Local Grand Canonical Monte Carlo Simulation Method for Confined Fluids

Area of Science:

• Computational physics
• Physical chemistry
• Materials science

Background:

• Simulating fluids in porous materials is crucial for understanding various physical and chemical processes.
• Existing methods for simulating confined fluids, such as traditional grand canonical simulations and Widom's particle insertion, face limitations in accuracy and efficiency.
• Accurately determining electrostatic potentials in confined charged fluids is essential for applications in electrochemistry and nanotechnology.

Purpose of the Study:

• To introduce a novel local grand canonical Monte Carlo method for simulating fluids in pores in chemical equilibrium with a bulk phase.
• To demonstrate the method's superior accuracy and efficiency compared to existing techniques for Lennard-Jones fluids in various pore geometries.
• To extend the application of the method to confined charged fluids for determining local electrostatic potentials referenced to the bulk.

Main Methods:

• Development of a local grand canonical Monte Carlo method utilizing a penalty potential to establish a gas-liquid equilibrium within the pore.
• Application of grand canonical Monte Carlo moves in the gas phase, enabling particle "diffusion" to maintain chemical equilibrium.
• Extension of the method to simulate confined charged fluids and calculate local electrostatic potentials without explicit bulk simulations.

Main Results:

• The local grand canonical Monte Carlo method significantly outperforms traditional grand canonical simulations and Widom's particle insertion in accuracy and efficiency for Lennard-Jones fluids.
• The method successfully determines local electrostatic potentials in confined charged fluids, properly referenced to the bulk, eliminating the need for explicit Donnan potential calculations.
• Analysis revealed that finite cross-section pores induce a small, non-zero linear charge density, causing a potential difference with the bulk that decays logarithmically with pore length (∼1/ln(L)).

Conclusions:

• The developed local grand canonical Monte Carlo method offers a more accurate and efficient approach for simulating fluids in confined geometries.
• This new method provides a computationally inexpensive way to determine local electrostatic potentials in confined charged fluids, crucial for understanding electrochemical phenomena.
• The findings highlight the impact of pore geometry on electrostatic potential differences between confined and bulk fluids, offering insights into nanoscale electrochemistry.