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

Four sulfonate-containing gemini surfactants. C-7-M-0-C-7, C-7-M-1-C-7, C-7-M-2-C-7 and C-11-M-1-C-11, with long rigid or semi-rigid spacer have been synthesized. Fully rigid spacer of C-7-M-0-C-7 is composed two rigid benzene rings and two rigid carbonyl groups. Besides the rigid moieties as C-7-M-0-C-7, the spacers of C-7-M-1-C-7, C-7-M-2-C-7 and C-11-M-1-C-11 contain one or two flexible methylene units. The characterization by H-1 NM R, C-13 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 C-7-M-x-C-7 surfactants were unlike the traditional plot exemplified by SOBS. Above the critical micelle concentration (cmc), the surface tension of aqueous solutions of C-7-M-x-C-7 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 20 mN/m) values, but higher gamma(cmc) (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 C-7-M-x-C-7 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. (C) 2012 Elsevier B.V. All rights reserved.