I. Some Topics of Liquid Crystals yet to be Discussed

Abstract

Abstract The science of liquid crystals is a part of general crystallography, which is the physical chemistry of crystals and related objects. Nowadays it is generally accepted that the so-called “liquid crystals” are neither real crystals nor real liquids. Whereas the misunderstandings produced by the name “liquid crystals” may be considered as overcome, this name has become accepted as historically established, indicating the referred matter to all who may be interested. In future denominations closer to the real nature of these objects may become more popular. The name “mesomorphic states” is already widespread. There is no doubt about the accurate meaning of “meso”, the intermediate position between amorphous and crystalline, but “morphic” and “state” mostly indicate the relation to the structure only, the relation to the space coordinates, whereas in physics, physical chemistry and generalized crystallography we have to deal with the time coordinate as well, with processes such as dissolution and growth, with equilibrium and temperature. So, the relation to thermodynamics would be still better expressed by the word “mesophases”, since the definition of “phase” is: a homogeneous system in thermic equilibrium. It is symmetric in relation to time. Of these so-called “liquid crystals” only two types of phase structure are well known: the nematic and the smectic. In the real crystals the coordinate transfer from one molecule to the next is characterised by the periodicity of all three translations and all three rotations in space. In the smectic state only one translation period and two rotational periods exist. And in the nematic state only the two rotational periods remain. In other words, the smectic phases show a layered structure, and the nematic phases a fibrous one. Some authors1 maintain that certain substances exhibit a higher number of different phases. Principally it is not excluded that one substance may present more than one smectic or more than one nematic phase. For example in one smectic phase the translation period may be twice that in the other.