Adsorption layer structure formation at the air/water interface in aqueous mixtures of an anionic carboxylate gemini and a cationic surfactant

The surface adsorption of the O,O′-bis(sodium 2-lauricate)-p-benzenediol (C11pPHCNa) and dodecyltrimethylammonium bromide (C12TABr) aqueous mixtures was examined by surface tension method. Strong synergism between the two components was observed, which leaded to high effectiveness and efficiency in surface tension reduction. The maximum effectiveness and efficiency in surface tension reduction appeared at the mole fraction of C11pPHCNa in the bulk (α1) being 0.33, where the two components equal electrical mixing in solution. The corresponding minimum surface tension (γcmc) and the total concentration required to reduce 20 mN m−1 surface tension of water (C20,T) were, respectively 25.2 mN m−1 and 2.02 × 10−3 mmol L−1. The maximum apparent surface excess, Γmax was observed at α1 = 0.1 rather than at α1 = 0.33. The cause was attributed to two aspects: (1) the considerable fast diffusion rate of C12TABr in comparison with C11pPHCNa and (2) significantly high hydrophobicity of the latter. The unusually small apparent area of the adsorbed molecule suggested the surface aggregates formation. The pattern of the surface aggregates was further described as the double layers structure in the subsurface by analyzing the data of the total area of all adsorbed molecules.