Molecular simulations of structures and solvation free energies of passivated gold nanoparticles in supercritical CO2

Abstract

The interfacial structures and solvation free energies of gold nanoparticles passivated by self-assembled monolayer (SAM) of fluorinated alkanethiols in supercritical CO(2) (scCO(2)) have been studied by using classical molecular dynamics simulation. A fragment-based free-energy perturbation approach was developed here, in which the solvation free energy of passivated metal nanoparticles was partitioned into the contributions from the inner metal core and the outer SAM. This is a first-time attempt to directly simulate the solvation free energy of nano-objects in supercritical fluids. The simulation result suggests that the nanoparticles can be thermodynamically soluble at lower scCO(2) density but insoluble at higher density. We have demonstrated that this density dependence of solvation free energy can be ascribed to the effect of the surface SAM in scCO(2). The presence of solvent molecules greatly affects the morphology of SAM on nanoparticle. It was observed that increasing the chain length in SAM makes nanoparticles more solvophilic at lower scCO(2) density or more solvophobic at higher density. This solvation thermodynamics behavior has been correlated with the specific solvation structure of scCO(2) around the passivated nanoparticles.