Barnhill William C, Qu Jun, Luo Huimin, Meyer Harry M, Ma Cheng, Chi Miaofang, Papke Brian L
Materials Science and Technology Division, Oak Ridge National Laboratory , P.O. Box 2008, MS-6063, Oak Ridge, Tennessee 37831-6063, United States.
ACS Appl Mater Interfaces. 2014 Dec 24;6(24):22585-93. doi: 10.1021/am506702u. Epub 2014 Dec 2.
Our previous work suggested great potential for a phosphonium-organophosphate ionic liquid (IL) as an antiwear lubricant additive. In this study, a set of five ILs were carefully designed and synthesized, with identical organophosphate anions but dissimilar phosphonium cations, to allow systematic investigation of the effects of cation alkyl chain length and symmetry on physicochemical and tribological properties. Symmetric cations with shorter alkyl chains seem to increase the density and thermal stability due to closer packing. On the other hand, either higher cation symmetry or longer alkyl moieties induce a higher viscosity, though the viscosity index is dependent more on molecular mass than on symmetry. While a larger cation size generally increases an IL's solubility in nonpolar hydrocarbon oils, six-carbon seems to be the critical minimum alkyl chain length for high oil miscibility. Both the two ILs, that are mutually oil miscible, have demonstrated promising lubricating performance at 1.04% treat rate, though the symmetric-cation IL moderately outperformed the asymmetric-cation IL. Characterizations on the tribofilm formed by the best-performing symmetric-cation IL revealed the film thickness, nanostructure, and chemical composition. Results here provide fundamental insights for future molecular design in developing oil-soluble ILs as lubricant additives.
我们之前的工作表明，鏻 - 有机磷酸离子液体（IL）作为抗磨润滑添加剂具有巨大潜力。在本研究中，精心设计并合成了一组五种离子液体，它们具有相同的有机磷酸阴离子但鏻阳离子不同，以便系统研究阳离子烷基链长度和对称性对物理化学和摩擦学性能的影响。具有较短烷基链的对称阳离子似乎由于堆积更紧密而增加了密度和热稳定性。另一方面，较高的阳离子对称性或较长的烷基部分会导致更高的粘度，尽管粘度指数更多地取决于分子量而非对称性。虽然较大的阳离子尺寸通常会增加离子液体在非极性烃油中的溶解度，但六个碳似乎是实现高油混溶性的关键最小烷基链长度。两种互溶的离子液体在1.04%的添加比例下均表现出良好的润滑性能，不过对称阳离子离子液体略优于不对称阳离子离子液体。对性能最佳的对称阳离子离子液体形成的摩擦膜进行的表征揭示了膜厚度、纳米结构和化学成分。此处的结果为未来开发油溶性离子液体作为润滑添加剂的分子设计提供了基本见解。
