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

Weakly basic alkyldimethylamine oxide (C(n)DMAO, n = 12, 14, and 16) molecules can be protonated to form a cationic surfactant, C(n)DMAOH(+), by an acidic fluorocarbon surfactant, 8-2-fluorotelomer unsaturated acid (C7F15CF=CHCOOH), 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 L alpha-phase for C(12)DMAO/C7F15CF=CHCOOH and C(14)DMAO/C7F15CF=CHCOOH systems at 25.0 +/- 0.1 degrees C. However, the birefringent La-phase of C(16)DMAO/C7F15CF=CHCOOH system exist at 60.0 +/- 0.1 degrees C. The birefringent L alpha-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 cryotransmission 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 L alpha-phase consisting of vesicles could be induced by the strong electrostatic interaction between the cationic hydrocarbon C(14)DMAOH(+) and the anionic fluorocarbon C7F15CF=CHCOO-, C(14)DMAOH(+) is produced through acid-base reaction of C(14)DMAO and C7F15CF=CHCOOH. 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 H-1 and F-19 NMR spectroscopy.