Phase diagrams liquid crystalline-solvent

In the following subsection detailed phase diagrams of three solvent/surfactant mixtures will be presented. In two of those phase diagrams, i.e. the C50/water and the C50/formamide system, the lyotropic SmC analog phase can be found. Furthermore, as a counterexample for a C50 system in which the lyo-SmC phase does not occur, the C50/iV-methylformamide system was investigated. Characteristic textures of the individual phases will be displayed to document the correct assignment of the phases. Additionally, the other liquid crystalline phases which appear in the phase diagrams will be characterized briefly. 

If there are included among the diluents mixed with the crystalline polymer some which are sufficiently poor solvents, the phase diagram may then exhibit liquid-liquid phase separation, in addition to the liquid-crystal boundary curve. Examples are shown in Figs. 133... 

Recalling the previous assertion that efficient fractionation requires liquid-liquid phase separation, we conclude that nitrobenzene and amyl acetate should be satisfactory solvents from which to fractionate polyethylene by successively lowering the temperature and that the better solvent xylene should be avoided for this purpose. The character of the phase diagram may, in fact, be used as a criterion of the efficacy of a given solvent for fractionation (see Chap. VIII, p. 344). If the curve representing the precipitation temperature plotted against concentration rises monotonically, crystalline separation is clearly indicated if it passes through a maximum at a low concentration, liquid-liquid separation is virtually assured, and the solvent may be assumed to be a satisfactory one to use for fractionation. 

Prinsen et al. [23] and Warren et al. [31] used dissipative particle dynamics to simulate dissolution of a pure surfactant in a solvent. Tuning surfactant-surfactant, surfactant-solvent, and solvent-solvent interactions to yield an equilibrium phase diagram similar to Fig. 1 at low temperatures except for the absence of the V i phase, they found that the kinetics of formation of the liquid crystalline phases at the interfaces was rapid and that the rate of dissolution was controlled by diffusion, in agreement with the above experimental results. 

In order to analyze the dependence of the liquid crystalline transition properties on temperature (i.e. on the solvent quality), it is necessary to introduce the attraction of rods parallel to their steric repulsion. This has been done by Rory9 . The classical phase diagram of Rory for the solution of rods (see Fig. 2) agrees well with experimental results from the qualitative point of view1 . However, the Rory theory cannot give adequate answers to all the questions connected with the orientational ordering in the system of rigid rods. Indeed ... 

The existence of liquid crystalline phases of rodlike molecules in solution becomes obvious if we simply consider the binary phase diagram of rigid rod-like low molar mass molecules or macroinolecules A in a suitable solvent S (Figure 2). Because of its chemical... 

In this connection let us consider a fragment of a schematic phase diagram in the region of high concentrations of a polymer capable of forming the liquid crystalUne phase (Fig. 2). In a crystalUne polymer containing no solvent (100% polymer, vf), the transition from the crystalline state (c) to the Uquid crystalline state (Ic) must take place at the temperature T -.ic and further transition into isotropic state (i), at the temperature Such transitions are called thermotropic, and the system formed at I, is called the thermotropic liquid crystal. The transition to the Uquid crystalline state can also occur by adding to a polymer a solvent at a temperature below T -. c-... 

Fig. 12. Schematic phase diagram for a rigid-chain polymer-solvent system involving the formation of the liquid crystalline phase and crystallosolvate (according to...

The existence of a lamellar phase with lecithin has also been demonstrated for ethylammonium nitrate [82], For lecithin and formamide, /V-mclhylformamidc, or /V, /V- di m e ill y I lb mi i m i dc, the full phase diagrams have been determined [83] showing a gradual disappearance of liquid crystalline phases with increasing methylation of the solvent (Figure 6.3). The lamellar phase is stable with /V-mclhyl ionnamidc, but disappears with MA-dimethylformamidc. 

FIG. 1 Phase diagram for Aerosol OT (AOT)-water-octane system. The boundaries of each individual phase were determined with 50 mM phosphate + 50 mM acetate buffer as an aqueous component (—). (From Ref. 2.) LI, L2 normal and reverse micelles of surfactant, respectively D, F liquid crystalline mesophases with lamellar and reverse hexagonal packing of surfactant molecules, respectively. Concentrations of all components are expressed as %(w/w). Cross-section of a type shows an example of the variation of water content at constant surfactant-to-organic solvent ratio cross-section of p type shows an example of the variation of organic solvent content at constant water-to-surfactant molar ratio. 

It is well known that colloidal suspensions can share many features with simple molecular systems such as gas, liquid, and solid crystalline and amorphous glass phases. This is particularly true when the colloid is nearly monodisperse for then the interparticle interactions, which are usually size dependent, are nearly all the same and hence the phase boundaries, which depend on the interactions, are distinct. Indeed, as the size distribution of a colloid narrows, one could claim that the colloidal suspension transforms into a solution, just as the different particles, by becoming alike or even identical, are transforming to molecules. Unlike simple molecular systems, which by their definition have no variety and are not dissolved in a medium, particle colloids and solutions can vary the interactions via changing size, surface groups, solvent, etc., and thereby change the phase diagram. 

Fig. 7 (a) Phase diagrams calculated for DP = 100 and 200. (b) Plot of solvent-polymer interaction parameter (x) values and phase transition temperatures versus acid activity at a given polymer volume fraction of 0.45. / Isotropic phase, B biphasic region, LC liquid crystalline phase. Reprinted with permission from [80]. Copyright 1997 Elsevier... 

The phase transitions of liquid crystals in solutions occur normally through two mechanisms, i.e. lyotropic and thermotropic transitions. The lyotropic liquid crystal occurs upon addition of solvent into the crystalline phase, while the thermotropic liquid crystal occurs upon heating the crystalline phase, as illustrated by the two arrows in Fig. 10.3, respectively. The phase diagrams for the transition from the homogeneous solution to the liquid crystal are formed by two almost parallel curves, reflecting the concentration gap between the two coexisting phases. 

Fig. 3.9 Theoretical phase diagram of a lyotropic liquid crystal. The phase transition from one lyotropic liquid crystalline phase into another mainly depends on the solvent concentration...

The last investigated surfactant/solvent system is composed of the surfactant C60 and water. Its schematic phase diagram is shown in Fig. 5.7. As different phases appear while heating and while cooling, schematic phase diagrams for both processes were measured. The only enantiotropic stable liquid crystalline phase is the lamellar L phase. In contrast to the other investigated systems, this phase also exists as SmA phase in the neat state. Most likely this is due to the elongated... 

Up to now, only the structural demands on the surfactant were considered. But as the solvent is the second important component of the lyotropic liquid crystalline system, the nature of the solvent has to be reflected, too. Thus, to learn more about the role of the solvent, contact preparations of C50 and diverse hydrophilic solvents were screened. The schematic phase diagrams obtained will be discussed in the following section. 

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