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.

中文翻译：

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亲水和疏水自组装膜在水中的纳米级摩擦

自组装单分子层（SAM）可以通过提供滑动的低剪切强度界面来减少边界润滑触点中的摩擦。但是，对于SAMs低摩擦的纳米机理仍未完全了解，特别是在其中疏水性或亲水性影响摩擦的液体环境中。为了了解这种效果，使用原子力显微镜实验和分子动力学模拟测量了SAM在水中的摩擦力，其中亲水性或疏水性由链烷硫醇的末端基团确定。在实验和模拟中，亲水性SAM上的摩擦力都比疏水性SAMs上的摩擦力大，但这种趋势无法用尖端与SAM之间的粘合力强度来解释。代替，对水和SAM的总摩擦力的分析结果表明，疏水性和亲水性SAM之间的差异可以通过滑动过程中尖端与水之间的相互作用来解释。亲水性SAM上的尖端水力更大，这归因于在滑动过程中被置换的致密水层以及水分子和亲水性SAM之间形成的氢键，然后在尖端滑动时被尖端破坏，从而导致更高的摩擦力。