Effective interactions between a pair of nanoparticles

Abstract

We investigate the effective interactions between two nanoparticles (or colloids) immersed in a solvent exhibiting two-phase separation. Using a non-local density functional theory, we determine the dependence of the effective potential on the separation of the nanoparticles when the solvent is near bulk two-phase coexistence. If identical nanoparticles preferentially adsorbing phase α are inserted into phase β, thick wetting layers of the preferable phase α develop at their surfaces. At some particular separation hb of the nanoparticles, the wetting layers connect to form a single bridge, and the induced effective potential becomes strongly attractive for all distances h < hb. The bridging is a first order capillary condensation like transition for all radii of the nanoparticles greater than the critical radius Rc, the value of which was estimated to be approximately Rc ≈ 20σ for a temperature T/Tc ≈ 0.9, where σ is the size of the solvent (square-well) particles. For radii R < Rc the process of bridging is continuous. If the same particles are inserted into the preferable phase α, the only effective interaction between them is induced by the short-ranged depletion potential. If the nanoparticles have opposite adsorption preferences, only a single wetting layer forms around one of the nanoparticles and the effective interaction is strongly repulsive in both phases. The repulsion, induced by a disruption of the wetting film by the presence of the second particle, is larger and slightly longer-ranged in a low density state.