Alkyl cubic phases

Cyclic carbohydrates with two alkyl chains (e.g. 1,2-dialkyl (or 1,2-diacyl) glycerol 8 a (sug=Glcp, Galp) present structural similarities with glycerophospho-lipids. They form complex mesophases such as bicontinuous cubic phases, inverted hexagonal phases or myelin figures [58-61]. Other dialkyl derivatives... 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

Dendritic molecules, forming predominately hexagonal columnar and micellar cubic phases, have been introduced and intensively investigated by Percec et al. and this field has recently been reviewed [143], The mesophases of these dendrons can also be stabilized and modified by replacing alkyl chains by fluorinated chains selected examples will be discussed in the next sections. 

An example of a special kind of mesogenic compound is provided by A,A -bis (4-alkoxybenzoylhydrazines) (e.g., compound 132a,) known to form bicontinuous cubic phases [247-249]. It is interesting that fluorination of only one chain of these compounds (compound 132b) removes the cubic phase which is restored if the second alkyl chain is also fluorinated (compound 132c, see Fig. 35a) [122]. 

The preparation of a cubic phase with supramolecular chirality was achieved using a branched folic acid derivative incorporating glutamic acid residues (Fig. 11) as the source of chirality [93]. The pterin rings of folic acid residues are able to form a cyclic tetramer as a result of two hydrogen bonds between the components. Depending on the number of carbon atoms in the alkyl substituents, the compounds form columnar phases over a wide temperature range, and for 8 and 9 form cubic phases at temperatures above 130 °C. Addition of sodium triflate stabilises the cubic phase for 7, and the salt is incorporated into the other mesophases. It was implied that the cation resides between stacked tetramers. Supramolecular chirality is expressed for both the columnar and the cubic phases, as revealed by vari-... 

Systems with fluorinated alkyl chains were also investigated [42]. A bilayer smectic phase, SmA, was reported for didodecyldimethylammonium-benzene-hexacarboxylate [35] while a bicontinuous cubic phase was observed for 1,3,5-tri-alkoxybenzyltrimethylammonium tetrafluoroborate [49]. 

However, a most significant observation is in the comparison between the two phase diagrams where it is seen that the smectic phases present in the triflate complexes are retained with dodecylsulphate as the anion. This is unusual if the alkylsulphate is considered as a lateral alkyl chain, where it would normally be the case that smectic phases were suppressed. Further, the phase diagram of the dodecylsulphate complexes shows, for seven homologues, a cubic phase between the Sc and Sa phase. Both of these points deserve comment. 

Fig. 5.25 Dependence on length of the lat xil chain and rigid core, mesophases sequence formed by T-shaped bolapolyphiles with nonpolar (alkyl or semiperfluoroalkyl) lateral chains [9, 137, 138], Abbreviations SmA smectic A phase Col rectangular columnar phase Col g square columnar phase Col/ hexagonal columnar phase Lcarti, Lanifi, Lamg laminated phases with different ordta of the rodlike units Cub bicontinuous cubic phase, branched axial rod-bundles Col/ f hexagonal columnar phase formed by non-branched axial rod-boundles...

Thienyl-4,6-diamino-l,3,5-triazines 94 with three flexible alkyl chains at one end of the elongated rodUke core, terminated by a rather bulky polar 4,6-diamino-1,3,5-triazine apex at the opposite end (Fig. 5.32) form micellar cubic phases. It is remarkable that such molecules with such a long aromatic unit and the bulky 4,6-diamino-substituted triazine apex, both contributing to a truncated shape with relatively small cone angles, are still able to form micellar cubic phases instead of... 

Whilst the early reports of intermediate phases concerned systems with reversed curvature [73-76] these were for surfactants where some residual short range order in the polar groups was probably present. There are few definitive reports of fully molten intermediate phases with reversed curvatures. In fact the pattern of how intermediate phases replace the normal bicontinuous cubic phase as alkyl chain size increase only became recognized as systematic studies on homologous series were carried out [37, 66], Here it has required a combination of microscopy, multinuclear NMR and X-ray diffraction to elucidate the structures. Such studies on reversed phases have yet to be carried out, particularly where small variations in alkyl chain structure are made. 

---