Relationship Between Molecular
Contact Thermodynamics
and Surface Contact Mechanics
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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><sub>a</sub>) for the
formation of 1:1 hydrogen-bonded complexes and the pull-off force <i>F</i><sub>a</sub> 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><sub>a</sub> and τ
enables the prediction of <i>K</i><sub>S</sub> 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><sub>a</sub>. 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