Classic L_α Phases as Opposed to Vesicle Phases in Cationic−Anionic Surfactant Mixtures

Abstract

It is now generally believed that stable vesicles form spontaneously in cationic−anionic surfactant mixtures. We show in this investigation that the vesicles that are observed in such mixtures may be the result of shearing forces that occur during mixing of the two components. When the two components are mixed but shear is avoided during the process the result is a classic Lα phase. The Lα phase can then be transformed into vesicles by the shearing forces that occur when samples with the Lα phase are turned upside down a few times. The stacked Lα phase is prepared by solubilization of methyl laurate in tetradecyltrimethylammonium laurate and the subsequent hydrolysis of the ester by the tetradecyltrimethylammonium hydroxide. When the sample is prepared in this way, the shear during the solubilization process can have no effect on the final structures because the hydrolysis occurs much slower than the mixing process (within a few hours). On the other hand, when the final products are produced by mixing tetradecyltrimethylammonium hydroxide and lauric acid or tetradecyltrimethylammonium bromide and sodium laurate, vesicle phases are obtained. The different structures in the mesophases can easily be distinguished on the basis of their different appearance between crossed polarizers. The vesicle phases are practically isotropic and show little birefringence while the stacked bilayer phases show the typical domainlike "Schlieren" texture. The different structures have also been demonstrated by FF-TEM micrographs. It is furthermore shown that the vesicle phases that are produced by the combination of the cationic and anionic surfactants have different macroscopic properties from the systems that were prepared from the acid and the cationic hydroxide. In the latter situation, the vesicle phases contain no excess salt and the ionic charges on the vesicles are not shielded. As a consequence, the vesicular solutions are strongly viscoelastic and have a yield stress.