In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids
In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids
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DOI：
10.1007/s11249-015-0465-x
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发表时间：
2015-02-01
影响因子：
3.2
通讯作者：
Spikes, H. A.
中科院分区：
文献类型：
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作者：
Campen, S.;Green, J. H.;Spikes, H. A.
近一个世纪以来，脂肪酸及其衍生物一直被用作模型有机摩擦改进剂，但关于它们在摩擦表面上形成的边界膜的性质仍然存在争议。在这项研究中，使用原位液体细胞原子力显微镜（AFM）来监测脂肪酸烷烃溶液到云母表面的边界膜的自组装。由于云母表面完全浸入溶液中，因此可以直接研究这些薄膜的形态和摩擦随时间以及加热和冷却过程的变化。研究发现，硬脂酸和反油酸能够采用线性分子构型，在云母上形成直径数十至数百微米、厚度通常为1.6 nm的不规则岛，对应于倾斜的单层区域。在相对较高浓度（0.01 M）下，十六烷中的硬脂酸形成几乎完整的单层，但在较低浓度下，在十二烷溶液和反油酸溶液中，这些薄膜在长时间浸泡超过一天后仍然不完整。在接触模式 AFM 中重复扫描时，云母上的脂肪酸形成的薄膜会发生位移，但使用轻敲模式 AFM 可以在不损坏的情况下成像。滑动球盘测试中摩擦过的石英表面也使用 AFM 进行了异位扫描，结果表明硬脂酸在宏观摩擦实验中在石英上形成了与云母上发现的类似的单层岛膜。油酸溶液的行为与硬脂酸和反油酸完全不同，在云母和摩擦石英表面上形成不规则的球状薄膜。这被认为是因为其顺式双键几何结构意味着，与反式异构体反油酸或饱和硬脂酸不同，它无法采用线性分子构型，因此不太能够形成密堆积单层。
Fatty acids and their derivatives have been used as model organic friction modifiers for almost a century, but there is still debate as to the nature of the boundary films that they form on rubbed surfaces. In this study, in situ liquid cell atomic force microscopy (AFM) is used to monitor the self-assembly of boundary films from solutions of fatty acids in alkanes on to mica surfaces. Because the mica surfaces are wholly immersed in solution, it is possible to study directly changes in the morphology and friction of these films over time and during heating and cooling. It has been found that stearic acid and elaidic acid, which are able to adopt linear molecular configurations, form irregular islands on mica that are tens to hundreds of microns in diameter and typically 1.6 nm thick, corresponding to domains of tilted single monolayers. At a relatively high concentration of 0.01 M, stearic acid in hexadecane forms an almost complete monolayer, but at lower concentrations, in dodecane solution and for elaidic acid solutions, these films remain incomplete after prolonged immersion of more than a day. The films formed by fatty acids on mica are displaced by repeated scanning in contact mode AFM but can be imaged without damage using tapping mode AFM. Rubbed quartz surfaces from a sliding ball-on-disc test were also scanned ex situ using AFM, and these showed that stearic acid forms similar monolayer island films on quartz in macro-scale friction experiments as are found on mica. Oleic acid solutions behave quite differently from stearic acid and elaidic acid, forming irregular globular films on both mica and rubbed quartz surfaces. This is believed to be because its cis-double bond geometry means that, unlike its trans-isomer elaidic acid or saturated stearic acid, it is unable to adopt a linear molecular configuration and so is less able to form close-packed monolayers.