THE EFFECT OF SOLUTES ON THE ORIENTATIONAL ORDERING OF LIQUID-CRYSTALLINE SOLVENTS

The effect of solutes on the orientational ordering of the liquid-crystalline solvents 4-n-heptyl-4'-cyanobiphenyl (7CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) has been studied by C-13 NMR. The technique used was a combination of variable-angle spinning (VAS) and a two dimensional method called separated local field (SLF) spectroscopy. 7CB exhibits only a nematic phase. The effects of four different solutes (carbon tetrachloride, cyclohexane, benzene, hexane) with mole fractions of c. 0.08 as well as different mole fractions of hexane were studied. 8CB has a nematic phase and a smectic A phase, and the effect of hexane with mole fraction of c. 0.07 was investigated. It was found that the effect of solutes in reducing the nematic to isotropic temperature (T(NI)) has the order CCl4 almost-equal-to cyclohexane > benzene > hexane. The incorporation of hexane in 8CB reduce T(NI) more significantly than T(SN), the smectic to nematic transition temperature. On the other hand, at the same reduced temperature (T/T(NI) for 7CB an T/T(SN) for 8CB), there is no significant change in the order parameters of either the core or the chain of the host liquid crystals, except for a fairly high concentration of hexane (mole fraction of 0.16) in 7CB. This indicates that the solute molecules do not have appreciable preferential association with any special part of the solvent molecules.