Physicochemical properties of anionic triple-chain surfactants in alkaline solutions

Physicochemical properties of two types of anionic triple-chain surfactants of disodium 2,10-didecyl-6-dodecyl-3,6,9-triazaundecanedioate (3C12dtda) with three hydrocarbon chains and two carboxylate headgroups, and trisodium tris(N-2-ethylamino-N'-2-dodecylcarboxylato)azane (3C12tata) with three hydrocarbon chains and three carboxylate headgroups were characterized by several techniques such as static surface tension, fluorescence spectroscopy, and dynamic and static light-scattering in alkaline solutions at pH 13. 3C12dtda shows a much lower critical micelle concentration (cmc) and higher efficiency in lowering the surface tension than 3C12tata. 3C12dtda also provides a very small occupied area per molecule as compared to 3C12tata and packs closely at the interface. The fluorescent intensity ratio of the first to third band in the emission spectra of pyrene for two triple-chain surfactants starts to decrease gradually from around the cmc obtained by the surface tension method, and decreases sharply from the concentration of 10-18 fold cmc. It is also found that the polarity of the triple-chain surfactant micelle is very much lower than that of the single-chain surfactant micelle. The results of dynamic and static light-scattering measurements show the difference behavior in the aggregation process for two triple-chain surfactants; that is, 3C12dtda forms large aggregates, and 3C12tata forms two aggregates of normal micelle size and a larger one in solution. This indicates that the aggregation states of triple-chain surfactants differ by the number of carboxylate groups, and the large aggregates are derived from the strong attractive interaction between multiple hydrocarbon chains.