| Abstract
| - 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 investigatedat 25 °C by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmissionelectron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 formsvesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentrationthen rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlikemicelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % andprogressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity dataclearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was stronglyadsorbed by the vesicles. Spectrophotometry and DLM showed that, under the conditions used, the vesicleswere nearly eliminated at above 0.7 wt % 12-2-12. The cryo-TEM observations were performed on samplesvitrified two months after preparing the mixtures. The progressive increase of the 12-2-12 content in themixture resulted first in vesicle growth (0.1 wt %), followed by vesicle breakage into smaller vesicles (0.26wt %), the formation of disklike micelles (0.4−0.75 wt %), then of ring-like micelles and short elongatedthreadlike 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. Thesame structures were observed when the mixtures for cryo-TEM observation were vitrified 5−7 days or twomonths 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 5−7days after preparation the mixtures showed long rigid rodlike micelles and irregular ribbons. Those structuresdisappeared when the mixtures were allowed to equilibrate for two months. The results are discussed and amodel is presented to explain the observed behavior.
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