Effects of Introduction of Mesogenic Units in Ammonium Amphiphiles on the Aggregation Behavior in Water

Abstract

A number of novel single- and double-chained ammonium amphiphiles with biphenyl, azobenzene, and stilbene mesogenic moieties at the terminus of one of the hydrophobic tails have been synthesized. The single-chained compounds form less developed bilayers or micellelike structures. The aggregate stability increases from biphenyl- to stilbene- to azobenzene-containing amphiphiles. This increased stability does not correlate with increased hydrophobicity or dipole moment, but correlates with increasing enthalpic contributions. These are attributed to favorable π-π stacking interactions between the mesogenic units. The double-chained compounds form stable bilayer vesicles upon dispersion in water. In the bilayers the mesogens from molecules of opposite leaflets of the bilayer are interdigitated. Bilayers from the azobenzene containing double-chained amphiphiles exhibit a gel-to-liquid crystalline phase transition. Also a blue-shifted UV-absorption maximum is observed, indicating that the mesogens are stacked into H-aggregates. The single-chained amphiphiles readily form aggregates with sodium dodecyl sulfate. These so-called ion-pair amphiphiles form unstable bilayer vesicles.