Structural Transformations in Lyotropic Liquid Crystals
The broad lines of the polymorphism of assemblies of amphiphilic molecules in presence of water are now well drawn. They can be found in basic review articles about phase diagrams[1,2] and structures[3,4]. These presentations are limited to descriptions of phase diagrams and structures; they do not consider the processes by which one structure transforms into another when the thermodynamical parameters of the phase diagram, water content and temperature, vary. In order to approach this problem we shall first analyze the structures described in the above classical works in purely geometrical terms. We shall consider them as organizations of space in two media, aqueous and paraffinic, of various connectivities. This will lead us to distinguish two classes of structural transformations: one corresponding to transformations without change of connectivity in the two media; the other corresponding to transformations with changes of connectivity. In the first case there is only growth, deformation and ordering of aggregates of amphiphilic molecules, without topological change. Thermodynamical models of aggregation and ordering, inspired from those developed for micellar aggregation and ordering of molecular crystals and thermotropic liquid crystals can give account, and in a few examples predict, some aspects of the phase transformations. In the second case the appearance of new connections between the aggregates when the transformation takes place changes the topology of the system. Such changes cannot be predicted by the above models and require specific treatments. This imposed the search for a more general frame within which the essential features of these models might be used. We shall propose a geometrical approach which consists in looking at the various structures as solutions to “frustrations” in the bilayers of the lamellar phases. Finally, we shall describe a few recent experimental studies which lead to the revision of some aspects of the classical descriptions, in agreement with the above proposition.
KeywordsPhase Diagram Liquid Crystal Lamellar Phase Amphiphilic Molecule Lyotropic Liquid Crystal
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