Micellar and lyotropic behavior of three isomeric bola-amphiphiles with pyridinium groups

The micellar and lyotropic phase behavior of three isomeric bola-amphiphiles 1, 2 and 3 based on para-, meta- and ortho-phenylene derivatives with hydrophilic pyridinium headgroups was investigated. The micellar behavior was characterized by surface-tension and conductivity measurements: while the para-substituted amphiphile 1 and the ortho isomer 3 aggregated to micelles, the meta-substituted amphiphile 2 exhibited stepwise, but discontinous self-association. This is explained by the mass-action and phase-separation model. The existence of “premicellar” aggregates and the discontinous aggregation of 2 is evidenced in addition by a difference-spectro-photometric measurement. The low surface activity and the calculated areas per molecule of 100–200 A2 at the air-water interface agree with a loop-like conformation of all three bola-amphiphiles 1–3. Lyotropic mesophases were found in the high-concentration regime only for the para isomer 1, while 2 formed crystalline-isotropic phases and 3 isotropic phases only. The mesophase of 1 was characterized by X-ray measurements as lamellar. The addition of 1-hexanol as a co-surfactant led to the induction of a lyotropic mesophase for 2. The role of the cosurfactant consists of filling the wedge between the two hydrophobic chains, thus stabilizing the aggregates.