Respective Roles of Molecular Shape and Chemical Structure on the Symmetries of Mesophases

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Abstract
Analogies between thermotropic and lyotropic Liquid Crystals were underlined at the earliest time of their investigation, since “smectic” is an old concept shared by the two domains. However, during the last decades, the differences between thermotropic and lyotropic systems were emphasized more than their similarities. As an example, the structural description is completely different for the two classes of mesophases. On one hand, the thermotropic systems are considered as close packed assemblies of rigid molecules, rods or discs, while on the other, the lyotropic systems appear as assemblies of aggregates with various shapes which mainly depend on the chemical structures and associated properties of the molecules. Recently, important progress in the synthesis of mesogens have brought new results. Taking these into account, it appears that the gap between the two classes of materials is vanishing and that the primitive analogy might be viewed in a more general frame. The wide variety of structures might be considered within a unique description in terms of a two media organization (paraffinic/polar or aromatic) separated by interfaces, where the nature of the interactions (ionic force or chemical linkage) is an important factor of structural modulation.