Temperature dependence and enthalpy-entropy compensation of micellization of gemini surfactants in aqueous solutions

Experiments on electrical conductivity in aqueous solutions of cationic gem ini surfactants as a function of concentration and temperature were carried out, where two species of alkyldimethylammonium bromide [CmH2m+1N(CH3)(2) . Br, m = 8, 10, 12] were linked at the polar head groups via a hydrocarbon spacer [(CH2)(s), s =2,3,4,6]. Critical micelle concentration (cmc) was ob tained from the break point in the curve of conductivity kappa vs concentra tion c. The results indicate that the trend of micelle formation of the gem ini surfactants is significantly stronger than the corresponding "monomer", i.e. alkyltrimethylammonium bromide. The changes of Gibbs free energy, ent halpy and entropy in the process of micellization were evaluated based on a mass action model. The results show that micellization of the gemini surfa ctants is an entropy - driven process, which is similar to conventional sur factants. All plots of enthalpy - entropy compensation exhibit excellent li nearity. The intercept of compensation of the gemini surfactants decreases with the decrease of s from 6 to 2. This implies that the gemini surfactant s with short spacer chain tend to form stable micelles.