Clausius limits on cooling and heating through the liquid–glass range of three pharmaceuticals and one metal alloy—Annealing effects and residual entropy

The time-dependent, irreversible value of the heat capacity, Cp, measured in the glass-formation range, forbids one from estimating the entropy. To find out how much can the estimate differ from the real change in entropy, we determine the change in the CpdT and Cpd lnT integrals before and after isothermal annealings by DSC studies of three pharmaceuticals, acetaminophen, carbamazepine and griseofulvin and one metal alloy, Mg65Cu25Tb10. The Cpd lnT integrals for the cooling and the heating paths approach the same value when annealing is minimized, or else when the effect of annealing is taken into account. The Clausius limits determined using the Bestul and Chang method show that irreversibility has lesser effect on the estimate of residual entropy than found earlier, and its value can be reasonably estimated from the Cp data. We also discuss the relevant features of dynamic Cp, and conclude that entropy is an ensemble-averaged value.