Microstructures in aqueous solutions of mixed dimeric surfactants: Vesicle transformation into networks of thread-like micelles

The microstructures in mixtures of two dimeric (gemini) surfactants, the dimethylene-1,2- and eicosamethylene-1,20-bis(dimethyldodecylammonium bromide), referred to as 12-2-12 and 12-20-12, have been investigated at 25 degreesC by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmission electron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 forms vesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentration then rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlike micelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % and progressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity data clearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was strongly adsorbed by the vesicles. Spectrophotometry and DLM showed that, under the conditions used, the vesicles were nearly eliminated at above 0.7 wt % 12-2-12. The cryo-TEM observations were performed on samples vitrified two months after preparing the mixtures. The progressive increase of the 12-2-12 content in the mixture resulted first in vesicle growth (0.1 wt %), followed by vesicle breakage into smaller vesicles (0.26 wt %), the formation of disklike micelles (0.4-0.75 wt %), then of ring-like micelles and short elongated threadlike micelles (1 wt %), the growth of those threads (1.5 wt %), and finally the formation of a network (2 wt %), where threads and rings were interconnected. The network contained also a few isolated rings. The same structures were observed when the mixtures for cryo-TEM observation were vitrified 5-7 days or two months after mixture preparation, for the mixtures containing 0.5 wt % or less, and 1 wt % or more 12-2-12. Aging of the systems was observed for mixtures containing 0.65-0.75 wt % 12-2-12. When examined 5-7 days after preparation the mixtures showed long rigid rodlike micelles and irregular ribbons. Those structures disappeared when the mixtures were allowed to equilibrate for two months. The results an discussed and a model is presented to explain the observed behavior.