Vesicles from salt-free cationic and anionic surfactant solutions

Vesicles from salt-free cationic and anionic (catanionic) surfactant aqueous solutions were prepared. The phase behavior and the phase transition in aqueous solutions of 100 mM tetradecyltrimethylammonium hydroxide (TTAOH) with fatty acids (decanoic acid (DA), lauric acid (LA), myristic acid (MA), and palmitic acid (PA)) were investigated. In this case, the solutions do not contain excess salts because the counterions H⁺ and OH^- can form water. For the four systems of TTAOH/DA/H₂O, TTAOH/LA/H₂O, TTAOH/MA/H₂O, and TTAOH/PA/H₂O, one finds with increasing concentration of fatty acid a low viscous L₁ phase, a viscous L₁ phase, a L₁/Lα phase in which the birefringent Lα phase is on the top of the viscous L₁ phase, and finally a more or less transparent viscoelastic Lα phase with the typical feature of unilamellar and multilamellar vesicles. The microstructures and the rheological properties of the unilamellar and multilamellar vesicles were determined by using freeze-fracture transmission electron microscopy and rheological measurements. Both unilamellar and multilamellar vesicles coexist in the birefringent Lα phase. The unilamellar vesicles have diameters ranging from about 30 nm to more than 200 nm, and the multilamellar vesicles have diameters about 250 nm but are relatively rare. The complex viscosity (|η*|) with 100 mM TTAC_n (n = 10, 12, 14, and 16) at a frequency (v = 0.01 Hz) was found to be increasingly linear with the carbon number of the fatty acids.