Liquid crystals, surfactant

Firouzi, A Atef, F Oerth, AG Stucky, GD Chmelka, BE, Alkaline Lyotropic Silicate-Surfactant Liquid Crystals, Journal of the American Chemical Society 119, 3596, 1997. 

Both mesoporous materials are synthesized hydrothermally with a surfactant liquid crystal as the template (see synthesis section below). They exhibit very high surface areas and pore volumes, of the order of 1000m g and l.Scm g, respectively. 

In a further modification, the mesoporous materials are synthesized hydrother-mally with a surfactant liquid crystal as the template. 

Vartuli, J. C., Schmitt, K. D., Kresge, C. T., Roth, W. J., Leonowicz, M. E., McCullen, S. B. Hellring, S. D., Beck, J. S., Schlenker, J. L., Olson, D. H. and Sheppard, E. W. Effect of surfactant silica molar ratios on the formation of mesoporous molecular-sieves-inorganic mimicry of surfactant liquid-crystal phases and mechanistic implications,... 

Seddon JM, ed. Surfactant liquid crystals. Curr. Opin. CoU. Interface Sci. 2001 6 242-312. 

This phenomenon is known for surfactant liquid crystals, where the macroscopic shape can be induced by external forces. Thus, if an external orienting force is exerted on the crystallite coacervates and a sufficient time is allowed for the crystallization to be completed, a maCTOscopic homogeneous crystal may form. Otherwise, even the primary particles are expected to be heterogeneous. 

Firouzi, A. et al., Alkaline lyotropic silicate-surfactant liquid crystals, J. Am. Chem. Soc., 118, 3596, 1997. 

At higher surfactant concentration liquid crystalline phases may be formed. Surfactant liquid crystals can also solubilise appreciable amounts of oil into the non-polar regions made up of the surfactant tails. Thus, both binary surfactant-water systems and ternary systems with oil included can be formulated into liquid crystals. Such systems can also be used as media for organic synthesis. In fact, a reaction in a surfactant liquid crystal often runs very rapidly, considerably faster than in a microemulsion based on the same surfactant [19]. Figure 5.1 shows the reaction profiles of a typical substitution reaction of... 

However, for practical use as reaction media the liquid crystalline systems are hardly realistic. They need very high surfactant concentrations, which make the work-up procedure complex, and these systems are highly viscous, which make mixing and heat removal a problem. Surfactant liquid crystals are of more interest as templates for making meso-porous oxides and such materials, in the form of suspensions of small particles, are also of interest for overcoming compatibility problems in organic synthesis [20]. The profiles for reactions in such systems are also included in Fig. 5.1. 

After initially FSM-16 [1] and MCM-41 [2] as the hexagonal members and MCM-48 [3] as the cubic 1 3 member of the ordered mesoporous materials had been synthesized, a wide range of other structures became available quite rapidly, most of which had pure surfactant liquid crystal counterparts. Some, however, were unique structures only known for silica-surfactant composites. 

Due to their importance for life sciences, for example the formation of biological membranes, and their technical importance as surfactants and soaps, potential lyotropic properties of amphiphilic compounds are much better investigated than other ones of different molecular types the broad field of surfactant liquid crystals is discussed in Chapter VII of this volume. 

---