X-ray diffraction study of polyphilic smectic liquid crystals

4-substituted n-5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecan-1-yloxybenzene and n-5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-pentadecafluorododecan-1-yloxybenzene, the former with CO₂CH₃ and the latter with CO₂CH₃ and CO₂H substituents in para position, are polyphilic compounds exhibiting smectic liquid crystalline phases. The structure of these phases was studied by X-ray diffraction and modelling. The method consisted of reconstructing all possible one-dimensional electron density profiles from diffraction intensities and matching them with the profiles calculated from structural models. The method is shown to be useful in both determining molecular arrangement in the smectic phase and in phase identification itself. An S_Ad phase is found in the two esters, having interdigitated perfluorinated alkyl segments. In contrast, a double-layer S_C phase, also known as S_C2, is found in the carboxylic acid. The method used here is particularly applicable to polyphilic liquid crystals such as the present ones, which are characterized by a complex electron density profile and a high translational order parameter.