A Novel Cationic/Anionic Surfactant System from a Zwitterionic Alkyldimethylamine Oxide and Dihydroperfluorooctanoic Acid

Abstract

A new cationic/anionic surfactant system is studied that consists of the zwitterionic alkyldimethylamine oxide (C14DMAO) and the anionic dihydroperfluorooctanoic acid (C6F13CH2COOH, DHPFOA, pKa = 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 C14DMAO solutions one observes a L1-phase, a viscous L1-phase, a two-phase L1/Lα-region where a birefringent Lα-phase is at the bottom of the L1-phase, a single birefringent and slightly viscoelastic Lα-phase, and finally (≥72 mM DHPFOA) two separate phases (L1/LC) where the bottom phase consists of crystals of DHPFOA. The phases of the system have been characterized by rheological and conductivity measurements. The birefringent Lα-phase has been investigated by means of freeze-fracture transmission electron microscopy. The birefringent Lα-phase is composed of novel unilamellar and multilamellar vesicles and large vesicles that have small unilamellar vesicles enclosed. Lα-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 Lα-phase is produced without shear by hydrolysis of methylformate that forms hydrogen ions in a L1-phase consisting of C14DMAO and potassium hydroperfluorooctanoate (C6F13CH2COOK), the resulting Lα-phase is a classic Lα-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 Lα-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 L1-phase from the alkyldimethylamine oxide and the potassium perfluorooctanoate is mixed (sheared) with formic acid directly, one obtains again a viscoelastic Lα-phase with vesicles. It is thus shown that Lα-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.