.e. the effective net surface charge density, some distance away from the surface, when accounting for ion neutralization), outside the so-called Stern layer. These conditions can be described as "perfect surface charge neutralization". Despite observed quantitative differences, we demonstrate that it might be possible to mimic a dispersion containing charged colloidal metal particles by a simpler model system with charged non-conducting particles, using modified particle-ion interactions.