Self-assembly of onion-like vesicles induced by charge and rheological properties in anionic–nonionic surfactant solutions

Abstract

Surface and bulk properties of mixed nonionic tetraethylene glycol monododecyl ether (C₁₂EO₄) and dialkyl anionic sodium bis(2-ethyl hexyl)sulfosuccinate (AOT) mixtures in aqueous solution were studied and self-assembly of onions (multilamellar vesicles) with uncountable bilayers induced by charge were obtained. An attractive net interaction exists between the two surfactant molecules and a negative interaction parameter values (β^m and β^s) indicate a synergy in the non-ideal mixing behavior. The onion-like structures, i.e., the densely packed multilamellar vesicles with uncountable bilayers are directly observed in a bluish sample solution by charging the swollen nonionic lamellar phases with addition of anionic surfactant, AOT. The addition of ionic surfactants into the nonionic lamellar phase, through electrostatic repulsion of the ionic headgroups, will suppress the Helfrich undulation and induce the transition from planar lamellae to vesicles. Similarly, in an AOT-rich region, upon addition of the nonionic ones, the planar lamellar gel phases of AOT–H₂O mixture can also be transformed into closed vesicle gel-phases due to the reduction of the Coulombic force. Rheological measurements show that both of the storage modulus (G′) and the loss modulus (G′′) increase with the increase of membrane charge density. Interestingly, the addition of NaCl into the mixed systems causes the phase transition from micelles to vesicles. On increasing the amount of salts, the bilayers of the lamellar phase exhibit more flexibility due to the electrostatic shielding of electrolyte, along with a decrease of the solution viscosity, the G′, and the G′′. It is believed that the ionic charges play crucial roles in the flexibility of bilayer membranes. The obtained multilamellar architectures are excellent for the long-term storage of multiple components and can effectively slow their release.