Sodium and Calcium Laurylamidomethylsulfate: Aqueous Micellar Phases, Their Properties, and a Precipitate of Vesicles

A new anionic surfactant (M-LAMS) that is capable of forming intermolecular hydrogen bonds was investigated. Inverse solubilities of Na and Ca salts were found. Critical micelle concentration and aggregation behavior were determined by surface tension, light scattering, electric birefringence, and SANS measurements. It is found that the Na salt forms globular micelles while the Ca salt forms rodlike micelles. The phase behavior of the micellar solutions with increasing cosurfactant concentration was also studied. It is observed that 100 mM Na-LAMS solutions in the presence of 100 mM CaCl2 undergo several phase transformations with increasing n-hexanol concentration. We found not only the expected micellar L1 phase and a lamellar phase at concentrations quite low for this kind of system, but also a novel phase: At a cosurfactant/surfactant ratio xC of 1.2 a white precipitate is formed at the bottom of the sample. With increasing ratio xC the precipitate dissolves into a liquid crystalline Lα phase that at xC=3.2 is transformed into an L3 or sponge phase. Investigation by FF-TEM, light microscopy, and SANS shows that the precipitate consists of agglomerated polydisperse multilamellar vesicles. The vesicles consist of densely packed bilayers that contain little water. The bilayer thickness is about 20 Å and independent of its composition whereas the interlamellar distance is strikingly linked to concentrations of cosurfactant (surfactant/cosurfactant ratio) and electrolyte. With increasing cosurfactant content, the bilayers become less rigid and resulting thermal undulations force the membranes apart and weaken their interactions until a common Lα phase is formed. This transition is an example of a bonding–nonbonding transition of membranes.