A novel cationic/anionic surfactant system from a zwitterionic alkyldimethylamine oxide and dihydroperfluorooctanoic acid

A new cationic/anionic surfactant system is studied that consists of the zwitterionic alkyldimethylamine oxide (C(14)DMAO) and the anionic dihydroperfluorooctanoic acid (C6F13CH2COOH, DHPFOA, pK(a) = 3.35). In the investigated system, the cationic surfactant is produced by the protonation of the amineoxide by the perfluorooctanoic acid. As a result of this proton-transfer reaction, the mixed system does not contain excess salt as do other studied cationic/anionic surfactant systems. Otherwise, the studied system shows the typical phase behavior of catanionic surfactant systems. With increasing concentrations of DHPFOA, for 100 mM C(14)DMAO solutions one observes a L-1-phase, a viscous L-1-phase, a two-phase L-1/L-alpha-region where a birefringent L-alpha-phase is at the bottom of the L-1-phase, a single birefringent and slightly viscoelastic L-alpha-phase, and finally (greater than or equal to 72 mM DHPFOA) two separate phases (L-1/L-C) where the bottom phase consists of crystals of DHPFOA. The phases of the system have been characterized by theological and conductivity measurements. The birefringent L-alpha-phase has been investigated by means of freeze-fracture transmission electron microscopy. The birefringent L-alpha-phase is composed of novel unilamellar and multilamellar vesicles and large vesicles that have small unilamellar vesicles enclosed. L-alpha-phases with excess salt were produced by two different preparation routes. When the components are mixed directly, which involves shear forces, we obtained a vesicle phase. When, in contrast, the L-alpha-phase is produced without shear by hydrolysis of methylformate that forms hydrogen ions in a L-1-phase consisting of C(14)DMAO and potassium hydroperfluorooctanoate (C6F13CH2COOK), the resulting L-alpha-phase is a classic L-1-phase with stacked bilayers. This phase can easily be transformed into vesicles by shearing forces, for instance, by turning the samples upside down a few times. We thus demonstrate that the vesicles that are obtained by mixing of the compounds are not formed spontaneously but are the result of shear forces in the mixing process. The kinetically produced L-alpha-phase contains potassium formate in excess, and it could be argued that the different structures in the two phases could be the result of the excess salt. In the one phase, it is shown that this is not the case. When the L-1-phase from the alkyldimethylamine oxide and the potassium perfluorooctanoate is mixed (sheared) with formic acid directly, one obtains again a viscoelastic L-alpha-phase with vesicles. It is thus shown that L-alpha-phases can exist and form in two different states: in the classic state with stacked bilayers and in the vesicle state. Both states are stable for long times. It is believed that the classic state is the one which is in thermodynamic equilibrium. The vesicle state has different macroscopic properties than the classic stacked bilayer state. It is viscoelastic and can be optically isotropic.