Liquid crystals Middle phase

Whereas an ethoxylated alcohol with dodecyl tails (e.g. C12E5) forms middle-phase microemulsions, ionic surfactants with dodecyl tails, such as sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB), are too hydrophilic for formation of middle-phase microemulsions. Simply increasing the length of the hydrocarbon tail to compensate for the high hydrophilicity of the ionic head-groups favours the formation of viscous liquid crystal line phases rather than fluid microemulsion phases (36, 37). However, increasing the hydrophobicity by adding double tails to the surfactant, as for example with didodecyldimethylam-monium bromide surfactant (DDAB), suppresses some of the tendency to form liquid crystals, and allows for formation of oil-rich microemulsions (38). However, this surfactant is too hydrophobic, and is far from the... 

Figure 2.2 Condensed binary phase diagrams for (a) CgE + H2O (b) C10E6 + H2O and (c) C12E6 + H2O. A, two isotropic liquids B, one isotropic liquid C, middle phase D, ice + liquid E, crystals + liquid F, ice + crystals G, neat phase. Full lines experimental boundaries dotted lines interpolated boundaries. The estimated extent of the two phase co-existence region between phases B and C and B and G is shown by the thickness of the boundary line. From Corkill et al. [1, 10] with permission.

Liquid crystals (LC) are phase structures that are intermediate between liquid and crystal phases. They have also been mentioned as mesophases (Greek mesos = middle). Liquid crystals have an intermediate range of order between liquid and crystal phases (Soltis et al., 2004 Friberg, 1976). LC may be described as follows. If a pure substance, such as stearic acid, is heated, it melts at a very specific temperature. Heating a pure solid shows the following behavior ... 

FIG. 8.8 Schematic representations of surfactant structures in (a) viscous isotropic, (b) middle, and (c) neat liquid crystal phases. 

The cubic amphiphilic mesophases (Sic, Vi, and V2) from their interposition in the succession of mesophases Sic, Mi, Vi, G, V2, and M2, have generally been termed liquid crystalline like the optically anisotropic amphiphilic mesophases Mi, G, and M2. The cubic mesophases formed by non-amphiphilic globular molecules have however usually been termed plastic crystals. This nomenclature has obscured the fact that these plastic crystals are fundamentally liquid crystals rather than solid cyrstals and bear a relationship to the optically anisotropic non-amphiphilic smectic and nematic liquid crystals similar to that born by the amphiphilic cubic mesophases to the optically anisotropic neat (G) and middle (Mi and M2) liquid crystalline phases. 

The behavior of a series of polyoxyethylene alkyl ether nonionic surfactants is also illustrative. According to Figure 11 the dioxyethylene (A) compound does not form liquid crystals when combined with water. Its solutions with decane dissolve water only in proportion to the amount of emulsifier. The tetraoxyethylene dodecyl ether (B) forms a lamellar liquid crystalline phase and is not soluble in water but is completely miscible with the hydrocarbon. The octaoxyethylene compound (C) is soluble in both water and in hydrocarbon and gives rise to three different liquid crystals a middle phase, an isotropic liquid crystal, and a lamellar phase containing less water. If the hydrocarbon p-xylene is replaced by hexadecane (D), a surfactant phase (L) and a lamellar phase containing higher amounts of hydrocarbon are formed in combination with the tetraoxyethylene compound (B-D). 

The importance of a surfactant - rich phase, particularly a lamellar one, to detergency performance was noted for liquid soils such as C16 and mineral oil (3.6). Videomicroscopy experiments indicated that middle phase microemulsion formation for C12E04 and Cjg was enhanced at 30 °C, while at 18 °C, oil - in - water, and at 40 °C, water - in - oil microemulsions were found to form at the oil - bath interface (3.6). A strong temperature dependence of liquid soil removal by lamellar liquid crystals, attributed to viscosity effects, has been noted for surfactant - soil systems where a middle - phase microemulsion was not formed (10). 

A mesomorphic (liquid-crystal) phase of soap micelles, oriented in a hexagonal array of cylinders. Middle soap contains a similar or lower proportion of soap (e.g., 50%) as opposed to water. Middle soap is in contrast to neat soap, which contains more soap than water and is also a mesomorphic phase, but has a lamellar structure rather than a hexagonal array of cylinders. Also termed clotted soap . See Neat Soap. 

The selectivity of liquid crystals can also play a role in the separation of linear n-alkene isomers. On liquid crystal phases, alternation is pronounced in sepa rations of alkenes with a double bond in the middle of... 

This experiment will introduce you to some techniques of chemical synthesis based on acid-base reactions, purification by recrystallization, and brief characterization by melting point. Also on the characterization side, you will investigate the absorption of polarized light by matter. A liquid crystal is a state of matter neither liquid nor crystal but a state in-between. Liquid crystals are often called mesophases after the Greek mesos for middle. Normally when a crystal melts it forms an ordinary liquid phase. However, a substance which exhibits liquid crystalline behavior melts at least twice, first into the liquid crystalline or mesophase, and second, into the ordinary liquid. 

The middle diagram of Figure 11 illustrates phase behavior that may be more desirable for some applications. In this case the presence of a lamellar liquid crystal region in the phase diagram, although it will not be revealed by experiments confined to low surfactant concentrations, may be exploitable for the formation of very stable dispersions (82). 

The presence of high concentrations of salt changed the microemulsion region considerably, but had only limited effect on the transport properties. The main effect was that the duration of the liquid crystal presence was shortened and that an isotropic liquid middle phase was formed. 

