Temperature-dependent phase behavior and the crystal-forming nucleation process of ethyl 4-fluoro-2,3-dihydroxystearate monolayers

The phase behavior of enantiomeric compounds as well as mixtures of enantiopure and racemic diastereomers of ethyl 4-fluoro-2,3-dihydroxystearates has been investigated using surface pressure-area isotherms and Brewster angle microscopy (BAM). All mixtures exhibit a small plateau region within the surface pressure-area isotherm at 20 degrees C, whereas the enantiopure compound shows an isotherm behavior similar to that of fatty acids. Corresponding to the film balance measurements, the BAM images demonstrate different shapes of the domains within the coexistence region of the liquid-condensed/liquid-expanded phase. The domain structures of the monolayers were visualized after Langmuir-Blodgett transfer on mica sheets by scanning force microscopy (SFM). From the SFM images it becomes obvious that small crystallites are formed for all investigated compounds; however, their molecular assembly is diverse for different enantiomers. Variations in the phase behavior can be correlated with interactions between the polar molecular moieties and the subphase and altered intermolecular interactions. Molecular modeling calculations were applied to elucidate the structural organization of these intermolecular interactions. Ab initio calculations of the minima conformers of (S,S,R)- and (S,S,S)-ethyl 4-fluoro-2,3-dihydroxystearates have been performed to predict with the HARDPACK program the two-dimensional lattice structure based on the P1 space group. These calculations showed that intermolecular hydrogen bridges are crucial for the interactions within and between the molecules.