Ion specific tuning of nanoparticle dispersion in an ionic liquid: a structural, thermoelectric and thermo-diffusive investigation

Abstract

Dispersions of charged maghemite nanoparticles (NPs) in EAN (ethylammonium nitrate) a reference Ionic Liquid (IL) are studied here using a number of static and dynamical experimental techniques; small angle scattering (SAS) of X-rays and of neutrons, dynamical light scattering and forced Rayleigh scattering. Particular insight is provided regarding the importance of tuning the ionic species present at the NP/IL interface. In this work we compare the effect of Li⁺, Na⁺ or Rb⁺ ions. Here, the nature of these species has a clear influence on the short-range spatial organisation of the ions at the interface and thus on the colloidal stability of the dispersions, governing both the NP/NP and NP/IL interactions, which are both evaluated here. The overall NP/NP interaction is either attractive or repulsive. It is characterised by determining, thanks to the SAS techniques, the second virial coefficient A₂, which is found to be independent of temperature. The NP/IL interaction is featured by the dynamical effective charge ξ^eff₀ of the NPs and by their entropy of transfer Ŝ_NP (or equivalently their heat of transport ) determined here thanks to thermoelectric and thermodiffusive measurements. For repulsive systems, an activated process rules the temperature dependence of these two latter quantities.