Environmental Responsiveness of Microgel Particles and Particle-Stabilized Emulsions

Abstract

Using stimulus-sensitive microgel particles as an emulsifier, we have prepared a new type of emulsion responsive to pH, ionic strength, and temperature changes. Each of these environmental changes can trigger a volume phase transition in poly(N-isopropylacrylamide) (PNIPAM) microgel particles containing some carboxylic groups. Depending on their hydrophobicity and charging state, such PNIPAM microgel particles can adsorb to the droplets of an octanol-in-water emulsion and provide excellent stability against coalescence and ripening. We have studied in detail the correlation between the particles' response to changes in the solution conditions and the corresponding response of particle-decorated emulsion droplets. In their swollen, hydrophilic state, the microgel particles consistently stabilize the octanol droplets, but inducing a microgel collapse usually results in a destabilization of the emulsion and eventually in phase separation. A notable exception was found at high pH where particles are highly charged: in this regime emulsions remain stable even upon a temperatureinduced collapse of the microgel particles and prove sensitive only to high levels of screening ions. Microscopy studies of toluene-in-water emulsions stabilized by compact polystyrene particles of variable surface charge further suggest an intimate connection between the charge and packing density of interfacial particles and hint at a charge-induced interparticle attraction.

Stability Proxies for Water-in-Oil Emulsions and Implications in Aqueous-based Enhanced Oil Recovery

“…Ngai et al [45] used stimulus-sensitive microgel particles as emulsifier. The resulting emulsions were responsive to pH, ionic strength and temperature changes which lead to changes in particles charge and hydrophobicity.…”

“…The higher the work required the more stable the particle-stabilized emulsion is. In microgel dispersions, high salt concentration reduces particle activity at the interface, leaving oil droplets unprotected [45]. On the other hand, Ngai et al [45] argue that reduced average emulsion drop size caused by increased concentration of microgel particles can be explained by the augmented coverage of the surface.…”

The Development of Hydrophobic and Superhydrophobic Cementitious Composites

“…Micro-particles of silica fume or metakaolin were used to stabilize (Binks & Lumsdon, 2000;Aveyard et al, 2003;Ngai et al, 2006) and modify the emulsion using two different approaches: "shell" and "core" as described in CFIRE 07-03 (Sobolev et al, 2013) and explained in detail in Flores-Vivian et al (2013). For external applications of superhydrophobic emulsions (E 1S and E 1SR ), the concentrations of surfactant, siloxane, and silica fume or metakaolin were 3.485, 25, and 5% by the weight of the emulsion respectively for E 1S and emulsion E 1SR was produced by diluting 1 part of E 1S emulsion with 4 parts of DI water (Table 1).…”

Preparation of Double Emulsions using Hybrid Polymer/Silica Particles: New Pickering Emulsifiers with Adjustable Surface Wettability

“…1,2 A wide range of particles, such as polystyrene latexes, 3,4,5 clay nanoparticles 6 ,7 ,8 and inorganic sols 9,10 have been reported to be effective Pickering emulsifiers. A particularly important class of Pickering emulsifiers is silica nanoparticles, 11 which have been used to form pH-responsive 12 and thermo-responsive 13,14 Pickering emulsions, as well as microcapsules known as colloidosomes. 15,16,17 It is well known that particle wettability is an important parameter in determining the nature of Pickering emulsions.…”