| A new equation of state, termed the Energetic Free Volume (EFV) model, is proposed. The model development is motivated by the observation that the temperature and density dependence of the thermodynamic properties of fluids are functionally linked. Statistical mechanics demonstrate that the temperature dependence of the Helmholtz free energy is highly non-linear at low densities, but becomes increasingly linear as density increases. The EFV model is formulated to match the low density thermodynamic properties of the square-well fluid. The model is then extended to higher densities by introducing an energetic free volume, a measure of the extent to which attractive and repulsive intermolecular forces determine the temperature dependence of the Helmholtz free energy.; A first approximation for the energetic free volume is taken from classical free volume theory. Square-well fluid simulation data are then used to refine the energetic free volume expression. The EFV model proves to be capable of representing the thermodynamic properties of the high-density square-well fluid without changing the functionality of the model. The resulting equation of state gives more accurate thermodynamic property predictions for the square-well fluid than previously proposed models.; The utility of the EFV model is demonstrated by predicting the enthalpy departure, liquid molar volume, and vapor pressure of methane. Prediction of these properties requires that the model be capable of representing the temperature and density dependence of the Helmholtz free energy and its derivatives with respect to temperature and volume. For methane, using just two adjustable constants, the EFV model yields average errors of 2.0% for enthalpy departure, 4.0% for liquid molar volume, and 1.6% for vapor pressure.; Finally, the extension of the EFV model to more complicated molecules is discussed. |
