Main Aspects and Terms of Translational Diffusion

Looking at translational diffusion in liquid systems, at least two elementary categories have to be taken into consideration self-diffusion and mutual diffusion [1, 2]. 

In a liquid that is in thermodynamic equilibrium and which contains only one chemical species, the particles are in translational motion due to thermal agitation. The term for this motion, which can be characterized as a random walk of the particles, is self-diffusion. It can be quantified by observing the molecular displacements of the single particles. The self-diffusion coefficient is introduced by the Einstein relationship 

If a liquid system containing at least two components is not in thermodynamic equilibrium due to concentration inhomogenities, transport of matter occurs. This process is called mutual diffusion. Other synonyms are chemical diffusion, interdiffusion, transport diffusion, and, in the case of systems with two components, binary diffusion. 

The description of mass transfer requires a separation of the contributions of convection and mutual diffusion. While convection means macroscopic motion of complete volume elements, mutual diffusion denotes the macroscopically perceptible relative motion of the individual particles due to concentration gradients. Hence, when measuring mutual diffusion coefficients, one has to avoid convection in the system or, at least has to take it into consideration. 

Mutual diffusion is usually described by Pick s first law, written here for a system with two components and one-dimensional diffusion in the z-direction  

Equation 4.4-2 describes the flux density JI (in mol m 2 s 1) of component l through a reference plane, caused by the concentration gradient dcjdz (in mol m 4). The factor D (in m2 s 1) is called the diffusion coefficient Most mutual diffusion experiments use Fick s second law, which permits the determination of D, from measurements of the concentration distribution as a function of position and time  

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