NaBr induced strong adsorption of bis-(quaternary ammonium) gemini surfactants with long polymethylene spacer chain at air/water interface

The adsorption of alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) (referred to as 12-s-12, s = 12, 14 and 16) at the air/water interface in the absence and the presence of 10 mmol L−1 NaBr has been investigated using surface tension measurements. The addition of salt strongly promoted the adsorption of 12-s-12, resulting in the formation of the double layers in the subphase below the main adsorption layer. In the double layers, the alkyl tails of surfactants were wrapped in the two layers of the heads and the long and flexible spacer chains were bent to insert into the alkyl tails. The π–A isotherm and surface dilational elasticity measurements supported this hypothesis. The reason may be due to the high hydrophobicity of the 12-s-12 with a long and flexible spacer chain. With addition of salt, the polarity of the solution environment was increased, which expelled the 12-s-12 from the bulk solution. The 12-s-12 molecules concentrated in the domain close to the interface formed the core–shell structure in order that the hydrocarbon chains could escape from the contact with water.