Chirality, twist and structures of micellar lyotropic cholesteric liquid crystals in comparison to the properties of chiralic thermotropic phases

Phase chirality in disk-like lyotropic cholesteric phases (ChD) was investigated, which was induced by addition of center and axial chiral dopants to achiral lyotropic nematic host phases (ND). In a lyotropic nematic matrix of the disk-like ND phase in the ternary system hexadecyldimethylethyl ammonium bromide (C16Me2EABr)/water/n-decanol, a disk-like lyotropic cholesteric phase ChD was induced by addition of the axial optically active compound R(−)-1,1′-binaphthalene-2,2′-diyl-hydrogen-phosphate (BDP). The helical twisting power (HTP) of the BDP is generally lower than the HTP value of inducing substances with center chirality as cholesterol, prednisolone and taurocholic acid. At constant composition of the ND phase, the helix lengths were determined in dependence on the BDP and steroid concentration by means of evaluation of the ‘spaghetti-like’ texture using polarizing microscopy. The reciprocal helix lengths are changing linearly with rising BDP concentration. The properties of the ChD phase (textures, helix lengths, structural parameters of the micelles) induced by the chiral compounds and changed by the composition of host phases can give information to the mechanism of chirality transfer from the molecular level to that of the micellar aggregates and finally, to the liquid crystalline superstructure. Furthermore, the matrix influence of the ND phase on the helix formation was examined at constant BDP and steroid concentration. The structure in the ChD phase was described in terms of micelle parameters. Finally, the inducing properties of a center chiral optically active compound such as cholesterol, prednisolone and taurocholic acid were compared with those of the axial chiral compound BDP. Last but not least, the situation of the theoretical and structural background for helix formation in liquid crystals, e.g. the explanation of chiralic transfer between micelles is analyzed and discussed. Two main conditions are necessary to build up the helix in the ChD phase: the formation of H-bridges; and the existence of a specific chiralic interaction energy between neighboring micelles in the cholesteric superstructure.