Ionic aqueous solutions

centrifugation

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Ionic aqueous solutions

#1  Postby The_Metatron » Jun 19, 2013 12:47 pm

When common salt dissolves in water, the salt molecules actually break apart into sodium and chlorine atoms, which get mobbed by clumps of water molecules. If I have it right, the oxygen ends of the water molecules stick to the sodium atoms, and the hydrogen ends of the water molecules stick to the chlorine atoms. What you end up with is neutral charged clumps of water molecules around equal numbers of sodium and chlorine atoms.

The atomic mass of chlorine is about a third heavier than sodium.

So, what prevents us from dissolving the salt, centrifuging the solution until the chlorine/water clumps are on the bottom of the solution and the sodium/water clumps are on the top. Decant off the top half of the solution, let the water evaporate, and collect your sodium.

I asked a team at Vespr, after watching Tyler DeWitt's presentation on What Happens when Stuff Dissolves?, this same question:

I wrote:Superb work, Tyler. A quick question on ionic aqueous solutions... Would I be able to separate sodium from chlorine by centrifuging saltwater?

The team replied:

What a great question! So theoretically, yes. Centrifugation can be used to separate things of different masses. In saltwater, the Sodium and Chloride ions weigh about 2 to 3 times more than the water molecules. Unfortunately, however, the masses that we're dealing with are SO TINY that you'd have to spin a bottle of water at an incredibly fast speed in order to separate them out. Like, I'm just totally making this up here, but I'm guessing that it's about a million times faster than anything has ever been spun in the world. So could you actually do it? Maybe. But there would be an insane amount of energy required to power your centrifuges, and you'd be better off using other methods, like plain old distillation.

Why do you suppose it would take such insane centrifugation? It seems to me that if the sodium and chlorine water molecule clumps are indeed neutrally charged, the only real difference they would have then is their weight.

What's up with my idea?
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Re: Ionic aqueous solutions

#2  Postby Arcanyn » Jun 19, 2013 12:59 pm

They're not neutral. In order for them to become neutral would require that at some point a chloride ion will transfer an electron to a sodium ion, which just isn't going to happen. All you've done by dissolving sodium chloride is that you've added some water molecules to the sodium ions and the chloride ions to effectively make larger ions; i.e. instead of Na+ you might have something along the lines of Na(H2O)6+. The reason why it's so hard to separate them is simple electrostatic attraction; it takes a lot of energy to separate oppositely charged ions.
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Re: Ionic aqueous solutions

#3  Postby Rumraket » Jun 19, 2013 1:11 pm

Well, I don't know why it is so, but I reckon it has to do with the weight and size of the ions you want to separate from the water. Water attracts the ions of course, so if their weight difference is low, it takes a much greater force to overcome their mutual attraction.

I had a job extracting and purifying virus particles from cell cultures, and the final part of the procedure required "ultracentrifugation". Basically, a solution containing cell debris(proteins and lipds) and virus particles could not be chemically separated without destroying the virus, so instead they were separated by centrifugation. The virus solution was mixed with a fully saturated Caesium133-Chloride solution and subsequently centrifugated at >60.000 rpm for up to 24 hours. Despite this relatively large centrifugation speed (at least compared to anything else I've worked with) and the large atomic weight of the Caesium ions, Caesium did not separate from the water molecules, but they simply "stuck together" as the bottom (densest, but still dissolved) layer in the centrifuge tube.
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Re: Ionic aqueous solutions

#4  Postby The_Metatron » Jun 19, 2013 2:10 pm

Rumraket wrote:Well, I don't know why it is so, but I reckon it has to do with the weight and size of the ions you want to separate from the water. Water attracts the ions of course, so if their weight difference is low, it takes a much greater force to overcome their mutual attraction.

I had a job extracting and purifying virus particles from cell cultures, and the final part of the procedure required "ultracentrifugation". Basically, a solution containing cell debris(proteins and lipds) and virus particles could not be chemically separated without destroying the virus, so instead they were separated by centrifugation. The virus solution was mixed with a fully saturated Caesium133-Chloride solution and subsequently centrifugated at >60.000 rpm for up to 24 hours. Despite this relatively large centrifugation speed (at least compared to anything else I've worked with) and the large atomic weight of the Caesium ions, Caesium did not separate from the water molecules, but they simply "stuck together" as the bottom (densest, but still dissolved) layer in the centrifuge tube.

Ahh, but you did push all that cesium solution to the bottom. All that would be left then is to decant off the top layers, and evaporate that water with the dissolved cesium in it, no?
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Re: Ionic aqueous solutions

#5  Postby The_Metatron » Jun 19, 2013 2:13 pm

Arcanyn wrote:They're not neutral. In order for them to become neutral would require that at some point a chloride ion will transfer an electron to a sodium ion, which just isn't going to happen. All you've done by dissolving sodium chloride is that you've added some water molecules to the sodium ions and the chloride ions to effectively make larger ions; i.e. instead of Na+ you might have something along the lines of Na(H2O)6+. The reason why it's so hard to separate them is simple electrostatic attraction; it takes a lot of energy to separate oppositely charged ions.

