Structural evolution of oleyl betainate aggregates: in situ formation of small unilamellar vesicles

Betaine esters prepared from long-chain alcohols are a class of hydrolyzable cationic surfactants that is interesting both because the compounds can be designed to give harmless products on degradation and that the hydrolysis products can induce potentially useful changes in the properties of systems where such surfactants are present. In this work, the evolution in structure of aggregates formed by oleyl betainate during hydrolysis of the compound has been investigated using (1)H NMR and cryo-transmission electron microscopy (cryo-TEM). With an increasing extent of hydrolysis, and thus an increasing fraction of oleyl alcohol in the aggregates, the aggregate structure changes in a sequence consistent with an increase in the average packing parameter of the surfactant-alcohol mixture, from spherical micelles, via wormlike micelles, to vesicles. An important result from this work is that it demonstrates a means of in situ production of small unilamellar vesicles with a rather narrow size distribution.