Calorimetric study of ethylene glycol and 1,3-propanediol: configurationalentropy in supercooled polyalcohols

The heat capacities of crystalline, liquid, supercooled liquid, and glassy states of ethylene glycol (EG), and 1,3-propanediol (PD) were measured with an adiabatic calorimeter in the temperature range between 13 and 300 K. Th e glassy ethylene glycol was prepared by quenching of a liquid sample doped with about 5% of glycerol to suppress crystallization. The effect of the d oping on thermodynamic quantities was corrected for by assuming the additiv ity of the heat capacities of ethylene glycol and glycerol. The configurati onal entropy S-c was calculated for EG, PD and several other alcohols using the present and literature data. The temperature dependence of S-c for the supercooled-liquid polyalcohols was well reproduced by a model which assum es that all the hydroxyl groups are engaged in hydrogen bonds and that the configurational entropy is governed by the reorientation about the C-C or C -O single bonds and OH ... O hydrogen bonds. The relation between the confi gurational entropy and the structural relaxation times is discussed in the framework of the Adam-Gibbs theory and Vogel-Tammann-Fulcher equation. (C) 1999 Elsevier Science B.V. All rights reserved.