I'm not sure you understood my setup. The sodium and chlorine atoms are already separated in the aqueous solution by the water molecules. I think you are right, it would end up with something like Na(H2O)6 and Cl(H2O)6.

The Cl(H2O)6 would surely be heavier than the Na(H2O)6, no?
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Re: Ionic aqueous solutions

#6  Postby Pulvinar » Jun 19, 2013 2:25 pm

The_Metatron wrote:The Cl(H2O)6 would surely be heavier than the Na(H2O)6, no?


I think I see the problem: you can't ignore that they're still ions: Na(H2O)6+ and Cl(H2O)6-. Try to separate them and you'll build an electric field which will pull them back together again.
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Re: Ionic aqueous solutions

#7  Postby Rumraket » Jun 19, 2013 3:24 pm

The_Metatron wrote:
Rumraket wrote:Well, I don't know why it is so, but I reckon it has to do with the weight and size of the ions you want to separate from the water. Water attracts the ions of course, so if their weight difference is low, it takes a much greater force to overcome their mutual attraction.

I had a job extracting and purifying virus particles from cell cultures, and the final part of the procedure required "ultracentrifugation". Basically, a solution containing cell debris(proteins and lipds) and virus particles could not be chemically separated without destroying the virus, so instead they were separated by centrifugation. The virus solution was mixed with a fully saturated Caesium133-Chloride solution and subsequently centrifugated at >60.000 rpm for up to 24 hours. Despite this relatively large centrifugation speed (at least compared to anything else I've worked with) and the large atomic weight of the Caesium ions, Caesium did not separate from the water molecules, but they simply "stuck together" as the bottom (densest, but still dissolved) layer in the centrifuge tube.

Ahh, but you did push all that cesium solution to the bottom.

All that would be left then is to decant off the top layers, and evaporate that water with the dissolved cesium in it, no?

Ahh sorry, that was not really an accurate description. What happens is that the Cesium concentration is just higher at the bottom than at the top, so there's a gradient. It looks like this after centrifugation:

virpur.jpg
virpur.jpg (461.33 KiB) Viewed 2427 times

So there's still Cesium everywhere in the solution, the density is just highest at the bottom.
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Re: Ionic aqueous solutions

#8  Postby The_Metatron » Jun 19, 2013 3:48 pm

Pulvinar wrote:
The_Metatron wrote:The Cl(H2O)6 would surely be heavier than the Na(H2O)6, no?

I think I see the problem: you can't ignore that they're still ions: Na(H2O)6+ and Cl(H2O)6-. Try to separate them and you'll build an electric field which will pull them back together again.

Yes, I understand what you mean. I think.

What I think is happening then, is that since water molecules are polar, and there are enough of them in close proximity, they provide a medium to which the sodium and chlorine atoms can stick. Give a sodium atom the choice of sticking to one chlorine atom, or a bunch of water molecules, one charge is as good as the next, the salt can break apart.

But, the Na(H2O)6+ will remain an ion, as the water molecules have a net neutral charge.

Centrifuging them then, will try to mash the heavier Cl(H2O)6- ions to the bottom. But, their individual negative charges will remain, holding them away from each other.

The centrifugal force would have to overcome electrostatic repulsion.

Unlikely.
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Re: Ionic aqueous solutions

#9  Postby The_Metatron » Jun 19, 2013 3:49 pm

Rumraket wrote:
The_Metatron wrote:
Rumraket wrote:Well, I don't know why it is so, but I reckon it has to do with the weight and size of the ions you want to separate from the water. Water attracts the ions of course, so if their weight difference is low, it takes a much greater force to overcome their mutual attraction.

I had a job extracting and purifying virus particles from cell cultures, and the final part of the procedure required "ultracentrifugation". Basically, a solution containing cell debris(proteins and lipds) and virus particles could not be chemically separated without destroying the virus, so instead they were separated by centrifugation. The virus solution was mixed with a fully saturated Caesium133-Chloride solution and subsequently centrifugated at >60.000 rpm for up to 24 hours. Despite this relatively large centrifugation speed (at least compared to anything else I've worked with) and the large atomic weight of the Caesium ions, Caesium did not separate from the water molecules, but they simply "stuck together" as the bottom (densest, but still dissolved) layer in the centrifuge tube.

Ahh, but you did push all that cesium solution to the bottom.

All that would be left then is to decant off the top layers, and evaporate that water with the dissolved cesium in it, no?

