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

Abstract

The excess molar enthalpies H_m^E of [xN₂O+(1-x)C₈ H₁₈] 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.

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