The biexponential road to Fickian yet non-Gaussian diffusion in kerogen's microporosity

Area of Science:

• Geochemistry
• Materials Science
• Chemical Physics

Background:

• Kerogen, an amorphous carbonaceous material, exhibits complex microporosity.
• Adsorbed fluids in kerogen can experience adsorption-induced swelling, suggesting strong fluid-structure coupling.
• Fickian yet non-Gaussian diffusion (FnGD) has been observed in other complex systems like colloids in biofilament networks.

Purpose of the Study:

• To investigate the diffusion dynamics of adsorbed fluids within kerogen's microporous structure.
• To characterize the transition from non-Gaussian to Fickian diffusion in kerogen.
• To explore the influence of confinement and microstructure rigidity on diffusion behavior.

Main Methods:

• Atomistic molecular dynamics simulations of kerogen's microporosity.
• Analysis of the mean squared displacement (MSD) and its logarithmic derivative.
• Characterization of displacement distributions using stretched Gaussian functions.

Main Results:

• Adsorbed fluids in kerogen exhibit Fickian yet non-Gaussian diffusion (FnGD).
• A successive biexponential relaxation process describes the transition to the Fickian regime in the MSD.
• Displacement distributions follow a stretched Gaussian form, deviating from exponential tails seen in other systems.
• Increased confinement and microstructure rigidity enhance non-Gaussian behavior and reduce diffusion rates.

Conclusions:

• Kerogen's complex pore space facilitates FnGD, characterized by a unique relaxation pathway.
• The findings suggest FnGD and a universal diffusion pathway may be prevalent in similar nanoporous materials.
• This research enables the development of simplified models for diffusion dynamics in heterogeneous media.