Fick's laws and bound-water diffusion

Fickian diffusion involves random movement of total population of molecules. Diffusion rate varies with partial pressure which varies with concentration (C). Both bound-water diffusion (B) and evaporation (E) involve only a small temperature-dependent population. Rates of B and E vary with population of mobile molecules, which varies with vapor pressure, not C, C-based forms of Fick's laws are thus shown to be invalid for B, and approximate validity only under isothermal conditions in the lower part of the hygroscopic range. With vapor-pressure-driven diffusion, diffusion coefficient is a function of MC only, and is independent of temperature. -- AATA