Phase behavior, rheological properties, and vesicles of alkyldimethylamine oxide and fluorinated acidic surfactant mixtures

Abstract

Weakly basic alkyldimethylamine oxide (CnDMAO, n = 12, 14, and 16) molecules can be protonated to form a cationic surfactant, CnDMAOH+, by an acidic fluorocarbon surfactant, 8-2-fluorotelomer unsaturated acid (C7F15CFCHCOOH), to produce the salt-free cationic and anionic (catanionic) surfactant mixtures in aqueous solution. These comparative studies on phase behavior and rheological properties of these salt-free catanionic hydrocarbon–fluorocarbon surfactant mixtures in detail clearly indicate the existence of a birefringent Lα-phase for C12DMAO/C7F15CFCHCOOH and C14DMAO/C7F15CFCHCOOH systems at 25.0 ± 0.1 °C. However, the birefringent Lα-phase of C16DMAO/C7F15CFCHCOOH system exist at 60.0 ± 0.1 °C. The birefringent Lα-phase which consists of uni- and multilamellar vesicles, and oligovesicular vesicles is independent of the hydrocarbon chain of CnDMAO at controlled temperatures. The vesicles were demonstrated by cryo-transmission electron microscopy (cryo-TEM) and negative-staining TEM images, in which pliability of the bilayer membranes decreases with the increasing length of hydrogenated chains of hydrocarbon surfactants. The formation of the salt-free catanionic Lα-phase consisting of vesicles could be induced by the strong electrostatic interaction between the cationic hydrocarbon C14DMAOH+ and the anionic fluorocarbon C7F15CFCHCOO−, C14DMAOH+ is produced through acid–base reaction of C14DMAO and C7F15CFCHCOOH. The rheological properties of micelles and vesicles in the three mixture systems were measured, which provided much more information about the hydro- and fluorocarbon surfactant mixtures. The size distribution and structural transition of these similar systems but having different length chains of hydrocarbon surfactants were studied by dynamic light scattering (DLS) and 1H and 19F NMR spectroscopy.