A Vesicle Phase That Is Prepared by Shear from a Novel Kinetically Produced Stacked Lα-Phase

Abstract

We have studied a new cationic and anionic surfactant system which consists of tetradecyldimethylamine oxide (C14DMAO) and dodecylethoxysulfonic acid [CH3(CH2)11(CH2CH2O)2.5SO3H, Texapon N70-H] where no salt in the mixed solutions is formed from the combination of the two surfactants. First, we mixed C14DMAO and Texapon N70-H where the total concentration was 100 mM and the mole fraction varied from 0 to 1 for each component. With increasing mole fractions of Texapon N70-H we observe a L1-phase, a viscous L1-phase, a two-phase L1/Lα-region where the low birefringent Lα-phase is on the top of the L1-phase, and after the two-phase L1/Lα-region a single low birefringent but viscoelastic Lα-phase, and finally at almost equal mole fraction a precipitate. On further increasing Texapon N70-H mole fractions, the sequence of the phases is reversed again. In the Lα-phase, the rheological measurements show that the complex viscosities (|η*|, ν = 0.01 Hz) are much higher than those in viscous L1-phase, and furthermore show that the Lα-phase has a more or less frequency-independent storage modulus. Freeze-fracture electron micrograph results show that small unilamellar and multilamellar vesicles coexist in the Lα-phase. The size range of the small unilamellar vesicles is from 20 nm to the big ones with diameters larger 1.0 μm, and the multilamellar vesicles are relatively small in quantity (the largest multilamellar vesicles are 3.5 μm or so). It is also demonstrated that a classic Lα-phase as opposed to a vesicle phase is produced in the new cationic and anionic surfactant mixed solutions by a different preparation route in which the Lα-phase is prepared without shear by mixing a small amount of methyl formate to a L1-phase from C14DMAO and sodium dodecylethoxysulfate [CH3(CH2)11(CH2CH2O)2.5SO3Na, Texapon N70]. By this route, one obtains the stacked Lα-phase and no vesicles. In the second pathway, methyl formate hydrolyses to formic acid that protonates the amine oxide headgroups of C14DMAO to the cationic surfactant (C14DMAOH+) and thus triggers the transformation of the L1-phase to the Lα-phase. The Lα-phase formed by the simple hydrolysis has stacked bilayers and can be transformed into vesicle by the shearing forces that occur when the samples with the classic Lα-phase are turned upside down a few times. The Lα-phase and vesicle phase have different macroproperties. This has been demonstrated by rheological measurements, SANS and FF-TEM. Our experimental results show that spontaneously formed vesicles that are often formed in cationic and anionic surfactant mixtures may be the result of shearing forces that occur during the mixing process of the two components. It is furthermore shown that the Lα-phase and the vesicle phases that are formed by the chemical reaction have different macroscopic properties from the systems that were prepared from the zwiterionic surfactant (C14DMAO) and Texapon N70-H acid. In the latter situation, the vesicle phases do not contain excess salt and the ionic charges on the vesicles are not shielded. As a consequence, the vesicle phases are strongly viscoelastic and have a yield stress that is large enough to suspend small dispersed air bubbles in the solutions.