I am looking for analytical technique(s) that can be employed for the determination of diffusion rate/coefficient of a small molecular weight gas (such as O2, water vapor) or liquid (organic solvents..) in a solid matrix. Solid matrix is composed up of amorphous polymer embedding small molecule organic crystals or amorphous composite of small and large organic molecules. The diffusing fluid can enter externally to the matrix or already present in trace amount in the matrix itself.
Ideally, a simpler method applicable from ambient temperature/pressure to slightly higher temperature is desirable.
I have seen NMR relaxometry used in most of instances. In addition, are there rather faster and simpler methodology (based on vibration spectroscopy, vapor sorption, depth profiling).
Thanks in advance!
Best Regards,
Amrit
Dear Amrit
For studying the diffusion of gases or vapours with a sufficiently high solubility, you can use gravimetric methods. In yours case, they should be suitable for organic vapours, maybe water. There it depends on the exact properties of you material.
To see how the diffusion coefficient can be extracted from gravimetric data, see the following paper:
http://pubs.acs.org/doi/abs/10.1021/je300916f
https://www.researchgate.net/publication/263983191_Experimental_Study_of_Sorption_and_Diffusion_of_n-Pentane_in_Polystyrene
Additionally, you could you permeation methods (permeation cells) or volumetric (pressure decay) methods. However, in the first case you have to suitably pre-process your sample, which might be challenging in the case of composites.
In the case of liquid sorption, you can use a method commonly used for polymer membranes, where the sample (in a compact = nonporous form) is placed into the liquid and left to equilibrate. Then you take the sample out, gently dry it with a filtration paper and monitor the decrease of the sample mass. This method is mostly used for obtaining equilibrium solubility by extrapolation to zero time (where it works ok), but I think it can also provide an estimate of the diffusion coefficient.
Regards,
Josef
Article Experimental Study of Sorption and Diffusion of n-Pentane in...
Dear Josef
Many thanks for your answers. They are of great help. The solid materials of my interests are generally composites of amorphous and/or crystalline materials in a compact. That means, they inherently have some microstructures, pores, voids, defects etc. Is there some way to probe the rate and extent of diffusion of small molecules (vapors liquid) within the solid.
On the other hand, if we have crystals with defined channels, voip, or surface defects, are there established experimental methods to reasonably determine the diffusion of small molecules through them?
Amrit
Dear Amrit
If you use a porous material for the gravimetric methods, it works ok, but you only have to keep in mind that the diffusion coefficient that you detemine is the so called "effective diffusion coefficient", which is a function of both material properties and pore structure.
Then with crystallinity. If you would change the amount of crystalline material, then I would expect the diffusion coefficient to also change. So you have to be careful when using or generalizing your results. If you will have samples with the same chemical composition but different porosities and crystallinities, you can have very large differences in the diffusion coefficient.
There is a lot of literature on this topic for polyethylene, which is semi-crystalline polymer and is usually produced is in the form of porous particles. Sadly I dont have any good paper downloaded, but you can try to find something from prof. Hedenqvist.
Regards,
Josef
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