diffusionmediated

Diffusion-mediated describes processes in which diffusion is a central determinant of rate and mechanism. In diffusion-mediated processes, the transport of molecules by random motion governs encounters, delivery, or exchange, and diffusion can be the rate-limiting step. The term is used across chemistry, biophysics, materials science, and pharmacology.

Kinetic framework: The diffusion of reactants is modeled by Fick's laws. When reaction occurs quickly upon encounter, the overall rate is diffusion-controlled (diffusion-limited). The classic Smoluchowski model provides encounter rate constants for diffusion-controlled reactions; in dilute aqueous solutions, realistic bimolecular rate constants can approach 10^9–10^10 M^-1 s^-1. If reaction steps are slower than encounter, the process is diffusion-mediated but not diffusion-controlled, i.e., reaction-controlled.

Contexts and examples: diffusion-mediated release from polymer matrices or hydrogels; diffusion-controlled crystallization and Ostwald ripening in materials science; diffusion-mediated signaling in cells and tissues; self-assembly of nanoparticles driven by diffusion of components; transport through membranes.

Methods and modeling: experimental approaches include measuring diffusion coefficients with NMR or FRAP, or diffusion-weighted imaging; for polymers, PFG-NMR. Theoretical descriptions rely on Fick's laws, diffusion equations, and sometimes more complex transport with degradation or crowding. Terminology varies; diffusion-mediated is sometimes hyphenated, related to diffusion-controlled and diffusion-limited usage.

See also: diffusion coefficient, Fick's laws, Smoluchowski model.