The latent heat of fusion and of a solid-state transition and the heat capacities of crystalline, liquid, and vitreous Ca(NO3)2·4H2O, Cd(NO3)2·4H2O, and Mg(OAc)2·4H2O have been determined by differential scanning calorimetry. In Ca(NO3)2·4H2O and Cd(NO3)2·H2O over 80% of the entropy of fusion has been lost before the state of the supercooled liquid is frozen in at the glass transition. By contrast, only 30% is lost in the case of Mg(OAc)2·4H2O, a substance which is unusual in several respects. The ratio Tg/T0, 1.07, is unusually small among glass-forming liquids for the calcium and cadmium hydrates, implying that under hypothetical equilibrium conditions the change of heat capacity cannot be much less sharp than that observed at the experimental glass transition. The Tg/T0 ratio for the Mg(OAc)2· 4H2O system, however, is approximately 1.30, a value commonly quoted for organic liquids and polymers. The data have been used to estimate "ideal glass" temperatures, T0(cal), at which S(internally equilibrated liquid) = S(crystal). T0(cal) is found to be 200 ± 4 K for Ca(NO3)2·4H2O and 198 ± 4 K for Cd(NO3)2·4H2O, in good agreement with the respective T0(transport) values of 202 ± 3 K and 194 ± 3 K salts obtained from analysis of the temperature dependence of liquid mass transport processes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry