Order Parameters of Two Phase Transitions in Mixtures of Ellipsoidal and Spherical Molecules

Abstract

AbstractThe model is the ES mixture of ellipsoidal (E) and spherical (S) molecules dispersed on a b.c.c. lattice with a ratio of concentrations different from 1:1. Minimising numerically the effective thermodynamic potential, obtained in a “modified” mean‐field approximation, the variation of the orientational and positional order parameters for different values of concentration, molecular parameter (of ellipsoidal molecules), and interaction constants are studied. The coupling effects arising from the interplay between orientational and positional degree of freedom are also checked. Determining the critical temperatures for two kinds of transitions the phase diagram (the critical temperature‐concentration diagram) is obtained, where the transition lines define four regions corresponding to the completely disordered mixture, the only positionally ordered mixture, the only orientationally ordered mixture, and the positionally and orientationally ordered mixture. Thus, in large lines, the model of the ES mixture reproduces qualitatively some features of the behaviour of plastic and liquid crystals.