The evolution of multicomponent systems at high pressures:: I. The high-pressure, supercritical, gas–liquid phase transition

Abstract

The thermodynamic stability of n-octane has been investigated as a function of temperature, pressure, and degree of molecular clustering at supercritical temperatures. At low pressures, the free enthalpy is shown to be always lowest in the unassociated, gas state, and the system is, in that regime, robustly resistant to clustering. At high pressures, the free enthalpy of the unassociated, gas state exceeds that of the clustered, liquid state. At the pressure at which the values of the free enthalpies of the gas and liquid states become equal, the system becomes abruptly unstable, and will then spontaneously cluster into effective `cluster-polymers', and undergo a phase transition to a liquid state. This phenomenon is a geometric effect, and occurs even at supercritical temperatures. The gas–liquid phase transition reported here is closely related to the Alder–Wainwright gas–solid phase transition, the onset of which is applied to approximate the optimal clustering parameter. This phase transition is of the class of entropically-driven phase transitions, characterized by an increase in spatial order accompanied by an increase in entropy, and manifests an inverted latent heat of transformation, analogous to adiabatic demagnetization.