HEAT-CAPACITIES OF 3-COMPONENT AND 2-COMPONENT FLUID SYSTEMS IN THE CRITICAL REGION

We derive the correlations between the different kinds of heat capacities (at constant pressure, at constant volume, and at saturation) for two-phase systems. We also establish the connection to measurable quantities. Our discussion relates to any one-component system or to a binary or multicomponent system near a critical point. In particular, we treat vapour-liquid equilibrium in a system containing one or two components. We investigate the signs of the relevant quantities and the behaviour of these quantities in the critical region. In the experimental part we present the results of our calorimetric measurements of heat capacities in the gas-liquid critical region of the fluids nitrous oxide (N2O), carbon dioxide (CO2), ethane (C2H6), and of the fluid mixtures N2O + CO2 and N2O + C2H6 (each at two compositions) at temperatures near 300 K. As to be expected, the heat capacity of the saturated vapour is negative, while the other heat capacities are positive. All heat capacities show abrupt changes (in the form of lambda transitions) near a critical point. Whether or not the heat capacities tend towards infinity at a critical point, cannot be safely concluded from our measurements.