Dynamic adsorption of opposite-charged Gemini surfactant mixture at air/water interface

Dynamic surface tension (DST) of the mixed aqueous solution of opposite-charged gemini surfactants C9pPHCNa with C10-2-E2-C10·2Br was measured using the maximum bubble pressure technique. The mole fraction of C9pPHCNa (α1, on a surfactant-only basis) in the bulk solution was from 1 to 0.6. Plateaus were observed in the DST curves of the mixtures at total concentration of 0.8 mmol L−1 except the case with α1 = 0.6. This was attributed to the C10-2-E2-C102+/C9pPHC2− ion-pairs with high surface activity adsorbing significantly, which led to desorption of the monomer molecules already at the surface. The micelles in the solution also affected the dynamic adsorption since all the investigated concentrations of mixed systems were above their cmcs. Dissociation of micelles offered both the monomers and the ion-pairs to adsorb at the surface, resulting in the DST curve moving down when the total concentration of the system was increased.