Diffusion coefficient:

A thickness of the film for non equilibrium becomes significant because the molecules have to travel thickness to reach to the interface for equilibrium. This means a solute in a thin film of liquid will be closer to equilibrium than that penetrated deep within the thick film. If a diffusion coefficient of a solute within the film is smaller the molecules will diffuse slower. Therefore, C_S can be expressed as

C_S   = f_S(k')d²f/D_S                                                                                                                   

C_S = coefficient of mass transfer in stationary phase,

fs(k´) = complex function of retention factor k´,

d_f  = thickness of liquid film (cm), and

D_S = diffusion coefficient of the solute in the film

Likewise, we can consider the mobile phase mass transfer coefficient (C_M). It is a complex function of the retention factor k´, i.e. f_M(k´), is directly proportional  to the square of the particle diameter  of the packing d²p  and inversely  proportional  to the diffusion of coefficient (D_M) of the solute in the mobile phase.

Thus, C_M is expressed as

C_M  = f_M(k')d²p/D_M