Interactions between fullerene C60 molecules in water were measured by force spectroscopy. Fullerene molecules were covalently connected to bifunctional water-soluble poly(ethylene glycol) (PEG) linkers and subsequently tethered to the substrate and to the tip of the atomic force microscope to facilitate single molecule detection and avoid spurious surface effects. The distributions of rupture forces for substrates prepared with different incubation times of C60-PEG-NH2 exhibit high rupture forces that cannot be explained by the theoretical distribution of single molecule binding. Moreover, the relative amplitude of the high force peak in the histogram increases with incubation time. These observations are explained by attributing the measured high forces to the rupture of multiple bonds between fullerene molecules. Force spectroscopy data analysis based on the most probable forces gives significantly different dissociation rates for samples that exhibit different amplitudes of the high force peak. An approximate analytical model that considers ruptures of two bonds that are simultaneously loaded by tethers with different lengths is proposed. This model successfully fits the distributions of the rupture forces using the same set of kinetic parameters for samples prepared with different grafting densities. It is proposed that this model can be used as a common tool to analyze the probability distributions of rupture forces that contain peaks or shoulders on the high force side of the distribution.

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 C 60 molecules in water. The C 60 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 mostprobable 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 C 60 -C 60 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.

Hydrolysis is an important component of the aging of cellulose, and it severely affects the insulating performance of cellulosic materials. The diffusion behavior of water molecules in amorphous cellulose and their destructive effect on the hydrogen bonding structure of cellulose were investigated by molecular dynamics. The change in the hydrogen bonding structure indicates that water molecules have a considerable effect on the hydrogen bonding structure within cellulose: both intermolecular and intramolecular hydrogen bonds decreased with an increase in ingressive water molecules. Moreover, the stabilities of the cellulose molecules were disrupted when the number of intermolecular hydrogen bonds declined to a certain degree. Both the free volumes of amorphous cells and water molecule-cellulose interaction affect the diffusion of water molecules. The latter, especially the hydrogen bonding interaction between water molecules and cellulose, plays a predominant role in the diffusion behavior of water molecules in the models of which the free volume rarely varies. The diffusion coefficient of water molecules has an excellent correlation with water molecule-cellulose interaction and the average hydrogen bonds between each water molecule and cellulose; however, this relationship was not apparent between the diffusion coefficient and free volume.

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