Alchemical Osmostat for Monte Carlo Simulation: Sampling Aqueous Electrolyte Solution in Open Systems.

Area of Science:

• Computational chemistry
• Materials science
• Physical chemistry

Background:

• Molecular simulations typically fix ion amounts, which is inaccurate for nanoporous materials where electrolyte concentration is unknown.
• Nanoporous materials like MOFs and zeolites exchange ions and water with bulk electrolytes, requiring chemical potential calibration.
• Determining confined electrolyte concentration is crucial for understanding ion transport and storage in these materials.

Purpose of the Study:

• To develop a novel algorithm for Monte Carlo (MC) simulations of electrolytes in contact with a saline reservoir.
• To enable fluctuating ion numbers in simulations, accurately reflecting conditions in nanoporous materials.
• To validate the method by calculating solvation free energies and exploring electrolyte thermodynamics.

Main Methods:

• Utilized nonequilibrium candidate Monte Carlo (NCMC) moves to adapt the Widom insertion technique for chemical potential calibration.
• Developed an alchemical osmostat in the semi-grand-constant volume and temperature ensemble (Δμ, N, V, T).
• Implemented the osmostat in MC simulations starting with bulk water, driving them towards electrolyte solutions.

Main Results:

• Successfully calibrated chemical potential by alchemically transmuting water into ions using NCMC.
• Validated the NCMC-based osmostat by determining water solvation free energy.
• Demonstrated the osmostat's capability to simulate electrolyte insertion into nanoporous materials.

Conclusions:

• The proposed alchemical osmostat is a promising tool for MC simulations of electrolytes in nanoporous materials.
• This method accurately samples electrolyte insertion by allowing fluctuating ion numbers and reservoir contact.
• Advances the simulation of ion behavior in confined environments, relevant for energy storage and separation technologies.