Transport: A Nonequilibrium Process

Abstract

So far, the focus of this book has been on systems at equilibrium, where they experience no net flux of heat, work, or matter. Classical thermodynamics treats these systems easily. As we pointed out earlier, the greatest value of thermodynamics is that the behavior of a system can be predicted, even when the mechanistic details are not known. Homogeneous systems, at constant temperature and pressure, such as the solutions of electrolytes and macromolecules described so far, are composed of molecules that individually experience a variety of forces, both orienting and randomizing. On an instantaneous time scale, this might lead to net movements of mass or energy; however, the time average of the forces leads to the steady-state condition of equilibrium. The activity of a component is the reflection of the time-average molecular forces acting in a system at equilibrium. There are cases in which the time average of a force or forces acting on a system results in the flow of material. When these events occur, transport phenomena result. Transport phenomena and the principles associated with non-equilibrium behavior are extremely important in biological systems because, as we have already suggested, true equilibrium states are achieved only in death. Steady-state systems, which have constant fluxes, are common. These systems are treated by nonequilibrium methods.