Reverse hexagonal phase

Conversely, the high-pressure hexagonal phase is one of high entropy for which the reverse inequality is true. In lower -paraffins, the occurrence of the rotator phase at atmospheric pressure is due to the fact that the orthorhombic-hexagonal (or rotator) transition temperature, is lower than the temperature of fusion of... 

Bicontinuous cubic phase Lamellar phase Bicontinuous cubic phase Reverse hexagonal columnar phase Inverse cubic phase (inverse micellar 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).

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. 

All processes on the simulation grid are modeled through the change of labels on the nodes. For example, adsorption of A on an empty site on the 1x1 phase is modeled as (, S) —> (A, S). Desorption of A is simply the reverse reaction. For reactions involving more than one site, all possible configurations have to be specified. We shall illustrate this with the reaction Aads + Bads AB + 2. For this reaction, eight possibilities have to be specified a Bads on the square phase can react with an Aads in four directions, and Aads can be on the 1 x 1 or on the hexagonal phase. 

Typical surfactant-water-phase diagrams are shown in Fig. 3.4 for single-chained ionic, and non-ionic surfactants respectively. Below a "Krafft" temperature characteristic of each surfactant, the chains are crystalline and the surfactant precipitates as a solid. Increased surfactant concentration (Fig. 3.4) results in sharp phase boundaries between micellar rod-shaped (hexagonal), bilayer (lamellar) and reversed hexagonal and reversed micellar phases. (The "cubic" phases, bicontinuous, will be ignored in this section and dealt with in Chapters 4,5 and 7.)... 

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).

The experimental data associated with this surface are actually more revealing than we have suggested (Gibbs et al. 1990, Abernathy et al. 1992). On the basis of systematic X-ray reflectivity studies, it has been deduced that a sequence of phases are found as a function of temperature these phases are shown schematically in fig. 9.12. In particular, two related pseudohexagonal phases have been foimd which differ by a small relative rotation in addition to a high-temperature disordered phase in which the pseudohexagonal symmetry is lost. However, despite the apparent abruptness and reversibility of the transition between the two distorted hexagonal phases, it remains unclear whether or not these structures constitute true equilibrium phases. 

Similarly, lyotropic phases may be further subdivided into nematic (N), lamellar (L), micellar cubic (I), bicontinuous cubic (V) and hexagonal (H), with the cubic and hexagonal phases being further classified as normal (oil-in-water, e. g. Vj) or reversed (water-in-oil, e. g. V2). 

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. 

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