How to make sure there is water in Ionic Liquid?

2019-03-27 09:22:56 adman 37

There is an ionic liquid user asked:

"I opened a bottle of [EMIM][TFSI] in a glove box, and then collected 4mL of it for my experiments. I used in once to make a solution in ambient temperature and then sealed it and kept in a nitrogen purge desiccator. How much water it would have absorbed? How should I degas it?"

And there are some answers as folloing:

As mentioned, this ionic liquid is hydrophobic (not miscible with water). But it is also hygroscopic and can absorb a certain amount of water from the air, so depending on the application, care should be taken in handling. From what you have mentioned, you have taken appropriate precautions (glove box and minimizing the time in the air) and the intake of water should be negligible. If in doubt, you can confirm this using Karl Fischer.

Practically ionic liquids containing TFSI anion are hydrophob. In most instants they are likely to not adsorb water. Notably, water is used as a means of purification during their synthesis, where the impurities are dissolved in water phase and are separated by decanting. This is contrary to hydrophobic ionic liquids with other anions and hydrophilic ionic liquids, where the level of water content is rather is high
So, basically ionic liquids with TFSI anion are not highly hygroscopic, contrary to other ionic liquids. However, it is not easy to predict the level of water content in ionic liquids of any type and it should be supported by experiment and observation. The most reliable method to determine water content is the Karl Fischer. Please see the following article.  
M.H. Ghatee, M Bhrami, N. Khanjari, J. Chem. Thermodynamics 65 (2013) 42–52

Ionic liquids with this anion are not very hygroscopic and will only absorb small amounts of water from the ambient. So water absorption will not be as problematic as with other ILs, unless you are measuring a property or carrying out a process that is highly sensitive to water as still they will absorb small water amounts. The best method to determine water content is Karl Fischer titriation (although you have to be careful because as this method usually involves certain manipulation of the sample, if you don't perform it with some precautions it may be that during measurement the IL is exposed to the ambient for a long time and absorbs far more water than it originally had), and a simple method to reduce water content is to use a vacuum oven connected to a dry nitrogen line.

You can investigate whether there is water content in your Ionic Liquid or not accurately and simply by using an ATR, FTIR spectrometer. Collect a sample spectrum for a drop of your Ionic Liquid for this test in mid-IR region. If you observed a broad peak near wavenumber between 3200-3500 cm-1 (which corresponds to OH stretch of water molecule H-bond), then it means that there is a water content in your sample. On the other hand, the absence of this peak reveal the lack of water content in the tested sample i.e.  your Ionic Liquid
If you are going to apply this method, then please make sure that you do not mix-up between the different OH stretch from different functional groups (i.e. carboxylic acid OH stretch). You can get some more information from IR tables such as”