In general, deep eutectic solvents based on choline chloride and dicarboxylic acids are LMMs, but not liquids at room temperature, with one exception: a 1 : 1 molar mixture of malonic acid and choline chloride. Therefore, the starting point of this study was the decrease of the melting point of one of the components, namely the dicarboxylic acid, which is succinic, glutaric or adipic acid. For this purpose, one of the two protons of the acidic group was exchanged by a bulky unsymmetrical choline cation. The resulting ionic liquids (ILs) were still solid at room temperature, but have a reduced melting temperature compared to the corresponding acids. In the second step, mixtures of these ILs with choline chloride were prepared. It turned out that choline glutarate–choline chloride mixtures are liquids at room temperature at compositions containing 95–98 wt% of choline glutarate. Finally, urea was added as another hydrogen bond donor. Density, conductivity and viscosity measurements were performed for all obtained mixtures. Moreover, a Walden plot was drawn which indicates that all mixtures are liquids with fully dissociated ions moving independently. Therefore, they are considered as ‘‘good’’ ionic liquids and, thus, for example they can be used to exchange more toxic or less biodegradable ILs in application processes. A brief outlook containing application possibilities is given. It is demonstrated that choline dodecylsulfate is readily soluble in these mixtures, forming aggregates in the LMM at temperatures exceeding 55 1C.

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Article Synthesis and Characterization of Choline Chloride Based Bin...

Your choline chloride may be very wet. Choline chloride is highly hygroscopic, urea is not. So, if you don't dry ChCl thoroughly, the amount you weigh may contain loads of water and if you don't account for the water you may be adding too much urea and missing the eutectic composition. To be safe, best thing is to vaccum dry ChCl until it becomes a loose powder and then prepare the mixture in a glovebox. If you don't have a glovebox, you may try weighing the ChCl at ambient atmosphere but be quick: it will start to deliquesce almost immediatly. I always prepare my mixtures in the glovebox but a colleague of mine has done it outside because water contamination is not a problem for her purpose and she dries the ChCl very thoroughly, then weighs it quickly and so far it has worked very well. Hope it works for you

PS - it may be worth it to run a FTIR-ATR of your ChCl and urea just to make sure there is no contamination/degradation. 

Firstly, I would agree with Catarina's point above. You should check that your choline chloride is not very hydrated, because even a slight change to the eutectic ratio will drastically alter the melting point of the mixture.

Does your DES form a homogeneous liquid phase once mixed at 80 C? If so, and if your ChCl is pure, and if the mixture is only solidifying at room temperature, then it is nothing to really worry about. It is becoming increasingly apparent that pure ChCl-Urea is only a room-temperature liquid in warmer countries, because the melting point is closer to 30 C. You can read the attached and rather interesting article which discusses this issue; for ChCl-Urea to melt at the originally suggested 12 C, it has probably absorbed quite a lot of water from the atmosphere.

http://dx.doi.org/10.1039/c5nj02677f

Thanks to all of you for the information on how to make CC:Urea (1:2) DES.  I found that my CC:Urea powder mixture melted at 80C and has remained a liquid at room temp (ca. 28C).  I store it under Argon and tightly sealed.  However, the CC did require overnight vacuum drying at 85C to get it dry enough to form the DES.

Catarina, to find the contamination, FTIR will not help. What you would like to see is water, isn't it? Either the bending mode or stretching mode of water is overlapped with the DES. So it is impossible to see water in the DES with FTIR.

Usually, my sample was dried in the vacuum over at 90C for 24 hours, that basically make sure your ChCl is dried. And sadly, there is no way to dry urea. We found urea will decompose after heating even at ~80C. The process will produce a tiny peak through FTIR. So we just mix urea and ChCl directly in a vial with vortex mixture and sonicator, which is no-heat applied. For reline, it will take ~30 min to form clear liquid.

