Interaction Energies of Ionic Liquids with Metallic Nanoparticles: Solvation and Stabilization Effects

The interaction energies and solvation structure of ruthenium nanoparticles (RuNPs) in ionic liquids are studied here by titration calorimetry and by molecular simulation. The size of metallic nanoparticles synthesized in situ in ionic liquids can be controlled, and the resulting suspensions are stable without additional surface-active molecules. However, little is known about the energetics and mechanisms of solvation of nanoparticles in these complex, structured solvents. Ionic liquids were added into a suspension of RuNPs in [C(1)C(4)Im][NTf2], and the heat effect was recorded. The background heat of mixing of the two ionic liquids was measured separately. The interaction energy of [C(1)C(n)Im][NTf2] (n = 6, 8, 10) with RuNPs is larger than that of [C(1)C(4)Im][NTf2] indicating that longer alkyl side chains enhance the interactions with RuNPs. [C(1)C(2)Im][NTf2] also has stronger interactions with the nanoparticles, but this cation does not possess a significant nonpolar moiety. Ionic liquids with lesser propensity to form H-bonds such as [C(1)C(1)C(4)Im][NTf2] or [C(1)C(4)Pyrro][NTf2] interact less favorably with RuNPs. No significant effect of the anion structure was observed when changing the ionic liquid from [C(1)C(4)Im][NTf2] to [C(1)C(4)Im][PF6]. Structural information from molecular simulation shows that the charged head groups of both the cations and the anions are in contact with the nanoparticle, with only small charge separation at the interface. Alkyl side chains tend to point away from the nanoparticle but are still within interaction range. The overall picture results from a balance between electrostatic, van der Waals, and H-bond forces, which changes between different ionic liquids.