Researchers have been studying ionic liquids for many years to find applications that can benefits from their unusual properties. One application that is receiving a lot of attention is the use of ionic liquids as lubricant additives. Once their benefits are proven in this application, their use as base fluids might be more practical. A number of researchers have examined this approach, and it was the subject of several papers at the Lubmat Conference held in Bilbao, Spain, in June.

Maria Taige of Iolitec Ionic Liquids Technologies GmbH, in Heilbronn, Germany, investigated the properties that ionic liquid additives produce in lubricants. Markus Kronberger from the Laboratory for Tribology and Interface Nanotechnology, at the University of Ljubljana, Slovenia, reported on the use of ionic liquids as additives in biodegradable base ionic liquids. In addition, Jun Qu of Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, United States, presented results of ORNLs work on ionic liquids last October at the Directions in Engine-Efficiency and Emissions Research (DEER) Conference in Detroit. Under the sponsorship of the U.S. Department of Energy Vehicle Technologies Program, ORNL is working with a major automaker to determine the usefulness of ionic liquids as base oils or additives in engine oils.

Why Ionic Liquids?

Taige said that ionic liquid is a generic term for materials consisting entirely of ions that are liquid at temperatures below 100 degrees C. If they are liquid at room temperature, they are called room temperature ionic liquids.

Taige explained that ionic liquids can be formulated from a wide variety of ions. As a result, the fluids can be tailored to produce specific properties such as high conductivity, high thermal and electrochemical stability, low vapor pressure and good solubility. This combination of properties makes them particularly suitable for dispersing nanoparticles.

According to Qu, ionic liquids are currently being used as environmentally friendly solvents in chemical synthesis and separation, electrochemistry, catalysis, food science, solar energy and battery manufacture. He explained that the properties making these liquids attractive as lubricants or additives are inherent polarity, high thermal stability, negligible volatility, and nonflammability.

In testing at Oak Ridge, ionic liquids displayed superior lubricating properties compared to fully formulated engine ionic liquids, and progress is being made in developing the liquids for lubricant applications. Low-viscosity ionic liquids are being examined as neat lubricants where they reduce viscosity, increase operating temperature range, and improve wear protection compared to conventional engine ionic liquids. As base stocks, they have been shown to reduce friction and wear, and operate at temperature up to 500 degrees C. They are also compatible with specialty bearing components. And, because these fluids are nonflammable, they are safer to transport and store.

Oil-soluble ionic liquids are under study at Oak Ridge as lubricant additives where they have the potential to provide multiple properties, including scuffing and wear resistance under boundary lubrication, reduced friction under hydrodynamic lubrication, and corrosion inhibition, among other properties. In fact, research indicates that it may be easier and more cost effective to introduce ionic liquids into the lubricants market as additives.

Because of their makeup, ionic liquids can be used as ashless an-tiwear agents, friction modifiers, antioxidants, and detergents. In particular, they may potentially replace or reduce the use of zinc dialkyldithiophosphate (ZDDP). Engine oil formulators have long leaned on ZDDP as an effective and economical additive for inhibiting wear and corrosion, but the chemicals contain ash-forming zinc and have fallen out of favor in recent years after complaints that they fouled new emissions control technologies. Finally, ionic liquids boost the performance of lower viscosity base oils to match that of more viscous engine oils, thereby improving fuel economy.

Qus research shows that, in extreme pressure conditions, chemical reactions between the ions and metal surfaces form a protective boundary film that provides antiwear properties. When ionic liquids are used in non-extreme pressure conditions, a layer of anions adsorbs onto the metal surface and attracts a secondary layer of cations. This structure functions as a friction modifier to reduce friction and wear.

Picking the Right Ions

According to Iolitecs Taige, the primary reasons ionic liquids are being investigated as lubricant additives are low volatility and non-flammability. In addition, she said, Some ionic liquids display interesting rheological properties, high thermal and chemical stability and high surface tension. Another property making the fluids potential candidates for lubricant additives is low cavitation potential.

Taige said that the usefulness of an ionic liquid as a lubricant additive depends on the nature of the ionic composition. For example, she said, testing shows that phosphonium-based liquids have better tribological properties than ammonium-and sulfonium-based liquids. She added that long-chain alkyls improve lubricating efficiency; however, they also decrease thermal stability. In addition, hydrophobic ionic liquids generally show better tribological performance than hydrophilic liquids.

Applications being investigated for ionic liquids are lubricants used in high vacuum or at extreme temperatures. In addition, Taige said that ionic liquids have been shown to reduce wear and friction in diesel engines compared to an SAE 15W-40 engine oil.

Some ionic liquids have advantages for use as lubrication additives, said Taige, because their viscosity can be tuned precisely to the application requirements by changing the ionic structure. Also, the conductivity of ionic liquids allows the design of lubricants with electrically switchable properties.

Thermal stability depends on the nature of the cation and the anion. Ionic liquids with bis(trifluoromethylsulfonyl)imide (Tf2N) and triflate anions generally have higher thermal stability.

Taige emphasized that the purity of ionic liquids used as additives impacts their effectiveness. Highly purified [ionic liquids] provide significant lubricity, while reagent grade [liquids] provide [only] acceptable lubricity. She noted that a major problem during synthesis of ionic liquids is contamination with inorganic salts. To ensure the purity of their ionic liquids, Iolitec developed methods to accurately analyze liquids using ion chromatography.

Finally, Taige reported that ionic liquids can be used to create stable, easy-to-handle and safe dispersions of nanomaterials for use as lubricant additives. This combination allows tailoring tribological and tribo-chemical behavior, and the creation of novel materials with enhanced properties.

Ionic Liquids and Biofluids

Kronberger said that most research has focused on using ionic liquids as additives in non-polar base ionic liquids like polyalphaolefin or mineral oil. One aim of his work was to determine the capabilities of the fluids as additives in highly biodegradable base oils such as rapeseed oil and polypropylene glycol.

Kronberger reported that tests in rapeseed oil show that liquids containing methyl sulfate anions reduce friction and wear at room temperature and at 100 degrees C.Room temperature results for methyl sulfonate anions are in the same range as the base oil, while their antiwear behavior decreases significantly as temperature increase.

It is also interesting that the mixture of base oil and a small quantity of ionic liquid can reduce friction to a value lower than that for either the base oil or the ionic liquid, said Kronberger. At 100 degrees C, differences in friction among ionic liquids become more pronounced. Values for MeSO3 increase, while those for methyl sulfate and Tf2N decrease.

Kronberger said, Most ionic liquids provide a beneficial effect on wear prevention. Wear values for rapeseed oil with ionic liquids as additives are up to 50 percent lower than those for the non-additized base oil.

Tests with polypropylene glycol base fluids, however, were not as promising, Kronberger reported. Both friction and wear were higher than with rape-seed oil. He attributed the results to the differences in base oil polarity and functional groups, which govern the surface reactions. However, to overcome this drawback, promising results have been seen with twin cationic structures, which form stronger electrostatic bonds than monocationic structures.