Modeling Structural Flexibility in 3D Carbon Models: A Hybrid MC/MD Approach to Adsorption-Induced Deformation

Abstract

Predicting adsorption-induced deformation in nanoporous carbons is crucial for applications ranging from gas separations and energy storage to carbon capture and enhanced natural gas recovery, where structural changes can significantly impact material performance and process efficiency. The interplay between adsorption and material deformation presents both challenges and opportunities, particularly for CO₂-CH₄ displacement processes in geological structures where matrix swelling can alter reservoir permeability. We investigate adsorption-induced deformation of nanoporous carbons using an original hybrid Monte Carlo/Molecular Dynamics (MC/MD) simulation approach that couples adsorption sampling with structural relaxation. By studying CH₄ and CO₂ adsorption on 3D carbon structures of varying densities (0.5-1.0 g/cm³), we demonstrate characteristic non-monotonic deformation behavior, with initial contraction at low pressures followed by expansion at higher pressures. A key contribution is the direct calculation of isothermal compressibility of adsorbate saturated porous structures from the volume fluctuations during NPT-MD simulations, which reveals dramatic mechanical property changes during adsorption. In the process of adsorption, carbon structures exhibit initial softening followed by substantial hardening, with a dramatic increase of the volumetric modulus in denser carbons. Using elastic theory relationships, we estimate the adsorption stresses reaching 175 MPa, that provides crucial insights into potential material degradation mechanisms. For binary CH₄/CO₂ mixtures, increasing CO₂ content amplifies both contraction and expansion effects due to stronger fluid-wall interactions. The iterative MC/MD methodology enables direct observation of the structural evolution and quantitative estimates of the mechanical properties, which are difficult to measure experimentally, advancing our understanding of coupled adsorption-deformation processes in nanoporous materials.