Bond lattice or excitation model analysis of the configurational entropy of molecular liquids

The temperature dependence of the configurational entropy S_c for 16 molecular liquids plus the elemental liquid Se in the temperature range between the glass transition temperature and the melting point has been analyzed using the bond lattice or excitation model. The model contains three adjustable parameters - the excitation enthalpy ΔH⁰ and entropy ΔS⁰ for the excitable degrees of freedom, plus the number n of excitable degrees of freedom per molecule. The model gives a reasonably accurate fit to the temperature dependence of S_c for all 17 liquids, providing something of a resolution to the Kauzmann paradox. Not unexpectedly, however, the model falls somewhat short of matching precisely the temperature dependence of the excess liquid heat capacity ΔC_P. The parameter n obtained from the fits shows a positive correlation with the number of conformational degrees of freedom f assessed from the structures of the molecules, providing some insight into the sources of the configurational entropy at the molecular level. In addition, it is found that the parameter n controls the shape of the S_c vs T curve, providing, via the Adam-Gibbs equation, some correlation between the molecular structures and the kinetic fragilities m of the liquids.