Transport driven by equilibrium

Essentially all transport is of such a nature that equilibrium is approached. Therefore, equilibrium dictates the direction of transport. Equilibrium considerations, however, do not detail the nature and the rate of the displacements toward equilibrium. These matters are extremely important. For one thing, the speed of separation is related to the speed of the elementary displacement steps leading toward equilibrium. For example, it is known that the motion of macromolecules through liquids is inherently slow. Accordingly, the speed of separation of macromolecules by chromatography or other methods is almost always slower than the speed of separation of small molecules. 

In another vein, we must recognize that separation (e.g., electrophoresis) is often achieved on the path to equilibrium. Here an important question unanswered by equilibrium thermodynamics concerns the nature of the concentration profiles that evolve in the drift toward equilibrium. One must determine if they are suitable for separation and, if not, how they can be made so. 

The issues raised above lie in the realm of transport phenomena. Below we develop elements of this discipline sufficient to address these issues for a majority of analytical separations. 

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