Monte Carlo simulation studies of diffusion in crowded environments

Anomalous diffusion has been observed in protein solutions and other multi-component systems due to macromolecular crowding. Using Monte Carlo simulation, we investigate mechanisms that govern gradient-driven diffusive transport, phase separation, and microstructural evolution in a crowded mixture. We consider a multi-component lattice gas model with "tracer" molecules diffusing across a density gradient in a solution containing sticky "crowder" molecules that cluster to form dynamically evolving obstacles. Simulation results show that tracer diffusivity is non-monotonically dependent on both temperature and crowder density. Interaction of diffusive flux of tracers with phase separation of the crowder species gives rise to complex microstructure. At low temperature, crowders aggregate to form slowly diffusing obstacles, and resulting tracer diffusivity shows scaling behavior near the percolation threshold with the same scaling exponent as the random resistor network model. Though highly idealized, this simple model reveals key mechanisms that couple gradient-driven diffusion, phase separation, and microstructural evolution in crowded solutions.