Solvation and Stabilization of Metallic Nanoparticles in Ionic Liquids

Abstract

Some room-temperature ionic liquids can hold stable suspensions of nanoparticles without additional surface-active agents [1] through mechanisms of solvation and stabilization that are not understood at present, particularly for metallic nanoparticles. These systems are relevant for applications in catalysis, lubrication, electrochemical devices, and chemical processes. We address this issue by studying the interactions and ordering of ionic liquids around metallic nanoparticles using molecular dynamics simulations, which is a suitable tool because the arrangement of the ions around a 2 nm particle is difficult to observe experimentally. The fundamental obstacle to modeling resides in the description of the interactions between metals and ionic fluids, a problem not only for nanometer-scale objects but for extended surfaces as well. In this work we devised an original strategy to represent accurately the molecular interactions and gain insight into the solvation and stabilization mechanisms of nanoparticles in ionic liquids.Experimental studies of metallic nanoparticles in ionic liquids provide different clues about the stabilization of the colloid. Some postulate an electric double layer (the Deryagin-Landau-Verwey-Overbeek model) in which a first solvation shell of anions surrounds the metal cluster, followed by a less ordered layer of cations, and so on.[2] Other studies present evidence of close interactions of the nanoparticles with the cations, through deuterium exchange on positively charged imidazolium rings [3] and through surface-enhanced Raman spectroscopy on gold nanoparticles in imidazolium liquids.[4] Correlations have been established between the size of metallic nanoparticles synthesized in situ with the anion volume.[5] Still other studies suggest that nanoparticles are solvated in nonpolar regions formed by aggregation of the hydrophobic alkyl side chains of the ions, as there is a relationship between the length scale of the structural heterogeneities of the ionic liquid [6] and the size of nanoparticles synthesized therein. [7] Measurements of the thickness of the electrostatic double layer of ionic liquids at metal surfaces have been performed [9] yielded an interfacial layer with one-ion thickness of 3.3 to 5 . This is consistent with the Debye length of the order of 1 estimated for an electrolyte with a concentration around 4 or 5 m, such as a pure ionic liquid, and constitutes an argument against DLVO-type stabilization. However, measurements on macroscopic flat surfaces may not be immediately transposed to nanoparticle suspensions.Suspensions of metallic nanoparticles in an ionic liquid are governed by three kinds of molecular interaction: ion-ion, metal-metal, and metal-ion, which are all nontrivial and each offers its own difficulties to a description. We adopted an atomistic description for both the nanoparticle and for the ionic liquid, providing a high level of detail regarding the interactions and conformations. We considered a ruthenium nanoparticle in [C 4 C 1 im][Ntf 2 ], 1-buty...

Transition from non-monotonic to monotonic electrical diffuse layers: impact of confinement on ionic liquids

“…While many studies have reported on alternating layers of opposite ions near the surfaces [13][14][15][16][17][18][19][20][21][22][23][28][29][30][31][32][33][34][35] it was recently conjectured through a revision of empirical methods 24 that RTILs are not much different from dilute electrolytes. Using spatial dynamics methods, we have demonstrated the nature of decaying charge distributions near the surfaces are in fact enslaved by the bulk and thus, EDLs in RTILs can exhibit both ion layering and a standard decaying behavior as in dilute electrolytes.…”

Ionic Liquids as Size‐ and Shape‐Regulating Solvents for the Synthesis of Cobalt Nanoparticles

“…The results from a recent theoretical study on 2 nm model Ru(0) particles in [C 4 C 1 Im][BTA] indicated that transition metal particles are preferentially solvated by the charged moieties of the ions with a one-ion-thick interface layer [34]. In this study by Pensado et al, both types of ions (i.e., anions and cations) were assumed to interact with the metal surface.…”

Imidazolium Salt Ion Pairs in Solution

“…[17] Moreover, metallic nanoparticles in ILs are solvated preferentially by the charged moieties of the ions, with an interface layer that is one ion thick with both cations and anions in close contact with the metal surface. [18] Finally, the presence of water (even in very low concentrations) affects the aggregation of polar and non-polar domains and modulates physicochemical properties such as diffusion and viscosity. [15,19] However, the structural organization and properties of ILs containing diluted solutions ( Figure 3) has received much less attention.…”