How domain segregation in ionic liquids stabilizes nanoparticles and establish long-range ordering – a computational study

Due to their physical properties including high thermal stability, very low vapor pressure and high microwave absorption, ionic liquids have attracted great attention as solvents for the synthesis of nanomaterials, being considered as greener alternatives to traditional solvents. While usual solvents often need additives to avoid nanoparticles coalescence, some ionic liquids can dispense their use, being able to stabilize nanoparticles in dispersion. In order to quantify how the ionic liquids can affect both the aggregation thermodynamics and kinetics, molecular dynamics simulations were performed to simulate the evolution of concentrated dispersions and to compute the potential of mean force between nanoparticles of both hydrophilic and hydrophobic nature in two imidazolium-based ionic liquids, which differ from each other by the length of the cation alkyl-group. Depending on the nature of the nanoparticle, structured layers of the polar and apolar regions of the ionic liquid can be formed close to its surface and those layers lead to activation barriers for dispersed particles to get in contact. If the alkyl group of the ionic liquid is long enough to lead to domain segregation between ionic and apolar portions of the solvent, the layered structure around the particle becomes more structured and propagates to several nanometers away from its surface. The leads to stronger barriers close to the contact and also in multiple barriers at larger distances that result from the unfavorable superposition of solvent layers of opposing nature when the nanoparticles approach each other. Those long-range solvent-mediated forces not only provide kinetic stability to dispersions, but also affect their dynamics and lead to a long-range ordering between dispersed particles that can be explored as a template for the synthesis of complex materials.