Fickian Mechanism

A molecule colliding with the pore wall is reflected in a specular manner so that the direction of the molecule leaving the surface has no correlation with that of the incident molecule. This leads to a Fickian mechanism, known as Knudsen diffusion, in which the flux is proportional to the gradient of concentration of partial pressure. The Knudsen diffusivity is independent of pressure and varies only weaMy with temperature ... 

Criteria 1-3 are the cardinal characteristics of Fickian diffusion and disregard the functional form of D(ci). Violation of any of these is indicative of non-Fickian mechanisms. Criterion 4 can serve as a check if the D(ci) dependence is known. As mentioned, it is crucial that the sorption curve fully adhere to Fickian characteristics for a valid determination of D from the experimental data. At temperatures well above the glass transition temperature, 7 , Fickian behavior is normally observed. However, caution should be exercised when the experimental temperature is either below or slightly above 7 , where anomalous diffusion behavior often occurs. 

A drug release mechanism from compressed cellulose ether matrices using dimensionless analysis has been discussed (17). A non-Fickian mechanism could be dismissed when calculating the Deborah and the Swelling interface numbers from relaxation, penetrant diffusion, swelling and drug diffusivity data (17). 

Graft copolymer of GG with acrylamide by crosslinking with glutar aldehyde Verapamil and Nifedipine Hydrogel Microspheres Drug release followed non-Fickian mechanism. [47,66]... 

It was demonstrated that concentration profiles of different components can be obtained utilizing the partial least squares method. With concentration profiles readily measurable, it is possible to obtain other quantities such as diffusion type, that is, Fickian versus non-Fickian mechanism of diffusion. The application of in situ ATR FT-IR spectroscopic imaging has been employed to investigate polymer interdiffusion of PVP and polyfethylene glycol) (PEG) under high pressure CO2 [74]. The diffusion mechanism of the system was described based on the spectroscopic imaging data, and it was found that CO2 molecules dissolved in the polymeric system greatly enhanced the interdiffusion process. 

It was found in several glassy polymers that a gas permeability depended onp and that sorption must be interpreted as occurring by two mechanisms the usual Fickian mechanism described above and a second mechanism resembling a Langmuir-type isotherm. The total concentration in the polymer in this duai-sorption model was then expressed as [30]... 

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