Monday, November 8, 2010: 3:35 PM
Canyon C (Hilton)
Pickering emulsions are distinguished by excellent stability against coalescence and their ability to disperse high discontinuous phase volume fractions. They utilize colloidal particles as emulsifiers, typically at concentrations of approximately 1 wt%. We created silica nanoparticulate emulsifiers with poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes grafted from their surfaces by atom transfer radical polymerization (ATRP). Similarly, poly(ethylene oxide) (PEO) star polymers with a dense cross-linked divinylbenzene core and emanating PEO arms were synthesized via ATRP and tested as emulsifiers. Both silica–PDMAEMA particles and PEO stars were highly surface active and efficient emulsifiers individually, affording extremely stable oil-in-water emulsions at concentrations of just 0.05 wt% and 0.005 wt% respectively. Emulsions thus prepared have been stable against coalescence for many months. This presentation focuses on the roles of chain grafting density and degree of polymerization, solvent quality of water and the selected oil for the grafted chains, and the phase (oil or water) in which the nanoparticles were dispersed prior to emulsification in determining emulsion type and emulsification efficiency. Oil solvent quality is varied by changing the identity of the oil, either xylene or cyclohexane. Water solvent quality is manipulated by varying pH and temperature. Moderate chain grafting densities provide optimum emulsification efficiency, due to the greater freedom for chain relaxation at the oil/water interface. Silica-PDMAEMA particles provide thermally responsive emulsions. The possibility for emulsification synergism in mixtures of PEO stars and silica-PDMAEMA nanoparticles was investigated, but only a modest synergy was obtained.