Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids
Linking Molecular Structure and Lubrication Mechanisms in Tetraalkylammonium Orthoborate Ionic Liquids
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DOI：
10.1007/s11249-023-01714-7
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发表时间：
2023-06-01
影响因子：
3.2
通讯作者：
Mangolini，Filippo
中科院分区：
文献类型：
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作者：
Yan，Jieming;Lien，Hsu-Ming;Mangolini，Filippo
虽然离子液体（ILs）由于其独特的物理化学性质和有前途的润滑行为而作为能够满足下一代摩擦学系统要求的潜在替代润滑剂获得了广泛的兴趣，但我们对ILs减少摩擦和/或磨损的机制的理解仍然是难以捉摸的。在这里，我们结合联合收割机宏观摩擦学实验与表面分析测量，阐明一类无卤素离子液体，即四烷基铵原硼酸离子液体，在钢/钢滑动接触的润滑机制。摩擦学结果表明，这些离子液体的减摩性能的改善作为连接到铵阳离子的烷基链的长度的增加。X射线光电子能谱分析提供了进一步的证据的润滑机制的四烷基铵原硼酸盐离子液体的IL结构上的依赖。在具有含有长烷基链的不对称铵阳离子的四烷基正硼酸铵离子液体的情况下，在钢表面上没有形成牺牲摩擦膜，从而表明这些离子液体的减摩能力源于它们在滑动界面处经历压力诱导的形态变化的倾向，这导致产生润滑的固体状层状结构。相反，在用含有更多对称铵阳离子和短烷基链的四烷基铵原硼酸盐IL进行的摩擦学测试中观察到的更高的摩擦响应被提出是由于该IL由于阳离子烷基链之间的降低的货车德瓦尔斯相互作用而不能产生瞬态界面层。滑动表面之间产生的硬/硬接触被提议导致在水的存在下硼-氧键的裂解以形成然后吸附到钢表面上的物种，包括来自原硼酸根阴离子分解的三价硼酸酯和草酸，以及来自由原硼酸根分解反应期间释放的氢氧化物引起的烷基铵阳离子降解的叔胺。这项工作的结果不仅建立了一类无卤离子液体的分子结构、润滑性能和润滑机理之间的联系，而且还为离子液体的润滑性能提供了多种机制的证据。图形摘要
While ionic liquids (ILs) have gained wide interest as potential alternative lubricants able to meet the requirements of next-generation tribological systems owing to their unique physico-chemical properties and promising lubricating behavior, our understanding of the mechanisms by which ILs reduce friction and/or wear is still elusive. Here, we combine macroscale tribological experiments with surface-analytical measurements to shed light on the lubrication mechanisms of a class of halogen-free ILs, namely tetraalkylammonium orthoborate ILs, at steel/steel sliding contacts. The tribological results indicate an improvement of the friction-reducing properties of these ILs as the length of the alkyl chains attached to ammonium cations increases. X-ray photoelectron spectroscopy analyses provide further evidence for the dependence of the lubrication mechanism of tetraalkylammonium orthoborate ILs on the IL structure. In the case of tetraalkylammonium orthoborate ILs with asymmetric ammonium cations containing a long alkyl chain, no sacrificial tribofilms were formed on steel surfaces, thus suggesting that the friction-reducing ability of these ILs originates from their propensity to undergo a pressure-induced morphological change at the sliding interface that leads to the generation of a lubricious, solid-like layered structure. Conversely, the higher friction response observed in tribological tests performed with tetraalkylammonium orthoborate ILs containing more symmetric ammonium cations and short alkyl chains is proposed to be due to the inability of this IL to create a transient interfacial layer owing to the reduced van der Waals interactions between the cationic alkyl chains. The resulting hard/hard contact between the sliding surfaces is proposed to lead to the cleavage of boron-oxygen bonds in the presence of water to form species that then adsorb onto the steel surface, including trivalent borate esters and oxalic acid from the decomposition of orthoborate anions, as well as tertiary amines from the degradation of alkylammonium cations induced by hydroxides released during the orthoborate decomposition reaction. The results of this work not only establish links between the molecular structure of a class of halogen-free ILs, their lubricating performance, and lubrication mechanism, but also provide evidence for the existence of multiple mechanisms underpinning the promising lubricating properties of ILs in general.Graphical Abstract