Controlling adsorption density of polymer-stabilised metal nanoparticles at the oil-water interface.

Emulsion droplets offer an alternative to solid supports as templates for the deposition of metallic nanoparticles. An emulsion interface provides the opportunity to exploit both sides of the nanoparticles and to utilise the liquid core as a microreactor in addition to forming a scaffold for encapsulation. However, despite the extensive literature studying a very broad range of factors influencing the characteristics of particle-stabilised (Pickering) emulsions, most reports focus on particles of diameters >100 nm and a very small proportions consider particles of diameters <10 nm. For catalytic purposes of course, the latter species are of utmost interest. Here, we report the synthesis of poly(vinyl pyrrolidone) (PVP) stabilised platinum nanoparticles, where the platinum core ranges between 3 and 5 nm in diameter and their subsequent use as emulsifiers for the oil-water interface where they form a densely packed layer. The nanoparticle density at the interface is quantified by both measuring the remaining concentration of nanoparticles in the aqueous phase after adsorption and also directly at the oil-water interface via cryo-TEM. The effect of electrolyte concentration and of addition of excess PVP in the bulk aqueous nanoparticle dispersion prior to emulsification on the resulting nanoparticle density at the oil-water interface is also determined.