Formation of lyotropic mesophases

The chemical potentials measured so far do not allow the formulation of thermodynamic criteria for the formation of lyotropic mesophases. Some qualitative remarks, however, can be made. Of particular interest are Ekwall s studies of the relations between the water binding of the mesophases, their ionization, x-ray parameters, and vapor pressures (4). For common soaps at room temperature mesophases can be observed only in the presence of amounts of water that hydrate the ionic and polar groups. Hydration is therefore characteristic of aqueous lyotropic mesophases as well as micellar systems (1, 2, 3). The binding of counterions to the micelles and to the mesoaggregates seems to be of a similar electrostatic nature. The addition of NaCl greatly affects the lamellar phase D and, to a lesser extent, phase E in these phases the counterions are more strongly bound than by micelles in the solution... 

If more surfactant is added above the cmc, the concentration of micelles increases (rather than the concentration of free surfactant) until the micelle concentration becomes so high that they themselves organize to form ordered arrays of lyotropic liquid-crystal phase. There are several well-characterized lyotropic liquid-crystal phases and a host of so-called intermediate phases whose characterization is not unequivocal. While cmc values are typically found in the range lO -lO moldm, formation of lyotropic mesophases typically starts at around 20wt.% of the surfactant in water. 

In dependence on how the liquid crystalline phase is formed, lyotropic and thermotropic mesophases can be distinguished. Thermotropic mesophases are induced by temperature and require an anisometric shape of the mesogen. In contrast, amphiphilic molecules (e.g. surfactants) are able to form lyotropic mesophases in the presence of suitable solvents. The formation of lyotropic mesophases depends on the concentration of the mesogen as well as on the temperature. Furthermore, certain molecules may form both thermotropic and lyotropic mesophases (e.g. phospholipids ). 

The formation of lyotropic liquid-crystal mesophase depends on the structure and properties of surfactant, solvent, and reaction conditions. Although studies on lyotropical liquid crystals have been carried out for many years, the structure and properties of some mesophases are still not very clear. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is difficult to analyse their structures and properties, the boundary in the phase diagram may be not accurate and the minor phase may be missed. 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

The photoinitiated polymerization of divinylbenzene (DVB) within four separate cubic phases of the system DVB/didodecyl dimethyl ammonium bromide (DDAB) is reported to yield retention of the lyotropic order during the course of the reaction [48], although the structure of the pure polymer matrix after removal of the template was not investigated. Similarly, polymerization of acrylamide within lamellar, hexagonal, and cubic phases of the surfactant Aerosol OT led to preservation of the parental mesostructure [49]. SAXS measurements showed similar diffractograms before and after polymerization, but again there was no report of characterization of the polymer matrix after surfactant removal. Hence, at least in these cases, the formation of a polymer phase within a lyotropic mesophase does not prevent the formation of lyotropic surfactant phases. 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Figure 7. Schematic Diagram of Viscosity versus Concentration Profile for a Lyotropic System Exhibiting the Characteristic Viscosity Decrease upon Formation of the Mesophase,...

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

In this chapter, the structural properties of thermotropic and lyotropic liquid crystals will be compared. In a first step, the driving forces for the formation of the mesophases, as well as the building blocks of the two types of liquid crystals will be analyzed. Afterwards, the structures and properties of the most important liquid crystalline phases will be described, as far as they are important in the context of this thesis. 

Even though lyotropic and thermotropic liquid crystals share the same state of matter, the driving forces for the formation of the mesophases differ substantially. To understand this, the molecules which form the respective liquid crystalline phases have to be examined in more detail. Figure 3.1 shows typical examples of such molecules. 

The closed SLC is exemplified in Figure 14(b) by a class B system when sites are internally compensated and no further growth accompanies the formation of the mesophase. The behavior of the closed SLC is thus indistinguishable from that of a molecular LC (Figure 14(a)). Relevant cases are DNA [146], adequately described by the theory of the molecular LC (Section II.C.2), and poly(p-benzamide) (PBA) in AA -dimethylacetamide/LiCl solutions. An assembly of seven PBA molecules with a side-by-side shift of one fourth the molecular length was detected in both isotropic and lyotropic solutions. Even the axial ratio of the assembly ( 104) was undistinguishable from the axial ratio ( 100) of molecularly dispersed PBA [147]. 

One of the main properties of cellulose derivatives is the fact that they can originate, under suitable conditions, liquid crystalline phases (mesophases). For each derivative, the solvent used and the critical concentration needed for the formation of a lyotropic phase depend on the type of lateral chain the interaction solvent/lateral chain is a key factor in the formation of a mesophase. Some cellulose derivatives never form a meso-phase with certain solvents and, in some cases, the liquid crystalline phase only forms after shearing [7-9] due to the alignment promoted by the flow of the molecules [10]. 

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