Polymorphism of racemic 1,2-diols with aliphatic chains: isostructural series of phases in dry and hydrated samples

Abstract

The different phases occurring in homologous dried n-alkane-1,2-diols (1,2-diols) and n-alkane-1,2,1′,2′-tetraols (1,2-boladiols), as well as in hydrated n-alkane-1,2-diols were characterised according to their X-ray diffraction patterns. The analysis of the X-ray data is mainly based on the small-angle region. The scattering in the wide-angle region, which may be regarded as fingerprint, is at present used for a qualitative discrimination of the phases. A homologous series of compounds is considered isostructural if the data show a linear relationship between the repeat distance d _L and the number of C atoms (Cm) in a chain. Assuming that a given lateral packing remains constant, two characteristic values can be obtained from the plot of d _L versus Cm. The slope b contains information about the tilt angle of the chains for the respective layer model and the intercept a represents the thickness of the hydrophilic part of the aggregate. A great variety of headgroup thicknesses and tiltings of the chains were found. Bilayers or double bilayers are the repeat unit in 1,2-diols, whereas in 1,2-boladiols the molecules are packed in monolayers. Water changes the packing of the chains and new packing modes with crossed chain axes were obtained. The ability to form hydrogen-bridged networks on both ends of the molecules forces the chains to occupy larger areas in dried 1,2-boladiols compared to the corresponding 1,2-diols.