This paper describes a class of water-based magnetic fluids that are specifically tailored to extract
soluble organic compounds from water. These magnetic fluids are prepared by precipitation and
consist of a suspension of ∼7.5 nm magnetite (Fe3O4) nanoparticles coated with a ∼9 nm
bifunctional polymer layer comprised of an outer hydrophilic poly(ethylene oxide) (PEO) region
for colloidal stability and an inner hydrophobic poly(propylene oxide) (PPO) region for
solubilization of organic compounds. The particles exhibit a high capacity for organic solutes,
with partition coefficients between the polymer coating and water on the order of 103−105, which
is consistent with values reported for solubilization of these organics in PEO−PPO−PEO block
copolymer micelles. In bench-scale experiments, high-gradient magnetic separation (HGMS) is
able to recover the nanoparticles with 98% efficiency. Process options for particle regeneration
in water purification applications are discussed.
Small-angle neutron scattering and mean-field lattice modeling were used to characterize a class of water-based magnetic fluids tailored specifically to extract soluble organic compounds from water. The fluids consist of a suspension of approximately 7 nm magnetite (Fe3O4) nanoparticles coated with a bifunctional polymer layer comprised of an outer hydrophilic poly(ethylene oxide) (PEO) region for colloidal stability and an inner hydrophobic poly(propylene oxide) (PPO) region for solubilization of organic compounds. The inner region of the polymer shell is increasingly depleted of water as the fraction of PPO side chains increases. The incorporation of PPO side chains also leads to a small increase in interparticle attraction. The lattice model predicted a shell structure similar to that of a PEO-PPO-PEO triblock copolymer (Pluronic) micelle, with equivalent levels of hydration but with more PEO present in the PPO-rich regions, as the side chains grafted to the surface are less able to segregate than when in free micellar systems.
The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene
oxide) (PEO−PPO−PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS),
ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium
swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92
(average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average
composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the
parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within
the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense
nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain
temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic
associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions
indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO
chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher
hydrophobicity of the resulting L92−PAA−EGDMA microgels and a larger presence of hydrophobic, densely
cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates
such as those formed in the F127−PAA−EGDMA microgels.
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