Lyotropic cubic phases

Cubic phases have long been known in the field of amphiphilic liquid crystals (surfactants [97] and lipids [98, 99], and have been studied extensively. For example, it has recently been proved that lyotropic, cubic phases play an important role in the control of biological processes such as membrane fusion, fat digestion and the ultra-structural organisation of cells [100]. They have also... 

Lyotropic cubic phases have been the subject of many structural studies [138,157-159]. Their structure is more complicated and less readily visualized than that of other phases. Almost all 3D fluid phases so far observed are of cubic symmetry, although rhombohedral, tetragonal, and orthorhombic phases of inverse topology have been detected in a few systems (based, for example, on SDS or lipids) [160]. There are two distinct classes of cubic phases [110] ... 

J.M. Seddon, N. Zeb, R.H. Templer, R.N. McElhaney, D.A. Mannock, An Fd3m lyotropic cubic phase in a binary glycolipid/water system. Langmuir 12, 5250-5253 (1996)... 

Most of the lyotropic cubic phases show a thermotropic behaviour, i.e. they melt at a characteristic temperature to an isotropic micellar phase without any elasticity and yield stress values. This melting process is connected with a heat of melting , and is reversible below the melting temperature, the system returns back to the cubic gel phase. An interesting behaviour is shown by the cubic phases in the (POEXAPOP)v(POE) /water systems these phases show a reverse thermotropic behaviour, i.e. they melt below a characteristic temperature and return to the stiff gel state above this temperature. This is due to the hydration of the hydrophobic POP blocks below the characteristic temperature, the POP units are hydrated and... 

Seddon, J. and Robins, J., Inverse micellar lyotropic cubic phases, in Foams and Emulsions, Sadoc J. F. and Rivier, N. (Eds), Kluwer, Dordrecht, The Netherlands, 1999, pp. 423-430. 

Figure 2a shows a schematic phase diagram for lyotropic liquid crystals. This figure shows the formation of micelles, cubic phases, bicontinuous cubic phases, and lamellar phases as the concentration of surfactant increases. Also shown in this figure is a schematic diagram of an ordered bicontinuous cubic phase (Fig. 2b). Another interesting example in...

MAS has been applied to a highly viscous cubic phase of a lyotropic LC formed by 1-monooleolyl-rac-glycerol and water in order to obtain liquid-like and 13C spectra.330 Deuterium, sodium, and fluorine NMR spectroscopy have been applied to study the phase behaviour of several dilute lamellar systems formed by low concentrations of an ra-hexadecylpyridinium salt, a sodium salt (e.g., NaBr, NaCl, or sodium trifluoroacetate), 1-hexanol, and D20.331 The 2H, 19F, and 23Na splittings were used to monitor the phase equilibria. The last two studies are motivated by the search of new lyotropic LC for the alignment of biomolecules. 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

Anderson DM, Gruner SM, Leibler S (1988) Geometrical aspects of the frustration in the cubic phases of lyotropic liquid-crystals. Proc Natl Acad Sci USA 85 5364—5368... 

Delacroix, H., Gulik-Krzywicki, T., and Seddon, J.M. (1996). Freeze fracture electronmicro-scopy of lyotropic lipid systems quantitative analysis of the inverse micellar cubic phase of space group Fd3m (Q227). J. Mol. Biol. 258, 88-103. 

Amphiphilic lipopeptides with a hydrophobic paraffinic chain containing from 12 to 18 carbon atoms and a hydrophilic peptidic chain exhibit lyotropic meso-phases and good emulsifying properties. The X-ray diffraction study of the mesophases and of dry lipopeptides showed the existence of three types of mesomorphic structures lamellar, cylindrical hexagonal and body-centred cubic. Two types of polymorphism were also identified one as a function of the length of the peptidic chain and the other as a function of the water content of the mesophases. The emulsifying properties of the lipopeptides in numerous pairs of immiscible liquids such as water/ hydrocarbons and water/base products of the cosmetic industry showed that small amounts of lipopeptides easily give three types of emulsions simple emulsions, miniemulsions and microemulsions. 

The polymerization of one or more components of a lyotropic liquid crys in such a way as to preserve and fixate the microstructure has recently been successfully performed. This opens up new avenues for the study and technological application of these periodic microstructures. Of particular importance are the so-called bicontinuous cubic phases, having triply-periodic microstructures in which aqueous and hydrocarbon components are simultaneously continuous. It is shown that the polymerization of one of these components, followed by removal of the liquid components, leads to the first microporous polymeric material exhibiting a continuous, triply-periodic porespace with monodisperse, nanometer-sized pores. It is also shown that proteins can be immobilized inside of polymmzed cubic phases to create a reaction medium allowing continuous flow of reactants and products, and providing a natural lipid environment for the proteins. 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

Similar to some zeolite syntheses, short-chain surfactants are used as templates in order to synthesize MCM. In aqueous solutions these surfactants form micellar phases, where the dissolved silicate species is made to condense. This way amorphous silicate walls, about 1 nm thick, originate between the micelles. Depending on the concentration of the surfactant, lyotropic, lamellar, hexagonal, and cubic phases develop which leads to varying siHcate structures. After thermal removal of the surfactant by calcination, materials with free pore systems are obtained. 

The photoinitiated polymerization of divinylbenzene (DVB) within four separate cubic phases of the system DVB/didodecyl dimethyl ammonium bromide (DDAB) is reported to yield retention of the lyotropic order during the course of the reaction [48], although the structure of the pure polymer matrix after removal of the template was not investigated. Similarly, polymerization of acrylamide within lamellar, hexagonal, and cubic phases of the surfactant Aerosol OT led to preservation of the parental mesostructure [49]. SAXS measurements showed similar diffractograms before and after polymerization, but again there was no report of characterization of the polymer matrix after surfactant removal. Hence, at least in these cases, the formation of a polymer phase within a lyotropic mesophase does not prevent the formation of lyotropic surfactant phases. 

Recently the polymerization of styrene within lamellar and cubic phases of the surfactant DODAB (dioctadecyldimethylammonium bromide) was studied [50]. After polymerization, the polystyrene/water/DODAB system showed the same phase behavior as the binary water/DODAB system, a result suggesting a phase separation during polymerization into a polymer-rich (with M 400,000) and a lyotropic phase. 

A theoretical phase diagram for a lyotropic system is shown in Figure 17 and reveals, in addition, the Vj and V2 phases which are bicontinuous cubic phases (normal and reversed, respectively) whose structure can be described by models involving interpenetrating rods or periodic minimal surfaces. Note also that each pair of phases is separated, at least in principle, by a cubic phase (a, b, c, d in Figure 17), and with a biphasic interface (two phases coexisting). 

---