diffuusionvakio

Diffuusionvakio, commonly referred to in English as the diffusion coefficient, is a parameter that quantifies the rate at which particles spread through a medium due to random molecular motion. It describes how quickly a substance diffuses and appears in models of mass transport such as Fick's laws. In Finnish, diffuusionvakio is the term used for this quantity.

Mathematically, Fick's first law states that the diffusive flux J is proportional to the negative gradient of concentration: J = -D ∂C/∂x, where D is the diffusion coefficient, C is concentration, and x is position. Fick's second law describes how concentration changes with time: ∂C/∂t = D ∂^2C/∂x^2 in one dimension (with analogous forms in higher dimensions). The diffusion coefficient thus links spatial gradients to flux and time evolution of concentration.

D depends on temperature, the diffusing species, and the medium. In gases, D generally increases with temperature and decreases with pressure. In liquids, D increases with temperature and decreases with solvent viscosity. Diffusion often follows an Arrhenius-type relation, D = D0 exp(-Ea/RT), where Ea is the activation energy for diffusion.

Types include self-diffusion (the diffusion of molecules of the same species in a pure substance) and tracer or mutual diffusion in mixtures. In multicomponent systems, more advanced descriptions such as the Maxwell–Stefan equations are used. In solids and porous media, diffusion can be much slower and may exhibit non-Fickian behavior under certain conditions.

Commonly used units are square meters per second (m^2/s). Typical values span several orders of magnitude: gases around 10^-5 m^2/s, liquids for small solutes around 10^-9 to 10^-10 m^2/s, with larger molecules diffusing more slowly.

Diffusion coefficients are measured by tracer methods, pulsed-field gradient NMR, or diffusion cells in membranes. They are essential in fields such as chemical engineering, pharmacology, environmental science, and materials research.