Molecular Dynamics Simulation Study on the Stability of Gold Nanoparticles in Ionic Liquids

Ionic liquids (ILs) have been used as solvents to prepare metallic nanoparticles (NPs), but it is still unclear what physical mechanism renders NPs stable and, consequently, which cation and anion combination is best. In this study, two gold nanoparticles (Au-NPs) immersed in four distinct ILs based on the cations 1-ethyl-3-methylimidazolium [EMIM] and 1-octyl-3-methylimidazolium [OMIM], along with the anions tetrafluoroborate [BF4] and bis(trifluoromethylsulfonyl)imide [NTf2], are investigated using classical molecular dynamics (MD) simulation. The Au-NP model was chosen in order to achieve an overall attractive interaction that is similar to that predicted by the Hamaker constant for gold. The potential of mean force (PMF) as a function of the Au-NPs separation was used to assess the solvation force brought on by the presence of the ILs. All of the IL that was studied resulted in an oscillatory pattern on the PMF. Short distances between the Au-NPs cause the PMF in [EMIM][BF4] to drop to negative values, suggesting that this IL is unable to prevent aggregation. In [OMIM]-based ILs, the PMF increases to highly positive values when the Au-NPs distance is less than ∼4.0 nm. The strength of the interaction energy between the cation or anion and the Au-NP interaction is unrelated to the different PMF behaviors among the ILs under investigation. The improved action of [OMIM]-based ILs in preventing Au-NPs from aggregating is related to a layer of alkyl chains that forms between the Au-NPs at a small separation.