Salt effects on the aggregation behavior of tripolar zwitterionic surfactants with different inter-charge spacers in aqueous solution

Abstract

Salt effects on the aggregation behavior of tripolar zwitterionic surfactants in aqueous solutions have been investigated using surface tension, dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (FF-TEM), and 1H NMR. The tripolar zwitterionic surfactants with different inter-charge spacers are [C14H29(CH3)2N+CsN+(CH3)2CH2CH2CH2SO3 −]Br− (C14CsTri, Cs = –(CH2)2–, –(CH2)6–, –(CH2)10–, and p-xylyl). It is found that the critical micelle concentration (CMC) values of the corresponding traditional zwitterionic surfactant C14H29(CH3)2N+CH2CH2CH2SO3 − (TPS) are almost constant with the increase of the NaBr concentration. However, the CMC values of C14CsTri decrease sharply at a lower NaBr concentration and then level off at a higher NaBr concentration. Moreover, the decreasing extents of the CMC values for C14C2Tri, C14C6Tri, and C14CpxTri are very close, but more significant than that for C14C10Tri, suggesting that the self-assembly ability of the tripolar zwitterionic surfactants with a longer inter-charge spacer is less sensitive to NaBr. The DLS and FF-TEM results reveal that C14C2Tri, C14C6Tri, and C14CpxTri form micelles without NaBr and that the size slightly increases with the increase of NaBr concentration, whereas micelles and vesicles coexist for C14C10Tri and TPS without NaBr and then transfer to micelles upon the addition of NaBr. The salt-induced morphological transition for C14C10Tri is further studied using 1H NMR. The addition of NaBr reduces both the electrostatic repulsion between the same charged ammoniums and the electrostatic attraction between the oppositely charged ammonium and sulfonate. Thus, the longer inter-charge spacer of C14C10Tri tends to be more bended and the sulfonate group becomes available to contact the ammonium, which promotes micellization.