Measurements and Models of Reversible Adsorption of Nanoparticles at Oil-Water Interface

We observed the reversible adsorption and desorption of 5 nm diameter gold nanoparticles at the oil-water interface. The extent of adsorption can be controlled by changing the pH or the ionic strength of the aqueous solution. Imaging experiments using optical microscopy also demonstrated the spontaneous formation of particle-laden drops. The number of drops decreases with increasing pH of the aqueous solution (desorption of the particles from the interface) and increase with the number of particles in the system. We developed a robust model based on perturbation theory to explain both the spontaneous formation of drops and the reversible adsorption/desorption of the nanoparticles from the interface. The model, which is in terms of the total Helmholtz free energy change in the system, allows us to relate the extent of adsorption to the particle-particle interaction potential, as well as to predict the physicochemical conditions where emulsification is favorable and we verify these predictions in our experiments.