Synthesis of Polystyrene/Poly[2-(Dimethylamino)ethyl Methacrylate- s tat -Ethylene Glycol Dimethacrylate] Core−Shell Latex Particles by Seeded Emulsion Polymerization and Their Application as Stimulus-Responsive Particulate Emulsifiers for Oil-in-Water Emulsions
Surfactant-stabilized polystyrene (PS) latex particles with a mean hydrodynamic diameter of 155 nm were prepared by aqueous emulsion polymerization using 2,2'-azobis(2-amidinopropane) hydrochloride as a cationic radical initiator. Seeded aqueous emulsion copolymerizations of 2-(dimethylamino)ethyl methacrylate (DMA) and ethylene glycol dimethacrylate (EGDMA) were conducted in the presence of these PS particles to produce two batches of colloidally stable core-shell latex particles, in which the shell comprised a cross-linked P(DMA-stat-EGDMA) overlayer. Both the PS and PS/P(DMA-stat-EGDMA) latexes were characterized in terms of their particle size, morphology, and composition using dynamic light scattering, electron microscopy, and FT-IR spectroscopy, respectively. Using the PS/P(DMA-stat-EGDMA) latex particles as a pH-responsive particulate ('Pickering'-type) emulsifier, polydisperse n-dodecane-in-water emulsions were prepared at pH 8 that could be partially broken (demulsified) on lowering the solution pH to 3. These emulsions were characterized in terms of their emulsion type, mean droplet diameter, and morphology using electrical conductivity and Mastersizer measurements, optical microscopy, and scanning electron microscopy (using critical point drying for sample preparation).
"Synthesis of sterically stabilized polystyrene latex particles using cationic block copolymers and macromonomers and their application as stimulus-responsive particulate emulsifiers for oil-in-water emulsions. Langmuir 2004, 20 (11) "
[Show abstract][Hide abstract] ABSTRACT: The interactions of two 2 mm pendant oil droplets grown in the presence of an aqueous solution of poly(glycerol monomethacrylate) stabilized polystyrene latex particles was observed using a high-speed video camera. The coalescence behavior was monitored as a function of oil type (n-dodecane vs. sunflower oil), particle size (135 nm vs. 902 nm) and in the presence and absence of an oil soluble cross-linker (tolylene 2,4-diisocyanate-terminated poly(propylene glycol)). The damping coefficient of the coalescing n-dodecane droplets increases in the presence of the latex, demonstrating particle adsorption. Coalescence times increased when changing the oil phase from n-dodecane to sunflower oil, due to the much higher viscosity of the latter oil. In addition, increasing the adsorbed particle size from 135 nm to 902 nm, leads to longer coalescence times due to the greater distance separating the oil droplets. Coalescence times observed in the presence of the larger 902 nm particles indicate that two different modes of contact can occur prior to a coalescence event (bilayer or bridging monolayer of particles in the film). Addition of an oil-soluble surface-active cross-linker to the sunflower oil phase to react with the hydroxy groups of the particle stabilizer reduces the interfacial elasticity, and ultimately prevents coalescence after cross-linking for 20 minutes at 25 °C. Such giant colloidosomes can remain in contact for several hours without undergoing coalescence, which demonstrates their high stability. Furthermore, coalescence is prevented even if the cross-linker is only present in one of the pendant droplets. Finally, evidence for cross-linker diffusion from one pendant droplet to another was indicated by a visible filament connecting the two droplets upon retraction.
"With the addition of minerals, certain substances were leached. These substances neutralized or decreased the surface potential of oil microdroplets, and thus facilitated flocculation    . Besides, the addition of minerals could also provide cores for the coalescence of gathered oil microdroplets. "
[Show abstract][Hide abstract] ABSTRACT: This study developed a novel method to destabilize emulsions and recycle oils, particularly for emulsified wastewater treatment. Natural minerals were used as demulsifying agents, two kinds of emulsions collected from medical and steel industry were treated. The addition of natural minerals, including artificial zeolite, natural zeolite, diatomite, bentonite and natural soil, could effectively destabilize both emulsions at pH 1 and 60 °C. Over 90% of chemical oxygen demand (COD) can be removed after treatment. Medical emulsion can be even destabilized by artificial zeolite at ambient temperature. The mechanism for emulsion destabilization by minerals was suggested as the decreased electrostatic repulsion at low pH, the enhanced gathering of oil microdroplets at elevated temperature, and the further decreased surface potential by the addition of minerals. Both flocculation and coalescence were enhanced by the addition of minerals at low pH and elevated temperature.
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