I have a few questions about the diffusion terms in the General Rate Model (GRM).
The k_f term is called film diffusion but it is not the coefficient of a second derivative term in the model. Is this truly a diffusion mechanism or is it a mass transfer term?
Secondly, what exactly is surface diffusion, in a physical sense? While pore diffusion is the movement of the mobile phase within the pores, is surface diffusion the movement of the solid phase while attached within the pores? Is this a mechanism that actually happens in reality? I would have thought that maybe the solid phase would enter the mobile phase, diffuse, then go back to the solid phase instead of diffusing while in the solid phase.
In chromatography modeling, external mass transfer (between bulk volume and particles pores) is also called film diffusion even though the mechanism differs from molecular diffusion within the particle pores. The model assumes the stagnant liquid film around the particles is infinitely thin. In reality, molecular diffusion takes place in a film with finite thickness.
Thermodynamic equilibria can be quite unstable at the molecular level. Surface diffusion is a model assumption/ simplification that assumes molecules can move in adsorbed state. Reality is rather a rapid combination of desorption, movement and re-adsorption. The speed of the associated adsorption and desorption processes separates surface diffusion from pore diffusion. The molecules stay very close to the surface. Molecules with multple binding sites can also move from on ligand directly to another without fully desorbing, particulary in grafted resin.