Molecular simulations of sliding on SDS surfactant films

TL;DR

This study uses molecular dynamics simulations to analyze how SDS surfactant films influence frictional behavior on gold surfaces, revealing load-dependent regimes and the effects of film density and structure.

Contribution

It provides new insights into the friction regimes of SDS surfactant films and their dependence on load and film density, using molecular simulations.

Findings

Friction exhibits two regimes: load-dependent at low loads and load-independent at high loads.

Friction force increases monotonically with film density at high loads.

Friction coefficient varies with surface concentration, showing a minimum at intermediate levels.

Abstract

We use molecular dynamics simulations to study the frictional response of the anionic surfactant sodium dodecyl sulfate (SDS) monolayers and hemicylindrical aggregates physisorbed on gold. Our simulations of a sliding spherical asperity reveals two friction regimes: At low loads, the films show Amontons' friction with a friction force that rises linearly with normal load. At high loads, the friction force is independent of load as long as no direct solid-solid contact occurs. The transition between these two regimes happens when only a single molecular layer is confined in the gap between the sliding bodies. The friction force at high loads on a monolayer rises monotonically with film density and drops slightly with the transition to hemicylindrical aggregates. This monotonous increase of friction force is compatible with a traditional plowing model of sliding friction. At low loads, the friction coefficient reaches a minimum at intermediate surface concentrations. We attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing.