Keep in mind that the presence of the salt will likely make your solution more dense than its pure solvent. Fortunately, I've often found salt solutions to be less miscible with polar solvents than the pure solvents. For instance brine and acetone are not miscible while water and acetone are. So you might be able to use water on the bottom (with a density > 1 g/cc because of the salt), a relatively heavy non-polar in the center such as toluene or even chlorobenzene, then a light polar on top, maybe an alcohol.

Maybe acetone?

TOP

Acetone (d = 0,79 g cm-3)

Cyclooexane (d= 0,81 g cm-3)

Water (d=1,0g cm-3)

BOTTOM

That really depends on what you are doing. I suspect that the solvent layers are not merely there to dissolve NaCl and has other purpose. But Mathias will be right, if you just want to have a separate layers, then you will need 3 solvents of different density with the top polar (least dense), mid non polar and bottom polar (most dense).

Keep in mind that the presence of the salt will likely make your solution more dense than its pure solvent. Fortunately, I've often found salt solutions to be less miscible with polar solvents than the pure solvents. For instance brine and acetone are not miscible while water and acetone are. So you might be able to use water on the bottom (with a density > 1 g/cc because of the salt), a relatively heavy non-polar in the center such as toluene or even chlorobenzene, then a light polar on top, maybe an alcohol.

Low density solvent - with minimum salt concentration (soluble in water)

2-propanol (0.785 g/ml) or acetonitrile (0.786 g/ml)

Middle - no salt concentration (insoluble in water)

Xylene (ortho, meta, para - 0.897, 0.868, 0.861 g/ml)

High density solvent - more salt concentration (polar)

Ethylene glycol (1.115 g/ml) or glycerin (1.261 g/ml)

Would this help you? < http://en.wikipedia.org/wiki/Organic_solvent#Properties_table_of_common_solvents > You could also use a KI solution in water for the bottom layer. The solutions are very dense. As non-polar solvent you could use halogenated hydrocarbons. As a top layer water would work in that case. One would have to look closely to find a suitable halogenated hydrocarbon (toxicity, boiling point, density, solvent properties) for the middle, but there are quite a few to choose from.

I agree with most of what was said above. I would like to add you have be careful when selecting the holder in which you want to put the three solutions.

Polar - Least dense

Non Polar - Mid density

Polar - Most dense

If the holder is non polar on the sides (very clean deactivated glass maybe?) then it should keep the two polar solvents from mixing. (it prevents them from coming into contact with each other)

Also- do not let the polar solvents touch the side when you pour. This may create a channel that allows the two polar solvents to mix. You would also have to look at the surface tension of the different solvents.

I think that the middle solvent should have a very low surface tension compared to the other two (bottom and top). This would then help the middle solvent to bond to the container.

Pouring this will be very tricky!!

Good luck.

I think it should be a liquid comparable by density with density of salt solution for example such as aniline (phenylamine).

Actually, the simplest solution would be is the concentration of the salt is different in the top and bottom phase, i.e. lower in the top layer (e.g. NaCl at 0.1 M in water), a less polar, not necessarily non-polar solvent (e.g. methylene chloride) as the middle layer, and a concentrated salt solution (e.g. NaCl in water) as the bottom layer. Most hydrocarbons are not suitable since they are less dense that salt solutions and will float on the top.

Wow that was quick, thank you everybody for your suggestions :-)

Gelin, you are correct - my use of this requires that the bottom and top layers are conductive, while the middle layer is insulating, a bit like how a capacitor is configured - I am using the ions as charge carriers.

Onas - that is what I was finding difficulty with - that I could not simply use the textbook densities that Deiter has kindly linked to as the presence of the salt caused a significant density change.

Marc - this is another thing I have found. I have been using thin sodaglass tubes cut to an appropriate size and have found exactly what you describe to happen. I will try to apply the solutions directly from the center using a needle.

Bela - I shall attempt this also, it seems like a simple solution, hopefully the differences in conductivity will not effect my application negatively.

Thanks again, I will report back if I find a suitable solution - It may be some time as if I do not manage to get it done tomorrow I shall be out of the lab for a few weeks.

You could also use water for both the top layer and the bottom layer. If you use a solvent a little bit more denser than water (as chlorobenzene, chloromethane or something similar) , for the bottom solution a saturated solution of the salt and for the top one, a diluted solution of the salt.

And if you use sulphuric acid instead of the salt, you could increase the density of the bottom layer to use more common solvents as dichloromethane, or chloroform...

Good luck, and I would like to know what was finally your solution to the problem.

