Nanoscale Friction of Hydrophilic and Hydrophobic Self-Assembled Monolayers in Water
Nanoscale Friction of Hydrophilic and Hydrophobic Self-Assembled Monolayers in Water
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DOI：
10.1007/s11249-020-01301-0
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发表时间：
2020-04
影响因子：
3.2
通讯作者：
Quanpeng Yang;Warren A. Nanney;Xiaoli Hu;T. Ye;A. Martini
中科院分区：
文献类型：
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作者：
Quanpeng Yang;Warren A. Nanney;Xiaoli Hu;T. Ye;A. Martini
自组装单分子膜(SAM)通过提供低剪切强度的滑动界面来减少边界润滑接触中的摩擦。然而，自组装膜低摩擦的纳米机理仍不完全清楚，特别是在疏水性或亲水性影响摩擦的液体环境中。为了了解这种影响，通过原子力显微镜实验和分子动力学模拟测量了自组装膜在水中的摩擦力，其中亲水性或疏水性由烷基硫醇的末端基团决定。在实验和模拟中，亲水性自组装膜上的摩擦力都大于疏水性自组装膜上的摩擦力，但这一趋势不能用针尖与自组装膜之间的粘附力的大小来解释。相反，对水和自组装膜对总摩擦力的贡献的分析表明，疏水和亲水自组装膜之间的差异可以通过滑动过程中尖端和水之间的相互作用来解释。亲水性自组装膜的尖端-水力要大得多，这归因于在滑动过程中置换的致密水层，以及水分子和亲水性自组装膜之间形成的氢键，然后在滑动时被尖端打破，导致摩擦力较大。
Self-assembled monolayers (SAMs) can reduce friction in boundary lubricated contacts by providing a low shear strength interface for sliding. However, the nanoscale mechanisms underlying low friction on SAMs are still not fully understood, especially in liquid environments in which hydrophobility or hydrophilicity affects friction. To understand this effect, friction of SAMs in water was measured using atomic force microscope experiments and molecular dynamics simulations, where hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. The friction on hydrophilic SAMs was larger than that on hydrophobic SAMs in both experiments and simulations, but this trend could not be explained by the strength of the adhesive force between the tip and the SAMs. Instead, analysis of the contributions of the water and SAMs to the total friction force revealed that the difference between the hydrophobic and hydrophilic SAMs could be explained by interactions between the tip and water during sliding. The much larger tip-water force on hydrophilic SAMs was attributed to a dense layer of water that was displaced during sliding as well as hydrogen bonds that formed between the water molecules and hydrophilic SAMs and were then broken by the tip as it slid, leading to higher friction force.