Using the Zeno line to assess and refine molecular models

Area of Science:

• Thermodynamics
• Fluid Mechanics
• Statistical Mechanics

Background:

• The Zeno line represents points where fluid compressibility factor (Z) equals one.
• Real fluids exhibit a straight Zeno line, but the underlying physical reasons remain unclear.
• Understanding the Zeno line's behavior is crucial for fluid property prediction and molecular modeling.

Purpose of the Study:

• To investigate the Zeno line's behavior and its relationship with the vapor-liquid coexistence curve.
• To analyze Zeno lines in simple model fluids: attractive square-well and Mie n-6 fluids.
• To identify parameters that lead to more linear Zeno lines in these models.

Main Methods:

• Simulation of attractive square-well fluids with varying well-widths (λ).
• Simulation of Mie n-6 fluids with different repulsive exponents (n).
• Analysis of the Zeno line's linearity and comparison with the vapor-liquid coexistence curve.

Main Results:

• Zeno lines for both square-well and Mie fluids were found to be curved, irrespective of parameter values.
• The most linear Zeno line for square-well fluids occurred at λ ≈ 1.8.
• The straightest Zeno line for Mie fluids was observed for n between 8 and 10.
• Models with the straightest Zeno lines showed good agreement with experimental data for fluids obeying the principle of corresponding states.

Conclusions:

• The Zeno line in simple model fluids is inherently curved.
• Specific parameters in square-well (λ ≈ 1.8) and Mie (n ≈ 8-10) fluids yield the most linear Zeno lines.
• The linearity of the Zeno line can serve as a valuable metric for assessing the accuracy of molecular models, complementing phase envelope data.