Optical microscope images of graphite oxide (GO) reveal the occurrence of fault lines resulting from the oxidative processes. The fault lines and cracks of GO are also responsible for their much smaller size compared with the starting graphite materials. We propose an unzipping mechanism to explain the formation of cracks on GO and cutting of carbon nanotubes in an oxidizing acid. GO unzipping is initiated by the strain generated by the cooperative alignment of epoxy groups on a carbon lattice. We employ two small GO platelets to show that through the binding of a new epoxy group or the hopping of a nearby existing epoxy group, the unzipping process can be continued during the oxidative process of graphite. The same epoxy group binding pattern is also likely to be present in an oxidized carbon nanotube and cause its breakup.

We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.

We employ fully quantum-mechanical molecular dynamics simulations to evaluate the force between two methanes dissolved in water, as a model for hydrophobic association. A stable configuration is found near the methane-methane contact separation, while a shallow second potential minimum occurs for the solventseparated configuration. The strength and shape of the potential of mean force are in conflict with earlier classical force-field simulations but agree well with a simple hydrophobic burial model which is based on solubility experiments. Examination of solvent dynamics reveals stable water cages at several specific methanemethane separations.hydrophobicity ͉ molecular dynamics H ydrophobicity is the molecular driving force behind numerous important biological processes, including protein folding and the formation of biological membranes (1-3). A quantitative understanding of hydrophobic interactions is crucial for modeling protein structures, protein functions, or manipulation of hydrophobic nanoparticles in aqueous solutions (4).Experimentally, the strength of the hydrophobic effect (hydration potential) can be measured by the solubility of hydrocarbons (5, 6). However, the detailed shape of the potential of mean force (PMF) between two hydrocarbon molecules has only been probed indirectly (7,8). Considerable effort has been expended in studying hydrophobic interactions and hydration by using classical Lennard-Jones potentials and various water models (9-11). The model parameters were typically chosen for consistency with bulk thermodynamic quantities. However, the hydrophobic effect for dissolved molecules originates largely from the hydrogen-bond network in the first solvation shell (12), and the properties of interfacial water differ substantially from those of bulk water. Indeed, hydrogen bonding remains quite difficult to represent effectively with simple (atom-atom) molecular-mechanics force fields (13).In first-principles molecular dynamics (FPMD) (14), interatomic forces are derived directly from quantum-mechanical calculations. FPMD has been successfully applied to ice (15), water clusters (16), bulk liquid water (17), and water in the solvation shell of a dissolved ion (18) or methane (19). Here, we report determination of the PMF between a pair of methane molecules in water by FPMD. In classical simulations, the general features of the PMF are a stable free-energy minimum at contact separation, with a second but pronounced free-energy minimum at a distance where the two methanes are separated by a single layer of solvent (20). However, rather small changes in the classical methane-water interaction parameters can lead to reordering of the stability of these two minima (21). Our quantum-mechanical simulation reveals an effective hydrophobic surface tension. The result is a stable configuration of two methanes near contact separation with only a shallow potential minimum for the solvent-separated configuration. The depth of the stable potential minimum is roughly in accord with solubility measuremen...