Near-equilibrium chemical force microscopy

Molecular force spectroscopy experiments probe the stochastic kinetics of individual intermolecular bond rupture under external loading. While there are numerous examples describing rupture kinetics under fast-loading, far from equilibrium conditions, bond rupture at slow loading rates near equilibrium conditions is seldom explored. We use an analytical and numerical approach to show that rupture forces in this qualitatively different regime reach an equilibrium plateau value that is a function of the probe stiffness and the free energy difference between the bound and dissociated state of the bond. Chemical force microscopy measurements of the interaction between a small number of well-defined COOH functional groups confirm these predictions and show the expected scaling of the force plateau values with the square root of the probe stiffness. Finally, we discuss the implications of these results for the interpretation of force spectroscopy experiments.