Thermodynamic Stability of the Nematic Phase and Configurational Entropy of Mesogens: A Molecular Simulation Approach

Abstract

The thermodynamic stability of the nematic phase and the configurational entropy of mesogens-p-n-pentylbenzoic acid (5BAC) and p-n-hexylbenzoic acid (6BAC)-have been studied. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrodinger perturbation theory along with multicenteredmultipole 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. The observed difference in free energy of the molecules between the mesophase and the crystal phase during in-plane interactions suggests the thermodynamic stability of the nematic phase. Further, the comparable values of translational entropy during stacking and in-plane interactions are helpful to understand the relative flexibility/stability of one configuration over the other.