(553f) Sponge to Lamellar Transition In a Mixture of a Double Tail Cationic Surfactant and a Protic Ionic Liquid

The self-assembly of didodecyldimethylammonium bromide (DDAB) in a protic ionic liquid, ethylammonium nitrate (EAN), in the high concentration regime, is studied using five different experimental techniques. A thermo-reversible first order sponge (L₃) to lamellar (L_a) transition occurring at [DDAB] > 80 wt% was evidenced by (1) a sharp increase in the elastic and viscous moduli, (2) a transition peak recorded by differential scanning calorimetry, (3) formation of Maltese cross birefringence textures observed via polarizing optical microscopy, (4) a decrease in the inter-bilayer mean distance measured by small angle neutron scattering and (5) an abrupt increase in the conductivity obstruction factor. In contrast to aqueous DDAB solutions, this surfactant forms a stable L₃ phase in EAN in a wide window of compositions and temperatures, which is potentially useful for the synthesis of nano-porous material. To the best of our knowledge, this is the first evidence of the formation of the L₃ phase in an ionic liquid.