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.