Interpretation of Thermodynamic Derivatives Near the Liquid Crystal Phase Transition in<i>p</i>-Azoxyanisole

Alben, R.

doi:10.1080/15421407008083483

Cited by 26 publications

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References 7 publications

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“…Most of them (7)(8)(9)(10) applied the Frenkel heterophase fluctuation theory (11) within 100 of the nematic-isotropic transition. A general theory of phase transition described by a single order parameter is proposed by Alben (12). Chandrasekhar et al discussed the thermodynamic conditions of nematic stability on the basis of the molecular statistical theory of orientational order (13).…”

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The Significant Structure Theory Applied to a Mesophase System

Eyring²

1975

Proc. Natl. Acad. Sci. U.S.A.

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The significant structure theory of liquids is extended to the mesophase system with p-azoxyanisole as an example. This compound has two different structures, a nematic phase and an isotropic phase, in its liquid state. In this study the nematic phase is treated as subject to a second volume and temperature-dependent degeneracy formally like that due to melting. The isotropic phase is treated as a normal liquid. The specific heat, thermal expansion coefficient, compressibility, volume, entropy of transitions, and heat of transitions are calculated and compared to the observed values. This analysis differs from previous ones in including the volume dependence as well as the temperature dependence in one explicit expression for the Helmholtz free energy. The significant structure theory, which was first proposed by Eyring et al. (1), has been successfully applied to many different types of liquids, such as normal liquids, molten metals, fused salts, liquid hydrogen, water and heavy water, liquid mixtures, two-dimensional liquids and solid adsorbants, plastic crystals, hydrocarbons and halobenzenes (2-4), and high polymers (5). In this study this theory is extended to systems in which the same molecules are arranged in more than one structure. The liquid crystal p-azoxyanisole was chosen for theoretical investigation in the present paper ( Fig. 1).X-ray data indicate that the stable crystalline form of p-azoxyanisole belongs to space group P12/a (c2h5) with four molecules per unit cell (6). This compound is a thermotropic liquid crystal. It changes from a solid structure to a turbid liquid (nematic phase) at its melting point and is converted into a true liquid (isotropic phase) at a higher temperaturethe clearing point. For liquid crystal compounds there is a rapid change in the density, the specific heat, the thermal expansion coefficient, the compressibility, the viscosity, the dielectric constant, and a number of other physical properties near their mesophase transitions.Theoretical interpretations of the thermodynamic properties of p-azoxyanisole have been made by several investigators. Most of them (7-10) applied the Frenkel heterophase fluctuation theory (11) within 100 of the nematic-isotropic transition. A general theory of phase transition described by a single order parameter is proposed by Alben (12). Chandrasekhar et al. discussed the thermodynamic conditions of nematic stability on the basis of the molecular statistical theory of orientational order (13). The approach of significant structure theory differs from these approaches in at least two important aspects:(1) Its application can be extended over a range from the melting point to the boiling point of the material under investigation.(2) It gives the volume dependency as well as the temperature dependency of the Helmholtz free energy and therefore of the thermodynamic properties of liquids that depend on both volume and temperature. THEORY According to the significant structure theory, there are three important contributions to liquid s...

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