Azane - 10-12-2014 at 23:33
One of the things I've noticed whenever I research ionic liquids is that 1-ethyl-3-methylimidazolium is mentioned much more often than
1,3-dimethylimidazolium. Is there any particular reason for this? Is EMIM easier or cheaper to make than DMIM? Does EMIM impart generally favorable
qualities to its compounds versus DMIM (viscosity, hydrophobicity, resistance to depolarization under basic conditions)?
Also, I've read of both EMIM AlCl4 and EMIM FeCl4 having poor or effectively no solubility in water. Why is this so? It doesn't
make a whole lot of sense to me. Would this make it reasonable to assume that most if not all tetrachlorometallate ionic liquids would have similarly
poor solubility in water?
WGTR - 11-12-2014 at 07:53
I second some of your questions. Is there any kind of a reference that gets into the different properties of the common types of ILs? I do have one
reference that I can try and dig up later. It takes about a dozen or so ILs and compares things like electrochemical window and hydrophobicity. The
overall body of data is limited, though, and doesn't cover the thousands (?) of types that are floating around out there.
One thing that I have learned with 1-butyl-3-methylimidazolium hexafluorophosphate is that although an ionic liquid may not be water soluble, it can
still be extremely hygroscopic. Battery applications can allow no more than about 20ppm of water in the electrolyte, and BMIM-HFP can absorb 1-2%.
Getting under 200ppm, even heated under vacuum, can be quite laborious. BMIM-HFP tends to hydrolyse very slowly in water to a certain percentage,
giving free HF.
One way that I dry BMIM-HFP is to put a vacuum chamber on a hot plate. The hot plate heats (improves viscosity) the chamber bottom and stirs the
beaker of IL in the chamber, while a vacuum is pulled. Electrolysis is commenced under vacuum once the IL stops out-gassing, using a platinum anode.
Ionic liquid stability is relative. The electrochemical window is often spec'd using a 1mA test current. This is a really high current, and the CV
curves don't give very useful information when considering typical battery leakage currents, etc. The actual window may be about a volt or two lower
for those low-leakage current applications than expected, considering that there will always be trace amounts of moisture present.
In other words, the electrochemical window shifts depending on how much moisture is present, and what the test current is. Since BMIM-HFP hydrolyses
slowly in water, exceeding the electrochemical window during drying can release free HF into the vacuum. Background current will quickly increase,
and this change is permanent. The only way to fix it is to add extra HF to it, boil the IL back down, and re-dry it carefully, taking care not to
exceed its breakdown potential again. This last point would seem easy, but like I said, that potential shifts with changes in moisture. Drying at a
fixed under-potential would take a very, very, long time.
I suppose all of this doesn't really answer your questions, but perhaps it shows some considerations that go into what makes an ionic liquid useful,
and what some common limitations are.
Also, the ionic liquids that you refer are reactive with water. Here is an MSDS:
http://www.iolitec-usa.com/Download-document/1019-MSDS-EP-00...
[Edited on 12-11-2014 by WGTR]