α, ω‐Dipolar amphiphiles: Influence of rigid and flexible units on aggregation behavior

Abstract Mono‐ and α,ω‐dipolar amphiphiles with hydrophilic pyridinium head groups, and flexible and rigid hydrophobic parts have been synthesized. Surface tension and conductivity measurements proved that micellar aggregates for amphiphiles 1–4 are formed. The incorporation of rigid units leads to a decrease in the critical micellar concentration (CMC): the rigid monopolar amphiphile 2 aggregates at lower concentration than the flexible monopolar amphiphile 1. A similar decrease was observed when chain ends were connected: the flexible α,ω‐dipolar amphiphile 3 has a lower CMC than the flexible monopolar amphiphile 1. The more flexible amphiphiles 1–3 allow the formation of micelles of different shape leading to both CMC and Ct values. For the α,ω‐dipolar amphiphile 4 with a rigid azoxybiphenylene core, the possibility of different micellar shapes is reduced, leading only to a CMC value (CMC = Ct). For the flexible α,ω‐dipolar amphiphile 3, only a small, but linear viscosity increase with concentration is found. Very viscous solutions were measured for the rigid α,ω‐dipolar amphiphile 4 above a certain concentration. Phase diagrams of amphiphiles 3 and 4 were determined using polarization‐microscopy and X‐ray measurements. Flexible α,ω‐dipolar amphiphile 3 shows a lyotropic mesophase only at very high concentration (83 mass %). In contrast, rigid analog 4 forms stable lyotropic mesophases at 16 mass %. Both nematic and higher ordered mesophases (probably lamellar) are found for the rigid α,ω‐dipolar amphiphile 4.