Thermotropic amphiphilic systems

The biaxial nematic (NJ phase was first identified by Yu and Saupe in a ternary amphiphilic system composed of potassium laurate, 1-decanol and D2O. In such systems the constituent units are molecular aggregates, called micelles, whose size and shape are sensitive to the temperature and concentration the phase was found to occur over a range of temperature/concentration. There are obvious advantages in having a single-component, low-molar-mass thermotropic phase. The sugges-... 

The different driving forces lead to LC phases in these low molecular compounds. Rod-like systems display LC behaviour in a certain range of temperature therefore, they are thermotropic. Amphiphiles are lyotropic LCs they show LC behaviour in a certain range of concentration. 

This chapter presents a summary of manuscripts published in the perissod of June 2011-June 2012 focusing on the use of NMR techniques to elucidate the microstructure and dynamics of self-assembling systems. In section 2 reviews and articles on general methods and models have been included. In section 3 the papers on thermotropic and lyotropic liquid crystals, phospholipids, vesicles and bicelles have been covered. Section 4 has been devoted to micellar solutions including ionic and non ionic surfactant systems, polymer amphiphiles and mixed amphiphiles systems. 

The similar approach seems to be fhiitful to create thermotropic mesophases based on amphiphilic systems where neither mesogenic structure nor strong shape anisotropy but the microphase separation process is responsible for the mesomorphic behavior as such. The main objective of this paper is to give some examples which show the correctness of this idea. 

There are some specific amphiphile systems which form exceptionally stable pancake-like micelles which, as one might expect, give anisotropic solutions analogous to discotic nematic phases. Conversely, there are other lyotropic systems containing elongated stacks of aromatic molecules, i.e., chromonic systems, which are analogous to thermotropic calamitic nematic phases. 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Order and Mobility are two basic principles of mother nature. The two extremes are realized in the perfect order of crystals with their lack of mobility and in the high mobility of liquids and their lack of order. Both properties are combined in liquid crystalline phases based on the selforganization of formanisotropic molecules. Their importance became more and more visible during the last years in Material science they are a basis of new materials, in Life science they are important for many structure associated functions of biological systems. The main contribution of Polymer science to thermotropic and lyotropic liquid crystals as well as to biomembrane models consists in the fact that macromolecules can stabilize organized systems and at the same time retain mobility. The synthesis, structure, properties and phototunctionalization of polymeric amphiphiles in monolayers and multilayers will be discussed. 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

Thermotropic liquid crystals and also lyotropic liquid crystals generate functional molecular assemblies. lyotropic liquid crystalline phases are exhibited by amphiphilic molecules in appropriate solvents. They form nano-segregated structures because the molecular structures consist of hydrophilic and hydrophobic components. In Chapter 6, Gin and co-workers describe how lyotropic liquid crystals may be used to form functional materials. Lyotropic liquid crystals can act as templates for inorganic materials, ion conductors, catalysts, drug delivery systems, and nanofilters. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

Lyotropic liquid crystals are formed by aggregation of micelles. They are multicomponent systems. Normally they consist of an amphiphilic substance and a solvent. In contrast, thermotropic liquid crystals are individual compounds. 

The second example is a system composed of water and an ionic amphiphile which incorporates several ethylene imine units and hydroxyl groups [26]. The phase diagram is shown in Fig. 1.8. The lyotropic SmC analog phase is stabilized over a quite broad concentration range. To prove the correct phase assignment of the lyotropic SmC analog phase, the authors provided X-ray dififaction data as well as texture images, which exhibit the characteristic schlieren texture known from thermotropic SmC phases cf. inset of Fig. 1.8). 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

Just as for thermotropic cholesterics, the twist increases with increasing enantiomeric excess of a chiral surfactant. The same holds for the dopant concentration. The capacity of solubilization within the micelles depends on the physico-chemical nature of the dopant and host phase especially the size and amphiphilicity of the dopant are essential. The course of the twist versus dopant concentration x is linear for small x the initial slope /cIx) q) is defined as helical twisting power (HTP) as for thermotropics. Figures 14.10 and 14.11 show typical experimental data for a system consisting of chiral surfactants and for a guest/host system, respectively. For the chemical structures of the dopants of Figure 14.11, see Figure 14.9. 

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