What’s up with ionic liquids?

Don Smolenski | TLT Machinery June 2021

The chemistry of ILs is so radically different from typical current oil additives.

The chemistry of ILs is so radically different from typical current oil additives that health and safety—both in new condition and during use—should be thoroughly evaluated.

If you are not familiar with them, ionic liquids (ILs) are a wide departure from typical lubricant additives. ILs are salts in a liquid state. The term is sometimes restricted to salts with a melting point below a selected temperature such as 100 C. ILs are comprised of ions. Most have organic cations such as ammonium-, phosphonium-, imidazolium- or pyridinium-based, with a large variety of anions. ILs are used in many applications, but our focus is on lubricants, which use oil-soluble ILs as oil additives.1 ILs exhibit good metal surface adsorption, low flammability, high thermal stability, good friction and wear reduction and lower changes in viscosity with temperature compared to conventional oil lubricants.

During my tenure at General Motors, we worked with Oak Ridge National Laboratory (ORNL) to investigate one ionic liquid in particular as a possible lubricant additive: trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P666,14] [DEHP].2 It has the following structure:

We found that this IL as an additive could be multifunctional. Initial focus was on antiwear, and it outperformed the conventional zinc dialkyl dithiophosphates (ZDDPs). ILs also have shown antioxidant and friction modifier potential. They contain phosphorus, so it would be expected to have adverse effects on emission control devices, although less so than ZDDPs even at the same phosphorus concentration.3 ILs were confirmed in engine tests to show improved wear protection and fuel economy.1,4 ILs have shown synergistic performance with conventional antiwear additives and friction modifiers.1,5 Good extreme-pressure performance and strong protection of rolling contact fatigue cracking make gear oils another good potential application for ILs.4,6

Talking recently with STLE member Jun Qu of ORNL, I understand that there is still much research being conducted on ILs. So why have ILs not gained more traction—no pun intended? ILs had initially been considered as base oils, but their high cost ($50-150/kg) made that impractical. As engine oil and gear oil additives, they are competing against very well-established and cost-effective additives, so cost and a general resistance to changing what works well is problematic. Since the chemistry of ILs is so radically different from typical current oil additives, health and safety—both in new condition and during use—should be thoroughly evaluated. Supply sufficiency of ILs also would have to be confirmed.

Aside from automotive, there are other potential applications, such as hydraulics and cutting fluids (particularly for Minimum Quantity Lubrication [MQL]), that would be a good topic for a future column. Stay tuned.

1. Zhou, Y. and Qu, J. (2017), “Ionic liquids as lubricant additives – A review,” ACS Appl. Mater. Interfaces, 9 (4), pp. 3209-3222, https://doi.org/10.1021/acsami.6b12489.
2. Qu, J., Bansal, D.G., Yu, B., Howe, J., Luo, H.M., Dai, S., Li, H., Blau, P.J., Bunting, B.G., Mordukhovich, G. and Smolenski, D.J. (2012), “Antiwear performance and mechanism of a oil-miscible ionic liquid as lubricant additive,” Appl. Mater. Interfaces, 4, pp. 997-1002.
3. Xie, C., Toops, T., Lance, M., Qu, J., Viola, M., Lewis, S., Leonard, D. and Hagaman, E. (2016), “Impact of lubricant additives on the physicochemical properties and activity of three-way catalysts,” Catalysts, 6 (4), p. 54.
4. Qu, J., Viola, M.B., et al. (2020), “ORNL-GM final report: Development of ionic liquid-additized, GF-5/6 compatible low-viscosity oils for automotive engine and rear axle lubrication for 4% improved fuel economy.”
5. Li, W., Kumara, C., Meyer, H.M., Luo, H.M. and Qu, J. (2018), “Compatibility between various ionic liquids and an organic friction modifier as lubricant additives,” Langmuir, 34 (36), pp. 10711-10720.
6. Stump, B.C., Zhou, Y., Luo, H.M., Leonard, D.N., Viola, M.B. and Qu, J. (2019), “New functionality of ionic liquids as lubricant additives: Mitigating rolling contact fatigue,” Appl. Mater. Interfaces, 11, pp. 30484-30492.
Don Smolenski is president of his own consultancy, Strategic Management of Oil, LLC, in St. Clair Shores, Mich. You can reach him at donald.smolenski@gmail.com.