Abnormal interfacial tension behavior of alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) gemini surfactants

The interfacial property of cationic ammonium gemini surfactants with the structure of [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (S = 3, 4, 6, 10, and 12), referred to as C12CSC12Br2, has been studied at the n-heptane/water interface as a function of the surfactant concentration. The interfacial tension vs surfactant concentration curves show an apparent minimum for the homologues with S ≥ 6. This behavior may be resulted from conformational and location changes of the spacer adsorbed at the interface due to its flexibility as well as the hydrophobic environment provided by the sublayer composed of gemini surfactant aggregates. In addition, the interfacial tension at the interface between n-heptane and the aqueous solution of C12C6C12Br2 with polyacrylamide (PAM) and hydrophobically modified polyacrylamide (HMPAM) has been studied to aid the understanding on the adsorption mechanism proposed for the abnormal interfacial behavior of C12CSC12Br2 surfactants. The interfacial tension is reduced significantly by the C12C6C12Br2/PAM mixture at very low concentration of C12C6C12Br2. For C12C6C12Br2/HMPAM, no minimum is shown in the interfacial tension curve and poorer efficiency in reducing interfacial tension is observed.