Molecular simulation of interaction between passivated gold nanoparticles in supercritical CO2

Molecular dynamics simulations have been performed to study the potential of mean force (PMF) between passivated gold nanoparticles (NPs) in supercritical CO(2) (scCO(2)). The nanoparticle model consists of a 140 atom gold nanocore and a surface self-assembled monolayer, in which two kinds of fluorinated alkanethiols were considered. The molecular origin of the thermodynamics interaction and the solvation effect has been comprehensively studied. The simulation results demonstrate that increasing the solvent density and ligand length can enhance the repulsive feature of the free energy between the passivated Au nanoparticles in scCO(2), which is in good agreement with previous experimental results. The interaction forces between the two passivated NPs have been decomposed to reveal various contributions to the free energy. It was revealed that the interaction between capping ligands and the interaction between the capping ligands and scCO(2) solvent molecules cooperatively determine the total PMF. A thermodynamic entropy-energy analysis for each PMF contribution was used to explain the density dependence of PMF in scCO(2) fluid. Our simulation study is expected to provide a novel microscopic understanding of the effect of scCO(2) solvent on the interaction between passivated Au nanoparticles, which is helpful to the dispersion and preparation of functional metal nanoparticles in supercritical fluids.