Molecular design of stable graphene nanosheets dispersions.

Graphene sheets, one-atom-thick layers of carbon atoms, are receiving enormous scientific attention because of extraordinary electronic and mechanical properties. These intrinsic properties will lead to innovative nanocomposite materials that could be used to produce novel transistors and thermally conductive polymeric materials. Such applications are currently hindered by the difficulty of producing large quantities of individual graphene sheets and by the propensity of these nanoparticles to agglomerate when dispersed in aqueous and/or organic matrixes. We report here molecular dynamics simulations for pristine and functionalized graphene nanosheets of 54 and 96 carbon atoms each dispersed in liquid organic linear alkanes (oils) at room conditions. For the first time, our results show that, although pristine graphene sheets agglomerate in the oils considered, graphene sheets functionalized at their edges with short branched alkanes yield stable dispersions. We characterized the simulated systems by computing radial distribution functions between the graphene sheets centers of mass, pair potentials of mean force between the graphene sheets in solution, and site-site radial distribution functions. The latter were used to determine the preferential orientation between approaching graphene sheets and the packing of the organic oils on the graphene sheets. Our results are useful not only for designing practical recipes for stabilizing graphene sheets in organic systems, but also for comparing the molecular mechanisms responsible for the graphene sheets aggregation to those that stabilize graphene sheets-containing dispersions, and for controlling the coupling between organic oils and graphene sheets used as fillers. In particular, we demonstrated that excluded-volume effects, generated by the branched architecture of the functional groups grafted on the graphene sheets, are responsible for the stabilization of small graphene sheets in the organic systems considered here.