Relationship Between Molecular Contact Thermodynamics and Surface Contact Mechanics

Abstract

Measurements have been made of the adhesion and friction forces between organic monolayers in heptane/acetone mixtures using an atomic force microscope (AFM). It has been found that the contact mechanics are best modeled by treating the friction force as the sum of a load-dependent term (attributed to “molecular plowing”) and an area-dependent term attributed to shearing (adhesion). The relative contributions of plowing and shearing are determined by the coefficient of friction, μ, and the surface shear strength τ. The transition from adhesion- to load-determined friction is controlled by the solvation state of the surface: solvated surfaces represent a limiting case in which the shear term approaches zero, and the friction-load relationship is linear, while in other circumstances, the friction-load relationship is nonlinear and consistent with Derjaguin–Muller–Toporov mechanics. A striking correlation has been observed between the concentration-dependence of the association constant (<i>K</i>_a) for the formation of 1:1 hydrogen-bonded complexes and the pull-off force <i>F</i>_a and surface shear strength τ for the same molecules when one partner is immobilized by attachment to an AFM probe and the other is adsorbed to a surface. Analysis of the concentration-dependence of <i>F</i>_a and τ enables the prediction of <i>K</i>_S with remarkably high precision, indicating that for these hydrogen bonding systems, the tip–sample adhesion is dominated by the H-bond thermodynamics. For mixed monolayers, H-bond thermodynamics dominate the interaction even at very low concentrations of the H-bond acceptor. Even for weakly adhering systems, a nonlinear friction-load relationship results. The variation in τ with the film composition is correlated very closely with the variation in <i>F</i>_a. However, the coefficient of friction varies little with the film composition and is invariant with the strength of tip–sample adhesion, being dominated by molecular plowing and, for sufficiently large concentrations of hydroxyl terminated adsorbates, the disruption of intramonolayer hydrogen bonding interactions