A Unified Picture of Diffusion

It is shown that if the equation[Formula: see text]where p is a pressure function and Ax a resistive or frictional force, is taken as the fundamental dynamical equation, it is possible to explain diffusion in solutions, in solid solutions, and in an adsorbing solid as well as the interdiffusion of two gases. In diffusion in both liquid and solid solutions the pressure function is the osmotic pressure; in diffusion of adsorbed gases the pressure function is the spreading pressure of the adsorbed layer. Since these pressure functions are thermodynamic properties of the system the pressure gradient is independent of the physical picture assumed for the system. To evaluate Ax, however, it is necessary to assume a specific physical mechanism for the movement of the molecules.