Liquid Crystalline Phases and Microemulsions

Apart from micelles, surfactants, block copolymers and polar lipids self-assemble to a wide range of liquid crystalline phases and microemulsions [Ij. These systems offer opportunities for increased solubilization of hydrophobic drugs. Similarly, due to their water compartments, some liquid crystalline phases (e.g. cubic) are also interesting delivery systems for proteins, peptides and other biomolecular drugs. Depending on its physicochemical properties, a drug incorporated in such self-assembly systems may localize preferentially in the oil or water compartment(s), or at the interface between these, thereby affecting the structure and stability of the self-assembled system. 

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IV. Aqueous Dispersions of Lyotropic Liquid Crystalline Phases and Microemulsions... 

Figure 3 shows a rich polymorphism upon the solubilization of OA and EA in the fully hydrated ME-based system different inverted-type self-assembled liquid crystalline phases and microemulsions are displayed [31]. 

Yaghmur, A., Rappolt, M., Ostergaard, J., Larsen, C. Larsen, S. W. (2012). Characterization of bupivacaine-loaded formulations based on liquid crystalline phases and microemulsions the effect of lipid composition. Langmuir, 28(5), 2881-9. 

The phase behavior observed in the quaternary systems A and B is also evidenced in ternary systems. Figure 4 shows the phase diagrams for systems made of AOT-water and two different oils. The phase diagram with decane was established by Assih (14) and that with isooctane has been established in our laboratory. At 25°C the isooctane system does not present a critical point and the inverse micellar phase is bounded by a two-phase domain where the inverse micellar phase is in equilibrium with a liquid crystalline phase, as for system B or system A when the W/S ratio is below 1.1. In the case of decane, a critical point has been evidenced by light scattering (15). Assih and al. have observed around the critical point a two-phase region where two microemulsions are in equilibrium. A three-phase equilibrium connects the liquid crystalline phase and this last region. 

FIGURE 9.12 High-salinity diffusion path for contact of composition D with oil (O) indicating an intermediate brine phase (b) and spontaneous emulsification in the oil phase (s.e.). Ic and w/o denote the lamellar liquid crystalline phase and a water in oil microemulsion, respectively. S/A denotes the surfactant/alcohol mixture in this pseudoternary diagram. (From Raney, K.H. and Miller, C.A., AIChE J., 33, 1791, 1987. With permission.)... 

Liquid-crystalline phase or microemulsion formation between surfactant, water, and oily soil accompanies oily soil removal from hydrophobic fabrics such as polyester (Raney, 1987 Yatagai, 1990). It has been suggested (Miller, 1993) that maximum soil removal occurs not by solubilization into ordinary micelles, but into the liquid-crystal phases or microemulsions that develop above the cloud point of the POE nonionic. 

Microemulsions and surfactant-stabilized (macro) emulsions are distinctively different with respect to thermodynamic stability and, therefore, while most significant for both types of systems, the role of studies of phase behavior is different in the two cases. For emulsions we are con-eemed with two- or multi-phase regions in the phase diagrams, and for microemulsions with one-phase regions. Beeause of that micro emulsion studies are closely related to studies of other thermo-dynamically stable phases, notably liquid crystalline phases and micellar solutions. Structural models of microemulsions have to a considerable extent been advanced on the basis of our understanding of other stable phases the formation and stability of a micro-emulsion phase for a certain surfactant results from the comope-tition with alternative phases. The principal differences between micro emulsions and emulsions, together with the related nomenclature, is bound to lead to considerable confusion for example, the persistence in literature of emulsion-based structural pictures of microemulsions can be traced to the related names. However, the term microemulsions is kept for historical reasons. 

A number of other NMR-probed w/o microemulsions have appeared in recent literature. The diffusion coefficients in water/SDS/pentanol and ammonium hydro-xide/SDS/pentanol microemulsions investigated by Olsson et al. [35] estabhshed that replacement of water by ammonium hydroxide destabilizes the liquid crystalline phase and reduces the size of the colloidal association structure in the isotropic liquid region, Olsson and Schurtenberger [36] worked on nonionic microemulsions prepared from D2O, pentaethylene glycol dodecyl ether and decarie. Discrete oil-swollen micelles have been evidenced by NMR self-diffusion measurements the preparations are in conformity with the hard-sphere model. The NMR self-diffusion measurements on a water/octyl glucoside/pentanol/decane microemulsion system advocated a progressive decrease in the mean curvature of the surfactant film with water addition at a constant level of the oil [37]. It was concluded that the... 

