Sponge phases liquid crystals

Keywords lipid-water phases liquid crystals swelling sponge phase small-angle X-ray scattering SANS micelles glycerol monooleate octyl glucoside... 

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.

The effect of adding various lipids to monoolein was discussed above. These lipids are soluble in either water (bile salt) or oil (triglyceride) or hardly soluble at all (lecithin). If a substance that is soluble in both water and oil, e.g., propylene glycol, is added to the monoolein-water system, the cubic liquid crystal undergoes a transition to a sponge or L3 phase [13], as shown in Fig. 5. The structure of the sponge phase has been described as a melted bicontinuous cubic phase [14]. 

Some unique mesostructures, somehmes identified as ill-defined or defected lyotropic liquid crystals, such as ribbon (Ri), sponge (L3), and low-viscosity, modified micellar cubic (QJ, and hexagonal (H2) phase, have been detected [29, 30, 33, 35, 36]. 

In an additional study by Yuli-Amar et al., in order to achieve low-viscosity reverse hexagonal phases at room temperature, ethanol and diethylene glycol monoethyl ether (Transcutol) were added to the ternary GMO/TAG/water mixture [29], These studies were based on findings showing that alcohols can destroy liquid-crystal phases, and ethanol and PEG were shown to form discontinuous micellar cubic and sponge phases instead of bicontinuous phases (49-51). It was shown that the addition of Transcutol or ethanol to the GMO/TAG/water mixture enabled the formation of a room temperature fluid Hn phase. 

In spite of these difficulties, with one notable exception discussed in Section 6.12, simple diffusion is quite common, as discussed in this section and next. A similar analysis, as shown previously for a drop, can be made when the oil is present as a thin layer on a solid surface (Lim and Miller, 1991b), a more interesting situation for detergency. Moreover, the intermediate phase need not be the lamellar liquid crystal. Depending on the phase behavior and the initial compositions of the oil and aqueous phases, it could also be another liquid crystalline phase, a microemulsion, or the Lg (sponge) phase discussed in Chapter 4. Finally, we note that sometimes more than one intermediate phase is seen. An example is given in the next section. 

The structure of the BPIII phase actually resembles the L3 (so-called sponge) phase of lyotropic liquid crystals/ and the smectic blue phases observed and studied recently. Both the sponge and smectic blue phases are optically isotropic, and the smectic blue are optically active as well. Theoretical arguments show that the reason for the defect structure is the negative value of tire saddle-splay elastic constant, K24, which makes the defects energetically favorable. A sketch of the sponge phase and of the smectic blue phase is shown in Figure 6.24. 

In the Winsor III state, the oil and water phases represent weak molecular solutions of the surfactant in the solvents. On the other hand, the structure of the surfactant phase is quite intriguing. Because the spontaneous curvature is small, the siufactants associate into structures of zero, or low, curvature, which are either lamellar liquid crystals with = H2 — 0, or a sponge-like bicontinuous microemulsion phase, where the surface has a locally saddle shape Hi —H2. It is believed that lamellar phases are favored for rigid surfactant monolayers, with K 10 keT, while bicontinuous microemulsion phases are favored for flexible monolayers, with k 1 AbT."... 

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