Ionic structure of cationic surfactant at the air/water interface

The adsorption of ionic surfactants and counter-ions at the air/water interface is important for many industrial processes. In the literature, the surface tension and surface potential data showed a complicated relationship that cannot be explained by the conventional models, either fully associated or fully dissociated models. Alternatively, ionic surfactants can have a partial association at the air/water interface. The degree of association varies with surface concentration and generates a distribution of counter-ions near the surface. This study reconciles the partial association and ionic distribution by a new quantification model. The model successfully described the surface tension and surface potential data for CTAB (cetyltrimethylammonium bromide). The model shows that the counter-ion association degree plays a deterministic role in limiting the adsorption capacity of ionic surfactants. The proposed structure is also consistent with X-ray surface reflectivity, in which CTAB demonstrated a condensed layer of counter-ion. The molecular insights can be extended to other ionic surfactants and oil/water interfaces.