Liquid-crystal lamellar phase

Micellar solutions are isotropic microstructured fluids which form under certain conditions. At other conditions, liquid crystals periodic in at least one dimension can form. The lamellar liquid crystal phase consists of periodically stacked bilayers (a pair of opposed monolayers). The sheetlike surfactant structures can curl into long rods (closing on either the head or tail side) with parallel axes arrayed in a periodic hexagonal or rectangular spacing to form a hexagonal or a rectangular liquid crystal. Spherical micelles or inverted micelles whose centers are periodically distributed on a lattice of cubic symmetry form a cubic liquid crystal. 

Fig. 9. Temperature-composition isobaric phase diagram for the fully hydrated dipal-mitoylphosphatidylcholine/dipalmitoyl-phosphatidylethanolamine system constructed using the temperature gradient method. The notation used is that of Luzzati [8] and is as follows Lc, lamellar crystalline (also referred to as the subgel phase) L f, lamellar gel phase with hydrocarbon chains tilted with respect to the bilayer normal P)el, ripple phase L. lamellar liquid crystal phase. Insert bold line in graph as indicated...

More and more disk-like micelles-that could be the promoters of the lamellar liquid-crystal phase-are formed with further addition of the cosurfactant, up to phase separation. In this region, then, the medium mode is the predominant one. 

DDAB films are in a lamellar liquid crystal phase at 25 C. The fluidity of this phase facilitates movement of the protein during voltammetry. In thick films, normal pulse voltammetry (NPV) limiting currents, a direct measure of [11,17] gave small NPV limiting currents for Mb-DD AB films at temperatures below the gel-to-Iiquid crystal phase transition temperature (T. ), where they are... 

Figure 3.4 The temperature-concentration phase diagrams of BPS-m (m=5,10,20, and 30) with BmimPFj. The phase abbreviations are as follows micellar phase (1 ), discontinuous cubic liquid crystal phase (1 ), hexagonal liquid crystal phase (H ), lamellar liquid crystal phase (L ), lamellar gel phase (L ), reverse micellar phase (L ), ionic liquid phase (IL), and two-phase separation (II). The chemical structure of j0-sitosterol ethoxylates as a typical example of BPS-m is also shown in this figure. Reproduced from Sakai et al. [37] with permission from Japan Oil Chemists Society.

The hexagonal and lamellar liquid crystal phases noted above require high surfactant concentrations. However, surfrctants can be forced to form a structured phase at relatively low surfactant concentrations using a technique known as salting-out At the required electrolyte concentration a surfiictant rich phase prec itates in die form of spherulites. The spherulites contain a significant amount of water, and a stable SSF can be formulated at a relatively low surfactant concentration. A schematic of an SSF is shown in Figure 2. 

Analogous phase behavior has been observed at the liquidus boundary of the DOACS water system. (DOACS is dioctadecylammonium cumenesulfo-nate, a thermally stable surfactant that also exists as a thermotropic lamellar liquid crystal phase at temperatures above its crystal phase.) A phase study of this system revealed that the temperature of this liquidus also increases as water is added [26]. Water would be expected to stabilize C12MG, provided no hydrolytic cleavage reactions (such as amide hydrolysis) occur [27], No... 

The next more concentrated liquid crystal (past the hexagonal phase) is a cubic phase, and the next is the lamellar phase. One may infer from its position that the cubic phase has a bicontinuous structure [35], but this was not established experimentally. The lower boundary of the lamellar liquid crystal phase region has a classical form, except that the region is truncated at the 100% border of the diagram. 

In soap bar processing free fatty acid is usually added in formulations to create so-called super-fatted soap. An acid-soap complex with a fixed stoichiometric ratio between alkaline soap and the fatty acid is formed. For example, the ratio of potassium acid soap is 1 1 while sodium soap forms acid soaps with various ratios. The fixed ratio complex exits not only in anhydrous crystalline phase but also in a hydrous liquid crystalline phase (11, 12). Oleic acid and its potassium soap form a 1 1 complex acid soap when equal molar acid and soap are mixed. Above the Krafft boundary, the acid soap in water forms a lamellar liquid crystal phase at low surfactant concentration, from a few percent, and the lamellar liquid crystal phase extends to ca 60% surfactant concentration. A hexagonal liquid crystal phase is formed after the lamellar liquid crystal phase with further increasing the surfactant concentration. This phase behavior is different from the soap and water phase behavior, in which the hexagonal liquid crystalline phase is formed first followed by the lamellar liquid crystalline phase. Below the Krafft boundary the acid soap complex forms a solid crystal and separates from water (4). 

An acid soap of 2 1 complex ratio was discovered recently between triethanolamine stearate and stearic acid (13). In water the acid soap forms a lamellar liquid crystal phase at high temperatures, above 60°C, and transforms to a lamellar gel phase on cooling. The gel phase, however, is not stable at ambient temperature due to the occurrence of the hydrolysis reaction which converts the soap back to stearic acid which precipitates in the triethanolamine aqueous solution. A polymorphism of C, E, and possibly A forms of stearic acid crystals were found resulting from the hydrolysis reaction (8). 

Figure 3 A prototypical phase diagram of a surfactant whose Krafft boundary lies above 0 C and which displays typical hexagonal and lamellar liquid-crystal phases. The temperature regions within which the crystal, the hexagonal liquid-crystal, and the lamellar liquid-crystal solubility boundaries exist are shown. The crystal solubility boundary (below the temperature of the Krafft eutectic) is the Krafft boundary. The magnitude of the solubility below the knee is greatly exaggerated in this figure for the sake of clarity.

Figure 8 The iV-dodecanoyl-iV-methylglucamine-water system, which illustrates the classical sequence of changes in the solubility boundary with temperature found in many nonionic and ionic surfactants. This compound is somewhat unusual in that the lamellar liquid-crystal phase extends to 100% and so exists as a thermotropic liquid crystal, which melts to an isotropic liquid at about 121 C. Physical studies in this temperature region are compromised by chemical instability. (From Ref. 91.)...

Diglycerol esters of samrated fatty acids have recently been extensively studied by Shrestha and co-workers in both aqueous [83] and nonaqueous [84, 85] systems. The phase behaviour of caprate (CIO) and laurate (C12) esters in water were found to be quite different from the solution behaviour of the myristate (C14) and palmitate (Cl6) esters (see Figure 1.5). In the former, a lamellar liquid crystal phase is present in the surfactant-rich region and it absorbs a substantial amount of water. The melting temperature of this phase is practically constant in a wide range of compositions. For the more hydrophobic surfactant the phases with solids and the extent of water solubilization are increased. 

Ishikawa et al. [201] established triangular phase diagrams of the LiDS-al-cohol-water and LiFOS-aleohol-water systems, for fluorinated alcohols (C F2 +iCH20H, n - 1, 2, 3) and hydrogenated alcohols (C,H2+iOH, n = 3, 4, 5,6). When alcohol was added, the phase boundaries of hexagonal and lamellar liquid-crystal phases assumed an outline that is convex to the water side. The alcohol promoted the formation of these mesophases and this effect increased with their increasing carbon number of the alcohols. A phase separation between the hydrocarbon and fluorocarbon components did not occur. 

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