Ngo Dien, He Xin, Luo Huimin, Qu Jun, Kim Seong H
Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States.
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Langmuir. 2020 Jul 7;36(26):7582-7592. doi: 10.1021/acs.langmuir.0c01197. Epub 2020 Jun 16.
Oil-soluble ionic liquids (ILs) have been proved as effective additives in lubricant oils through tribological experiments and post-test analytical analyses. In this study, surface structures of lubricant base oil, oil-soluble ILs, and their mixtures at the air/liquid and solid/liquid interfaces have been studied using sum frequency generation (SFG) vibrational spectroscopy. At the air/base oil and air/IL interfaces, the alkyl chains of the studied compounds were shown to be conformationally disordered and their terminal methyl groups point outward at the liquid surface. The base oil dominates the air/(base oil + IL) interface due to its higher surface excess propensity and larger bulk concentration. At the solid (silica) surface, ILs adopt a structure with their charged headgroups in contact with the silica surface, while their alkyl chains are more conformationally ordered or packed compared to the air/IL interface. At the interface between silica and (base oil + IL) mixtures, ILs also preferentially adsorb to the silica surface with their layer structures somewhat different from those of ILs alone. These results showed that ILs can adsorb onto the solid surface even before tribological contacts are made. The insights obtained from this SFG study provide a better understanding of the role of ionic liquids in lubrication.
通过摩擦学实验和测试后分析，油溶性离子液体（ILs）已被证明是润滑油中的有效添加剂。在本研究中，使用和频振动光谱（SFG）研究了润滑油基础油、油溶性离子液体及其混合物在气/液和固/液界面的表面结构。在空气/基础油和空气/离子液体界面，所研究化合物的烷基链呈构象无序，其末端甲基在液体表面向外指向。基础油由于其较高的表面过剩倾向和较大的本体浓度，在空气/（基础油+离子液体）界面占主导地位。在固体（二氧化硅）表面，离子液体采取带电头基与二氧化硅表面接触的结构，而与空气/离子液体界面相比，其烷基链的构象更有序或堆积更紧密。在二氧化硅与（基础油+离子液体）混合物的界面处，离子液体也优先吸附到二氧化硅表面，其层状结构与单独的离子液体略有不同。这些结果表明，即使在进行摩擦接触之前，离子液体也能吸附到固体表面。从这项和频振动光谱研究中获得的见解有助于更好地理解离子液体在润滑中的作用。
