Reversible Partitioning of Nanoparticles at an Oil–Water Interface

The reversible adsorption of nanoparticles (NPs) to oil-water interfaces has been observed experimentally, however, models capable of interpreting and predicting the equilibrium partitioning of particles between bulk media and fluid interfaces are still lacking. Here we characterize the adsorption of 5 nm gold NPs functionalized with ion-pair ligands at the toluene-water interface. Partitioning of the NPs between the bulk aqueous phase and the interface is measured via absorbance spectroscopy for two different aqueous-phase pH values (11.0 and 11.7) and over several orders of magnitude of aqueous phase NP concentration. The surface pressure of the interfacial film in equilibrium with the bulk aqueous phase is measured using the pendant drop method. We determine the range of surface pressure where the adsorption is reversible as well as conditions under which the adsorbed NPs are irreversibly adsorbed at the oil-water interface. We analyze together the adsorption and surface pressure isotherms to obtain the two-dimensional equations of state (EOS) for the NPs in equilibrium with the bulk aqueous phase. The experimental data are then compared to the Frumkin models. We find that the adsorption isotherm and the equation of state show good agreement at low coverage with the Frumkin equations; however, both curves cannot be described with the same parameters. We also show that the low-coverage portion of the EOS can also be described by a wetting model. We hypothesize that deviations from models at higher coverage are likely due to nonequilibrium effects and possible coadsorption.