An integral equation theory for inhomogeneous molecular fluids: the reference interaction site model approach

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Statistical Mechanics

Background:

• Understanding molecular liquids with varying densities is crucial for chemical processes.
• Existing theories often struggle to accurately describe inhomogeneous molecular systems.
• The reference interaction site model (RISM) is a powerful tool for liquid structure but traditionally applied to homogeneous systems.

Purpose of the Study:

• To develop an integral equation theory for inhomogeneous molecular liquids.
• To extend the RISM equation to systems with varying densities.
• To investigate the hydration structure around ions in such systems.

Main Methods:

• Proposed the inhomogeneous reference interaction site model (RISM) equation.
• Developed approximations for closure relations and intramolecular susceptibility in inhomogeneous liquids.
• Applied the theory to study the hydration structure of lithium, sodium, and potassium cations.
• Utilized the Percus trick to relate local solvent density to solute-solvent pair correlation functions.
• Analyzed hydration structure using triplet correlation functions derived from inhomogeneous pair correlations and local solvent density.

Main Results:

• The inhomogeneous RISM theory successfully calculates pair correlation functions in different density regions.
• Triplet correlation functions for cations indicate a strong relationship between hydration shell thermal fluctuation and solute ion size.
• Significant differences were observed between the current theory's triplet correlation functions and those from the Kirkwood superposition approximation, particularly for lithium ions.

Conclusions:

• The developed inhomogeneous RISM theory provides a robust framework for studying molecular liquids with density variations.
• The theory accurately captures the influence of ion size on hydration shell dynamics.
• The findings highlight the limitations of approximations that ignore system inhomogeneity, such as the Kirkwood superposition approximation.