Effect of Bolaform Counterions on the Adsorption of Sodium Dodecyl Sulfonate at the Air/Water Interface

The effect of quaternary ammonium-based bolaform counterions (Bolan-series where n is the number of carbon atoms between the heads) on the adsorption of sodium dodecyl sulfonate (SDDS) at the air/water interface has been studied using surface tension measurements. The results showed that the Bolan counterions strongly interacted with SDDS even at very low concentrations. This reduced the CMC and C ₂₀ in the presence of Bolan counterions. The SDDS packed more tightly at the interface under the inducement of Bolan counterions and thus the occupied area per surfactant molecule decreased. Another bolaform counterion series (Bolanph) with a benzene ring attached on each side of the quaternary ammonium heads could further enhance the interaction with SDDS. This is due to the fact that the attached aromatic rings penetrate into the gaps between the surfactant headgroups, which provide an additional association force besides the electrostatic attractions. The present results suggested an approach to construct gemini-like surfactants depending on the electrostatic attraction between the bolaform counterions and the conventional ionic surfactants or an additional hydrophobic force made by the attached aromatic rings.