Excess molar enthalpies of nitrous oxide–octane in the liquid and supercritical regions

Abstract

The excess molar enthalpies H _m ^E of [xN ₂ O+(1-x)C ₈ H ₁₈ ] have been measured in the liquid and supercritical region over the whole concentration range. Mixtures at 308.15 K and 7.64, 9.48 and 12.27 MPa show moderately endothermic and exothermic mixing in the octane-rich region and nitrous oxide-rich region, respectively. The exothermic-mixing region is confined to a smaller range of x, as pressure increases, and disappears completely at 308.15 K and 15.00 MPa. Mixtures at 318.15 K and 7.64 and 9.48 MPa show very exothermic mixing. The magnitude of the minimum observed at 318.15 K decreases as the pressure increases and mixtures at 318.15 K and 12.27 and 15.00 MPa exhibit endothermic mixing in the octane-rich region. The temperature and pressure effects on the H _m ^E values are both large and of opposite sign. These effects are discussed in terms of a liquid–vapour equilibrium and critical constants for nitrous oxide–octane. Excess enthalpies for nitrous oxide–octane mixtures are also calculated using the Peng–Robinson equation of state and the resulting H _m ^E values are compared with experiment.