Solute separation by centrifuge

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SUMMARY

The discussion centers on the feasibility of separating solutes from solvents using centrifugation. It is established that while centrifuges can effectively separate isotopes, such as U-235 and U-238 for uranium enrichment, not all solute-solvent combinations are separable, particularly those with similar densities like phenol and water or KOH in water. The potential for molecular disassociation at extreme RPMs is noted, but practical limitations exist, as centrifuges may break before achieving significant separation. The principle of creating density gradients through centrifugation is also highlighted, particularly in biological applications like DNA replication studies.

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  • Understanding of molecular density and solubility
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  • Familiarity with biological applications of centrifugation
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Researchers in chemistry and molecular biology, engineers designing centrifuge systems, and professionals involved in isotope separation processes will benefit from this discussion.

neanderthalphysics
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If energy (in)efficiency was not a factor, is it possible to separate any solute from any solvent by centrifuge? At extreme RPMs, is molecular disassociation possible?
My view is that it is possible to separate any solute from any solvent by centrifuge. Likewise it is possible to separate heavy water from H1 water by centrifuge.

Molecular disassociation is probably a borderline possibility, especially if we consider the molecular disassociation of your centrifuge by too high RPMs! But maybe, if you have a long chain molecule with two heavy atoms on the ends, and it is forced to bend under the rotational forces, then it might break the bonds.
 
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neanderthalphysics said:
Summary:: If energy (in)efficiency was not a factor, is it possible to separate any solute from any solvent by centrifuge? At extreme RPMs, is molecular disassociation possible?

My view is that it is possible to separate any solute from any solvent by centrifuge. Likewise it is possible to separate heavy water from H1 water by centrifuge.

Molecular disassociation is probably a borderline possibility, especially if we consider the molecular disassociation of your centrifuge by too high RPMs! But maybe, if you have a long chain molecule with two heavy atoms on the ends, and it is forced to bend under the rotational forces, then it might break the bonds.
No. You can find solvents and solute which are not separable by centrifuge. For example, different variants of hydrocarbons with same density.
Also, the centrifuge itself will break long before any gravity-chemical effects would be noticeable.
 
trurle said:
No. You can find solvents and solute which are not separable by centrifuge. For example, different variants of hydrocarbons with same density.
Also, the centrifuge itself will break long before any gravity-chemical effects would be noticeable.

Thanks for your inputs.

Can you give some examples of solvents and solute which are not separable by centrifuge?

If we go for something basic (excuse the pun) like KOH in water where the former is highly soluble in the latter, will you get an increasing concentration gradient as you spin the centrifuge faster?

What happens if you have a saturated or even supersaturated solute in a solvent, and then you spin it in a centrifuge? Will you start getting crystallization?

Tom.G said:
It can however be used to separate various isotopes of elements. For instance that is how enriched Uranium is obtained for reactor fuel, and for bombs.

https://science.howstuffworks.com/uranium-centrifuge.htm

I guess with a stretch of imagination one could view the U-235 and U-238 isotopes in a uranium centrifuge as a "solvent" and "solute" both of which are infinitely miscible with the other with interchangeable definitions of solvent and solute.
 
neanderthalphysics said:
Thanks for your inputs.

Can you give some examples of solvents and solute which are not separable by centrifuge?

If we go for something basic (excuse the pun) like KOH in water where the former is highly soluble in the latter, will you get an increasing concentration gradient as you spin the centrifuge faster?

What happens if you have a saturated or even supersaturated solute in a solvent, and then you spin it in a centrifuge? Will you start getting crystallization?
I guess with a stretch of imagination one could view the U-235 and U-238 isotopes in a uranium centrifuge as a "solvent" and "solute" both of which are infinitely miscible with the other with interchangeable definitions of solvent and solute.
For example, phenol and water. Densities are very similar.
Saturated solution under centrifuge may become super-saturated if difference in density is large enough.
You need rotational speed about ~100 (m/s)/(g/cm3)
 
The Uranium isotope separation mentioned earlier is one example. Of course you don't get some drastic all-or-none separation, your get quantitative enrichment, and a cascade of processes is needed (fortunately).

The principle has a classical application in molecular biology which is in all the textbooks. Centrifuge a solution containing a heavy ion fast enough, long enough you will get an equilibrum density gradient which is just the same principle as the gradient of air density of the atmosphere. Will be a shallow gradient but that is okay for the application, which is to separate macromolecules of closely similar density.

The question was when the (double-stranded) DNA molecule replicates in cellular duplication, how does the original DNA distribute between daughter cells? It was known that the original DNA was not destroyed - DNA is radioactively labelled (e.g. by growing bacteria in radioactive thymidine) all the DNA radioactivity is still found in the DNA of the daughter cells. But what way was it distributed? I.e. if we represent an existing strand as | and a newly produced strand as | , so the original DNA is || , when the cell has duplicated are progeny || and || (called 'conservative replication') or are they || and || (called 'semi conservative replication')? (They are not || and || (nonconservative) from what was mentioned above, the parent DNA strands are conserved.)

Cells were grown in a medium where their nitrogen source was the (nonradioactive) nitrogen isotope 15N. This gives a DNA slightly more dense than that with the normal 14N isotope. So || , || and || have three different densities and will eqiilibrate settling at different heights in the centrifuge tube in which a gradient of CsCl is established, and in this way it was possible to verify that replication produced || i.e. is semiconservative.

See any textbook or look up 'semiconservative replication' or 'Meselson & Stahl experiment'.
 
Last edited:
Tom.G said:
It can however be used to separate various isotopes of elements. For instance that is how enriched Uranium is obtained for reactor fuel, and for bombs.

In gaseous phase, not in solution.
 
Surely whether it is in the gaseous or liquid phase just affects separation by a matter of degree?
 
neanderthalphysics said:
Surely whether it is in the gaseous or liquid phase just affects separation by a matter of degree?
Yes.
gaseous phase U235:U238 separation: known (I believe as their hexafluorides)

Liquid phase U235:238 separation: unknown
 

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