Phase behaviour and structure of zwitanionic mixtures of perfluorocarboxylates and tetradecyldimethylamine oxide—dependence on chain length of the perfluoro surfactant

Abstract

Phase behaviour and the mesoscopic structure of zwitanionic surfactant mixtures based on the zwitterionic tetradecyldimethylamine oxide (TDMAO) and anionic lithium perfluoroalkyl carboxylates have been investigated for various chain lengths of the perfluoro surfactant with an emphasis on spontaneously forming vesicles. These mixtures were studied at a constant total concentration of 50 mM and characterised by means of dynamic light scattering (DLS), electric conductivity, small-angle neutron scattering (SANS), viscosity, and cryo-scanning electron microscopy (Cryo-SEM). No vesicles are formed for relatively short perfluoro surfactants. The extension of the vesicle phase becomes substantially larger with increasing chain length of the perfluoro surfactant, while at the same time the size of these vesicles increases. Head group interactions in these systems play a central role in the ability to form vesicles, as already protonating 10 mol% of the TDMAO largely enhances the propensity for vesicle formation. The range of vesicle formation in the phase diagram is not only substantially enlarged but also extends to shorter perfluoro surfactants, where without protonation no vesicles would be formed. The size and polydispersity of the vesicles are related to the chain length of the perfluoro surfactant, the vesicles becoming smaller and more monodisperse with increasing perfluoro surfactant chain length. The ability of the mixed systems to form well-defined unilamellar vesicles accordingly can be controlled by the length of the alkyl chain of the perfluorinated surfactant and depends strongly on the charge conditions, which can be tuned easily by pH-variation.

Synergism and formation of vesicle gels in salt-free catanionic hydrocarbon/fluorocarbon surfactant mixtures

“…33 Thus, it can be concluded that the molecular structure of the uorocarbon carboxylic acid has a pronounced effect on the phase behavior and properties of the mixed systems. Catanionic hydrocarbon/uorocarbon surfactant mixtures have also been reported by other research groups such as Gradzielski et al [35][36][37][38][39] In this study, we examined the surface and bulk properties of salt-free catanionic hydrocarbon/uorocarbon surfactant mixtures that are composed of C 14 DMAO and two novel partially uorinated surfactants, i.e., C 6 F 13 C 2 H 4 OC 2 H 4 COOH (TFOPA) and C 8 F 17 C 2 H 4 OC 2 H 4 COOH (HFPDA). We speculate that the incorporation of oxyethylene groups in TFOPA and HFDPA will enhance their solubility and exibility.…”

Concentration dependent pathways in spontaneous self-assembly of unilamellar vesicles

“…10,14,15 In this work, we first investigated the influence of the surfactant mixing ratio on the final state reached by the system within a few tens of seconds, where the phase behavior and structure at higher concentration have been studied previously. 38,39 ULVs are observed over an extended range in the phase diagram and the size of vesicles increases with increasing content of TDMAO. When TDMAO is in excess, the scattering curves display weaker features of shell scattering and the characteristic form factor minimum is shifted to lower q, indicating larger sized vesicles, which is in agreement with other observations at a higher concentration.…”

“…This is reasonable because LPFO and TDMAO chains mix well and no segregation of chains is expected, the interaction parameter b for this surfactant mixture has been determined to be À12 kT, 39 i.e., these are strongly synergistic mixtures. In addition, due to their low contrast, the scattering contribution of counterions (both Li + and OH À ) can be neglected.…”

Probing the Self-Assembly of Unilamellar Vesicles Using Time-Resolved SAXS

“…The experiments presented in this chapter are performed using dilute solutions of anionic lithium perfluorooctanoate (LPFO) and zwitterionic tetradecyldimethylamine oxide (TDMAO) surfactants, which form unilamellar vesicles over a broad concentration range [7,8]. Similar unilamellar vesicles can also be formed in the equivalent anionic and cationic surfactant mixtures, but due to the stronger head group interaction the formation process occurs more rapidly [16].…”

“…The lower compatibility of fluorocarbon and hydrocarbon chains makes the hydrophobic part bulkier while the head groups strongly attract each other [7]. As a result, vesicle phase can be observed over a wide concentration range of the two components [8]. Time-resolved SAXS can be used to follow the structural dynamics underlying the formation of unilamellar vesicles upon rapid mixing of dilute solutions of surfactants by means of a stopped-flow device [9].…”