Interesting Carlos,

I did not consider H2SO4, presumably this would require high concentration or even fuming sulfuric to achieve? I have Chloromethane and Chlorobenzene so I will try as both yourself and Bela have suggested with the differing salt solutions first.

I shall feedback when I have succeeded in achieving my three layers. As I have said it may be a few weeks as I am out of the lab in September (with other engagements + the arrival of the new undergraduates).

Dichloromethane is a good choice as a nonpolar solvent

Semi-facetiously, one might emulate a bartender making a multi-layered pousse-café. Keep specific gravity uppermost in mind.

Oh and I did not see Mathias' first reply sorry - I attempted this, thank you for the suggestion, however with the salts dissolved in the appropriate layers the top and bottom layer sank below the cyclohexane.

Also the solubility of salt is unfortunately pretty low in acetone.

Lookinf at the answer above I would try something like.

TOP

Diluted water+NaCl solution (d < 1,33 g cm-3)

Dichloromethane (d = 1,32)

Saturated water+NaCl solution (d = 1,33 g cm-3)

BOTTOM

Thanks again :-)

Hi everyone,

Working late again - (I should really learn when and how to go home, lol) - I decided to perform a quick try with things I have in my spectroscopy / XRD lab - rather then the chem labs and had some success.

I managed to stack as follows:-

KCl 0.1M

Propylene Carbonate

Sat. KCl soln.

This worked, however I must try again as I seem to get conduction across the PC layer (it does not insulate as required) - and thus causing electrolysis to occur at the electrodes - which I wish to avoid.

It was a quick trial however so I shall be attempting this further when I have the time :-)

For the bottom layer use a concentrated solution of CsX, where X = Cl, Br, or I. The solubility of CsCl is ~ 64 wt % and has a density of ~ 1.88 gm/cm^3. This high density will make the choice of an intermediate non-polar organic layer pretty flexible. For example, CCl4 has a density of "only" ~ 1.6 gm / ml. Any non-polar organic solvent would serve as well or better. The upper conductive layer could be most any dilute aqueous electrolyte.

What about using mercury as bottom layer - if I got it right your overall goal is to have two conductive liquids being separated by a non-conductive one?! This could be topped with an apolar solvent with a very high density and finally with an aqueous salt solution. I´m aware of the disadvantages of using mercury but for some applications (e.g. mercury porosimetry) its still the best choice.

You can use diochloromethane

would it be possible to use a solid non-conducting layer instead? This would make things easier and the final more stable. Several different things come to mind like using a fat or wax and melt it or a solution of a polymer or nonpolar compound with a volatile solvent. For the third layer you will have to add a wetting agent.

You could adjust the dense polar solution on the bottom by increasing the density using glycerol or another polar, non-ionic solute like sucrose. Both compounds are used in density gradient centrifugation or used to make sample density higher in sample application for electrophoresis.

So you have 3 liquids

a) Liquid containing NaCl

b) Liquid in which NaCl is NOT soluble

c) Liquid containing NaCl

Well. For the first and third layer you should use water or alcohol( methanol or propanol) and dichloromethane for the middle layer.

Hi again and thank you everyone for your additional ideas - they are most useful :-)

Vince - I shall see what we have in our stock room (as this is a complete ' ohh.. I wonder if this might do something cool' idea - which may be significant if it works but is not directly part of the projects I am involved with - so I cannot order new materials in)

Michael - Mercury is an interesting one which I would never have thought of and might be worth a try so see if my idea works in principal. Unfortunately I failed to add in my original question that if everything works as planned I require my liquids to be transparent enough to see through - we have a great quantity of mercury though so an attempt is not out of the question :-)

Titus - indeed a solid would be easier to produce the required, however it is essential that the layers are all liquid - hence the issue.

Ralf - That is really useful thank you - meaning I can use the same ionic concentrations on top and bottom :-)

I think you can use this formula, it should be worked

1-Saturated sol of NaCl

2- Isopropanol + 10 % of an organic solvent (eg. Toluene)

3- Diluted sol of NaCl

Benzene orcabontetrachloride as nonpolar solvent,water

Thanks again for your further suggestions I shall feed back when I am next able to get in the lab to test this further :-)

Well Mr. Matthew, hope we understand that based on conductivity facts better suggession for you, the first layer might be sea water on the top most layer, follows on the second layer can be use saturated solution of acenapthene in ether (Slurry), which have nonconductive than toluene in an interface, and the final layer would be the mineral water is suitable choice for you.

The ideas for denser polar bottom phase and organic phase have great merit and simplicity. If for some reason the density or weigh becomes an issue, consider making top phase a conductive emulsion or slurry of polar and less dense solvent.

Slurry can be conductive without true solubility. Bet there is a lot of information on this from the world of battery technology.