Received: March 5, 2007

In Final Form: May 31, 2007

Abstract:

The hydrophobic effect is important for many biological and technological processes. Despite progress in theory, experimental data clarifying the structure of water and the interactions between hydrophobic solutes at the nanometer scale are scarce because of the very low solubility of hydrophobic species. This article describes single-molecule force spectroscopy measurements of interactions between single fullerene C₆₀ molecules in water. The C₆₀ molecules were tethered by flexible poly(ethylene glycol) linkers to AFM probes and substrates, removing the uncertainty of the aggregation state of solution-based approaches and spurious surface effects. Our analysis of the dependence of the measured most-probable rupture force on the most-probable loading rate considered the deviations from the conventional Bell-Evans model caused by the effects of the anharmonic tether, as well as by the finite depth and shape of the potential well. The kinetic parameters of the activation barrier width, the dissociation rate of the C₆₀-C₆₀ dimer, and the activation energy are reported. The measured values differ significantly from predictions based on molecular dynamics simulations, indicating that further advances in computer simulations are necessary for the development of a model showing quantitative agreement with experimental results.

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