Compounds amphotropics

Beside this, in the recent past some investigations on amphotropic materials of non-amphiphilic structure were made as well (see Further Molecular Architectures in this Chapter). Additionally, it is somewhat difficult to draw a sharp separation line between amphiphilic and nonamphiphilic compounds the ability to act as an amphiphile depends mainly on the size and the polarity of the hydrophilic and hydrophobic parts of a molecule. This internal equilibrium can be varied in a broad range. Thus, in this chapter, although mainly directed toward meso-gens of the amphiphilic type, some apolar mesogens will be discussed as well. 

Further milestones in the field of lyotropic liquid crystals are investigations of binary systems of soaps and detergents with water [21 ] and work on the lyomesomorphism of polypeptides and nucleic acids [22], In 1933 the first review on lyotropic liquid crystals appeared [23]. The author of this review investigated the amphotropic properties of various compounds. He suggested that in amphotropic materials raising the temperature or adding a solvent (water) lead to the same partial break up of the crystal lattice, however, a certain order is maintained. Furthermore, colloidal solutions showing partial supramolecular structures are also described here. But, due to different parameters of order these latter systems differ from lyotropic liquid crystals. 

Somewhat later, even biological systems were investigated from a liquid crystal research point of view (e.g., the tobacco-mosaic-virus) [26], see also Chap. VIII of this volume. The main progress of lyotropic liquid crystal research in these times is connected to works on soap/water mixtures [27, 28] and the investigation of thermotropic mesophases of soaps [29]. The amphotropic character of such compounds was also studied. In such systems, no continuous transitions were observed between thermotropic and lyotropic mesophases, but always biphasic regions could be seen [30, 31]. Thereafter, the interest in understanding biological cell membranes inspired research-... 

For amphotropic liquid crystals it is obvious that the so-called surfactants (including for example soaps) are the best investigated, as documented in two reviews [61, 62]. Therefore, an own chapter of this book is dedicated to that field of supramolecular chemistry (see Chapter VII of this volume). Since the lyotropic properties of this type of amphotropic liquid crystals are discussed there in detail, we focus now mainly on the thermotropic behavior of such compounds. 

Some other compounds may also have the potential to possess amphotropic behavior, but because of too high melting points these properties can not be observed, for example polyamides such as Kevlar show lyotropic behavior only, and are not discussed here. Furthermore, one has to take into account... 

Other compounds which might possess amphotropic behavior, but which have not been studied hitherto, can be found within tannines, saponines, gangliosides, naturally occurring membrane components or biopolymers. Also many synthetic surfactants have never been studied for their lyotropic and thermotropic liquid crystal properties. 

As a result, not all the compounds discussed in the following are already tested for both, lyo- and thermomesomorphism. Thus, in some cases their amphotropic behavior is not yet proved but nevertheless likely. On the other hand, it becomes more and more clear that the strong separation of both kinds of liquid crystallinity is fading [65]. The amphiphilic/amphotropic compounds seem to open a view on an uniform field of liquid crystals, regarding both lyo-and thermotropic behavior of compounds together as one intrinsic principle of nature. 

Other types of interesting, sometimes even curious, amphotropic materials with unusual molecular architectures have also been found. Whereas T-shaped [109] molecular structures have flexible wings, T-shaped compounds are made of rigid lipophilic units [166]. In the T-shaped case, smectic instead of columnar phases are observed. While mono-alkylated amphiphiles usually show only SmA phases, an interesting exception are so-called banana shaped amphiphiles of which 51 in Fig. 35 may be an example. This gentiobioside exhibits a bi-continuous cubic mesophase [129]. 

Hitherto, only monomeric, unimolecular compounds have been discussed in this overview article of which the amphotropic behavior is based on a balanced amphiphilic character. This is also true for polymeric materials [5, 11, 167, 168] which, however, are not included here. 

Amphotropic material - a compound which can exhibit thermotropic as well as lyotropic mesophases. 

The term amphitropic or amphotropic means compounds that can exhibit both types of liquid crystal formation, thermotropic as well as lyotropic. Three recent reviews have summarized the progress in this field. ... 

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