Boundary Friction of Aromatic Self-Assembled Monolayers: Comparison of Systems with One or Both Sliding Surfaces Covered with a Thiol Monolayer

The boundary friction of three aromatic thiol monolayers with different packing densities on gold was studied with friction force microscopy. The friction of each monolayer was measured against unfunctionalized silicon tips and thiol-functionalized gold-covered tips. The experiments were done in ethanol to significantly reduce the tip−substrate adhesion and therefore the dependence of the friction force on contact area. This allowed a direct, quantitative comparison of load- and velocity-dependent friction with tips of different radii. In close-packed systems, the friction force was lower when both surfaces were covered with a monolayer, and plateaus in the friction force vs velocity curves appeared at higher velocity, suggesting a more fluidlike sliding. A transition in the monolayers at high loads was found to be dependent on the tip radius and appeared at higher average pressure for more close-packed systems but did not depend on whether the system contained one or two monolayers.