In spite of these difficulties, with one notable exception discussed in Section 6.12, simple diffusion is quite common, as discussed in this section and next. A similar analysis, as shown previously for a drop, can be made when the oil is present as a thin layer on a solid surface (Lim and Miller, 1991b), a more interesting situation for detergency. Moreover, the intermediate phase need not be the lamellar liquid crystal. Depending on the phase behavior and the initial compositions of the oil and aqueous phases, it could also be another liquid crystalline phase, a microemulsion, or the Lg (sponge) phase discussed in Chapter 4. Finally, we note that sometimes more than one intermediate phase is seen. An example is given in the next section. 

The liquid crystals described above which have been studied with X-rays usually are made from surfactants and water. We expect that those found in microemulsion systems will give similar results when studied by X-rays but this has yet to be demonstrated. Other surfactant-water systems which are not liquid crystalline also give X-ray results which are different from those of the liquid crystalline phases and these will be discussed now. They are of two sorts as far as we know, neither have been oriented. 

Figure 6 is a plot of specific conductance against mole ratios of methanol to bis(2-ethylhexyl) sodium sulfosuccinate. Like the viscosity data, there are three regions. In the first region, a rapid rise in conductance occurs, which indicates the formation of a microemulsion. It is in this region that the swollen micellar solution and liquid crystalline phase of methanol in bis(2-ethylhexyl) sodium sulfosuccinate is breaking with the formation of microspheres that constitute the microemulsion (13). 

Fatty acid esters would be predicted to have little irritation or toxic effects. Ex vivo permeability studies conducted in porcine buccal mucosa showed significant permeation enhancement of an enkephalin from liquid crystalline phases of glycerine monooleate [32]. These were reported to enhance peptide absorption by a cotransport mechanism. Diethylene glycol monoethyl ether was reported to enhance the permeation of essential oil components of Salvia desoleana through porcine buccal mucosa from a topical microemulsion gel formulation [33]. Some sucrose fatty acid esters, namely, sucrose laurate, sucrose oleate, sucrose palmitate, and sucrose stearate, were investigated on the permeation of lidocaine hydrochloride [34], with 1.5% w/v sucrose laurate showing a 22-fold increase in the enhancement ratio. 

Yaghmur, A., de Campo, L., Sagalowicz, L., Leser, M.E., and Glatter, O. (2005). Emulsified microemulsions and oil-containing liquid crystalline phases. Langmuir. 21, 569-577. 

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. 

The first part of the book discusses formation and characterization of the microemulsions aspect of polymer association structures in water-in-oil, middle-phase, and oil-in-water systems. Polymerization in microemulsions is covered by a review chapter and a chapter on preparation of polymers. The second part of the book discusses the liquid crystalline phase of polymer association structures. Discussed are meso-phase formation of a polypeptide, cellulose, and its derivatives in various solvents, emphasizing theory, novel systems, characterization, and properties. Applications such as fibers and polymer formation are described. The third part of the book treats polymer association structures other than microemulsions and liquid crystals such as polymer-polymer and polymer-surfactant, microemulsion, or rigid sphere interactions. 

The current state OF THE ART of various aspects of macro- and microemulsions is reflected in this volume. The symposium upon which this volume is based was organized in six sessions emphasizing major areas of research. Major topics discussed include a review of macro- and microemulsions, enhanced oil recovery, reactions in microemulsions, multiple emulsions, viscoelastic properties of surfactant solutions, liquid crystalline phases in emulsions and thin films, photochemical reactions, and kinetics of microemulsions. 

The general pattern of microemulsion phase behavior described above is seen when the amounts of water and hydrocarbon present are comparable. However, a hydrocarbon-free mixture of surfactant and water (or brine) near optimal conditions is typically not a simple micellar solution but either the lamellar liquid crystalline phase or a dispersion of this phase in water. Starting with such a mixture and adding hydrocarbon, we sometimes find that the system passes through several multiphase regions before reaching the microemulsion/oil/water equilibrium characteristic of optimal conditions. 

Liquid soil is usually removed by roll-up, emulsification, direct solubilization, and possibly formation of microemulsion or liquid crystalline phases. The oil emulsification capability of the surfactant solution and the oil-water interfacial tension are relevant physicochemical parameters. 

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