Moleculesdiffuse

Moleculesdiffuse is the term used to describe the process by which molecules spread within or between media due to random thermal motion, moving from regions of higher concentration to regions of lower concentration. It operates in gases, liquids, and solids, with rates that depend on temperature, molecular size, and the properties of the surrounding medium.

The rate and direction of diffusion are described by Fick's laws. Fick's first law states that the diffusive flux J is proportional to the negative gradient of concentration, J = -D ∇C, where D is the diffusion coefficient. Fick's second law predicts how concentration changes in time: ∂C/∂t = D ∇^2 C, assuming a constant D. The diffusion coefficient reflects molecular motion and interactions with the medium; in gases it is relatively large, in liquids smaller, and in solids often very small. The microscopic basis is Brownian motion, as described by the Einstein relation D = μ kT, linking diffusion to thermal energy and mobility.

Factors affecting diffusion include temperature (higher T generally increases D), molecular size and shape, solvent viscosity, medium structure (porosity, tortuosity), and chemical interactions (binding, solvation). Diffusion can occur alone (molecular diffusion) or be assisted by carriers or channels (facilitated diffusion). Mixed transport often includes advection or convection, which is not diffusion but can confound measurements.

Applications span drug delivery, environmental transport of pollutants, food science, materials engineering, and soil science. Measurement methods include tracer experiments, NMR diffusometry, FRAP, and molecular dynamics simulations to estimate D and visualize diffusion pathways.