Mixed micelles of cationic 12-2-12 gemini with conventional surfactants: the head group and counterion effects

Mixed micelle formation by cationic 12-2-12 gemini with conventional single chain surfactants were studied with the help of conductivity, turbidity, and NMR measurements. The influence of steric contributions in the head group region and counterion effects on the mixed micelle formation have been explored. Favorable interactions are responsible for the mixed micelle formation of 12-2-12 with hexadecylpyridinium bromide (HPyBr), hexadecylpyridinium chloride (HPyCl), benzylhexadecyl dimethylammonium chloride (BHDACl), and benzyltetradecyl dimethylammonium chloride (BTDACl), whereas ideal mixing leads to the mixed micelles of 12-2-12 with hexadecyltrimethylammonium bromide (HTAB) and hexadecyltributylphosphonium bromide (HTBB). Specific interactions between the cationic head group of 12-2-12 and aromaticity of co-surfactant (CS) head groups were mainly attributed to the favorable micellization in the former case. Transmittance (turbidity) measurements identified the structure transitions of 12-2-12 with respect to the concentration, and which were in excellent agreement with those reported by Zana from cryo-TEM studies [Adv. Colloid Interface Sci. 97 (2002) 205]. The presence of small amount of cosurfactant drastically reduces the structure transitions of 12-2-12 and the reduction effect is maximum in the presence of HTAB and HPyCl. NMR measurements are in line with those of conductivity. They show that the intercalation of pyridinium and benzylic bulky head groups along with Cl- counterions induce maximum steric effects in the mixed micelles with 12-2-12 gemini in comparison to all other conventional surfactants. (C) 2004 Elsevier B.V. All rights reserved.