Sorption Hysteresis Is a Deformation Problem: An Atomistic Free-Energy Perspective

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Gas sorption hysteresis in flexible amorphous materials is crucial for engineering applications but lacks a clear mechanistic explanation.
• Existing models often overlook the role of material deformation during gas sorption cycles.

Purpose of the Study:

• To elucidate the fundamental thermodynamic origin of gas sorption hysteresis in flexible amorphous microporous solids.
• To develop a computational framework capable of simulating continuous adsorption-desorption cycles in deformable materials.

Main Methods:

• Developed a stepwise hybrid Grand Canonical Monte Carlo/Molecular Dynamics (GCMC/MD) simulation framework.
• Simulated continuous adsorption-desorption cycles in a deformable microporous carbonaceous matrix under equilibrium conditions.
• Introduced a two-route alchemical free energy perturbation (FEP) protocol to quantify host deformation work.

Main Results:

• Identified adsorption-induced swelling and persistent micromechanical rearrangements as the cause of hysteresis.
• Observed systematically larger pore volumes and altered pore size distributions (PSDs) on the desorption branch.
• Quantified higher deformation free energy density during desorption compared to adsorption, indicating irreversible structural evolution.

Conclusions:

• Branch-asymmetric deformation free energy provides a thermodynamic explanation for sorption hysteresis in flexible amorphous solids.
• Sorption-induced deformation and metastable free energy states must be integrated into predictive models for gas storage and separation.