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Barrier technology - Kinetics: diffusion

7 important questions on Barrier technology - Kinetics: diffusion

What happens to the permeability P in the Langmuir absorption?

It decreases with increasing water activity.

Which model can you use if the permeability is independent of the water activity gradient?

The Henry sorption

How is the thickness of the film related to the mass flux?

The thinner the film, the higher the mass flux through the film.

What is the diffusivity in polymeric materials strongly dependent on?

The amount of permeant in the matrix

Water permeation --> the more water, the higher the diffusion coefficient
Diffusivity of water in non-polymeric media (such as lipids) --> less dependent on the amount of water present. Glassy material, diffusivity extremely low.

Are glassy materials suitable as barrier?

Not really. If a small amount of water is absorbed in the material, the glass transition may be crossed, and the diffusivity suddenly rises to normal values. It can therefore only be used when the water activities remain low at both sides of the film. Further, glassy materials usually are brittle, which makes them fragile and difficult to handle.

Which 3 different models for in-homogeneous layers exist?

Parallel model
Series model
Maxwell model

What is the effect of the crystalline fraction on a lipid film?

A high crystalline fraction reduces the permeability significantly. However, it may also influence the mechanical behaviour: if the crystalline fraction is very high, the film may become brittle, and may show defects. In addition, if you have a crystalline lipid film and need to subject the total product to a temperature treatment, the film may melt and loose its coherence.