Nematic surfactants

This remarkable scaling property, which is shared by some liquid crystalline polymers (see Section 11.3.4), by nematic surfactant solutions (Section 12.4.2), and by some particulate suspensions, is a consequence of the lack of an intrinsic relaxation time. In the case... 

Shearing nematic surfactant solutions has revealed complex transient responses... 

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. 

Caputo FE, Ugaz VM, Buighardt WR, Berret JF (2(X)2) Transient 1-2 plane smtdl-angje X-ray scattering measurements of micellar orientation in tdigning tmd tumbling nematic surfactant solutions. J Rheol 46(4) 927-946... 

Caputo FE, Burghardt WR, Berret JF (1999) Tumbling dynamics in a nematic surfactant solution in transient shear flows. J Rheol 43(3) 765-779... 

Lyotropic liquid crystalline nanoparticles have also been described. Concentrated solutions of gold nanorods in water in the presence of a surfactant (cetyltrimethyl-ammonium bromide) display a nematic mesophase stable up to 200 °C [74[. The N mesophase was identified by optical microscopy by their typical nematic droplets texture. 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

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. 

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.)...

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