Particle adsorption at the oil–water interface studied with second harmonic generation

Abstract

In this work, energetics of the adsorption of polystyrene nanoparticles at the hexadecane–water interface was studied with second harmonic generation. The adsorption of positively and negatively charged nanoparticles at the oil–water interface induced a decrease and an increase in the SHG emission from the interface, respectively. This change in the SHG emission, which is similar to that upon the adsorption of ionic surfactants at the hexadecane–water interface, which we reported previously, was then used as an indicator of particle adsorption at the interface. The adsorption free energies of the particles with a diameter of 20 nm at the hexadecane–water interface were found to be −14.7 ± 0.5 kcal mol⁻¹, −14.4 ± 0.4 kcal mol⁻¹ and −15.1 ± 0.3 kcal mol⁻¹ for the amidine, carboxyl and sulfate latex beads, respectively. This result implied that the van der Waals interaction between the oil phase and the polystyrene particles is capable of driving negatively charged particles to the negatively charged hexadecane–water interface. The principle of like dissolves like played a major role in the adsorption of polystyrene particles from the aqueous phase to the oil–water interface. The origin of the SHG emission from the oil–water interface was also discussed.