Self-assembly of cationic gemini surfactants, alkanediyl-bis-(dimethyldodecyl-ammonium bromide), in cyclohexane: effects of spacer length on their association into reverse lyotropic liquid crystalline or reverse vesicles

Herein, homogeneous solutions of cationic gemini surfactants, alkanediyl-alpha,omega-bis(dimethyldodecylammonium bromide), referred to as 12-s-12 where s = 2, 4, 6, 8, and 10, in cyclohexane have been prepared with the help of sodium hexanoate (SH) or sodium laurate (SL). These surfactants self-assembled in cyclohexane to form various aggregating structures, which were characterised by small-angle X-ray scattering (SAXS) together with polarised microscopy observations. The results showed that 12-2-12/SH, where the gemini had the shortest spacer among this series, formed an inverse micellar cubic liquid crystalline phase of the Fd3m structure. 12-s-12/SL, including s = 4, 6, and 8, which contained an adequate length spacer, formed an inverse hexagonal liquid crystalline phase packed by the cylindrical assemblies of surfactants. 12-10-12/SL, in which the gemini had a longer spacer, formed dispersed reverse vesicles (a lamellar structure). The rheological properties of liquid crystalline phases in a linear visco-elastic regime were studied. 12-2-12/SH formed a hard gel, whereas 12-s-12/SL formed soft gels. Dynamic light scattering and steady-state viscosity analyses were performed for the reverse vesicle solutions formed by 12-10-12/SL. In all these systems, the amount of added water W-0, denoting the mole ratio of water to the gemini surfactant, was demonstrated to influence the properties.