Lamellar liquid crystalline structure

The short bulky aromatic compound does not pack well in a lamellar liquid crystalline structure, the mutual stabilizing action of the straight hydrocarbon chains is lost, and instability results. 

Cosmetic creams and lotions may be formulated with ingredient designed to penetrate the outer layer of the skin (the stratum corneum), or explicitly not to enter this layer. Liposomes can be a useful means of delivering selected chemical species since they form lamellar liquid crystalline structures on the surface of skin, do not disrupt the outermost layer of skin and therefore do not cause skin irritation [236]. See also Section 14.2 on vesicles and drug delivery. 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

It is interesting to note, however, that a polymer foam of sufficient durability could be produced even at high surfactant concentrations if, for example, a formulation of polymerising surfactant such as sodium co-acrylamidoundecanate or oleic alcohol is used [134]. This can be realised only in the region of the phase diagram corresponding to the existence of a lamellar liquid-crystalline structure. 

The special restriction caused by tying low molecular mass liquid crystalline substances to a polymer chain was also illustrated with amphiphilic liquid crystals. A hexagonally close-packed structure of rod-like micelle cylinders of sodium 10-undecenoate with about 50% water lost during polyma-ization at 60 °C its structure and became isotropic. On cooling, a lamellar liquid crystalline structure, more suitable to accommodate the macromolecular backbone was found. Bas l on the discussions of Sect. 5.3.4 it is likely that with longer side-chain amphiphiles condis crystals could be grown in analogy to the soaps described in Sect. 5.2.3... 

Even though hydrotropes exhibit a resemblance to surfactants, a number of differences are obvious. The amount of hydrotrope needed to facilitate solubilization of the solute in water is usually much higher than that needed for surfactants. The reason for this is that the shorter carbon chain of the hydrotrope will result in a higher concentration for self-association, which is a requirement for solubilization (12). In the case of hydrotropes, the maximum solubilization will usually be higher than for surfactants. This might be explained by the fact that the micellar solution of a surfactant will be transformed into an inverse micellar solution via the formation of a lamellar liquid crystalline structure and the solubilization of the solute in water will be... 

TTiis is particularly the case with nonionic surfactants that produce a lamellar liquid crystalline structure in the film between the bubbles [24, 25]. These liquid crystals reduce film drainage as a result of the increase in viscosity of the film. In addition, the liquid crystals act as a reservoir of surfactant of optimal composition to stabilise the foam. 

Vesicles are ideal systems for cosmetic apphcations. They offer a convenient method for solubilizing active substances in the hydrocarbon core of the bilayer. They will always form a lamellar liquid crystalline structure on the skin and, therefore, they do not disrupt the structure of the stratum corneum. No facUitated trans-dermal transport is possible, thus eliminating skin irritation (unless the surfactant molecules used for making the vesicles are themselves skin irritants). Indeed, phospholipid liposomes may be used as in vitro indicators for studying skin irritation by surfactants [14]. 

The mono-olein-water phase diagram (Figure 15c) shows the formation of lamellar liquid crystalline structure at room temperature (20 °C) at water content between 2 and 20%. At higher water concentrations, a cubic phase is formed, which above 40% water exists in equilibrium with water. If the temperature of the cubic phase is increased above 90 °C, a hexagonal II phase is produced. 

The most useful liquid crystalline structures in personal care applications are those of the lamellar phase. These lamellar phases can be produced in emulsion systems by using a combination of surfeictants with various HLB numbers and choosing the right oil (emollient). In many cases liposomes and vesicles are also produced by using lipids of various compositions. Two main types of lamellar liquid crystalline structures can be produced oleosomes and hydrosomes (Fig. 1.21). 

Fig. 5.1 Schematic representation of lamellar liquid crystalline structures.

Fig. 5.12 Schematic representation of the lamellar liquid crystalline structure at the oil/water interlace.

As discussed above, for emulsion stabihzation in food systems lamellar liquid crystalline structures are ideal. At the interface, the liquid crystals serve as a viscous... 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

For mixtures of lecithin plus Na cholate it appears possible to infer the molecular arrangement in the dispersed micelles from the most likely structure of the liquid crystalline phase suggested by x-ray analysis. However, there are cases where dispersion is not possible because neither component is sufficiently hydrophilic to be dispersed even when alone in water. This is shown by the association of cholesterol and lecithin in the presence of water. The ternary diagram of Figure 4 is relative to these systems. Here only the lamellar liquid crystalline phase is obtained (region 1< in Figure 4). This phase is already given by lecithin alone, which can absorb up to 55% water. Cholesterol can be incorporated within this lamellar phase up to the proportion of one molecule of choles-... 

