About Complex Heat Capacities and Temperature-Modulated Calorimetry

With increasing use of temperature-modulated calorimetry, TMC, it is has been proposed to use a complex heat capacity for the description of the response of samples to periodic temperature changes. In this article the ramifications of this approach are discussed on the basis of irreversible thermodynamics. Experimental results are summarized which describe heat capacities of liquids and solids, as well as TMC during transitions. It is concluded that the complex heat capacity is of limited value. Solids and liquids have no dissipative (imaginary) contributions. In the glass transition, the thermal response is nonlinear, so that a detailed kinetic model (in real notation) is more advantageous to describe the heat capacity. The crystallization is often so far from equilibrium that it is not modulated. During melting and chemical reactions the heat flow is frequently so large, that steady state is lost and complex heat capacity is of questionable value, even if modulation is accomplished.