Thermodynamic properties of methyl esters of benzoic and toluic acids in the condensed state

The heat capacities between T = 5 K and T = 320 K and the enthalpies of phase transitions of methyl benzoate, methyl o-toluate, methyl in-toluate, and methyl p-toluate in the condensed state were measured in a vacuum adiabatic calorimeter. It was found that liquid methyl m-toluate supercooled by cooling it from T = 300 K at a rate of 0.02 K (.) s(-1) to 0.03 K (.) s(-1), formed a glass. The crystalline phase of methyl m-toluate was obtained and the residual entropy of the glass at T 0 was evaluated. Methyl o-toluate had two solid-to-solid transitions before fusion. Thermodynamic parameters of fusion for the compounds and solid-to-solid transitions for methyl o-toluate were determined and the molar thermodynamic functions in the condensed state were derived for the methyl esters under study. The enthalpy of formation of holes in liquid methyl m-toluate at the glass transition temperature was estimated on the basis of the heat capacity jump at the glass transition and the enthalpy of vaporization. The temperature dependencies of the configurational entropy and the number of molecules in a cooperative domain according to the Adam-Gibbs theory were evaluated for liquid methyl m-toluate. The residual entropy of the glassy methyl m-toluate was shown to be mainly caused by the orientational disorder of domains. (C) 2002 Elsevier Science Ltd. All rights reserved.