Reversed hexagonal phase

L micellar solution phase L lamellar liquid crystalline phase V viscous isotropic phase H2 reverse hexagonal phase... 

Solyom and Ekwall (20) have studied rheology of the various pure liquid crystalline phases in the sodium caprylate-decanol-water system at 20 °C, for which a detailed phase diagram is available. Their experiments using a cone-and-plate viscometer show that, in general, apparent viscosity decreases with increasing shear rate (pseudo-plastic behavior). Values of apparent viscosity were a few poise for the lamellar phase (platelike micelles alternating with thin water layers), 10-20 poise for the reverse hexagonal phase (parallel cylindrical micelles with polar... 

Fig. 1. Factors involved in the intramitochondrial transport of cholesterol. Left, membrane fusion stimulating reversed hexagonal phase formation right, permeation of cholesterol across membranes (from Ref. 25, with permission).

Figure 4.11 Plot of the approximate compositions for which surfactant/water mixtures can form monolayers versus the surfactant parameter of the surfactant. This plot is for chain lengths of 14A, which corresponds to hydrocarbons made up of about 12 carbon atoms. The notation for various mesophases is as follows Vi, V2 are bicontinuous cubic phases (the former containing two interpenetrating hydrophobic diain networks in a polar continuum, the latter polar networks in a hydrophobic continuum). Hi and H2 denote normal and reversed hexagonal phases. La denotes the lamellar phase, and Li and L2 denote isotropic micellar and reversed micellar phases (made up of spherical micelles).

If the third component is a water-insoluble alcohol (five carbons or more), amine, carboxylic acid, or amide, the phase topography is profoundly modified. The phase diagram shown in Figure 3.8b [7] shows in addition to LI and HI a very large lamellar phase, a narrow reverse hexagonal phase H2, and, even more important, a sector-like area of reverse micelles L2. This means that the solubility of n-decanol in a sodium octanoate-water mixture containing between 25 and 62% amphiphile is far more important (30 to 36%) than pure water (4%) and pure sodium octanoate (almost zero). This phase is essential to obtain water-in-oil (w/o) microemulsions. 

Figure 33. Phase diagram of sodium caprylate (octanoate)/decanol/water (reproduced from [40]). (B, mucous woven lamellar phase C, tetragonal phase F, reversed hexagonal phase G, isotropic phase otherwise as for Figs. 14, 17, 23 and 27.)...

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 11.12 shows the proposed scheme for the stractural evolution during the synthesis, based in a liquid-crystal templating (LCT) mechanism. Initially, the aqueous acid solution is solubihzed in the cyhndrical reverse micelles forming the reverse hexagonal phase. Nonhydrolyzed TEOS molecules and PDMS chains are mutually soluble and therefore constitute the hydrophobic continuous medium. The hydrolysis is probably driven by interfacial diffusion of the water towards the... 

L.B. Lopes, D.A. Ferreira, D. de Paula, M.T.J. Garcia, J.A. Thomazini, M.C.A. Fantini, M.V.L.B. Bentley, Reverse hexagonal phase nanodispersion of monoolein and oleic acid for topical delivery of peptides in vitro and in vivo skin penetration of cyclosporin A. Pharm. Res. 23, 1332-1342 (2006)... 

Figure 21.4. Schematic representations of (a) normal and (b) reversed hexagonal phases...

La, lamellar phase Hn, reversed hexagonal phase l2, bicontinuous reversed cubic phase. 

FIGURE 4.14 Phase diagram for the three component system sodium octanoate-water-decanol at 293°K. Symbols as in Figure 4.11, but with F reverse hexagonal phase. After Friman et al. (1982) with permission. 

The reversed hexagonal phase is basically the same as the hexagonal phase except that the micellar cylinders are reversed with the non-polar chains radiating outwards from the cylinders (see Figure 7.4). 

Water is contained within the cylindrical reversed micelles which have a typical diameter of 1 to 2 nm. The remaining space is occupied by the non-polar chains which overlap to leave the cylinders much closer together than in the hexagonal phase. The reversed hexagonal phase occupies much smaller regions of phase diagrams and is much rarer than the hexagonal phase. 

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