ABSTRACT
This chapter examines the nature of molecular diffusivity, introduced the concept of mass-transfer coefficients, and treated the problem of resistances occurring in series and parallel as material diffuses from one phase to another. Once the necessary D-values for advection, deposition, reaction, inter-compartment diffusion, volatilization, etc. are defined, it is then possible to write mass-balance equations for each medium. A more fundamental approach that seeks to understand the basic determinants of diffusion in terms of molecular velocities is pursued in most physical chemistry textbooks that demonstrate that such an equation form is correct. The rate of evaporation is controlled by diffusion through a thin air film immediately the water surface. Measurement of diffusion D-values usually involves setting up a system in which there is a known fugacity driving force and the capacity to measure rDiff, leaving the overall transport D-value as the only unknown in the flow rate equation.
