| Abstract
| - A molecular-based equation of state derived from perturbation theory is used to study the phase diagram andthermodynamic properties of water and ammonia in the fluid phase region. The molecular model that representsthese substances consists of spherical particles interacting via a square-well potential with an embedded pointdipole. The equation of state is an analytical expression, which depends explicitly on density, on temperature,and on the four adjustable parameters of the potential model: the particle's diameter, the energy depth andrange of the square-well interaction, and the dipolar moment strength. Although the associating behavior dueto hydrogen bonding that characterizes water and ammonia is not modeled by the perturbation approachfollowed in this paper, the theory is able to predict (except in the critical region) the vapor−liquid phasediagram and the saturation pressures, with accuracy close to that of theories that include a modeling of theH-bonding effects. A detailed comparison of experimental data from our theory and from three differentapproaches based on the Wertheim perturbation theory for associating fluids is presented. This comparisonindicates that none of the later theories is able to give an accurate prediction within experimental error of thephase diagram of water, for the whole region of densities and temperatures for the fluid phase. This paperpresents the ranges of values for density and temperature where each theory is accurate. Since the theorypresented gives comparable predictions to those of the other equations, the dipolar square-well potential usedin this work could be considered a good effective potential for associating polar fluids if the dipolar momentof the substance is taken slightly higher than its real value, an indication that by increasing the dipolar momentstrength it is possible to mimic, at least in relation to thermodynamic properties, the H-bonding effects. Thesimplicity of this model can be useful as an important ingredient in the building of better equations of statefor polar associating fluids.
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