Thermodynamic aspects of the vitrification of toluene, and xylene isomers, and the fragility of liquid hydrocarbons

The three xylenes, 1,2-, 1,3-, and 1,4-dimethyl benzene, provide a well-characterized family of isomeric molecules within which the melting points vary greatly while fluid-state properties such as boiling points and viscosities remain very similar. Because all pure isomers crystallize rapidly on cooling, their behavior at the low-temperature end of the liquid state, i.e., near the glass transition, has not been studied. We report here differential scanning calorimetry studies of phase relations for the three mixed isomer systems, and identify composition regions in which emulsified and also bulk mixtures can be vitrified at ordinary cooling rates. For one of the latter, 70% m-xylene + 30% o-xylene by volume, we determine the change in heat capacity through the glass transition. On the assumption that this is the same for pure m-xylene, we assess the Kauzmann limit on liquid behavior, T_K, for m-xylene to fall at 104.6 K, T_g/T_K to be 1.20, and the excess entropy at T_g to be 15.6 J/mol K or 30.5% of the entropy of fusion. Equivalent data are obtained for the better-known glass former toluene, for which we find T_g/T_K = 1.16. Comparisons of these results are made with literature data for some saturated analogs. From these comparisons emerges the simple rule that the excess heat capacity of supercooled liquid over crystal can be described as a hyperbolic function of temperature, the one parameter of which varies linearly with the carbon number of the molecule. The plot passes through the origin, and has a greater slope for aromatic than for paraffinic molecules. This is correlated both with the greater "fragility" of the aromatic liquids assessed from viscosity-temperature relations normalized by the calorimetric T_g, and with a simple "liquid range" index T_b/T_g, where T_b is the boiling point.