Emulsions liquid crystal stabilization

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

Liquid Crystal Stabilization. Davies 15) first proposed the formation of a viscous layer around emulsion droplets when he calculated that the electrical forces, interfacial tension, and solvation were not sufficient to explain the stability observed experimentally in many emulsions. Furthermore, he could not explain why many emulsions containing aromatic hydrocarbons formed flocculated clusters that resisted coalescence, whereas the same systems, in which aliphatic hydrocarbons were substituted for the aromatic hydrocarbons, displayed no such stability against coalescence. Davies reasoned that his postulated thick, viscous layers arose from interaction between the liquid molecules and the emulsifier. 

Friberg et al. (17) demonstrated that a sudden increase in emulsion stability arose in the concentration range where a liquid-crystalline phase could be separated from the emulsion. They postulated, with good reason, that this liquid-crystalline phase was the viscous layer around the emulsion droplets that Davies had been seeking. Further, the liquid-crystal stabilization hypothesis explained the difference in stability between emulsions of aromatic and aliphatic hydrocarbons, because aromatic hydrocarbons, because of their large polarizability, are more prone to form lyotropic liquid-crystal structures than aliphatic hydrocarbons. 

Eccleston [19] reviewed the emulsion stability factors on the basis of the HLB and of the DLVO theory. He emphasized the Friberg school results which show that the presence of liquid crystals stabilizes the emulsions by delaying the film-thinning process and, consequently, by reducing the rate of coalescence. He also mentioned stabilization by the formation of a gel network (surfactant-fatty alcohol-water system). Dahms [20] explained the role of fatty alcohol as a viscosity modifier on the basis of lamella-phase generation. 

Kavaliunas DR, Frank SG. 1978. Liquid crystal stabilization of multiple emulsions. J Colloid Interface Sd 66 586-588. 

In the case of emulsions with three liquids the presence of the third phase results in a reduction of the energy input for the emulsification process, whereas the emulsion with a liquid crystal as the third phase shows interesting stabilization mechanisms. Finally, the emulsion with added panicles illustrates the importance of liquid-solid wetting for stability. 

The introduction of liquid crystals as stabilizing elements for emulsions occurred in 1969 when it was found that the sudden stabilization at emulsifier concentrations in excess of 2.5% of a water—p-xylene emulsion by a commercial octa(ethylene glycol) nonylphenyl ether was due to the formation of a liquid crystalline phase in the emulsion (26). Later investigations confirmed the strong stabilizing action of these structures (27). 

The final factor influencing the stability of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a liquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the liquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 Xm to emulsion droplets of 5 im if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 Jim thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% liquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

Dynamic processes are prominent among potential applications of lyotropic liquid crystals. It is known, for example, that emulsion stability is greatly enhanced when liquid crystal forms at the surfaces of the drops (4). However, emulsion formation is invariably a dynamic process, involving fluid flow and transport of the surfactant from one liquid to the drop surfaces and the other liquid. Since the time required to reach equilibrium in systems containing liquid crystals can sometimes be quite long—weeks or months—the need for knowledge about dynamics of the process is evident. 

In view of the importance of macroscopic structure, further studies of liquid crystal formation seem desirable. Certainly, the rates of liquid crystal nucleation and growth are of interest in some applications—in emulsions and foams, for example, where formation of liquid crystal by nonequilibrium processes is an important stabilizing factor—and in detergency, where liquid crystal formation is one means of dirt removal. As noted previously and as indicated by the work of Tiddy and Wheeler (45), for example, rates of formation and dissolution of liquid crystals can be very slow, with weeks or months required to achieve equilibrium. Work which would clarify when and why phase transformation is fast or slow would be of value. Another topic of possible interest is whether the presence of an interface which orients amphiphilic molecules can affect the rate of liquid crystal formation at, for example, the surfaces of drops in an emulsion. 

Understanding surfactant phase behavior is important because it controls physical properties such as rheology and freeze-thaw stability of formulations. It is also closely related to the ability to form and stabilize emulsions and microemulsions. Micelles, vesicles, mi-croemulsions and liquid crystal phases have all been used as delivery vehicles for perfumes or other active ingredients. 

