Nanocrystals in Molten Salts and Ionic Liquids: Experimental Observation of Ionic Correlations Extending beyond the Debye Length

Abstract

The nature of the interface between the solute and the solvent in a colloidal solution has attracted attention for a long time. For example, the surface of colloidal nanocrystals (NCs) is specially designed to impart colloidal stability in a variety of polar and nonpolar solvents. This work focuses on a special type of colloids where the solvent is a molten inorganic salt or organic ionic liquid. The stability of such colloids is hard to rationalize because solvents with high density of mobile charges efficiently screen the electrostatic double-layer repulsion, and purely ionic molten salts represent an extreme case where the Debye length is only ∼1 Å. We present a detailed investigation of NC dispersions in molten salts and ionic liquids using small-angle X-ray scattering (SAXS), atomic pair distribution function (PDF) analysis and molecular dynamics (MD) simulations. Our SAXS analysis confirms that a wide variety of NCs (Pt, CdSe/CdS, InP, InAs, ZrO2) can be uniformly dispersed in molten salts like AlCl3/NaCl/KCl (AlCl3/AlCl4 –) and NaSCN/KSCN and in ionic liquids like 1-butyl-3-methylimidazolium halides (BMIM+X–, where X = Cl, Br, I). By using a combination of PDF analysis and molecular modeling, we demonstrate that the NC surface induces a solvent restructuring with electrostatic correlations extending an order of magnitude beyond the Debye screening length. These strong oscillatory ion–ion correlations, which are not accounted by the traditional mechanisms of steric and electrostatic stabilization of colloids, offer additional insight into solvent–solute interactions and enable apparently “impossible” colloidal stabilization in highly ionized media.

Colloidal dispersions of oxide nanoparticles in ionic liquids: elucidating the key parameters

“…Only one group provided an overall picture about colloidal stability, however, mainly in molten salts. 13,14 Ueno et al 15 studied the stability of hydrophilic silica particles in 11 ionic liquids, which produced some well dispersed systems according to rheological measurements. The stability was attributed to specic interactions between the surface-bound OH and one of the ions of the ionic liquid.…”

Solvation of quantum dots in 1-alkyl-1-methylpyrrolidinium ionic liquids: toward stably luminescent composites

“…These values further lead to the number of IL-component pairs trapped by the single NP on its surface to be 200, 340 and 1900, respectively, (Tables S2-4). The solvation layer thicknesses estimated are also roughly translated to single, 1-2 and 4-5 ion-pair layers, respectively, which are apparently smaller compared to those measured by the surface force measurements on a flat surface [15][16][17] but in a similar range to that estimated based on the PDF analysis for NPs in an ionic liquid [14]. The surface force measurement is possible to induce a confinement effect, which enhances the ordering of ionic components confined between two surfaces [16,17].…”

“…However, no apparent peak corresponding to regular lamellar structure appears and three peaks characteristic to P 18 Tf 2 N are still prominent. It is therefore concluded that the IL components gave a limited change from the bulk microstructure to make a solvation layer on the NPs, which could be evaluated as the enhancement of ion-ion correlations by the careful PDF analysis [14]. The orientation of pyrrolidinium cations should change on the NPs but the interionic interactions and van der Waals forces between alkyl chains operate in the solvation layer in a similar range to the bulk phase.…”

“…The formation of solvation layer with an approximate thickness of 5 nm was also suggested for silica particles capped with fluorocarbon molecules by a combination of techniques including dynamic light scattering (DLS) and small-angle neutron scattering (SANS) measurements [13]. A recent work employing atomic pair distribution function (PDF) analysis of X-ray scattering data for an ionic liquid solution of semiconductor NPs unveiled the enhanced ion-ion correlations in the solvation shell [14]. The restructuring of ionic liquid with the oscillatory ion-ion correlations on the NPs was discussed to be responsible for the colloidal stability.…”

Colloidal Dispersions of Oxide Nanoparticles in Ionic Liquids: Elucidating the Key Parameters

“…Adding nanoparticles (NPs) to ionic liquids can further improve the systems properties. 6 The addition of charged nanoparticles, for example, can improve the thermoelectric properties compared to mentally determined by the same group using X-ray pair distribution analysis on nanoparticles in molten salts 14 . This layering at the interface was invoked to explain colloidal stability for three types of nanoparticle surfaces: bare (i.e.…”