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 then broken by the tip as it slid, leading to higher friction force.

Simulations of Friction Anisotropy on Self-Assembled Monolayers in Water

“…Particularly, AFM has been used to characterize friction on SAMs with a wide range of different tips (Si 3 N 4 , Au, and −CH 3 -or −OH-terminated SAMs), 16,22−24 substrates (Au, Si), 25−29 and environments (ethanol, hexadecane, water, humid air, and dry air). 26,30−35 The sliding systems studied using AFM have included SAM− SAM, 36 tip−SAM, and SAM−tip−water 26,35 combinations.…”

“…Friction has been shown to be higher on hydrophilic vs hydrophobic SAMs. 42,35 In addition, the presence of water molecules can change the magnitude of friction force on hydrophilic SAM. 26,35,41,42 For a hydrophilic silicon nitride tip or hydrophilic SAM sliding on hydrophilic SAM, water lowers friction due to the layering of the water molecules between the sliding surfaces, thus providing lubrication.…”

“…42,35 In addition, the presence of water molecules can change the magnitude of friction force on hydrophilic SAM. 26,35,41,42 For a hydrophilic silicon nitride tip or hydrophilic SAM sliding on hydrophilic SAM, water lowers friction due to the layering of the water molecules between the sliding surfaces, thus providing lubrication. 26,41 However, when a carbon tip is sliding over hydrophilic SAM, water increases friction due to hydrogen bonding (H-bonding) between the water molecules and the terminal −OH groups on the SAM, thus offering additional resistance to sliding.…”

“…26,41 However, when a carbon tip is sliding over hydrophilic SAM, water increases friction due to hydrogen bonding (H-bonding) between the water molecules and the terminal −OH groups on the SAM, thus offering additional resistance to sliding. 35 Both experiments and simulations have reported that friction on SAMs is anisotropic, 11,39,43 where anisotropy refers to the observation that friction is dependent on the sliding direction. Some studies have attributed friction anisotropy to the tilt angle (with respect to the surface-normal direction) and the orientation angle (with respect to the sliding direction) of the SAM molecules.…”

“…44 The same study proposed that subtle structural variations could affect friction due to steric constraints leading to different van der Waals interactions between the terminal groups and an interacting surface. 44 Although friction on SAMs in water has been studied with MD simulations, 35,45 friction anisotropy on SAMs in a liquid environment has not been modeled. In this work, we performed MD simulations of atomic-scale friction between a nanoscale AFM probe and alkanethiol monolayers terminated with −CH 3 (hydrophobic) or −OH (hydrophilic) groups in the presence of water to study friction anisotropy.…”