Influence of the microscopic shape anisotropy of the micelles on the pitch in a cholesteric lyotropic liquid crystal

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Abstract
The influence of the microscopic shape anisotropy of the micelles on the pitch of a cholesteric lyotropic liquid crystal is investigated by x-ray diffraction and optical techniques. The mixture studied is potassium laurate, decanol, water, and brucine sulfate. The relative molar concentration of the brucine sulfate is kept constant and the relative molar concentrations of the alcohol and the soap vary. The microscopic shape anisotropy of the micelles is controlled by the balance between the molar concentrations of alcohol and soap. For a fixed molar concentration of the chiral molecule, the greater the shape anisotropy of the micelles, the smaller the cholesteric pitch. This result is discussed in terms of the analogy between the local rotation induced by the chiral molecule and the electrostatic field in a dielectric medium. The relation between the optical birefringence and the shape anisotropy is also discussed.