Nanoscale Friction of Hydrophilic and Hydrophobic Self-Assembled Monolayers in Water

Abstract

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.