Figure I. Difference in the phase region of the lamellar liquid crystal (black) when an aromatic hydrocarbon (left) is replaced by an aliphatic one (right) demonstrates the sensitivity of the lyotropic liquid crystalline structure to weak intermodular forces. The emulsifier is a polyoxyethylene (9) nonyl phenol ether.

It is essential to realize that any thermodynamic evaluation of this solubility "maximum" with standard reference conditions in the form of the three pure components in liquid form is a futile exercise. The complete phase diagram. Fig. 2, shows the "maximum" of the solubility area to mark only a change in the structure of the phase in equilibrium with the solubility region. The maximum of the solubility is a reflection of the fact that the water as equilibrium body is replaced by a lamellar liquid crystalline phase. Since this phase.transition obviously is more. related to packing constraints — than enthalpy of formation — a view of the different phases as one continuous region such as in the short chain compounds water/ethanol/ethyl acetate. Fig. 3, is realistic. The three phases in the complete diagram. Fig. 2, may be perceived as a continuous solubility area with different packing conditions in different parts (Fig. 4). 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

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... 

A full analysis of interactions in multiple emulsions would obviously have to take account of forces of repulsion. The systems are too complex to (Fig. 7) allow any reasonable estimate of repulsive forces at this stage, although simplified models are being developed to allow an approach along this route. One complication resides in the possible lamellar nature of interfacial films or liquid crystalline structures, discussed earlier. 

The most important emulsifiers are polar lipids which interact with water to form liquid crystalline structures. These are often three dimensional stmctures which can assume different geometric configurations. The most active is referred to as the layered or lamellar phase. Figure 3.43 shows the lamellar form as a two dimensional structure. 

Formation of lamellar liquid crystalline phases at the O/W interface This mechanism, as suggested by Friberg and coworkers [37], proposed that surfactant or mixed surfactant film can produce several bilayers that wrap the droplets. As a result of these multilayer structures, the potential drop is shifted to longer distances, thus reducing the van der Waals attractions. A schematic representation of the role of Hquid crystals is shown in Figure 10.32, which illustrates the difference between having a monomolecular layer and a multilayer, as is the case with hquid crystals. 

When surface active agents are considered, a further complication may be encountered. Because of their surface active nature, the surfactants not only emich at the surfaces, but also form extended structures themselves. At low concentrations, the surfactants remain as dissolved monomers or asssociate to oligomers. However, when the critical micellization concentration (cmc) is surpassed, a cooperative association is activated to micelles (1 to 10 nm) consisting typically of some 50 to 100 monomers. At stiU higher concentrations, or in the presence of cosurfactants (alcohols, amines, fatty acids, etc.), liquid crystalline phases may separate. These phases have an infinite order on the x-ray scale, but may remain as powders on the NMR (nuclear magnetic resonance) scale. When the lamellar liquid crystalline phase is in equilibrium with the liquid micellar phase the conditions are optimal for emulsions to form. The interface of the emulsion droplets (1 to 100 pm) are stabilized by the lamellar liquid crystal. Both the micelles and the emulsions may be of the oil in water (o/w) or water in oil (w/o) type. Obviously, substances that otherwise are insoluble in the dispersion medium may be solubilized in the micelles or emulsified in the emulsions. For a more thorough analysis, the reader is directed to pertinent references in the literature. ... 

More lipophilic surfactants form larger, nonspherical micelles, vesicles, or lyotropic liquid crystalline phases at rather low concentrations in water. For example, at temperatures above those where the chains form crystalline structures, phospholipids and other surfactants with two relatively long hydrocarbon chains typically form the lamellar liquid crystalline phase consisting of many parallel surfactant bilayers separated by water layers. The hydrocarbon interiors of the bilayers are rather fluid as in micelles. Of course, in this case a true phase separation occurs beginning at a definite surfactant concentration. 

The formula for the dicarboxylic acid (Figure 2.9) has a hydrophilic/lipophilic balance similar to that of octanoic acid, but the influence of the two acids on amphiphilic association structures is entirely different, as shown in Figure 2.10 [93], The octanoic acid causes the formation of a liquid crystal when added to a solution of water in hexylamine. The size of the lamellar liquid crystalline region is large (Figure 2.10a). Addition of the dicarboxylic acid, in contrast, gives no liquid crystal, and it may be concluded that its action in concentrated systems is similar... 

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