Orientational Order Parameter in Alkoxy Benzoic Acids - Optical Studies

Alkoxy benzoic acids are well-known liquid-crystalline compounds with nematic phase of long thermal ranges. The refractive indices measurements are carried out using a wedge-shaped cell with the help of a modified spectrometer. The birefringence (n=ne-no) is obtained directly by using the method developed by Kuczynski et al. [10] in all the available compounds (alkoxy chain number n=3 to 12 and 16). The higher homologues exhibit the smectic-C phase along the nematic phase. The molecular polarizabilities are calculated from the refractive indices and the density using the well-known internal field models. The molecular anisotropy is estimated from the Lippincott -function model and molecular vibration method. The order parameter, S, is estimated (a) using molecular polarizabilities and the molecular anisotropy for both the models, (b) from Haller's extrapolation using molecular polarizabilities, and (c) directly from the birefringence, a method developed by Kuczynski et al. [10] along with the nematic phase. Further, the order parameter from n, without the consideration of any internal field model, to the nematic molecule is compared with S obtained from density due to Maier et al. [23]. The results are compared with one another and the advantages and disadvantages are discussed.