Vesicular precipitates from surfactant/cosurfactant mixtures

Abstract

A new anionic surfactant lauryl amidomethylsulfate (LAMS^-) was investigated. The aggregation behaviour was determined by small-angle neutron scattering (SANS) measurements. The Na-salt micelles are highly charged, while the Casalt micelles are almost uncharged. Na-LAMS (100 mM) solutions in the presence of 100 mM CaCl₂ undergo several phase transformations with increasing n-hexanol concentration. We found the expected micellar L₁ phase and a lamellar phase, but also a novel phase: a white precipitate is formed at the bottom of the sample. With increasing n-hexanol concentration, the precipitate dissolves into a liquid-crystalline L_ga phase. Investigation by freeze—fracture transmission electron microscopy, light micros-copy and SANS shows that the precipitate consists of agglomerated polydisperse multilamellar vesicles. The bilayer thickness is about 20 A and is independent of the composition, whereas the interlamellar distance is strikingly linked to the concentrations of cosurfactant (sur-factant/cosurfactant ratio) and electrolyte. With increasing cosurfactant content, the bilayers become less rigid and resulting thermal undulations force the membranes apart until a common L_ga phase is formed. This transition is an example of a bonding—nonbonding transition of membranes.