(The) oretical prediction of the thermodynamic properties of a liquid ctystal,4,4'-dimethoxyazoxybenzene

Abstract

The significant structure theory of liquids and the Bragg-Williams approximation of phase transition have been applied to the system of p-azoxyanisole (PAA, 4,4`` - dimethoxyazoxybenzene) which exhibits liquid crystal phase of the nematic type. Considering the dual properties of the nematic p-azoxyanisole we have constructed the partition function of the nematic p-azoxyanisole as the product of two terms, that is, $f_{L.C.}=f_{bgd}ㆍf_{conf}$. The term $f_{bgd}$ which can be obtained from the significant structure theory of liquids gives backgrounds of all thermodynamic properties of p-azoxyanisole. To obtain this term it has been assumed that the solid-like molecule has five-degree Einstein oscillators and rotates one-dimensionally having the field interaction effect. And the gas-like molecule which has also the field interaction effect rotates three-dimensionally. The term f conf which gives an explanation about the characteristics of phase transition is obtained using the Bragg-Williams approximation described by a single long-range order parameter. Another assumption which is used to obtain the order parameter is that the dipole of the nematic p-azoxyanisole molecule has only two states of up and down directions. In the isotropic range of PAA, it is assumed that there is no contribution from the $f_{conf}$. and no restriction such as the field interaction effect in $f_{bgd}$. Using these concepts, all thermodynamic properties of PAA are calculated in the temperature range of the anisotropic and isotropic phases. The calculated results of the molar volume, the vapor pressure, the heat capacity at constant pressure, the heat capacity at constant pressure, the thermal expansion coefficient, the compressibility, and the surface tension, etc. are compared with the experimental results. The agreement between the theory and experiment is satisfactory.