Brewster Angle Microscopy and X-ray GID Studies of Morphology and Crystal Structure in Monolayers ofN-Tetradecyl-γ,δ-dihydroxypentanoic Acid Amide

Abstract

First-order phase transitions from the low-density fluidlike phase to the condensed phase of monolayers of N-tetradecyl-γ,δ-dihydroxypentanoic acid amides at the air−water interface have been studied by π−A isotherms, Brewster angle microscopy (BAM), and synchrotron X-ray grazing incidence diffraction (GID). The thermodynamic differences between enantiomeric and racemic monolayers are too small to be measured. However, chiral discrimination is observable in the macroscopic domain morphology by BAM. The domains of the condensed phase grow dendritically. The characteristic shapes of domains of the pure S and R enantiomers are mirror images of each other. The domains of the racemic mixture are quite similar; however, they have no mirror symmetry. Synchrotron X-ray grazing incidence diffraction (GID) measurements reveal an oblique lattice with tilt direction of molecules close to one of the nearest neighbors for condensed-phase monolayers of both enantiomer and racemate. The molecules of the racemic monolayers are more densely arranged indicating a week heterochiral interaction of the molecules. The crystal structure changes only slightly with the surface pressure due to hydrogen bonds between the amide groups of molecules in the condensed phase. Thermodynamic behavior, morphological features, and two-dimensional crystal structure are discussed on the basis of the headgroup interactions, in particular the dominance of strong hydrogen-bonding chains of the amide groups only weakly affected by the hydroxyl groups and the chiral center.