A multi-scale approach to characterize pure CH4, CF4, and CH4/CF4 mixtures

Area of Science:

• Computational chemistry
• Physical chemistry
• Materials science

Background:

• Phase separation in methane (CH4) and tetrafluoromethane (CF4) mixtures is observed experimentally in liquid states near freezing points.
• Understanding the microscopic drivers of this phase separation is crucial for materials science and chemical engineering applications.

Purpose of the Study:

• To develop accurate intermolecular potentials for CH4, CF4, and CH4/CF4 systems using a novel ab initio method.
• To investigate the phase-separation phenomenon in CH4/CF4 systems at a microscopic level.
• To validate the developed potentials through comparison with experimental data and simulations.

Main Methods:

• Development of intermolecular potentials using a novel ab initio method.
• Monte Carlo (MC) simulations to verify the accuracy of the developed potentials for pure CH4.
• Reduction of six-dimensional (6D) ab initio potentials to one-dimensional (1D) effective potentials using the inverse Monte Carlo (IMC) technique.
• Analysis of potential energy landscapes for pure CF4 and CH4/CF4 systems.

Main Results:

• MC simulations for pure CH4 using ab initio energies accurately reproduced experimental liquid densities, internal energies, second virial coefficients, and radial distribution functions.
• The 1D effective potential derived via IMC also successfully reproduced experimental results for CH4.
• Ab initio potentials for pure CF4 and CH4/CF4 exhibited rough energy landscapes, hindering MC simulations and suggesting a potential cause for phase separation.
• The study explored the determination of 1D effective potentials using IMC and reweighting techniques for temperature dependence.

Conclusions:

• The developed ab initio intermolecular potentials are reliable for simulating CH4 systems.
• The roughness of CF4 and CH4/CF4 potentials suggests a microscopic driving force for the experimentally observed phase separation.
• The IMC technique is effective for reducing dimensionality and creating accurate effective potentials.
• Further investigation into the temperature dependence of 1D potentials using reweighting techniques is feasible.