Estimation of configurational entropy and stability of nematic phase in mesogens - Role of molecular interactions

The temperature dependence of configurational entropy for two mesogens p-n-Octylbenzoic acid (8BAC) and p-n-Nonylbenzoic acid (9BAC) 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. Further, an attempt has been made to analyze the stability of nematic phase at phase transition temperature. This study is essential in understanding the flexibility of a configuration in a particular phase and in employing molecular models to yield insight into structural organization of the compounds.