Lamellar liquid crystalline phase stability

A maximum in emulsion stability is obtained when three phases exist in equilibrium, and it was therefore proposed that the lamellar liquid crystalline phase stabilizes the emulsion by forming a film at the 0/W interlace. This film provides a barrier against coalescence. This is illustrated in Fig. 5.12 which shows that the lamellar liquid crystalline phase exhibits a hydrophobic surface towards the oil and a hydrophilic surface towards the water. These multilayers cause a significant reduction in the attraction potential cuid they also produce a viscoelastic film with much higher viscosity than that of the oil droplet. In other words, the multilayers produce a form of "mechanical barrier against coalescence. 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

However, experimental evidence has shown (7) that inverse micellar systems are rarely in equilibrium with aqueous micellar solutions but rather with a lamellar liquid crystalline phase. The presence of an electrolyte will influence the stability of both the inverse micelles and the lamellar liquid crystalline phase. This influence will be estimated now. 

The discussion of the relative stability of solutions with inverse micelles and of liquid crystals containing electrolytes may be limited to the enthalpic contributions to the total free energy. The experimentally determined entropy differences between an inverse micellar phase and a lamellar liquid crystalline phase are small (12). The interparticle interaction from the Van der Waals forces is small (5) it is obvious that changes in them owing to added electrolyte may be neglected. The contribution from the compression of the diffuse electric double layer is also small in a nonaqueous medium (II) and their modification owing to added electrolyte may be considered less important. It appears justified to limit the discussion to modifications of the intramicellar forces. 

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. 

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. ... 

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]. 

Recently, we have shown that the stability of the lamellar liquid crystalline phase is reduced considerably by replacing a monovalent counterion with a divalent one in anionic surfactant systems [1-4]. The dependence of the swelling capability of the lamellar liquid crystalline phase on the valency of the counter ion appears to be a general characteristic for the ionic surfactant systems. 

The rheological properties of monolayers of bineuy surfactant mixtures have been related to emulsion stability (see below) and to the structural properties of the lamellar liquid crystalline phases formed by the surfactant in water. It was suggested that the emulsifier molecules adsorbed at the 0/W interlace will adopt the same hydrocarbon... 

The transition between the lamellar liquid crystalline phase emd the gel pheise can be utilized to stabilize the emulsion, provided the actual gel phase is stable. If an aqueous dispersion of the emulsiher is hrst formed, and the emulsihcation is then performed under cooling, an emulsion is formed with the gel phase forming the 0/W interface. Such an emulsion has a much higher stability when compened with that produced with the lamellar phase at the 0/W interface. This is probably due to the higher mechanical stability of the crystalline lipid bilayers compared to bilayers with liquid chain conformation. 

Shrestha, L. K., Acharya, D. P., Sharma, S. C. et al. (2006) Aqueous foam stabilized by dispersed surfactant solid and lamellar liquid crystalline phase. J. Colloid Interface Sci., 301, 274-281. 

In this chapter, the characteristic properties of nano-emulsions and relevant applications have been described. A great deal of research effort in recent years has been focused toward the conditions required for nano-emulsion formation. Low interfacial tension values and the presence of lamellar liquid crystalline phases are among the factors that have been shown to be important for their formation. However, it has also been shown that the kinetics of the emulsification process plays a key role. Comprehensive knowledge of the fundamental aspects related to nano-emulsion formation and stability will allow improvement of established applications, such as those described in this chapter, and development of new ones. 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

Liquid crystals stabilize in several ways. The lamellar structure 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 liquid crystalline phase, coalescence takes place over a thin liquid 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 liquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the liquid crystal is highly viscous, and two droplets separated by a viscous film of liquid crystal with only a small compressive force exhibit stability against coalescence. Finally, the network of liquid crystalline leaflets (30) hinders the free mobility of the emulsion droplets. 

A position to the right of the microemulsion area means the presence of a lamellar liquid crystal as has been repeatedly demonstrated by Ekwall (19). The temporary appearance of liquid crystals when W/0 microemulsions are brought into contact with water have, with this result, been given a satisfactory explanation. The faster transport of the surfactant into the aqueous layers gives rise to temporarily higher surfactant concentrations and the stability limits for the water rich W/0 microemulsions phase are exceeded towards the liquid crystalline phase in water. 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

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. 

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