A study is presented of emulsification by silica nanoparticles with poly(2-(dimethylamino)ethyl methacrylate) brushes grafted from their surfaces (SiO(2)-PDMAEMA) by atom-transfer radical polymerization (ATRP). The grafted nanoparticles were used to stabilize xylene-in-water and cyclohexane-in-water Pickering emulsions. PDMAEMA is a water-soluble weak polyelectrolyte with a pH-dependent lower critical solution temperature (LCST). Accordingly, SiO(2)-PDMAEMA nanoparticles were thermally responsive, as shown by the fact that they displayed a critical flocculation temperature (CFT) when heated. ATRP provides a high degree of control over the brush grafting density and degree of polymerization, two of the principal variables examined in this study. The effects of the solvent quality of the "oil" for the PDMAEMA brush were studied in addition to the effects of aqueous pH, ionic strength, and temperature relative to the CFT. The preferred emulsion type was oil in water in all cases. The lowest grafting density particles (0.077 chains/nm(2)) proved to be the most efficient and robust emulsifiers, producing stable emulsions using as little as 0.05 wt % particles in the aqueous phase and successfully emulsifying over a broader range of solution conditions than for the higher grafting density particles (0.36 and 1.27 chain/nm(2)). Both good (xylene) and poor (cyclohexane) solvents could be emulsified, but the poor solvent could be emulsified over a broader range of conditions than the good solvent. Emulsions have been stable for over 13 months, and some have dispersed as much as 83 vol % oil in the emulsion phase. Thermally responsive emulsions were created with the SiO(2)-PDMAEMA particles such that stable emulsions prepared at low temperature were rapidly broken by increasing the temperature above the CFT.

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.

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Multiarm star copolymers with approximately 460 poly(ethylene oxide) (PEO) arms that have a degree of polymerization N = 45 were synthesized via atom transfer radical polymerization (ATRP) of PEO-methacrylate macromonomers in the presence of divinyl benzene cross-linkers. These are an example of molecular or nanoparticulate brushes that are of interest as steric stabilizers or boundary lubrication agents when adsorbed from solution to a solid/aqueous interface. We use ellipsometry to measure adsorption isotherms at the silica/aqueous interface for PEO star polymers and linear PEO chains having molecular weights comparable either to the star polymer or to the individual arms. The compactness of the PEO star polymers (molecular weight 1.2 × 10(6)) yields a saturation surface excess concentration that is approximately 3.5 times greater than that of the high molecular weight (1 × 10(6)) linear PEO. Adsorption of low molecular weight (6000) linear PEO was below the detection limit. Competitive adsorption experiments were conducted with ellipsometry, complemented by independent quartz crystal microbalance with dissipation (QCM-D) measurements. Linear PEO (high molecular weight) displaced preadsorbed PEO star polymers over the course of approximately 1.5 h, to form a mixed adsorbed layer having not only a significantly lower overall polymer surface excess concentration, but also a significantly greater amount of hydrodynamically entrapped water. Challenging a preadsorbed linear PEO (high molecular weight) layer with PEO star polymers produced no measurable change in the overall polymer surface excess concentration, but changes in the QCM-D energy dissipation and resonance frequency suggested that the introduction of PEO star polymers caused a slight swelling of the layer with a correspondingly small increase in entrapped water content.

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