moleculardiffusion

Moleculardiffusion is the net movement of molecules or ions from regions of higher concentration to regions of lower concentration, driven by the random thermal motion of particles. In the absence of bulk flow, diffusion tends to equalize concentrations over time and occurs in gases, liquids, and solids, though at different rates and mechanisms.

The process is described quantitatively by Fick's laws. Fick's first law states that the diffusive flux J is proportional to the negative gradient of concentration: J = -D dC/dx, where D is the diffusion coefficient. Fick's second law describes how concentration changes with time: ∂C/∂t = D ∂²C/∂x². At the molecular level, diffusion arises from Brownian motion, a random walk of particles that, on average, produces net movement down a concentration gradient.

The diffusion coefficient D depends on temperature, the medium’s viscosity, and the size and interactions of the diffusing species. Higher temperatures and smaller molecules generally yield larger D, while more viscous media and larger or more strongly interacting species decrease D. Typical values span several orders of magnitude: in liquids, small molecules often diffuse with D around 10^-10 to 10^-9 m²/s; in gases, D can be around 10^-5 m²/s or larger.

Moleculardiffusion is distinct from bulk advection, where overall fluid flow carries substances, and from facilitated diffusion or active transport across membranes, which involve proteins or energy input. It is a central concept in chemistry, biology, environmental science, and materials science, underpinning processes such as gas exchange, nutrient transport, chromatographic separation, pollutant spread, and phase formation in solids and porous media.