Other Lyotropic Phases

A third lyotropic mesophase which occurs frequently in surfactant-water systems is normally designated viscous isotropic. This phase is very viscous, sometimes brittle, but unlike neat and middle it is not bire-fringent. The structure of the viscous isotropic phase is still not known with certainty. In some systems x-ray studies have indicated that the structure consists of spherical units packed in a face-centered arrangement 4, 22). It has been proposed that the polar groups of the molecules cover the outside surfaces of the spherical units and that the hydrocarbon chains are essentially liquid in their arrangement inside the units. In this respect the structure is similar to one of the proposed middle phase structures (4). As in the other lyotropic phases, the solvent probably fills the voids among the spherical units of surfactant. 

Lyotropic nematic phases were first reported by Lawson and Flautt (62) for mixtures of Cg and Cio alkyl sulfates, together with their corresponding alcohols in water. They are somewhat less common than the mesophases discussed so far. When they do form, they occur at the boundary between an isotropic micellar phase (LQ and the hexagonal phase (Li/HQ, or between Li and the lamellar phase (Li/L ). As their name implies, they have a similar micellar order to that of the molecules in a thermotropic nematic phase. This long-range micellar orientational and translational order is lower than in the other lyotropic phases described above. Like the thermotropic phases, they are of low viscosity and can be aligned in a magnetic field. It is possible to identify nematic phases optically because of their characteristic schlieren optical texture. 

Other Lyotropic Phases 1.3.1 Lyotropic Nematic Phases... 

In the presence of water, surfactants and lipids give rise to a variety of phases referred to as lyotropic phases or mesophases.i The most important of these phases are the lamellar, hexagonal, cubic micellar, and cubic bicontinuous phases denoted by L, H and V, and Q, respectively (see Figure 1.11 in Chapter 1). The subscripts 1 or 2 attached to these phase symbols indicate that the phase is direct (water continuous) or inverse (discontinuous water domains). Many other lyotropic phases have been identified that differ from the main ones by the state of the alkyl chain (crystalline or disordered) and of the head group arrangement (ordered or disordered). In the particular case of the lamellar phase, additional variations come from the possible different orientations adopted by the alkyl chains with respect to the plane of the lamellae (angle of tilt of the chain) and also from the state of the surface of the lamellae that can be planar or rippled. Numerous detailed descriptions have been given for the equilibrium state of the various phases that surfactants and lipids can form in the presence of water. 

A very simple model that predicts lyotropic phase transitions is the hard-rod model proposed by Onsager (Friberg, 1976). This theory considers the volume excluded from the center-of-mass of one idealized cylinder as it approaches another. Specifically, if the cylinders are oriented parallel to one another, there is very little volume that is excluded from the center-of-mass of the approaching cylinder (it can come quite close to the other cylinder). If, however, the cylinders are at some angle to one another, then there is a large volume surrounding the cylinder where the... 

Additional structural order in the lyotropic phases and bulk films of PBLGlu-based block copolymers beyond the nanometre-length scale could be revealed by POM (Fig. 13, Losik [43]) and TEM/SFM (Pochan et al. [59]). Smectic, hexagonal, (twisted) cholesteric, and other phases could be observed depending on the chemical composition of the copolymer and the history and processing of the sample. A comprehensive picture of the processes involved in the formation of hierarchical superstructures does not exist yet. 

The phase behaviour of biomimetic polypeptide-based copolymers in solution was described and discussed with respect to the occurrence of secondary structure effects. Evidently, incorporation of crystallisable polypeptide segments inside the core of an aggregate has impact on the curvature of the corecorona interface and promotes the formation of fibrils or vesicles or other flat superstructures. Spherical micelles are usually not observed. Copolymers with soluble polypeptide segments, on the other hand, seem to behave like conventional block copolymers. A pH-induced change of the conformation of coronal polypeptide chains only affects the size of aggregates but not their shape. The lyotropic phases of polypeptide copolymers indicate the existence of hierarchical superstructures with ordering in the length-scale of microns. 

