Hexagonal liquid crystals, surfactant

These structures are commonly referred to as hexagonal liquid crystals. As the surfactant concentration is further increased, the tubules expand in a second direction to form large, stacked lamellar sheets of surfactants, commonly referred to as lamellar Hquid crystals. These Hquid crystals are very important in soap making. 

Of course, the surfactant molecules can also phase separate to form micelles within the solvent as shown in Fig. 10.30, in addition to other liquid aystal phases which can strongly influence particle nucleation and growth. Fot example, the formation of a hexagonal liquid crystal phase for 55% Tween surfactant dissolved in 45% water caused crystal growth of copper sulfate to be much inhibited, producing monosize dispersions of crystals around 2 pm in diameter. ... 

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). 

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.

A great many short-chain surfactants do not display either the hexagonal or lamellar liquid crystal as the separating phase above the Krafft eutectic they exhibit a cubic phase instead. Because the hexagonal liquid crystal is often found next to this cubic phase (at higher compositions), it may be presumed that this cubic phase has the discontinuous phase structure. The discontinuous cubic-phase structure consists of discrete micelle-like structural elements arrayed in various ways to form isotropic phase structures having cubic symmetry. A large family of such cubic structures exists [81,82]. 

Starting from an oil- and coemulsifier-free system, a 40% Cg j -APG/SLES mixture in water forms a hexagonal liquid crystal (11, ). The surfactant paste is highly viscous and nonpumpable at 25°C. 

The hexagonal liquid crystal rich in surfactants and the lamellar liquid crystal differ considerably in their reactions to the addition of oil. Whereas the hexagonal liquid crystal can only take up very small quantities of oil, the lamellar phase area extends far towards the oil comer. The capacity of the lamellar liquid crystal to take up oil clearly increases with increasing GMO content. 

The other type of porous glass that has cylindrical pores is mesoporous silicate (MPS) (14,15). The advantage of MPS is in its feasibility to make a small pore diameter, typically below 10 nm. A columnar-phase liquid crystal, formed from surfactant molecules with a long alkyl chain tail and silicate molecules, is calcined to remove hydrocarbons. At the end, a hexagonal array of straight and uniform cylindrical holes is created in a crystalline order. MPS is not available commercially either. 

The phase behavior of a-ester sulfonates has been studied in detail with methyl laurate and methyl palmitate [58]. In both cases, at higher temperatures, as the surfactant concentration increases, there is a transition from an isotropic solution to a hexagonal liquid crystalline phase and finally, at high surfactant concentrations, to a lamellar liquid crystal (Fig. 4). The crystal/liquid-crys-tal phase transition occurs at even higher temperatures as the chain length increases. On the other hand, chain length has practically no influence on the... 

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