Lamellar structure, liquid crystalline ionic liquids

The formation of layered assemblies can be induced by the addition of a small amount of water to isotropic ionic liquid l(10/Br ) [52, 53], A lyotropic liquid crystalline gel consisting of l(10/Br ) and water of 16wt% has been prepared. Addition of water to l(10/Br ) induces the formation of a lamellar structure with... 

Membrane lipids are invariably polymorphic that is, they can exist in a variety of kinds of organized structures, especially when hydrated. The particular polymorphic form that predominates depends not only on the stmcture of the lipid molecule itself and on its degree of hydration, but also on such variables as temperature, pressure, ionic strength and pH (see References 11 and 12 and article Lipids, Phase Transitions of). However, under physiologically relevant conditions, most (but not all) membrane lipids exist in the lamellar or bilayer phase, usually in the lamellar liquid-crystalline phase but sometimes in the lamellar gel phase. It is not surprising, therefore, that the lamellar gel-to-liquid-crystalline or chain-melting phase transition has been the most intensively studied lipid phase transition... 

Oils. SANS has been used to establish the effect of the addition of a hydrophobic guest (dodecane) on the behavior of liquid crystalline phases, in particular the lamellar and columnar phases of mixtures of the non-ionic surfactant C16E7 with D2O, as well as to determine the distribution of the hydrophobic guest in the microstructure. SANS showed that the presence of the hydrophobic guest molecule, in some cases, stabilized a particular phase structure, (for example lamellar phases formed at lower temperatures in the presence of dodecane) while in other cases it destabilized it, eventually (depending upon the concentration of dodecane added) causing the phase to disappear. In the lamellar phase, dodecane was found to be totally segregated in the center of the bilayer. 

Analogous gel matrices of liquid crystalline lamellar phases can also be formed with non-ionic mesogens, for example, with the combination of cetyl/stearyl alcohol and ethoxylated fatty alcohol, provided the hydrophilic and lipophilic properties of the surfactant molecules are more or less balanced to favor the formation of lamellar structures. 

Extensive recent studies has established that liquid crystallinity is not only induced by the presence of mesogenic groups (7). but also induced in certain molecules by the existence of distinct polar and non-polar moieties which segregate forming lamellar structures. These amphiphilic molecules (1.7-1S). both ionic and non-ionic, exhibit liquid crystalline phases in the melt, and form aggregates in solution which generate molecular organizates (19). 

A typical phase diagram of a ternary system of water, ionic surfactant and long-chain alcohol (co-surfactant) is shown in Figure 15.5. The aqueous micellar solution A solubilizes some alcohol (spherical normal micelles), whereas the alcohol solution dissolves huge amounts of water, forming inverse micelles, B. These two phases are not in equilibrium, but are separated by a third region, namely the lamellar liquid crystalline phase. These lamellar structures and their equilibrium with the aqueous micellar solution (A) and the inverse micellar solution (B) are the essential elements for both microemulsion and emulsion stability [3]. 

The solubility-temperature relationship for nonionic surfactants shows a different behavior from ionic surfactants. Figure 20.6 shows the phase diagram of Ci2E06-The nonionic surfactant forms a clear solution (micellar phase) up to a certain temperature (that depends on concentration) above which the solution becomes cloudy. This critical temperature, denoted as the cloud point (CP) of the solution, decrease with increase in surfactant concentration reaching a minimum at a given concentration (denoted as the lower consolute temperature) above which the CP increases with further increase in surfactant concentration. Above the CP curve the system separates into two layers (water -I- solution). Below the CP curve, several liquid crystalline phases can be identified as the surfactant concentration exceeds a certain limit. Three different liquid crystalline phases can be identified, namely, the hexagonal, the cubic, and lamellar phases. A schematic picture of the structure of these three phases is shown in Fig. 20.7. 

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