Liquid crystals Microemulsion

To overcome most of solubilization problems, colloidal surfactant systems (e.g. micelles, liquid crystals, microemulsions, vesicles, emulsions, etc.) are attracting a great deal of attention as alternative reaction media (Walde 1996 Holmberg 1997 Antonietti 2001). Their advantages are they possess micro- and nanostmctures consisting of well-defined hydrophilic and lipophilic domains separated by surfactant films with very large interfacial area, the exchange between chemical species... 

Ease of forming nonmicelle phases (liquid crystals, microemulsions) Low Low Moderate High Moderate Low... 

Keywords hydrophiUc, hydrophobic, cationic, anionic, non-ionic, amphiphiles, adsorption, micelles, aggregates, vesicles, liquid crystals, microemulsions. 

D. O. Shah and W. C. Hsieh, Microemulsions, Liquid Crystals and Enhanced Oil Recovery, in Theory, Practice, and Process Principles for Physical Separations, Engineering Foundation, New York, 1977. 

Liquid Crystal Third Phase. In addition to micelles and microemulsion droplets, surfactants may form Hquid crystals. A Hquid crystal is a separate phase, which comes out of solution, not like the micelles or microemulsion droplets, which are microscopic entities within the solution. 

From (62) and (70) it follows that the Lifshitz and tricritical points coincide giving the Lifshitz tricritical point [18,66] for 7 = 27/4. 7 = 27/4 can be considered, as a borderline value between the weak (7 <27/4) and the strong (7 >27/4) surfactants. For the weak surfactants the tricritical point is located at the transition between the microemulsion and the coexisting uniform oil- and water-rich phases, whereas for the strong surfactants the tcp is located at the transition between the microemulsion and the liquid-crystal phases. The transition between the microemulsion and the ordered periodic phases is continuous for p < Ps < Ps and first order for p > p[. 

A. Ciach. Bifurcation analysis and liquid-crystal phases in Landau-Ginzburg model of microemulsion. J Chem Phys 704 2376-2383, 1996. 

The fluorescence polarization technique is a very powerful tool for studying the fluidity and orientational order of organized assemblies (see Chapter 8) aqueous micelles, reverse micelles and microemulsions, lipid bilayers, synthetic non-ionic vesicles, liquid crystals. This technique is also very useful for probing the segmental mobility of polymers and antibody molecules. Information on the orientation of chains in solid polymers can also be obtained. 

Fig. 15.4 Schematic ternary-phase diagram of an oU-water-surfactant microemulsion system consisting of various associated microstructures. A, normal miceUes or O/W microemulsions B, reverse micelles or W/O microemulsions C, concentrated microemulsion domain D, liquid-crystal or gel phase. Shaded areas represent multiphase regions.

Microemulsions are microstructured mixtures of oil, water, emulsifiers, and other substances. Since their structures differ in many ways from that of ordinary emulsions, it will be described separately. Liquid crystals (LC) are substances that exhibit special melting characteristics. Further, some surfactant-water-cosurfactant mixtures may also exhibit LC (lyotropic crystal) properties. 

Ma ZN, Friberg SE, Neogi P (1988) Observation of Temporary Liquid-Crystals in Water-in-Oil Microemulsion Systems. Colloids Sirnf 33 249-258... 

Micelles are spontaneously formed by most surfactants (especially single-chained ones) even at fairly low concentrations in water, whereas at higher surfactant concentrations, with or without the addition of an oil (e.g. octane) or co-surfactant (e.g. pentanol), a diverse range of structures can be formed. These various structures include micelles, multibilayers (liquid crystals), inverted micelles, emulsions (swollen micelles) and a range of microemulsions. In each case, the self-assembled structures are determined by the relative amounts of surfactant, hydrocarbon oil, co-surfactant (e.g. pentanol) and water, and the fundamental requirement that there be no molecular contact between hydrocarbon and water. 

There are a number o-f processes which have not been discussed (because o-f space) in which mixture o-f sur-f actants are important. Among these are foaming, emulsion formation, liquid crystal formation, microemulsion formation, adsorption as 1iquid—1iquid interfaces, and phase partitioning of surfactants between immiscible liquid phases. These areas will also see increased interest in the use of surfactant mixtures. 

Modem scaling theory is a quite powerful theoretical tool (applicable to liquid crystals, magnets, etc) that has been well established for several decades and has proven to be particularly useful for multiphase microemulsion systems (46). It describes not just interfacial tensions, but virtually any thermodynamic or physical property of a microemulsion system that is reasonably dose to a critical point. For example, the compositions of a microemulsion and its conjugate phase are described by equations of the following form ... 

However, the formal differences between microemulsions and macroemulsions are well defined. A microemulsion is a single, thermodynamically stable, equilibrium phase a macroemulsion is a dispersion of droplets or particles that contains two or more phases, which are liquids or liquid crystals (48). 

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