what happens inside the pores in a column???

[rick1112]

i have been always curious regarding the events inside pores in a column....so what all happens in there??

i remember reading some were that its assumed that mobile phase is stagnant? is this true, if so then a change in mobile phase composition will never be refecletd in side the pores....

can someone plz calrify this....

thanking you

[tom jupille]

stagnant, but still in contact with the mobile phase. Therefore, things can diffuse in and out.

[danko]

I understand Rick’s objection to the conception of eternally stagnant mobile phase in the particle pores. And how could it be feasible? As Rick points out, it would be impossible to change mobile phase composition once an “initialâ€

[Uwe Neue]

The mobile phase in the pores is mechanically stagnant, i.e. it does not move. However, the distances are so short that diffusion in and out of the pores is so fast that for all practical purposes there is little difference in the composition inside and outside the particles, even in gradients.

The time for a solvent molecule to diffuse through an HPLC particle is in the order of milliseconds...

[danko]

I agree with Uwe’s formulation. With one minor modification: The turbulence originating from the interaction/friction between the moving liquid and the stagnant one will only add to the blending process. So nothing stands still in nature.

[Uwe Neue]

Turbulence is a specific phenomenon of macroscopic (compared to the level of molecules) mixing. There is no turbulence anywhere in any HPLC column, nor is there a need for such a thing, since diffusion is fast enough.

[danko]

There is no lower limit for number of molecules to be implicated in turbulence - unless of course there are only 2 of them (which naturally is unthinkably far from the reality). Even then the molecule with the higher kinetic energy will transfer some of its energy to the one with the lower energy upon contact (but this is a different situation, because the second molecule won’t have a third one to transfer some of its energy, so the chain reaction will stop there).
Whether there’s a need for it or not, is another question, but generally motion promotes mass transfer and that’s the reason for stirring a given liquid when dissolving a solute in it.

Best Regards

[AdrianF]

What is the evidence that the only interchange within the pores of a column is by diffusion and not flow through the pores? By analogy like water through a sponge.

In the case of monolithic columns where the column is cast as a rod, surely the only way for the mobile phase to travel is through the pores.

[danko]

The pores in the particles are not like channels, going through the whole particle, but more like cavities with a blind end. So the mobile phase and the anaytes are just wondering in and out - and I agree – mostly by diffusion. But as mentioned above, I can not see any way of complete absence of interaction between the flowing/passing by mobile phase and the stagnant one in the pores. Even more so, when one thinks of the rough edges around the pores.

Best Regards

[Uwe Neue]

The reason for the absense of flow through the particles is the large difference between the dimensions of the interparticle channels and the channels inside the particle (yes, the channels inside the particles are not dead end, they go through and through the particle).

For your standard column, the size of a channel between the particles is in the order of 1 micron. The average pore size of a packing is about 10 nm. The ratio in the channel dimensions outside the particle to inside the particle is about 100. The linear velocity is proportional to the square of the channel size, and 100 x 100 is 10000. Therefore the flow inside the particle is about 10 000 times slower than the flow outside the particles, i.e. essentially nonexistent, and MUCH slower than diffusion.

A monolith also has two types of pores, at least when one is talking about the Chromolith. There are the channels through which the liquid is flowing, just like through the interstitial spaces between the particles. And then there are the real pores, where the real surface area is and where the bonded phase is attached. The dimensions are about the same and the ratio is about the same as for particles, and there is no flow in the micropores of a monolith either.

[danko]

I’ve seen them (the pores) described and illustrated both as dead end cavities (in superficially porous particles) and as channels. I’m sure there are some “trappedâ€

[Uwe Neue]

I am aware of the rather standard sketch of a pore which looks like a bag. However, this is not what the pore space is like. You should think of it more along the lines of a complicated cave.

Porous particles are essentially made by gluing nanometer-sized pebbles together. At least, this is the first step. One can vary the size of the "nano-pebbles" to change the pore size. Or one can dissolve and reprecipitate the solids to change the size of the structure and thus the size of the cavities in between.

If you think about it this way, you will realize that it is close to impossible to make "closed" pores.

[Uwe Neue]

http://i30.tinypic.com/2e4g7is.jpg

http://i29.tinypic.com/nmn6lf.jpg

The first picture is one of a particle made from agglomerated smaller particles. The second one is from a particle that has been subjected to a treatment that formed the standard corral structure.

Both are from particles with much larger pores than standard HPLC particles. At the time when I took these pictures (about a dozen years ago), the structure of a particle with 100 Angstroem pores was outside the magnification that was available to me.

[danko]

Uwe,
Thanks

Best Regards

[HW Mueller]

Danko, we used to call stirring bulk movement, done to overcome the extreme slow diffusion over long distances. Diffusion is due to molecular motion, bulk movement is due to the simultaneous motion of whole arrays (groups) of molecules. Also, as stated by Uwe, diffusion over distances, lets say maybe 10 to 100 molecules, is extremely fast (one should take a look at Fick´s Laws of diffusion, and at diffusion controled reactions).

Now we have talked about turbulent flow in columns before. I wondered why there should be turbulent flow only in columns from this manufacturer who claims advantages for that (forgot their name at the moment). In other words, I still have a problem with understanding that knitting of a tube changes laminar flow to turbulent, but particles in a tube should not normally cause turbulent flow. Particles are too similar in size and packed too orderly??