Diffusion Coefficient

  • Diffusion coefficients in gases lie between 0.1 and 1 cm\(^2\)/s.

    • \(D_{\ce{O2}-\ce{He}} = 0.822\) cm\(^2\)/s

    • \(D_{\ce{O2}-\text{benzene}} = 0.101\) cm\(^2\)/s

  • Diffusion coefficients in liquid fall close to 10\(^{-5}\) cm\(^2\)/s. Diffusion coefficients in liquids are about ten thousand times slower than those in dilute gases.

    • \(D_{\ce{H2}-\text{water}} = 4.5\times10^{-5}\) cm\(^2\)/s

    • \(D_{\text{acetone}-\text{water}} = 1.21\times10^{-5}\) cm\(^2\)/s

    • \(D_{\text{ethanol}-\text{water}} = 0.84\times10^{-5}\) cm\(^2\)/s

  • Diffusion coefficients in solids will be lower than in liquids, about \(10^{-30}\) cm\(^2\)/s.

    • \(D_{\ce{H2}-\ce{Fe}} = 2\times10^{-9}\) cm\(^2\)/s

    • \(D_{\ce{C}-\ce{Fe}} = 6\times10^{-21}\) cm\(^2\)/s

    • \(D_{\ce{Zn}-\ce{Cu}} = 2\times10^{-38}\) cm\(^2\)/s

  • Diffusion coefficients in polymer and glasses lie between solid and liquid values, say about \(10^{-8}\) cm\(^2\)/s.

  • The low values of diffusion coefficient means that diffusion often limits the overall rate of processes occurring in liquids. Diffusion limits the rate of acid-base reactions. In physiology, diffusion limits the rates of digestion.