Aggregation Behavior Of Quaternary Ammonium Gemini Surfactants In Heptane Solution

Quaternary ammonium Gemini surfactants alkanediyl-a,ω-bis(dimethyl dodecylammonium bromide)(C12-s-C12?2Br) are made up of two cationic surfactants connected at the lever of the head groups by a hydrocarbon spacer chain. The distance between the head groups in C12-s-C12?2Br molecule can be controlled through designing and synthesizing different length of the spacer chain. This results in a change in the molecule shape and in the alkyl chain density of the surfactant molecule, and also in the change density of the ionic head groups and their hydration, which should directly affect self-association of surfactant molecules and structure of the aggregate. At present most studies are only focus on the self-association law of Gemini surfactant molecules in the aqueous solution, few studies are concentrated on the nonaqueous system. With the financial support form Nature Sciences Foundation of China (sanction: 20373012), the aggregation behavior of Gemini surfactants C₁₂-s-C₁₂·2Br (s=2,3,4,5,6,8,12) in heptane has been systematically investigated. The dissolution of C₁₂-s-C₁₂·2Br in heptane needs 1-hexanol as cosolvent, so we first study the law of 1-hexanol affecting the dissolution of Gemini surfactants in heptane, including the distribution of 1-hexanol between the micelle phase and the heptane phase and the ΔG_0→i⁰ of this process. And then the critical reverse micelle concentrations of C₁₂-s-C₁₂·2Br in heptane solutions, crmcI and crmcw, have been investigated by iodine probe spectrometric method and water solubilization method, respectively. And the crmc is compared with the cmc in aqueous solution. The influence of 1-hexanol affecting the crmc is also considered. From the water solubilization and conductivation we also can acquire the maximal water solubilizing capacity. Finally the W/O microemulsions of C₁₂-s-C₁₂·2Br have also been investigated by the dilution method, including the influence factors of water content W0 and the spacer chain s. The main results are as follows: 1 Gemini surfactants C12-s-C12?2Br are dissolved in the form of mixed reverse micelles composed of C12-s-C12?2Br and the cosolvent, 1-hexanol. The critical moles of 1-hexanol na required to dissolving ns mol Gemini in no mol heptane can be calculated from the formula n a = ano+bns, b inflects the average molecular number connected to per surfactant molecule and exhibits different variation laws with s being odd or even, which is attributed to larger bend for odd s than even s. The distribution coefficient K indicates that more 1-hexanol molecules joining to form mixed reverse micelles, and the ?G o 0 →iof the distributing process confirms this fact. 2 The critical reverse micelle concentrations, crmc, of C12-s-C12?2Br is minor than that of the critical micelle concentrations, cmc, of the corresponding surfactants in aqueous solution. The crmc has been found to go through a maximum for s=4 and this is interpreted in terms of conformation changes of the surfactants and progressive bend of the spacer incorporation to the hydrophobic chains upon increasing s. The increasing content of 1-hexanol in solutions can prevent the formation of reverse micelles for it can increase the polarity of the mixed solvent and so decrease the micellization driving force. 3 The water solubilization and conductivation investigation indicate that the solubilizing capacity of the C12-s-C12?2Br reverse micelle firstly increases and then decreases with increasing s, and go through a maximum for s=5. This change also can be interpreted in terms of conformation changes of the surfactants with increasing s the same as the above interpretation. So as, for s=5, the size of the surfactant head group is the largest, which leads to that this surfactant constructs reverse micelles with the maximal hydrophilic core. 4 For W/O microemulsions of C12-2-C12?2Br, the straight dilution lines indicate that the micelle shape is spherical. With the increasing of water content W0, the micelle growing up, the water pool radius Rw and the aggregation numbers N are all increase.5 For W/O microemulsions of C12-s-C12?2Br(s=2,3,4,5,6) at W0=20, the size of micelle, the water pool radius Rw and the aggregation number N are all decrease along with the increasing of s.