This question would be more transparent if the author presents his case with a specific material exposed to a  substance, whether it is in gaseous  or liquid states or even in the solid state having contact with a welting interface. 

What do you mean by the penetration enhancer?  

I mean the penetration enhancer of transdermal drug delivery system like oleic acid, isopropyl myristate and glycols from gel base.

Dear Maid Said,

Please refer Fick’s first law of diffusion for basic mechanism of diffusion.The drug diffusion through the skin is passive kinetic process that takes place down the concentration gradient from a region of high concentration to a region of

lower concentration. 

Different penetration enhancers have different mechanism of penetration enhancement.. for example ethanol; It shows permeation enhancement by various

mechanisms. As a solvent can increase the solubility of the drug in the vehicle, permeation of the ethanol into stratum corneum can alter solubility properties of the tissue with improvement for drug partioning into the membrane. It Modifies thermodynamic activity of drug and concentration of drug increase beyond saturated solubility by ethanol evaporation which provide a supersaturated state with

greater driving force for penetration..

Dear Somnath here the problem of diffusion is much  more complicated then simple Fickian diffusion because of the cross effects of enhances, which can't be described by concentration gradient as the primary driving force. One should talk about the chemical potential gradients of diffusion substances as well as  the potential gradient of enhances as cross term. This way you get two flux equations namely for the drug and for the enhancer.  Using Onsager's symmetry requirement which tells us that the mobility matrix symmetric. We should remember that each chemical activity coefficient  is not only the function of concentration of that chemical species but also depends on the rest as well.  Some times flux would be from the low concentration to high concentration side, which is called UP-HİLL diffusion.  Mostly cross terms may be neglected as first order approximation.

To solve a problem like this one needs very tedious  experimental studies to get the full information on the chemical activity coefficient dependence on concentrations  at various temperatures and may be at pressures as well. 

Dear Tarik Ogurtani,Dear Somnath Vibhute, thanks a lot for your helpful information but is there any articles explaine it more easily 

Dear Tarik Sir,

Thank you very much for your kind guidance...