The above discussion summarizes the main results obtained in the previous sections, emphasizing their relationship to the results of the preceding article which motivated this analysis. However, as motivated in the text, study of reactions in the droplet and other ordered lyotropic phases possesses enough practical applications to justify an independent study, and therefore we indicate briefly some effects which need to be accounted for in future work to impact on such applications. 

The potential of nuclear magnetic resonance spectroscopy for studying liquid crystalline systems is discussed. Typical spectra of nematic, smectic, and cholesteric mesophases were obtained under high resolution conditions. The observed line shape in the cholesteric phase agrees with that proposed on the basis of the preferred orientation of this phase in the magnetic field. The line shapes observed in lyotropic smectic phases appear to be the result of a distribution of correlation times in the hydrocarbon portions of the surfactant molecules. Thermotropic and lyotropic phase transitions are easily detected by NMR and agree well with those reported by other methods. The NMR parameters of the neat and middle lyotropic phases allow these phases to be distinguished and are consistent ivith their proposed structures. 

In addition to those formed by surfactant amphiphiles, two other types of lyotropic mesophases are generally recognized, neither of which exhibits a cmc. The first of these are lyotropic phases of rigid-rod polymers that can form mesophases in both aqueous and non-aqueous solvents " these mesophases are of the nematic or hexagonal type. Examples include polymeric metal acetylide complexes and DNA." The other type is usually formed from flat and largely aromatic molecules which stack to give lyotropic columnar phases, also referred to as chromonic phases." " This latter class is formed from systems with ionic or strongly hydrophilic peripheral functions, and forms mesophases... 

The first liquid crystals of disc-shaped molecules, now generally referred to as discotic liquid crystals, were prepared and identified in 1977. Since then a large number of discotic compounds have been synthesized and a variety of mesophases discovered. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked one on top of the other aperiodically to form liquid-like columns, the different columns constituting a two-dimensional lattice (fig. 1.1.8 (a)). The structure is somewhat similar to that of the hexagonal phase of soap-water and other lyotropic... 

The basic columnar structure is as illustrated in fig. 1.1.8(a) it is somewhat similar to the hexagonal phase of soai>-water and other lyotropic systems (fig. 1.2.2). However, a number of variants of this structure have been found. Fig. 6.1.2 presents the different two-dimensional lattices of columns that have been identified here the ellipses denote discs or, more precisely, cores that are tilted with respect to the column axis. Table 6.1.1 gives the space groups of the columnar structures formed by some derivatives of triphenylene. (These are planar space groups that constitute the subset of the 230 space groups when symmetry elements relating to translations along one of the axes, in this case the column axis, are absent.)... 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

The formation of intermolecular stmcture of the cylindrical bmshes described in the previous section is mainly governed by their anisotropic shape, which enables them to form even lyotropic phases. Other driving forces are of ionic and/or entropic... 

In addition to the staled aims of this thesis, the phase diagram of the selected surfactant mixed with iV-methylformamide as solvent was investigated to show that no lyotropic SmC analog phase occurs with a solvent that does not form a three-dimensional hydrogen bond network. However, two other interesting phases appear by the addition of this solvent. The first phase is a rare example of a re-entrant cholesteric phase and the second is a solvent-induced twist grain boundary phase, the first observation of this phase in a lyotropic liquid crystal. 

This section pertains to reports on oriented molecules in which phases other than the usual thermotropic nematics have been used. Studies in chiral, smectic, columnar, lyotropic and polymeric liquid crystals as well as other unusual phases have been presented. The use of carbon-proton heteronuclear selective refocusing 2D NMR experiment designed for the spectral analysis of enantiomers dissolved in weakly ordering chiral liquid crystal solvents has been proposed." The method permits the extraction of carbon-proton residual dipolar couplings for each enantiomer from a complex or unresolved proton-coupled... 

Many other cellulose derivatives were studied and, among them, acetoxyproylcellu-lose (APC) was found to develop a thermotropic cholesteric phase as well as a lyotropic phase, in several organic solvents, at room temperature. Gray et at [18] prepared this cellulose derivative by the acetylation of hydroxypropylcellulose (a schematic of the chemical reaction is shown in Figure 8.3). 

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