Mechanism of cationic surfactant adsorption – Effect of molecular structure and multiple charge

Influence of molecular structure on the adsorption of double chain, cationic surfactants: N,N-(didodecyl)-N,N-dimethyl ammonium bromide as well as gemini type N,N,N′,N′-tetramethyl-N,N′-di(dodecyl)-alkylenediammonium bromides at the free surface was investigated. The surface tension was measured by a pendant drop shape analysis method. The equilibrium surface tension isotherms of investigated ammonium salts solutions were determined. The molecular structure of investigated surfactants was elucidated by molecular mechanics simulations using AMBER99 force field and Hyperchem 8.0 chemical computation software. The extended “surface quasi two-dimensional electrolyte” (STDE) model was applied for the description of experimental data of surfactants adsorption. Surface activity of investigated gemini-type cationic surfactants, due to their multiple charge, was in concentration range between that of dialkyl and single chain surfactants. It was observed that cmc of dimeric surfactants first increases (up to (CH2)6 spacer chain) and then decreases with the length of the spacer. Using molecular dynamics simulation we confirmed that this dependence can be explained as result of flexibility of long spacer chain, which can be exposed out of the aqueous phase.