Metastable micelles and true liquid crystal behaviour of newly designed “cataniomeric” surfactants

Abstract

Jeffamines were used as molecular tools for the design of novel organized molecular systems based on so-called “cataniomeric” surfactants, which combine features of ionic liquids (ILs) and catanionic surfactants and which contain oligomeric building blocks. These systems show an original self-assembling behaviour in water, with a rich polymorphism: dynamic (or metastable) micelles and liquid crystals (cubic Im3m and Fm3m and hexagonal H₁ phases). “Cataniomeric” surfactants were prepared by a simple acid–base reaction between oligomeric Jeffamine ED900 and various fatty acids (lauric, myristic, stearic and oleic acid) in a stoichiometric ratio, similar to well known salt-free catanionic surfactants. The phase behaviour was rationalized in terms of hydrocarbon chain length of the fatty counterion (equivalent carbon atoms, ECN) and of headgroup electrostatic interactions and flexibility. All synthesized surfactants show a direct micellar cubic phase, whose stability increases with hydrophobicity. A transition to a hexagonal phase takes place when ECN is higher than 29, and thus independently of the symmetry of the molecule. Thermal stability of the hexagonal phase also increases with ECN. An epitaxial transition of H₁ → Im3m → Fm3m was also evidenced. These novel systems have straightforward applications in modern formulations or smart nanodevices, reversibly responsive to external stimuli.