Helical twist in cholesteric mesophases

We review the theoretical and experimental studies on the helically twisted supennolecular structure of cholesteric liquid crystals (CLCs). We discuss the problem of the nature of the forces responsible for the helical twist, generalize the results of studying the orientational order in CLCs, and treat the features of the supermolecuiar structure of CLCs in the region of pretransition phenomena. We pay special attention to the temperature- and concentration-dependences of the pitch of the helix in different cholesteric systems. We analyze critically the theoretical models that have been proposed to describe these dependences. We examine in detail the approach based on averaging the angle of twist in the mean-molecular-field approximation for various types of angular dependence of the intermolecular interaction energy (modified Keating-Böttcher theory). We undertake an attempt to treat from a unitary standpoint the types of cholesteric systems known up to now (cholesterics proper, chiral nematics, nematic-cholesteric mixtures, and systems of a nematic with an optically active additive) and other types of mesophases that possess helical twist (chiral smectics-C, cholesteric polymers, and lyotropic CLCs). We discuss the features of the supermolecuiar structure and of the intermolecular interaction in the stated systems. We treat in detail the nematic-cholesteric mixtures and cholesteric systems having nonmesogenic components, which are important in application.