Preparation, characterization and properties of anionic gemini surfactants with long rigid or semi-rigid spacers

Abstract

Four sulfonate-containing gemini surfactants, C7–M0–C7, C7–M1–C7, C7–M2–C7 and C11–M1–C11, with long rigid or semi-rigid spacer have been synthesized. Fully rigid spacer of C7–M0–C7 is composed two rigid benzene rings and two rigid carbonyl groups. Besides the rigid moieties as C7–M0–C7, the spacers of C7–M1–C7, C7–M2–C7 and C11–M1–C11 contain one or two flexible methylene units. The characterization by 1H NMR, 13C NMR, electrospray ionization/mass spectrometry (ESI-MS) and Fourier transform infrared (FT-IR) verified the structural characters of these new gemini surfactants. The basic surface properties of these novel gemini surfactants were investigated through measuring the relationship between the surface tension of water and the surfactant concentration, and compared with those of the conventional single-chain surfactant sodium dodecylbenzenesulfonate (SDSB). The curves of surface tension versus logarithm of concentration of C7–Mx–C7 surfactants were unlike the traditional plot exemplified by SDBS. Above the critical micelle concentration (cmc), the surface tension of aqueous solutions of C7–Mx–C7 did not become steady, but still obviously decreased with the concentration increase, which was supposed to be related with the long rigid or semi-rigid spacers. All these gemini surfactants showed lower cmc and C20 (surfactant concentration required for lowering the surface tension of water by 20mN/m) values, but higher γcmc (surface tension at cmc) value than SDBS. The gemini surfactants with semi-rigid spacers exhibited superior surface activity to that with fully rigid spacer. The aggregation behavior of C7–Mx–C7 in water was investigated using dynamic light scattering (DLS) and transmission electron microscopy (TEM). It was found that the gemini surfactants with long fully rigid spacer preferred to form vesicles, whereas the ones with semi-rigid spacers self-assembled into the mixture of micelles and vesicles at low surfactant concentration and formed vesicles at high surfactant concentration.