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

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⁺C_sN⁺(CH₃)₂CH₂CH₂CH₂SO₃ ⁻]Br⁻ (C₁₄C_sTri, C_s = –(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 C₁₄C_sTri 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 C₁₄C_pxTri 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 C₁₄C_pxTri 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.