Liquid-crystal discontinuous cubic phase

A great many short-chain surfactants do not display either the hexagonal or lamellar liquid crystal as the separating phase above the Krafft eutectic they exhibit a cubic phase instead. Because the hexagonal liquid crystal is often found next to this cubic phase (at higher compositions), it may be presumed that this cubic phase has the discontinuous phase structure. The discontinuous cubic-phase structure consists of discrete micelle-like structural elements arrayed in various ways to form isotropic phase structures having cubic symmetry. A large family of such cubic structures exists [81,82]. 

The discontinuous cubic phase is, without exception (so far as is known), the solubility boundary that exists just above the Krafft eutectic in zwitterionic surfactants. This phase is also the saturating phase at the solubility boundary in many quaternary ammonium salts (such as dodecyltrimethylammonium chloride). It is also prominent in polyfunctional quaternary ammonium surfactants such as dodecyl-tris(2-hydroxyethyl)ammonium chloride [23]. It is interesting that the cubic liquid-crystal solubility boundary is not often found when lipophilic proximate substituent groups do not exist on or near the primary hydrophilic group, as in alkylammonium chlorides (RNH3, Cl ). In contrast, structurally... 

Figure 3. Sketch of the principal phase behavior of amphiphilic compounds. Usual amphiphiles are represented by a vertical line in this scheme they exhibit only one type of mesophase. Extreme geometries of one of their molecular parts or the addition of solvents (linear alkanes or water) may lead to a deviation from the vertical orientation of that line, thus, amphiphilic compounds in such situations may form various types of liquid crystal phases T = temperature, SmB phase (rotator phase), Cubjn and Cub(,i = cubic discontinuous or cubic bicontinu-ous phases, respectively, Col, i = columnar hexagonal phase, Iso = isotropic phase.

According to Fig. 3, classical thermotropic smectic phases of amphotropic liquid crystals are (SmA ), colunmar hexagonal (Col ), bicontinuous cubic (Cub, i), or discontinuous cubic (Cubjis) [169]. All these meso-phases include a disclination surface between the hydrophilic and the lipophilic parts of the unordered molecules. This surface can be uncurved (SmA), curved in one direction (columnar), curved in two directions with the same sign (discontinuous cubic), or curved in two directions with opposite sign (bicontinuous cubic). 

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. 

Figure 3.4 The temperature-concentration phase diagrams of BPS-m (m=5,10,20, and 30) with BmimPFj. The phase abbreviations are as follows micellar phase (1 ), discontinuous cubic liquid crystal phase (1 ), hexagonal liquid crystal phase (H ), lamellar liquid crystal phase (L ), lamellar gel phase (L ), reverse micellar phase (L ), ionic liquid phase (IL), and two-phase separation (II). The chemical structure of j0-sitosterol ethoxylates as a typical example of BPS-m is also shown in this figure. Reproduced from Sakai et al. [37] with permission from Japan Oil Chemists Society.

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