Lyotropic liquid crystals phase sequence

A compound that has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule (as mentioned earlier). Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences, depending on the volume balances between the hydrophilic part and the hydrophobic part. These structures are formed through the microphase segregation of two incompatible components on a nanometer scale. Hand soap is an everyday example of a lyotropic liquid crystal (80% soap + 20% water). 

The lyotropic liquid crystals have been studied as a separate category of liquid crystals since they are mostly composed of amphiphilic molecules and water. The lyotropic liquid-crystal structures exhibit the characteristic phase sequence from normal micellar cubic (IJ to normal hexagonal (Hi), normal bicontinuous cubic (Vi), lamellar (1 ), reverse bicontinuous cubic (V2), reverse hexagonal (H2), and reverse micellar cubic (I2). These phase transitions can occur, for instance, when increasing the apolar volume fraction [9], or decreasing the polar volume fraction of the amphiphilic molecule, for example, poly(oxyethylene) chain length in nonionic poly(oxyethylene) alkyl (oleyl) or cholesteryl ether-based systems (10, 11). 

Phase Sequences of Thermotropic and Lyotropic Liquid Crystals... 

The appearance of specific fiquid crystalline phases and especially the sequence in which they occur is not random but follows certain rules. As the driving forces behind the formation of thermotropic and lyotropic liquid crystalline phases differ from each other, these rules are also different for the two types of liquid crystals. 

For lyotropic liquid crystals, the temperature plays a secondary role in the formation of the individual mesophases. The primary influence on the phase sequence is exerted by the solvent concentration. The solvent concentration is directly connected to the packing parameter and thus to the micellar shape (cf. Sect. 3.1), which largely determines the mesophase type. At low solvent concentrations lamellar phases are usually formed. By increasing the solvent concentration, columnar and nematic phases appear. At very high solvent concentrations an isotropic micellar solution dominates. An illustration of this phase behavior is shown in the theoretical phase diagram depicted in Fig. 3.9 (c [14]). The individual phases in Fig. 3.9 are separated by biphase regions. 

Lee W.B., Mezzenga R., Fredrickson G.H., Anranalous phase sequences in lyotropic liquid crystals. Physical Review Letters, 99, 187801 (2007)... 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

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