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Composition (wt% C^E6)

Figure 6.31 Phase diagram of the CH3(CH2)11(OCH2 CH2)6OH(C12E6)/H2O system: A, two isotropic liquid phases; B, micellar solution; C, middle or hexagonal phase; D, cubic phase; E, neat or lamellar phase; F, solid phase. The boundary between phases A and B is the cloud point. Modified from J. S. Clunie, J. F. Goodman and P. C. Symons, Trans. Farad. Soc., 65, 287 (1969).

for the polyoxyethylated nonionic surfactant C12E6 (CH3(CH2)n(OCH2CH2)6OH).

Liquid crystals are anisotropic; that is, their physical properties vary with direction of measurement. The middle phase, for example, will flow only in a direction parallel to the long axis of the arrays. It is rigid in the other two directions. On the other hand, the neat phase is more fluid and behaves as a solid only in the direction perpendicular to that of the layers. Similarly, plane-polarised light is rotated when travelling along any axis except the long axis in the middle phase and a direction perpendicular to the layers in the neat phase. Because of this ability to rotate polarised light, the liquid crystals are visible when placed between crossed polarisers, and this provides a useful means of detecting the liquid crystalline state.

Thermotropic liquid crystals

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name 'liquid crystal' was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular structure within this solution.

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