Nemato-Cholesteric Compositions

Since the microscopic structure of nematics and cholesterics is similar, these two phases are mixed like two nematics. Thus the thermodynamic behavior of nemato-cholesteric mixtures needs no special discussion (see Section 1.2.1). What is of interest, especially for liquid crystalline materials for technological applications, is the concentration dependence of the helical pitch and the physical parameters relevant to this dependence. 

The admixture of the chiral compound to a conventional nematic induces a macroscopic helical structure whose wave vector, that is, inversed pitch qo = 27t/Po depends on concentration (c), in general, nonmonotonically. Only in the case of the ideal mixture, without any short-range intermolec-ular interactions, we have the linear relationship 

The coefficient p may be called an induction power for a helix. For the ideal multicomponent mixture the waye vector may be calculated from the additity rule 

FIGURE 1.17. Phase diagram for a 80CB-DB70CN mixture showing smectic A mesomorphism x is the molar percentage of 80CB in the mixture) [43]. 

Equation (1.8) has been shown to be true for several cholesterol derivatives in the absence of pretransitional phenomena [12]. 

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