A perturbative density functional theory for square-well fluids.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Statistical Mechanics

Background:

• Accurate modeling of fluid behavior is crucial for understanding diverse physical and chemical processes.
• Existing theories often struggle to precisely capture the complex interactions in systems with both short-range repulsion and long-range attraction, such as square-well fluids.

Purpose of the Study:

• To develop and validate a perturbative density functional theory (DFT) for the quantitative description of structural and thermodynamic properties of square-well fluids.
• To provide a reliable theoretical framework for predicting fluid behavior under both bulk and inhomogeneous conditions, including confinement.

Main Methods:

• A free-energy functional was constructed by combining modified fundamental measure theory for short-range repulsion and a quadratic density expansion for long-range attraction.
• Long-correlation effects were incorporated using analytical expressions for direct correlation functions derived from the first-order mean-spherical approximation.
• The developed DFT was rigorously calibrated against extensive simulation data from existing literature and new simulations.

Main Results:

• The perturbative DFT accurately predicts the radial distribution function for bulk square-well fluids.
• The theory successfully describes density profiles of square-well fluids near spherical cavities and within slit pores.
• The model demonstrates good agreement with simulation results across a wide range of parameters and thermodynamic conditions.

Conclusions:

• The developed perturbative density functional theory offers a quantitatively accurate approach for studying square-well fluids.
• This theoretical framework is effective for describing fluid behavior in both bulk and confined environments.
• The validated DFT provides a valuable tool for researchers in physical and computational chemistry.