X-ray diffraction on smectic liquid crystals: Determining molecular arrangement from diffraction intensities

A quantitative method to determine molecular packing in smectic liquid crystals is proposed based not only on relative but also on absolute electron density profile. The method is based on normalization of diffraction intensities using wide-angle scattering as the norm, and on simultaneous fitting of several compounds and/or polymorphs. Electron density profiles from the constructed models and from layer reflection intensities are compared in both shape and normalized amplitude, aided by molecular cross-section profile calculation from the models. As an example, a case study is described on three compounds displaying five Smectic A and one Smectic B phase. The compounds contain a hydrophobic bicyclohexane or phenylcyclohexane mesogen and a hydrophilic oligo(ethylene oxide) chain ended with a cyclic carbonate. Interestingly, a reentrant Smectic-A is observed below the Smectic-B. An explanation of the intriguing smectic polymorphism is proposed based on the detailed molecular packing models derived from the fitting X-ray method described.