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Figure 6.29 Variation of cmc with temperature for: sodium dodecyl sulfate (CH3(CH2)ii SO- Na+) and pentaoxyethylene glycol monodecyl ether (CH3(CH2)9(OCH2CH2)5OH).

Modified from E. D. Goddard and G. C. Benson, Can. J. Chem., 35, 986 (1957) with permission.

Figure 6.29 Variation of cmc with temperature for: sodium dodecyl sulfate (CH3(CH2)ii SO- Na+) and pentaoxyethylene glycol monodecyl ether (CH3(CH2)9(OCH2CH2)5OH).

Modified from E. D. Goddard and G. C. Benson, Can. J. Chem., 35, 986 (1957) with permission.

micellar size and a corresponding decrease in cmc is noted for many nonionic surfactants (Fig. 6.29). The cloud point is very sensitive to additives in the system, which can increase or decrease the clouding temperature.

Temperature has a comparatively small effect on the micellar properties of ionic surfactants. The temperature dependence of the cmc of sodium lauryl (dodecyl) sulfate shown in Fig. 6.29 is typical of the effect observed.

6.4 Liquid crystals and surfactant vesicles

6.4.1 Liquid crystals Lyotropic liquid crystals

Surfactant solutions at concentrations close to the cmc are clear and isotropic; that is, the magnitudes of such physical properties as viscosity and refractive index do not depend on the direction in which these properties are measured. As the concentration is increased there is frequently a transition from the typical spherical micellar structure to a more elongated or rod-like micelle. Further increase in concentration may cause the orientation and close packing of the elongated micelles into hexagonal arrays. A new phase containing these ordered arrays separates out from the remainder of the solution, which contains randomly orientated rods, but remains in equilibrium with it. This new phase is a liquid crystalline state termed the middle phase or hexagonal phase. With some surfactants, further increase of concentration results in the separation of a second liquid crystalline state, the neat phase or lamellar phase. In some surfactant systems another liquid crystalline state, the cubic phase, occurs between the

Figure 6.30 Diagrammatic representation of forms of lyotropic liquid crystals.

Figure 6.30 Diagrammatic representation of forms of lyotropic liquid crystals.

middle and neat phases. The most common type of cubic phase is the micellar cubic phase formed by the close packing of spherical micelles; a more complex cubic phase, the bicontinuous cubic phase, occurs with some amphiphilic lipids such as glyceryl monooleate (see section 6.4.2). Finally, in all systems, surfactant separates out of solution. The liquid crystalline phases that occur on increasing the concentration of surfactant solutions are referred to as lyotropic liquid crystals; their structure is shown diagrammatically in Fig. 6.30. The phase diagram in Fig. 6.31 shows the transition from micellar solution to liquid crystalline phase and finally to pure amphiphile

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