Cross-linked poly(4‐vinylpyridine) particles for pH- and ionic strength-responsive “on–off” Pickering emulsions

Abstract

Poly(4-vinylpyridine) particles, P4VP, show a great potential from the environmental and economic perspective since they are cheap and extensively used for safer and greener organic syntheses. Moreover, their pH-sensitive properties would make it easy to obtain “on-off”, recyclable and reusable Pickering emulsions. The relationships between particle structures (size, shape, acidity and charge) and emulsion properties (type, droplet size and stability against environmental stresses) were investigated. The protonation increased the particles diameter to 147 nm compared to the pristine ones (88 nm) due to electrostatic repulsions which swell the particles. The ζ-potential was decreased with increasing acidity (from +60 to +30 mV) due to the accumulation of chlorides onto the surface. Thus, the acidic particles of 39 mmolH+/g and +57 mV were more effective in emulsifying heptane in water with long-term stability in storage conditions (up to one year) and unprecedented resistance against centrifugation and temperature (up to 5000 rpm and 60 °C). Nevertheless, destabilization can be easily achieved by pH or ionic strength changes facilitating the phase separation and the particles recovery. Contrary to the literature of responsive Pickering emulsions, which require relatively complex systems, the investigated one provides a simple route to obtain various “on-off” switches using P4VP cross-linked particles which possess changeable structures based on electrostatic repulsions. Consequently, the exploitation of such a system could be attractive in biphasic media in which the P4VP would act as a reagent, scavenger, and/or catalyst.