Ahh sorry, that was not really an accurate description. What happens is that the Cesium concentration is just higher at the bottom than at the top, so there's a gradient. It looks like this after centrifugation:

virpur.jpg

So there's still Cesium everywhere in the solution, the density is just highest at the bottom.

I just wanna pick that up and shake it.
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Re: Ionic aqueous solutions

#10  Postby Berthold » Jun 30, 2013 1:44 pm

Also (referring to the first sentence of the OP), there are no NaCl molecules. The lattice is made of ions. See here.
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Re: Ionic aqueous solutions

#11  Postby Acetone » Jul 03, 2013 2:09 pm

Centrifuges are used to separate atoms all the time. For instance in order to enrich uranium with 235 isotopes (which naturally occurs at less than 1%) up to 3% to make it used as a fuel source or much higher for weapons grade (I think it's 90% enriched) the 235 isotopes must be separated via centrifugion (because the isotopes are all similar in chemistry and weight). They are so close in mass that it requires something like 100,000 rpms to create a suitable gradient for enrichment.

The process is also used a lot for biochemistry (my major) as a poster mentioned above CsCl is frequently used to separate DNA molecules based on their basepairs. The Cs+ ions are heavier and thus form a higher concentration at the bottom. This takes ~100,000 rpms and at least 10 hours of centrifugion to achieve.

However, there's an important difference between CsCl and NaCl. The mass of Cs is MUCH higher than the mass of Cl. Relatively the mass of Na is very close to the mass of Cl. This means A LOT more force is required to effectively separate these two by mass using a centrifuge. Significantly more. Especially if at the end you want only Cl ions at the end. I'm not even sure how this would all work after because you'd have ionized chlorine gas. So you couldn't just let the water evaporate away I wouldn't think. I would try my hand at the force that would be necessary to generate but my physics is pretty rusty xD.

To answer everyone else though the reason that NaCl dissolves is really thermal energy causing the collision of NaCl lattice with the H2O molecules. The energy causes some of the lattice to break off and it is immediately surrounded by the H2O molecules. This surrounding effectively shields the ions from each other so they can not touch and don't really have an influence over one another. I'm too lazy to actually work out the actual influence they have on one another but I'm quite certain that it is minimal relative to mass for determining whether it's possible to centrifuge the solution.

A better way to separate NaCl IMO would be electrolysis of the molten salt (not in solution).
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Re: Ionic aqueous solutions

#12  Postby The_Metatron » Jul 03, 2013 3:27 pm

I've never melted salt. It never occurred to me to try it.
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Re: Ionic aqueous solutions

#13  Postby Arthur Methoxy » Jul 18, 2013 7:13 pm

The_Metatron wrote:When common salt dissolves in water, the salt molecules actually break apart into sodium and chlorine atoms, which get mobbed by clumps of water molecules. If I have it right, the oxygen ends of the water molecules stick to the sodium atoms, and the hydrogen ends of the water molecules stick to the chlorine atoms. What you end up with is neutral charged clumps of water molecules around equal numbers of sodium and chlorine atoms.

The atomic mass of chlorine is about a third heavier than sodium.

So, what prevents us from dissolving the salt, centrifuging the solution until the chlorine/water clumps are on the bottom of the solution and the sodium/water clumps are on the top. Decant off the top half of the solution, let the water evaporate, and collect your sodium.

I asked a team at Vespr, after watching Tyler DeWitt's presentation on What Happens when Stuff Dissolves?, this same question:

wrote:Superb work, Tyler. A quick question on ionic aqueous solutions... Would I be able to separate sodium from chlorine by centrifuging saltwater?

The team replied:

What a great question! So theoretically, yes. Centrifugation can be used to separate things of different masses. In saltwater, the Sodium and Chloride ions weigh about 2 to 3 times more than the water molecules. Unfortunately, however, the masses that we're dealing with are SO TINY that you'd have to spin a bottle of water at an incredibly fast speed in order to separate them out. Like, I'm just totally making this up here, but I'm guessing that it's about a million times faster than anything has ever been spun in the world. So could you actually do it? Maybe. But there would be an insane amount of energy required to power your centrifuges, and you'd be better off using other methods, like plain old distillation.

Why do you suppose it would take such insane centrifugation? It seems to me that if the sodium and chlorine water molecule clumps are indeed neutrally charged, the only real difference they would have then is their weight.

What's up with my idea?


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Re: Ionic aqueous solutions

#14  Postby DavidMcC » Jul 20, 2013 6:27 pm

The_Metatron wrote:When common salt dissolves in water, the salt molecules actually break apart into sodium and chlorine atoms, which get mobbed by clumps of water molecules. ...


No, they don't, they break apart into Na+ and Cl- IONS. Big difference, because water molecules are polar, and interact with charges.
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