Aggregation and Thermodynamic Properties of Some Cationic Gemini Surfactants

In this study, the gemini surfactants of the alkanediyl-alpha-omega-bis(alkyl dimethyl ammonium) dibromide type, on the one hand, with different alkyl groups containing m carbon atoms and an ethanediyl spacer, referred to as "m-2-m" (m = 10, 12 and 16) and, on the other hand, with n-C-16 alkyl groups and different spacers containing s carbon atoms, referred to as "16-s-16" (s = 2, 6, 10 and Ar (8)) have been synthesized, purified and characterized. The critical micelle concentration (CMC), micelle ionization degree (alpha) and Gibbs free energy of micellization (a dagger G (mic)) of these surfactants and the monomeric cationic surfactants DTAB and CTAB have been determined by means of electric conductivity measurements. In addition, the temperature dependence of the CMC was determined for the 10-2-10 gemini surfactant. The CMCs of the gemini surfactants are found to be much lower than those of the corresponding monomeric surfactants and the effect of the hydrophobic alkyl chain length is more important than that of the spacer. The CMC of 16-s-16 passes through a maximum of (or around) s = 6 and then decreases for s = 10. The presence of a maximum CMC is explained by the contribution of a change of conformation of the surfactant with increasing spacer chain length. The changes of alpha with s and m are found qualitatively similar to those found for CMC values. The values of a dagger G (mic) are more negative for the dimers than for the monomers and also change with an increasing spacer carbon number, as CMC values do. The thermodynamic parameters of micellization indicate that the micellization of 10-2-10 is enthalpy driven.