Cubic/hexagonal phase ratio

The formation of the cubic phase is related to the presence of phenyl groups under the same synthesis conditions, with TEOS as unique inorganic precursor, only hexagonal phases have been prepared. But it also depends on other parameters since for a given PTES/TEOS molar ratio, the hydrolysis step plays a major role. We have thus tried to better understand the parameters that control the formation of the cubic phase, by changing the PTES/TEOS as well as the EtOH/Si molar ratios. 

If ethanol, such as EtOH/Si = 1 1, is added to the CTAB solution, and then mixed with PTES and TEOS (1 4), a cubic phase is obtained. But when the same amount of ethanol is first added to PTES and TEOS, and then mixed with the CTAB solution, the hexagonal phase is formed. Samples have been prepared for different EtOH/Si ratios, with ethanol added to the PTES/TEOS mixture (Figure 3). 

The X-ray pattern of the sample prepared with no added ethanol corresponds to the cubic phase. As soon as ethanol is present in the PTES/TEOS mixture, one can notice differences in the relative intensities of the diffraction peaks of the related samples. Up to EtOH/Si = 0.75 1, hexagonal and cubic phases seem to be simultaneously present, while for EtOH/Si>l, the X-ray pattern shows one main peak, that could be assigned to a hexagonal phase. Indeed, one can observe a continuous increase in the amount of hexagonal phase with increasing EtOH/Si ratio. 

A sample has been prepared starting from phenyltrimethoxysilane (PTMS) and tetramethoxysilane (TMOS) in a 1 4 molar ratio. The corresponding X-ray pattern shows the presence of a hexagonal phase with no evidence of cubic phase formation. 

Generally, lipids forming lamellar phase by themselves, form lamellar lipoplexes in most of these cases, lipids forming Hn phase by themselves tend to form Hn phase lipoplexes. Notable exceptions to this rule are the lipids forming cubic phase. Their lipoplexes do not retain the cubic symmetry and form either lamellar or inverted hexagonal phase [20, 24], The lamellar repeat period of the lipoplexes is typically 1.5 nm higher than that of the pure lipid phases, as a result of DNA intercalation between the lipid bilayers. In addition to the sharp lamellar reflections, a low-intensity diffuse peak is also present in the diffraction patterns (Fig. 23a) [81]. This peak has been ascribed to the in-plane positional correlation of the DNA strands arranged between the lipid lamellae [19, 63, 64, 82], Its position is dependent on the lipid-DNA ratio. The presence of DNA between the bilayers has been verified by the electron density profiles of the lipoplexes [16, 62-64] (Fig. 23b). 

Cubic strut phases are common in the phase diagrams of two-tailed surfactants. These surfactants have a relatively high value of the vfaolc parameter, because the volume-to-length ratio v/i(. of the double tail is twice that of a single tail. A high value of v/aoic is consistent with the formation of type II bicontinuous and other inverse phases, such as the inverse hexagonal phase in Fig. 12-24. 

For cubic Laves-phase compounds with RpJR > 1-35, the Z2/rj ratio becomes less than 1. In this case, each g site has only one nearest neighbor lying at the adjacent hexagon. Such a transformation of the g-site sublattice may lead to a qualitative change in the microscopic picture of H jump motion the faster jump process is expected to be transformed into the back-and-forth jumps within pairs of g sites belonging to adjacent hexagons. The results of recent QENS experiments... 

However, even without structural studies, Friberg et al. [32], Shinoda [33], and others noted that the broad existence range with respect to the water/oil ratio could not be consistent with a micellar-only picture. Also, the rich polymorphism in general in surfactant systems made such a simplified picture unreasonable. It was natural to try to visualize microemulsions as disordered versions of the ordered liquid crystalline phases occurring under similar conditions, and the rods of hexagonal phases, the layered structure of lamellar phases, and the minimal surface structure of bicontinuous cubic phases formed a starting point. We now know that the minimal surfaces of zero or low mean curvature, as introduced in the field by Scriven [34], offer an excellent description of balanced microemulsions, i.e., microemulsions containing similar volumes of oil and water. 

Hydration of CA leads to the formation of two hexagonal hydrates CAHj and C2AHg. CAHjo is formed at lower temperatures, not exceeding 20 °C the ratio of C2AHg increases with temperature. At temperature above 30 °C both hexagonal hydrates transform to the only stable cubic CjAHg phase. 

Wide angle X-ray scattering (WAXS) provides information on the unit cells where the data obtained can be utilized in determination of the lattice parameters, hkl reflection planes and hence the lattice structure i.e. face centred cubic, hexagonal, etc. In addition to the unit cell information, the level of crystallinity can be determined by considering the polymer as a two-phase material, amorphous and crystalline. The ratio of the second moment of the data corresponding to the sharp... 

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