Domain Segregation in Ionic Liquids Induces Long-Range Oscillatory Forces between Nanoparticles and Surfaces

Ionic liquids have aroused great interest as solvents for the synthesis and stabilization of nanomaterials. The segregation between polar and apolar domains in ionic liquids with long alkyl groups provides kinetic stability for nanoparticle dispersions by rendering multiple free energy barriers for the aggregation. Similar effects also modulate the adsorption of nanoparticles over both liquid-vapor and liquid/solid interfaces. In this work, molecular dynamics simulations were performed to compute the potential of the mean force for the adsorption of spherical nanoparticles over solid substrates through films of imidazolium-based ionic liquids with different alkyl group lengths. While liquids with small alkyl groups produce simple profiles with barriers only close to the substrate, complex oscillatory forces arise between the nanoparticle and the substrate for ionic liquids with significant domain segregation. In addition, long-range solvent-mediated repulsive forces were also noted for liquids with an alkyl group long enough to display a smectic liquid crystal phase.