Your DMSO is wet. Probably very wet. An easy qualitative test for this is to put it in the fridge, if it is dry it should easily crystallise. If it is wet it will remain a liquid.

Distil some fresh and keep it in a schlenk under nitrogen with some 4A molecular sieves. Very dry DMSO is often a solid at room temperature (at least in winter, depending on where your lab is).

If you want to dry the system you already have then before adding other reagents you could try drying it overnight with a dean stark apparatus, exploiting the water/toluene azeotrope. Throw a few molecular sieves in the collecting arm to increase efficiency. You may have to determine the exact solvent ratios afterward via NMR.

In terms of miscible salts it depends on what your desired counter ions are. Tetrabutylammonium salts work well if you want a functional anion, and tetrafluoroborates and triflates are good for functional cations.

In general dissolution of salts will tend to make your system less miscible given the large difference in dielectric constants, but with heat and good stirring that might not matter anyway.

Hi Peter.

Is this occurring during the reaction or just when mixing the two solvents?

If its just the two solvents then are you certain the DMSO is completely free from water?

It happens directly after mixing. The DMSO is sealed with a septum, but quite old so there might be water in it.

Is there a way to separate the water from the mixture?

Would it make sense to mix the water-contaminated DMSO wit toluene and then dry this mixure? Hoping that the mixture is less hygroscopic?

You might be able to dry the mixture with something like anhydrous calcium sulphate or 3A molecular sieves. Much easier to dry the DMSO by itself with molecular sieves though.

OK, thanks for the hints, will try that!

Richard, could addition of inorganic salts cause sudden immiscibility of DMSO and Toluene?

Yes, that is also a possibility. Depends on the salt involved. Some may results in salt/DMSO phase that is immiscible with toluene while others might just precipitate out.

Have you heard of any salt that does neither cause precipitation nor an immiscible DMSO/salt phase?

After a quick search I couldn't find anything definite but perhaps some of the longer chain organic acid salts.

It looks normal to me.It is difficult to dissolve a polar solvent  into an unpolar one.(Same dissolve the same)

Tthese two should be miscible according to literature

In the Handbook of chemistry and physics it is said for solubility of DMSO : water, alcohol, diethylether and ethylacetate.

Your DMSO is wet. Probably very wet. An easy qualitative test for this is to put it in the fridge, if it is dry it should easily crystallise. If it is wet it will remain a liquid.

Distil some fresh and keep it in a schlenk under nitrogen with some 4A molecular sieves. Very dry DMSO is often a solid at room temperature (at least in winter, depending on where your lab is).

If you want to dry the system you already have then before adding other reagents you could try drying it overnight with a dean stark apparatus, exploiting the water/toluene azeotrope. Throw a few molecular sieves in the collecting arm to increase efficiency. You may have to determine the exact solvent ratios afterward via NMR.

In terms of miscible salts it depends on what your desired counter ions are. Tetrabutylammonium salts work well if you want a functional anion, and tetrafluoroborates and triflates are good for functional cations.

In general dissolution of salts will tend to make your system less miscible given the large difference in dielectric constants, but with heat and good stirring that might not matter anyway.

Repeat your experiment then add calcium chloride, calcium sulfate, sodium sulfate or magnesium sulfate,

These are nice propositions, will try this!

Eric, are triflates soluble in toluene purum or is DMSO stll needed as "emulgator"?

I effect, I need to get Calcium cations into solution but in a solvent which is not miscible with water (I chose toluene).

I don't know the exact solubility of calcium triflate in toluene.

however Ca(TMHD)2 is soluble in toluene. TMHD = 2,2,6,6-tetramethyl-3,5-heptanedionate.

"How to get Calcium cations into solution but in a solvent which is not miscible with water" -> You need simply ionic liquid not soluble in water.

Toluene is wrong choice  - in such non-polar media, calcium (ionic) concentration will be very low. DMSO in presence of soluble salts will pushed out from non-polar toluene (salting-out effect)

Andrei, thanks for the suggestion, I also thought about ionic liquids, but have to experience about their properties. I want the metal cation to be able to move to the aqueous phase which is formed during the the reaction. Do I need a phase transfer catalyst for this or can themodynamics (partition effect) be exploited?

Then, which combination of ionic liquid and phase transfer catalyst would you propose?

In such question formula I recommend use CaO powder as Ca(2+) source. Once water will formed, it will react to Ca(OH)2  - it is water-soluble source of ionic Ca-ions.

Calcium alcoholates are also promising because will react with water as well. By regulation of alkyl chain length in alcoholate, you can regulate solubility in organic solvents.

I think that Ca tert-butylate is sufficient by solubility in toluene (it can be prepared by exchange of commercial tert-BuOK and CaCl2 in tert-Butanol) - may be it is available from catalogs

These are interesting suggestions Andrei.

I found calcium methoxide is commercially avaiblable, but not calcium tert-BuO - maybe there are steric issues due to the rather extensive anion and calcium would need to bind two of them? If still possible, how would you propose to generate pure Ca tert-BuO? Does Ca have higher affinity to the anion than K?

Tert-BuOK + CaCl2 -> (tert-BuO)2Ca + KCl

KCl is insoluble in tert-BuOK - this is driving force of reaction

Also you can try available in any chemical institute reagents  : CaH2 + tert-BuOH - result (solubility) will be visible immidiately and directly.

Commercial samples of CaH2 contain various impurities like Ca metal or calcium nitride, but all of them produce sole final product. 

OK, I will try that. But how can I extract (tert-BuO)2Ca from tert-Butanol to then solve it in toluene?

Excess of tert-BuOH you should distill off ; vacuum rotary evaporator is very suitable for this.