Umbrella Sampling Simulations of Carbon Nanoparticles Crossing Immiscible Solvents

Abstract

We use molecular dynamics to compute the free energy of carbon nanoparticles crossing a hydrophobic–hydrophilic interface. The simulations are performed on a biphasic system consisting of immiscible solvents (i.e., cyclohexane and water). We solvate a carbon nanoparticle into the cyclohexane layer and use a pull force to drive the nanoparticle into water, passing over the interface. Next, we accumulate a series of umbrella sampling simulations along the path of the nanoparticle and compute the solvation free energy with respect to the two solvents. We apply the method on three carbon nanoparticles (i.e., a carbon nanocone, a nanotube, and a graphene nanosheet). In addition, we record the water-accessible surface area of the nanoparticles during the umbrella simulations. Although we detect complete wetting of the external surface of the nanoparticles, the internal surface of the nanotube becomes partially wet, whereas that of the nanocone remains dry. This is due to the nanoconfinement of the particular nanoparticles, which shields the hydrophobic interactions encountered inside the pores. We show that cyclohexane molecules remain attached on the concave surface of the nanotube or the nanocone without being disturbed by the water molecules entering the cavity.

Graphene Exfoliation in Binary NMP/Water Mixtures by Molecular Dynamics Simulations

“…In this context, molecular dynamics simulations have convinsingly verified that the favored reaction coordinate for the delamination of layered materials is on the shear direction [32] . In particular, steered molecular dynamics (MD) have shown that it requires essentially less force to pull the exfoliating graphene sheet parallel (shear) to the basal plane of the graphite than perpendicular to it [12,33,34] . Molecular simulations are capable to readily identify the favorable reaction coordinate and required force in transition processes like LPE.…”

“…[32] In particular, steered molecular dynamics (MD) have shown that it requires essentially less force to pull the exfoliating graphene sheet parallel (shear) to the basal plane of the graphite than perpendicular to it. [12,33,34] Molecular simulations are capable to readily identify the favorable reaction coordinate and required force in transition processes like LPE. Nevertheless, their primary advantage lies in offering microscopic insights and delving into the thermodynamic aspects that govern such processes, including evaluations of the free energy.…”

Solvent-Assisted Graphene Exfoliation from Graphite Using Umbrella Sampling Simulations

“…The subsequent stage involves dividing the range of ξ into discrete intervals, dξ, and introducing a biasing potential (harmonic potential) within each interval to improve the sampling further. This method is called umbrella sampling. − Umbrella sampling allows for getting as many sample points as needed at each location ξ along the interval of interest. We get relatively uniform sampling within each interval, and by doing so, we obtain excellent statistics even in transition regions that are rarely visited otherwise.…”