Abstract
The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO2 (scCO2). The potential of mean force (PMF) between two graphene nanosheets in scCO2 was simulated, and the effect of scCO2 density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO2 fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO2 molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO2 fluid density, there are more confined CO2 molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO2 solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.
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