Phase diagram lyotropics

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...

The difference between this system and an usual supersaturated solution is that two dimensions of the crystallites which are deposited are in the colloidal range, ca 10-200 nm. Only part of the phase diagram has been represented and exploited. For higher concencrations ( > 10 H ), gelation is complicated by some turbidity and existence of lyotropic mesophases. ... 

Figure 6 shows the phase diagrams plotting temperature T vs c for PHIC-toluene systems with different Mw or N [64], indicating c( and cA to be insensitive to T, as is generally the case with lyotropic polymer liquid crystal systems. This feature reflects that the phase equilibrium behavior in such systems is mainly governed by the hard-core repulsion of the polymers. The weak temperature dependence in Fig. 6 may be associated with the temperature variation of chain stiffness [64]. We assume in the following theoretical treatment that liquid crystalline polymer chains in solution interact only by hardcore repulsion. The isotropic-liquid crystal phase equilibrium in such a solution is then the balance between S and Sor, as explained in the last part of Sect. 2.2. 

The change of lyotropic l.c. behavior of a monomer in the hexagonal phase by polymerization has been described for the first time by Friberg et al. I07,108). A change from the hexagonal monomer phase to the lamellar phase of the polymer was observed and carefully identified. Complete phase diagrams of monomer and polymer, however, were not compared. 

The phase behavior of surfactant systems is particularly complex because of the existence of numerous lyotropic (solvent-induced) liquid crystal phases (3). These phases, like liquids and crystals, are discrete states of matter. They are fluids, but their x-ray patterns display sharp lines signifying the existence of considerable structure. They are often extremely viscous because of their high viscosities and for other reasons they are difficult to study using conventional methods. This is evident from the fact that serious errors in the presumably well-established classical aqueous phase diagrams of soaps, sodium alkyl sulfates, monoglycerides, and... 

The calculated results in the absence of electrolyte will be now compared with the experimental results obtained regarding a lamellar lyotropic liquid crystal SDS (sodium dodecyl sulfate)/pentanol/water/dodecane swollen in a mixture of dodecane and pentanol.24 The weight fraction water/surfactant was 1.552 from the dilution line in the phase diagram, we calculated that the initial concentration of pentanol in the oil-free system was 29 wt % and the concentration of pentanol in the dodecane-based diluant was 8 wt %. The experimental values for the repeat distance were obtained from the X-ray diffraction spectrum (Figure 2 in ref 24) for various dodecane concentrations. 

Figure 2 Illustrative lyotropic lipid-water phase diagram for phospholipid amphiphiles with transitions between the phase ranges driven by the water content. Hatched areas indicate two-phase regions (reproduced from (52) with permission from Elsevier).

Because biopolymers may have properties uncommon in synthetic systems, they can be very attractive as model systems to test specific ideas. An early example of this can be seen in the work on PBLG, a synthetic polypeptide. Although the motivation for its original synthesis failed, it provided a firm basis for many of the early studies on lyotropic liquid crystalline polymers. It was one of the first systems to have its phase diagram characterised, for comparison with Flory s predictions, and a study of its viscosity demonstrated that there is a non-monotonic increase in viscosity with concentration as the liquid crystalline phase is entered. 

Figure 4.13. (Left ) The binary phase diagram of AOT-water mixtures (after (17,18]). The lyotropic behaviour at room temperature is illustrated by the line ABC. (Right) The trajectory of die line ABC in the local/global domain.

Figure 8.6 Cj2E08-water lyotropic liquid-crystal phase diagram. Reproduced with permission from [63], Copyright (1983) Royal Society of Chemistry...

Other surfactants show the similar lyotropic liquid-crystal phase behavior and follow the same succession of phases, but not all of the phases are always present. Figure 8.7 shows a phase diagram for the CTAB-water binary system. CMC can also be classified CMC1 (spherical micelle) and CMC2 (rod-shaped micelle). 

Figure 8.7 The lyotropic liquid crystal phase diagram for CTAB-water system...

The formation of lyotropic liquid-crystal mesophase depends on the structure and properties of surfactant, solvent, and reaction conditions. Although studies on lyotropical liquid crystals have been carried out for many years, the structure and properties of some mesophases are still not very clear. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is difficult to analyse their structures and properties, the boundary in the phase diagram may be not accurate and the minor phase may be missed. 

SBA-1 and SBA-6 were synthesized by using different surfactants and from acidic and basic synthesis media, respectively. They have the same structure and show the similar XRD patterns. Their structure is similar with cubic Ij phase, spherical micelles packed in Pm3n symmetry, in lyotropic liquid-crystal phase diagram for surfactant-water systems. 

The common structure models for Iniim symmetry are shown in Figure 8.28. Among the well known lyotropic liquid-crystal mesophases, these are at least two mesostructures with Im3m symmetry one locates near the Ii region in the phase diagram, with a possible spherical micelle packed structure. Another one is close to the Vi region, and its most probable structure can be described by a P surface. 

The phase diagram of sodium dodecyl sulfate-water is representative of many ionic systems (Figure 3.7) [5], In Figure 3.7 Liquid is the aqueous micellar phase Ha is the hexagonal lyotropic liquid crystal, sometimes called the middle phase and La is the lamellar lyotropic liquid crystal, sometimes called the neat phase. On the surfactant-rich side, several hydrated solid phases are present. 

FIG. 3.14 Binary phase diagram of a water-ethoxylated nonionic amphiphile system, including lyotropic liquid crystal domains. (From Kalhweit, M. and Strey, R., Angew. Chem. Int. Ed. Engl., 24, 654, 1985. With permission.)... 

The closed loop is not the only characteristic of the nonionic surfactant-water binary phase diagram. Like the ionic surfactant-water mixture, nonionic surfactants, at higher concentration in water, exhibit lyotropic mesophases. Figure 3.14 shows a typical binary phase diagram exhibiting the full lyotropic mesophase sequence II, cubic isotropic phase HI, direct hexagonal phase (middle phase) VI, special cubic ( viscous phase) La, lamellar phase (neat phase). Note the presence of the two-phase domains surrounding each mesophase, the critical point on top of each, and the zero-variant three-phase feature. 

Winsor behavior is not the only characteristic of water-oil-nonionic amphiphile systems. The lyotropic mesophases appearing on the water-amphiphile binary phase diagrams expand to some extent in the Gibbs triangle (Figure 3.19). 

---