A Molecular Theory of the Cholesteric Phase and of the Twisting Power of Optically Active Molecules in a Nematic Liqud Crystal

Abstract The molecular properties of optically active molecules are used to extend the molecular statistical theory of the nematic phase in order to explain the helical structure of the cholesteric phase. In order to account for these properties the dispersion energy between two molecules is calcualted taking into account not only the dipole-dipole interaction but also the dipole-quadraupole interaction. The calculated twist thas the right order of magnitude. The theory moreover explains the twisting power of optically active solute molecules in a menatic solvent and also the concentration dependence of the induced twist angle. A consequence of this theory is that the helical structure only exists, if the distribution of orientations around the long molecular axes is not rotationally symmetric; one may expect this to be the case for e.g. planar molecules.