Surfactant nematic states

Figure 12.21 Phase diagram for the surfactants cetylpyridinium chloride and hexanol in 0.2 M NaCl at 30°C. Li is a disordered state of wormy micelles, Nc is the nematic state of oriented micelles, H is a state of hexagonally ordered cylindrical micelles, and L is a lamellar state. The hatched areas are coexistence regions. (Reprinted with permission from Roux et al.. Macromolecules 28 1681. Copyright 1995, American Chemical Society.)...

Mesomorphic Phase. A phase consisting of anisometric molecules or particles that are aligned in one or two directions but randomly arranged in other directions. Such a phase is also commonly referred to as a liquid-crystalline phase or simply a liquid crystal. The mesomorphic phase is in the nematic state if the molecules are oriented in one direction, and in the smectic state if oriented in two directions. Mesomorphic phases are also sometimes distinguished on the basis of whether their physical properties are mostly determined by interactions with surfactant and solvent (lyotropic liquid crystals) or by temperature (thermotropic Uquid crystals). See also Neat Soap. 

Nematic states occur also in nonionic monomeric and polymeric surfactant solutions, living amoeboid cells i and rigid or semiflexible polymeric rods, such as fd virus in water solutions. - We note that colloidal suspension of semiflexible virus particles also form nematic phases. - ... 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

Finally, a study of fairly monodisperse CdSe semiconducting nanorods functionalized with amphiphilic molecules in a suspension of cyclohexane has recently been published that shows preliminary results indicative of a typical nematic behavior [ 102]. Note that these nanorods have been synthesized by the pyrolysis of organometallic precursors of Cd and Se in hot surfactant mixture allowing a good control of the aspect ratio. This state of the art particle size and shape control method should allow one to customize accurately their semiconducting properties. 

The time needed to reach the stationary state, tsu varies inversely with the shear rate, indicating that whatever 7, steady state is reached after a constant deformation. For both CPCl/Hex and SDS/Dec this deformation was of the order of 300 strain units [284,301]. Nematic surfactant solutions of micelles can be considered to have forgotten their shear history after having been sheared for more than a few hundreds strain units. 

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