Designer Nanoparticles as Robust Superlubrication Vectors.

Weifeng Lin,Jacob Klein,Nir Kampf

doi:10.1021/acsnano.0c01559

Abstract

Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication. This is attributed to the hydration lubrication mechanism acting at the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each bilayer. Micelles exposing such phosphocholine groups could be an attractive alternative to liposomes due to their much easier preparation and structure control, but all studies to date of surfactant micelles have revealed that at relatively low normal stresses the surface layers rupture and friction increases abruptly. Here, we examine surface interactions between three kinds of phosphocholine-exposing micelles with different designed structures: single-tail surfactant micelles, homo-oligomeric micelles, and block copolymer micelles. Normal and shear forces between mica surfaces immersed in solutions of these micelles were measured using a surface force balance. The adsorbed layers on the mica were imaged using atomic force microscope, revealing surface structures ranging from wormlike to spherical micelles. The block copolymer micelles showed relatively low coverage arising from their stabilizing corona and consequently poor lubrication (μ ∼ 10–1). In contrast, the surfactant and homo-oligomeric micelles fully covered the mica surface and demonstrated excellent lubrication (μ ∼ O(10–3)). However, while the boundary layer of single-tailed surfactant micelles degraded under moderate pressure, the homo-oligomeric micellar boundary layer was robust at all applied contact pressures in our study (up to about 5 MPa). We attribute the difference to the much greater energy required to remove a homo-oligomeric molecule from its micelle, resulting in far greater stability under pressure and shear.

Highlights

Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication
Phosphatidylcholine (PC) lipids have been identified as exceptionally efficient lubrication molecules due to the high hydration level of their phosphocholine headgroups, together with their ability to form robust layers arising from the strong attraction between their acyl tails, and have been suggested as the main boundary lubricating elements in synovial joints.[4−6] Goldberg et al.[7] reported that hydrogenated soy phosphatidylcholine (HSPC) layers on mica, consisting of small unilamellar vesicles (SUVs), showed exceptionally low friction coefficients μ = 10−4−10−5
Hydration lubrication has emerged as a paradigm for lubrication in aqueous media based on subnanometer hydration shells which massively reduce frictional dissipation

Summary

Introduction

Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication. The friction force Fs versus the normal load Fn are summarized, revealing a linear dependence with friction coefficients μ = Fs/Fn ≈ 0.002 ± 0.001 between two C16PC micelle-coated mica surfaces up to an applied pressure of 2.9 ± 0.4 MPa. We can attribute this low friction coefficient to the hydration lubrication by the highly hydrated phosphocholine groups of the surfactants exposed at the micellar surfaces, and the closed-packed wormlike structure on the substrate that can support loads.

Results

Conclusion