Hybrid MC/MD Simulations of Carbon Deformation

Developed novel simulation approach combining Monte Carlo and Molecular Dynamics to predict how nanoporous carbons deform during gas adsorption

This project introduced an innovative hybrid Monte Carlo/Molecular Dynamics (MC/MD) simulation methodology to study how nanoporous carbons deform when exposed to gases - crucial for applications in carbon capture, gas storage, and enhanced natural gas recovery.

Key Innovations

  • Developed novel iterative MC/MD simulation approach combining RASPA and LAMMPS
  • First direct calculation of adsorbate-saturated structure compressibility from volume fluctuations
  • Revealed dramatic mechanical property changes during adsorption
  • Quantified adsorption stresses reaching 175 MPa
Schematic of the iterative GCMC/NPT-MD simulation approach for modeling gas mixture adsorption on flexible 3D carbon structures.

Technical Approach

  1. Structure Generation
    • Generated 3D carbon models using Annealed Molecular Dynamics
    • Created structures with varying densities (0.5-1.0 g/cm³)
    • Used EDIP/c potential for carbon interactions
  2. Hybrid Simulation Method
    • Iterative coupling of:
      • Grand Canonical Monte Carlo (GCMC) for adsorption
      • NPT Molecular Dynamics for structural relaxation
    • Custom scripts for seamless software integration
    • Equilibration protocols for reliable convergence

Key Findings

  1. Structural Response
    • Non-monotonic deformation behavior:
      • Initial contraction at low pressures
      • Expansion at higher pressures
    • Lower density carbons show greater flexibility
    • CO₂ presence amplifies deformation effects
  2. Mechanical Properties
    • Dramatic changes in compressibility during adsorption
    • Initial softening followed by substantial hardening
    • Volumetric modulus variations:
      • 0.5 g/cm³ structure: 5-30 GPa
      • 1.0 g/cm³ structure: 18-49 GPa
Generated carbon structures showing different densities and corresponding pore size distributions. Left: 0.5 g/cm³, Right: 1.0 g/cm³.

Adsorption and Deformation Behavior

Adsorption (upper left) and strain (lower left) isotherms for CH₄ at 298K, with molecular configurations shown at key points (right). The inset shows the percent change in adsorption on flexible vs. rigid structures.

Key observations:

  • Initial contraction at low pressures due to molecular “spring” effect
  • Transition to expansion as pores become crowded
  • Density-dependent deformation magnitude
  • Enhanced adsorption in flexible structures
  • Complex relationship between structural changes and adsorption capacity

Impact

This work advances our understanding of:

  • Material behavior during gas adsorption
  • Mechanical property evolution
  • Structure-property relationships
  • Binary mixture effects

Applications include:

  • Carbon capture optimization
  • Gas storage system design
  • Enhanced gas recovery
  • Material degradation prediction

Methods

  • Grand Canonical Monte Carlo (GCMC)
  • NPT Molecular Dynamics
  • EDIP/c potential
  • Custom integration scripts
  • PoreBlazer analysis

Computational Resources

  • LAMMPS molecular dynamics software
  • RASPA adsorption simulation package
  • Custom file conversion utilities
  • High-performance computing implementation

This research provides both fundamental insights and practical methodological advances for understanding and predicting the behavior of nanoporous materials under realistic operating conditions.

References