The formation and stabilization of 0/W emulsions prepared with mixed emulsifier systems has been extensively investigated. However, the mechanisms proposed differ greatly. One of the primary hypotheses attributes the enhanced stability to the formation of a molecular "complex" or layer at the oil/water interface (8-11). The mixture of emulsifier types increases the packing density of the adsorbed interfacial film. Several investigators have shown that more closely packed complexes produce more stable emulsions (9,12-14). Friberg, et al. (15-17) have attributed the enhanced stability of mixed emulsifier emulsions to the formation of liquid crystals at the oil/water interface, which reduce the van der Waals attractive forces. 

The presence of liquid crystal structures at both the w-o and o-w interfaces in multiple emulsions has been investigated by Kavaliunas and Frank (31). Microscopic examination of w/o/w emulsions between crossed polarizers revealed the presence of liquid crystal phases at both inner (w-o) and outer (o-w) interfaces in a w/o/w system composed of water, p-xylene and nonylphenol diethylene glycol ether. Liquid crystalline phases were also detected in o/w/o emulsions at both interfaces. The presence of these liquid crystal structures was found to improve the stability of the emulsions markedly. Matsumoto (32, 33) have concluded that the oil layers in w/o/w systems are likely to be composed of or contain,at least in proximity to the aqueous phase,multilamellar layers of the lipophilic surfactant used in the formulation this is postulated in part to explain the rate of volume flux of water through the oily layer. 

Emulsion stability is determined by the strength of the interfacial film and the way the adsorbed molecules in it are packed. If the adsorbed molecules in the film are closely packed, and it has some strength and viscoelasticity, it is difficult for the emulsified liquid droplets to break the film. In other words, coalescence is difficult. The emulsion is therefore stable. The molecular structure and the properties of the emulsifiers in the film affect the film s properties. The molecules in the film are more closely packed if the emulsifier has straight chains rather than branched chains. The film strength is increased if mixed emulsifiers are used rather than a single one. The reasons are that (1) the molecules in the film are closely packed, (2) mixed liquid crystals are formed between droplets, and (3) molecular complexes are formed in the interface by emnlsifier compositions. For example, an oil-soluble surfactant mixed with a water-solnble snrfactant works very well to stabilize emulsions (Kang, 2001). 

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. ... 

Another consequence of the addition of fatty alcohols to cationic surfactants is the formation, under the right conditions, of liquid crystal and gel networks [41-45] that can greatly increase viscosity and confer stability upon the emulsion. Formation of such liquid crystals has been observed even at low concentrations [44,45] the ready formation of these structures, along with low cost, improved stability, and compatibility with cosmetic ingredients are important reasons why long-chain alcohols are so ubiquitous in conditioning formulations. 

Liquid crystal structures are important not only in the viscosity modification of surfactant solutions, but also in the stabilization of foams and emulsions, in detergency, in lubrication (Boschkova, 2002), and in other applications. 

The presence of liquid-crystalline material at the emulsion interface has been shown by electron microscopy using the freeze-etching technique 18). Typical liquid-crystalline structures are shown in Figure 16. These liquid-crystalline compositions are viscous, and the lamellar phase displays pseudoplastic rheology. The lamellar phase is the most important of all liquid-crystalline phases for emulsion stability. The presence of a liquid-crystalline phase causes a reduction of the available London-van der Waals forces for coalescence 16). As a consequence of the reduction of the influence of these dispersion forces and the high viscosity of the liquid-crystal layer, the time for coalescence is increased dramatically. 

Apart from the already established formulations, researchers are trying to develop novel oil-based formulations to combat the poor solubility and bioavailablity of NCE. Shevachman et al. developed novel U-type microemulsions to improve the percutaneous permeability of diclofenac. Shah et al.2 2 used microwave heating for the preparation of solid lipid nanoparticles by microemulsion techniques, which resulted in improved particle characteristics. Ki et al. reported sustained-release liquid crystal of injectable leuprolide using sorbitan monooleate. Recently, various novel oil-based drug delivery technologies are reported, which includes tocol emulsions, solid lipid nanopar-ticles, nanosuspensions, Upid microbubbles, sterically stabilized phospholipid micelles, and environmentally responsive drug delivery systems for parenteral administration.25 259... 

Emulsions can be defined as a mixture of at least two inmiscible liquid or liquid crystal phases with the interface being stabilized by an adsorbed monomo-lecular or micellar layer of surface active agents as shown in Fig. 1. Depending on the nature of the dispersed and the continuous phase, different types of emulsions can be distinguished oil-in-water (0/W) or water-in-oil (W/0) emulsions. If the dispersed phase represents an emulsion itself the systems formed are called multiple emulsions. 

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