Reversed phases liquid crystals

On the other hand, studies with three-dimensional isotropic lamellar matrices have shown that Azone is a weakly polar molecule, which can occupy the interfacial region as well as the hydrocarbon interior of bilayers [86,87]. The contrasting observations of Azone promoting the assembly of reversed-type liquid-crystal phases (e.g., reversed hexagonal and reversed micellar) in simple model lipid systems [88-90], while also favoring the formation of lamellar structures in one of these mixtures [91], adds further confusion to the discussion [92]. This notwithstanding, the studies by Schiickler and co-workers [91] emphasize the differences in the calorimetric profiles of intact human stratum comeum (HSC) and model SC lipid mixtures Although these systems are clearly useful and versatile, extrapolation of inferences from model lipids to the intact membrane must be performed with caution. 

Figure 1 A phase diagram of a system composed oflipids extracted from cereal lipids and water. The weight fraction of each component is given in the diagram. L3, sponge phase LC, liquid crystal LC-L, lamellar liquid crystal and LC-H, reverse hexagonal liquid crystal.

Figure 4 A phase diagram of a system composed of monoolein and water. One representative structure of a bicontinuous cubic liquid crystalline phase is shown to the right. L, lamellar liquid crystal Hn, reverse hexagonal liquid crystal and Li, reverse micellar phase.

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

A further possibility is the formation of liquid crystals on contact with body fluids at the site of application. The initially applied drug solution interacts with body fluids such as plasma, tears, or skin lipids and undergoes a phase transition into a mono-or multiphasic system of liquid crystals (Fig. 15). For example, oily solutions of reverse micellar solutions of phospholipids, which solubilize additional drug, trans-... 

Mueller-Goymann, C.C., and Hamann, H.-J., Sustained release from reverse micellar solutions by phase transformation into lamellar liquid crystals, J. Contr. Rel., 23 165-174 (1993). 

Fig. 15.4 Schematic ternary-phase diagram of an oU-water-surfactant microemulsion system consisting of various associated microstructures. A, normal miceUes or O/W microemulsions B, reverse micelles or W/O microemulsions C, concentrated microemulsion domain D, liquid-crystal or gel phase. Shaded areas represent multiphase regions.

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap liquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

A nonaqueous reversed-phase high-performance liquid chromatography (NARP-HPLC) with refractive index (RI) detection was described and used for palm olein and its fractions obtained at 12.5°C for 12-24 h by Swe et al. (101). The objective of their research was to find the optimum separation for analysis of palm olein triglycerides by NARP-HPLC, and to find a correction factor to be used in calculating CN and fatty acid composition (FAC). The NARP-HPLC method used to determine the triglyceride composition was modified from the method of Dong DiCesare (88). Palm olein was melted completely at 70°C in an oven for 30 min prior to crystal-... 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

---