N-Octadecanoyl-l-alanine Amphiphile Monolayer at the Air/Water Interface and LB Film Studied by FTIR Spectroscopy

Surface pressure (π)−molecular area (A) compression/expansion isotherms of N-octadecanoyl-l-alanine reflect homochiral discrimination behavior of the enantiomeric monolayer. FTIR studies indicate that carboxylic acid groups form out-of-plane ring dimers between two adjacent N-octadecanoyl-l-alanine molecules in monolayer LB films and that the long hydrocarbon chains in the film matrix take a biaxial orientation. The enantiomeric molecules assemble regularly and twist from neighbor to neighbor, thus giving rise to chirality of the aggregate in the two-dimensional condensed phase. Prior to the phase transition, the transformation of the triclinic subcell packing of the hydrocarbon chains into a hexagonal packing occurs. The variable-temperature infrared spectra of LB films provide powerful evidence for the formation of a hydrogen-bonded structure between chiral headgroups.