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Transport of toluene in water and air

In an environment that is the same everywhere (homogeneous environment), a substance diffuses and distributes evenly if we wait long enough. This is not the case if the substance can travel through different materials. For example, toluene diffuses from water into air (or vice-versa) until the concentration of toluene is about four times higher in water than in air (Fig. 1, left and right). If we have soil and water present that can take up toluene, final (equilibrium concentrations) are again not the same. The situation is even more complex than in the case of water and air. The relation between equilibrium concentrations in soil and water is not linear.

Interpretation

It appears that toluene “prefers” to be in water rather than in air. So, if there are equal concentrations of toluene in water and air in contact, there still is a drive of toluene to migrate into water. We can explain this by introducing the concept of chemical potential. At equal concentration, the chemical potential of toluene is lower in water than in air. The difference of the chemical potentials of toluene in water and in air is interpreted as the drive (or driving force) for the diffusion of toluene. The observation tells us that the driving force for the flow of toluene cannot be differences in its concentration; rather, it is a gradient of the chemical potential of a substance. This is reflected in the dynamical case (Fig. 1, right).

The difference in the relations between equilibrium concentrations for toluene in water/air and in soil/water indicate that the form of the dependence of the chemical potentials upon the concentration of toluene is different for different environments (soil is different in this respect from water or air).