Yaowen, you are correct to point out that, once the ChCl:Urea DES is prepared, it is hard to spot water, because urea's bands overlap with those of water. It is certainly possible to detect it once you have a spectrum of dry DES - then, by comparison, you see distortions above 3300 cm-1 which are caused by hydration. However, that was not my suggestion. My suggestion is to check whether the pure ChCl is dry or not before preparing the DES, and this is pretty straightforward. In the picture in attachment there are three ATR spectra of ChCl - the top one with only vestigial water content, the middle one with some more and the last one pretty wet. You see that all the water goes in the 3300-3400 cm-1 region while ChCl's OH should be around 3220 cm-1. Urea is not hygroscopic, so it usually does not pose a problemregarding water contamination. We dry it in vacuuumfor a short time (1-2h), just to be sure, but it's really unnecessary. On the other hand, drying ChCl is of utmost importance, as well as preparing the mixture in a glovebox - when you do it in normal atmosphere you can see the choline powder become gooey and moist before you have the time to weigh it. Your sonicator idea sounds neat though. Best of luck in you endeavours and please promote your papers, they're good and people need to read them. Cheers!

Thanks for sharing, Catarina ! In our lab, we are studying the solvent structure and dynamics, so we are strict with moisture containing of samples. Forgive my picky to the spectrum.

I weight the CC and dropped it into a preheated beaker at 60 degrees. Added urea and gave it a stir. Melted into a liquid and stayed like that at room temp

Does anybody recommed to recristallize choline chloride in etanol?

Crystallization of ChCl in methanol is mentioned in Abbott paper. But is that necessary?

is the eutectic mixture hygroscopic? Because i left mine on the table in a beaker and the next day its volume seemed to increase.

@ Mukund Vadrev, yes the eutectic mixture is hygroscopic. You must dry the reagents, weigh them in inert atmosphere (glovebox) and store the mixture in a sealed vial, otherwise it will absorb water.

I think I have been able to make this using 1.39g of Choline Chloride (1 molar) and 1.2g of ethylene gycol (2 molar) @ 80 degrees after 20 mins. I see a lot of fuming even at 70 degrees and smells like the EG. I am doing this in open air. How do I verify the DES formed? Because today tried adding metal chlorides, to the DES. But upon cooling, it precipitated into a semisolid. Would you know why that might have happened?

Mukund Vadrev it is probable that you added less than 1 mol of CCl because it may be wet, I don't dry CCl because it is very difficult to remove all water, I usually measure first with Karl Fischer to recalculate what is much faster. EG may also have water but in less amount.

Deep eutectic solvents are not ionic liquids, are much less robust, the molecular solvent EG will for sure evaporate and choline is degraded into triethylammine above room temperature (you will note the fishy smell).

If you dry your compound at low temperatures, to avoid degradation you will obtain your DES. However, a DES is also "formed" in presence of water, but when water and DES ratio is 1:1, it is said that most of special properties of DES are lost.

Another method to prepare DES, safer, is to mix compounds in water and then lyophilize the mixture.

I hope I helped you, feel free to message me.

Yes, Crystallization of ChCl in methanol is necessary.

Firstly you should check purity by AgNO3 titration or by titration of OH-base after

passage through an anion-exchange column after that crystallise it from absolute EtOH.

Regards

Choline + urea (1:1) -->??

can anyone answer this case? please!

Dear Nguyen Thi Hong Hanh

1 ChCl + 1 Urea does not form a eutectic. 1ChCl+2Urea has lowest melting point.

For more information you can read my article:

Barzinjy, A. and Zankana, M.M., 2016. A novel application of the quartz crystal microbalance for determining the rheological properties of the highly viscous liquids. Acta Physica Polonica A, 130(1), pp.239-244.

Is there a eutectic material which allows way to turn choline into a nonhygroscopic solid, ideally a free flowing powder?

Catarina Fernandes Araújo is it possible to dry in a normal oven without vacuum. What are the pros or cons?

you can mix them in a test tube and heating over a heater tell the mixture becomes clear

Yes, along with hygroscopic nature of choline chloride, please check the purity of both the components of DES is. Choline chloride and Urea. The mixing of the components are due to H bonding interaction. From personal observations I can say that you should try in low temperature may be between 55-65°C and for 45min to get clear transparent liquid.

Sudam Sankar Padhi why 55-65 Celsius specifically? Is the final mixture/solvent temperature sensitive? I have observed that working with the mixture at higher temperatures 80-90 Celsius gives undesirable results in certain applications.

What about microwaving?