Análisis del comportamiento presión-temperatura y otras propiedades termodinámicas para los cristales líquidos PAA, 5CB y HOAOB utilizando el modelo convex peg y la teoría de los funcionales de la densidad en la transición isótropa-nemático

Abstract

In this work, we employed the Density Functional Theory to determine thermodynamic properties of isotropic and nematic phases of liquid crystals, based on a ``Convex Peg'' model. The Helmholtz-free energy of the molecular system, a convex hard core within an encircling spherical square-well (SW) potential, is obtained from a second-order perturbation theory for SW nonspherical particles, combined with the Parsons decoupling approximation of the translational and rotational degrees of freedom, and a long-range approximation for the evaluation of the perturbation terms. The theory is applied to predict the phase diagram and isotropic-nematic transition of p-azoxyanisole (PAA), 4 -- pentyl -- 4' - cyanobiphenyl (5CB) and 4 - 4'- bis (heptiloxy) azoxybenzene (HOAOB). To do this, an estimation of the volume and k of a Liquid Crystal molecule was derived using IPCM calculations; the volume obtained is mapped into a hard ellipsoid revolution volume of a Convex Peg molecule. A very good agreement in the prediction of the thermodynamic properties is obtained when they are compared with experimental data.