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

Another important factor is the salinity of the aqueous phase. The presence of high concentrations of electrolyte usually destabilizes a liquid crystalline phase (18) of a charged surfactant and a long chain alcohol the present results show the temporary liquid crystals to exist only for a few days, when the water was 1 M or 1.7 M NaCl solution. Figs. 14A,B. After that time the liquid crystal was replaced by an isotropic liquid middle phase (2,3). 

The first part of the book discusses formation and characterization of the microemulsions aspect of polymer association structures in water-in-oil, middle-phase, and oil-in-water systems. Polymerization in microemulsions is covered by a review chapter and a chapter on preparation of polymers. The second part of the book discusses the liquid crystalline phase of polymer association structures. Discussed are meso-phase formation of a polypeptide, cellulose, and its derivatives in various solvents, emphasizing theory, novel systems, characterization, and properties. Applications such as fibers and polymer formation are described. The third part of the book treats polymer association structures other than microemulsions and liquid crystals such as polymer-polymer and polymer-surfactant, microemulsion, or rigid sphere interactions. 

Vesicles are ordered fluids or liquid crystals, a fact which reflects well in those photoreactivities that are particular to vesicle solutions. Fatty acid derivatives 24 and 25, for example, show low quantum yields of fluorescence ((j)f) and high quantum yields of cis-trans isomerization (< )c) as well as short fluorescence life times (tf) in both methylcyclohexane and micellar SDS solutions (Table 1). In DPPC vesicles, on the other hand, cis-trans isomerizations are cumbersome and much slower, and the fluorescence yield and lifetime rise considerably (Table 1). For those stilbene derivatives which are embedded in the middle of a fatty acid backbone, isomerization is virtually eliminated in the low-temperature or gel phase of the bilayer. The vesicle thus plays the role of stabilizing trans configurations which fit into the frozen oligomethylene chain matrix. 

Use of liquid crystalline phases Surfactants produce liquid crystalline phases at high concentrations. Three main types of Hquid crystals can be identified hexagonal phase (sometimes referred to as middle phase) cubic phase and lamellar (neat phase). All of these structures are highly viscous and also show elastic responses. If produced in the continuous phase of suspensions, they can eliminate sedimentation of the particles. These Hquid crystalline phase are particularly useful for application in liquid detergents which contain high surfactant concentrations. Their presence reduces sedimentation of the coarse builder particles (phosphates and silicates). 

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. 

Over the past two decades, liquid crystal polymers (LCP s) have received a considerable amount of attention in both academic and industrial laboratories. Often termed mesomorphic (meaning having "middle form"), liquid crystalline phases have a degree of order between that of the zero ordered liquid and that of the three dimensional crystal lattice. Recent reviews of liquid crystal polymers have provided a fundamental understanding of the synthesis, classification, morphology, and rheology of this unique class of materials (52-541. 

Helpful tools for this structurization of liquid crystal research were temperature dependent X-ray investigations [36] of natural and synthetic lipids, and the discovery that mesophases may be identified by their different textures appearing in the microscope using crossed polarizers [37]. In the decade starting in about 1957 systematic screening of the concentration and temperature dependency of the major lyotropic mesophases was done and models of the molecular arrangement in the different phases were developed [38-45] (e.g., the so-called middle or neat phases [38], the cholesteric phase of polypeptides and nucleopep-tides [44]). 

The term liquid crystal is in some ways unfortunate, since materials in this state are not crystalline and they may but need not be liquid. A preferable term that is often used is mesophase, which means middle phase and attempts to indicate that the order in this state is between that of the liquid state and that of the crystalline state. Mesogens are then molecules that tend to form mesophases and the attribute of being liquid-crystallinelike is called being mesomorphic. 

It is worth noting that this description assumes implicitly that the = 1 case is associated with a zero-curvature C layer, which could be provided either by a lamellar liquid crystal structure with alternating O and W flat layers or a zero curvature surface of the Schwartz type or as a transient and fluctuating combination of Si and S2 structures. It is now well recognized that middle-phase microemulsions, which are in equilibrium with both oil and water excess phases, exhibit bicontinuous structures as shown in Fig. 7 [27] that are not far from the transient mixture of Si and S2 swollen micelles predicted by Winsor. 

Figure 1 Effect of changing the fi hexane conceiuration of the emulsion e< uilibration time for systems containing different proportions of phos Aaicd nonylphenol elhoxytate (PNE) and phosphated fatly alcohol eihoxylate (PFE). 9. System containing 9 1 PNE/ PEE O. system containing 1 9 PNE/PFE . system emnaining I I PNE/PFE. (a> Mixture consists of middle phase liquid crystal, (b) Mixture consists of gel phase, (c) Mixture consists of neat phase liquid crystal. (Frani Ref. 7.)...

Substances that form liquid crystals are often composed of rod-shaped molecules that are somewhat rigid in the middle. In the liquid phase, these molecules are oriented randomly. In the liquid crystalline phase, by contrast, the molecules are arranged in specific patterns as illustrated in A FIGURE 11.32. Depending on the nature of the ordering, liquid crystals are classified as nematic, smectic A, smectic C, or cholesteric. 

Figure 6 Schematic phase diagram of a water (W)/nonionic surfactant (S)/oil (O) system at the HLB temperature and self-organizing structures (a) normal" lamellar liquid crystal (b) normal vesicles (c) bicontinuous surfactant phase (middle-phase...

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