Liquid crystals in emulsions

FIG. 11. Transmission electron micrographs of freeze fractured oily droplets dispersed (a) in a hexagonal and (b) in a cubic liquid crystalline phase, bar 100 nm. From Mueller-Goymann, C., Liquid crystals in emulsions, creams and gels, containing ethoxylated sterols as surfactant, Pharm. Res. 1 154-158 (1984). 

Li Y, Friberg S. Course Manual for the American Chemical Society Short Course on Surfactant Micelles, Liposomes, and Liquid Crystals in Emulsions and Microemulsions. 2002. 

Mileller-Goymann, C. Liquid crystals in emulsions, creams and gels, containing ethoxylated sterols as surfactant. Pharm. Res. 1984,1, 154-158. 

The detection of liquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the liquid crystal in an emulsion. The liquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The structure of the liquid crystalline phase is also most easily identified by optical microscopy. Lamellar liquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

A dispersion of droplets of one liquid in another, immiscible liquid, in which the droplets are of colloidal or near-colloidal sizes. The term can also refer to colloidal dispersions of liquid crystals in a liquid. Emulsions were previously referred to as emulsoids, meaning emulsion colloids. See also Macroemulsion, Mini-emulsion, Microemulsion. 

There are some micelles, micellar aggregates, microemulsions, emulsions, and dispersed liquid crystals in the mixed phase. A proper match between the size of liquid crystals and size of small particles results in ultralow IFT. 

Surfactants of the latter kind, whieh are also not significantly soluble in the oil, such as lecithin, give rise to a lamellar liquid crystal at the smallest addition to the emulsion. The water eontent of the liquid erystal is approximately 50% and the pereentage of liquid crystal in the emulsion is easily ealeulated ... 

LoU P. Liquid crystals in cosmetic emulsions. In-Cosmetics, Preprints, Vol. I. Verlag fur Chem. Industrie H. Ziolkowski GmbH, Augsburg 1993 64-85. 

Ionic Liquids can be mixed with surfactants and other solvents to give all types of association colloids that are known with water and other surfactants [28]. Microemulsions can be made as well as different types of lyotropic liquid crystals and emulsions. The Ionic Liquids can play the role of the polar (pseudo-) phase, the apolar one, or of the surfactant. Even two of the three components can be Ionic Liquids. Possible advantages are in the wide temperature range over which such systems show a reasonable stability and the low vapor pressure. Further advantages are the reduced viscosity compared to pure Ionic Liquids (in the cases where the ILs are the inner phase) and the possibility to work in water-free systems [29-31]. 

Encapsulation [4] is brought about by emulsifying a liquid crystal in a waterborne polymer such as polyvinyl alcohol (PVA) the emulsion is then coated on a substrate and dried. The permeation method [5] is used for basic studies and the liquid crystals are permeated into the pores of a prefabricated polymer matrix. 

The interactions listed above represent the majority of synthesis pathways to mesoporous molecular sieves. However, pathways involving other structure directing agents, such as polymers, lyotropic liquid crystals, and emulsions are also possible. Similarly, a wide range of inorganic species may be used as discussed in Section III. 

Rydhag and Friberg [16] showed the stabilizing action of liquid crystals in a water-hydrocarbon nonionic system. The liquid-crystalline phase was characterized by a very high viscosity, which is thought to influence the emulsion stability. 

An emulsion of E7, a common nematic liquid crystal, in an aqueous solution of poly (vinyl alcohol) (PVA) was deposited onto the sur ce of an inditun-tin-oxide (ITO) coated substrate. The ITO coating provided a transparent conducting surface that formed the counter-electrode to the biased NSOM tip. Polarized light NSOM experiments were performed on both pure E7 droplets and those doped with the fluorescent probe molecule BODIPY.(Mei et al. 2000)... 

A further means of achieving efficient light scattering in a liquid crystal device is to disperse a nematic material in a plastic matrix [63, 64]. The plastic is chosen so as to have a refractive index which is equal to no of the liquid crystal. One fabrication route [63] involves forming an emulsion of liquid crystal in a water-borne polymer system, which is then coated onto a substrate and dried. The product is a plastic film containing many small, separate droplets of liquid crystal. Alternatively, the liquid crystal may be dissolved in a reactive monomer [65], which is polymerized in a thin layer. As the polymer forms, the liquid crystal is expelled from solution and forms a similar array of droplets. Another route uses a polymer in... 

Binary mixtures of a flexible polymer and a rigid rod-like molecule (nematogen or liquid crystal) play an important role in electro-optical devices, such as light shutters and displays. Since the miscibility or phase separation controls the performance of the materials, the phase behavior and phase separation kinetics have been of fundamental and practical interests. Liquid crystalline domains dispersed in a polymer matrix are called polymer dispersed-liquid crystals (PDLCs), or polymer-stabilized liquid crystals (PSLCs), where the polymer forces the liquid crystals to phase separate into droplets surrounding by the polymer matrix [2]. Practically, there are many ways to create PD LCs by mixing polymers and liquid crystals the emulsion method [37] and phase separation method [38], including polymerization-, thermally-, and solvent-induced phase separations. The reader is referred to text books [1, 2] for details of PDLC and a review [39] for the rheological and mechanical properties. 

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. 

Loudet JC, Poulin P (2003) Monodisperse Aligned Emulsions from Demixing in Bulk Liquid Crystals. 226 173-196... 

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