Concentration dependent pathways in spontaneous self-assembly of unilamellar vesicles

Abstract

We report on the structural dynamics underlying the formation of unilamellar vesicles upon mixing dilute solutions of anionic and zwitterionic surfactant solutions. The spontaneous self-assembly was initiated by rapid mixing of the surfactant solutions using a stopped-flow device and the transient intermediate structures were probed by time-resolved small-angle X-ray scattering. The initial surfactant solutions comprised of anionic lithium perfluorooctanoate and zwitterionic tetradecyldimethylamine oxide, where the mixtures form unilamellar vesicles over a wide range of concentrations and mixing ratios. We found that disk-like transient intermediate structures are formed at higher concentrations while more elongated forms such as cylinder-like and torus-like micelles are involved at lower concentrations. These differences are attributed to monomer addition mechanism dominating the self-assembly process when the initial concentration is well below the critical micellar concentration of the anionic surfactant, while at higher concentrations the process is governed by fusion of disk-like mixed micelles. This means that the pathway of vesicle formation is determined by the proximity to the critical micellar concentration of the more soluble component.

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

“…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]. Detailed studies of anioniczwitterionic system have revealed that disk-like transient mixed micelles are involved at higher concentrations above the cmc, while more elongated intermediate structures such as cylinder-like and torus-like mixed micelles are observed at lower concentrations below the cmc of the more soluble component [15]. The transient disk-like micelles formed at higher concentrations are not fully stable and they grow with time by fusion, and beyond a critical size disks close to form unilamellar vesicles [16].…”

“…The transient disk-like micelles formed at higher concentrations are not fully stable and they grow with time by fusion, and beyond a critical size disks close to form unilamellar vesicles [16]. At low concentration range, the process is more likely governed by a monomer addition mechanism by which initially formed elongated cylindrical micelles grow until a certain size, beyond that they close to form more globular torus-like shape which then transform to unilamellar vesicles [15]. For concentrations close to the cmc of both surfactants, torus-like mixed micelles are formed spontaneously, which grow predominantly by a monomer addition mechanism in a reservoir of excess soluble surfactant component and eventually transform to unilamellar vesicles.…”

“…Therefore, appropriate data acquisition strategies are required to circumvent the radiation damage problem. Using a sequential data acquisition scheme, the radiation damage can be minimized which then allowed probing the self-assembly process in dilute solutions in the vicinity of the critical micellar concentrations (cmc) of the individual surfactant components [15].…”

Practical applications of small-angle neutron scattering

“…11b shows snapshots of TR-SANS data taken during the formation of vesicles on mixing a zwitterionic surfactant, tetradecyl dimethyl amine oxide (TDMAO) and an anionic surfactant, lithium perfluorooctanesulfonate (LiPFOS). 8,138,139 Immediately after mixing, disk-like mixed micelles are formed, evidenced by the region (0.02 o Q o 0.1 Å À1 ) in which I(Q) scales with Q À2 .…”

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

“…23 The formation process via disc-like micelles has been well established for mixtures of surfactants above the cmc, while new investigations show that the mechanism procedes via a torus-like state, if the anionic surfactant is below the cmc. 24 Generally, in these surfactant mixtures long-time stable unilamellar vesicles are formed spontaneously which do not precipitate. In comparison the corresponding catanionic mixtures with tetradecyltrimethylammonium bromide (TTABr) show a tendency for precipitation after some time.…”