IONIC LIQUIDS

Q. I have heard about ionic liquids being used as a plating medium but I have found very little being written on the subject. Can you explain what they are and how the system works? J.P.

 

A. Your question is a good one. There has been a lot of interest in these materials, and I know of at least one research center primarily devoted to learning more about them.

To learn more about these materials, a good place to start is Wikipedia (http://en.wikipedia.org/wiki/Ionic_liquid). If you’re not familiar with Wikipedia, now is a good time to introduce yourself to this Web site. It’s basically an online encyclopedia that contains information about all sorts of things. The info is contributed by individuals like you and me, and is vetted by readers.
An ionic liquid is made up of only cations and anions. For you non-chemists out there, a cation is a positively charged species, for example, Cu2+, and an anion is a negatively charged ion, for example SO42-. In the case of ionic liquids, the cations are usually organic and the anions inorganic.

Again, looking at our example ions, we would write the formula for copper sulfate as CuSO4. CuSO4 is a molecule, but under the right conditions we can get the molecule to break apart into ions:

CuSO4 >> Cu2 + + SO42-

If CuSO4 exists essentially in the form to the right of the arrow, you would have an ionic liquid. In the case of CuSO4, a very high temperature would be required. This is considered a molten salt. With ionic liquids, room temperature is adequate. For example tetraethylammonium nitrate is made up of essentially 100% ions. The drawing below shows this material:

 

 

Many of these materials are sensitive to water and their properties will change if they are diluted with water.
What about the use of these ionic liquids in plating? Like many things in the development stage, “they are extremely promising.” They essentially replace water and increase chemical reactivity, which can lead to more efficient metal finishing processes. Since they are electrically conductive and non-flammable, they are a good replacement for many of the classic hydrocarbon-based solvents.

If you Google the term ionic solvents electroplating you will be amazed at the number of hits you get. One in particular, http://www.ionmet.org/fileadmin/ionmet/Dissemination/EIPC_PressRelease_042007.pdf, has an excellent but brief description of ionic liquids and potential use in metal finishing.