Tracing the critical loci of binary fluid mixtures using molecular simulation

Area of Science:

• Thermodynamics
• Computational Chemistry
• Physical Chemistry

Background:

• Critical loci of binary mixtures are essential for understanding phase behavior.
• Accurate prediction of mixture critical properties is crucial for chemical engineering applications.
• Previous simulation studies have faced challenges in quantitatively predicting complex mixture critical behavior.

Purpose of the Study:

• To determine mixture critical lines for binary mixtures using advanced Monte Carlo simulations.
• To investigate different classes of criticality, specifically Type I and Type IIIb.
• To assess the accuracy of simulation methods against experimental data and explore the impact of intermolecular potentials.

Main Methods:

• Grand canonical Monte Carlo (GCMC) simulations.
• Histogram reweighting and mixed-field finite-size scaling techniques.
• Investigation of various intermolecular potentials for methane and combining rules for methane-water interactions.

Main Results:

• Simulations quantitatively reproduced experimental critical loci for methane-ethane mixtures without adjustable parameters.
• Methane-ethane mixtures exhibited Type I criticality, demonstrating continuous mixing.
• Methane-water mixtures displayed Type IIIb criticality with a discontinuous critical locus.
• Simulations showed qualitative agreement for methane-water but failed to quantitatively capture the critical temperature increase with methane addition.

Conclusions:

• GCMC simulations with histogram reweighting and finite-size scaling are effective for predicting Type I critical loci.
• Predicting Type IIIb criticality, especially the abrupt changes in critical temperature, remains challenging.
• The choice of intermolecular potentials and combining rules significantly influences the accuracy of simulations for complex mixtures like methane-water.