Role of configurational entropy on ordering and phase organization of nematic liquid crystals-A molecular model

The temperature dependence of configurational entropy for two mesogens p-n-heptyloxybenzoic acid (7OBAC) and p-n-octyloxybenzoic acid (8OBAC) has been estimated. The atomic net charge and dipole moment at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrodinger perturbation theory along with multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a '6-exp' potential function has been assumed for short-range interactions. The interaction energy values obtained with respect to translational and orientational motions during the different modes of molecular interactions have been taken as input to estimate the configurational entropy and Helmholtz free energy at room temperature, nematic-isotropic transition temperature, and above transition temperature. An attempt has been made to understand the role of configurational entropy on ordering and phase organization of nOBAC (n = 7, 8) molecules. The flexibility of a particular configuration at different temperatures has been analyzed in the light of thermodynamic parameters introduced in this paper. These parameters are helpful to develop a new and interesting model for structure-phase stability relationship at the molecular level. (C) 2011 Elsevier B.V. All rights reserved.