Adsorption of Anionic or Cationic Surfactants in Polyanionic
Brushes and Its Effect on Brush Swelling and Fouling Resistance during
Emulsion Filtration
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Abstract
Atom transfer radical polymerization
of ionic monomers from membrane
surfaces yields polyelectrolyte brushes that swell in water and repel
oil droplets to resist fouling during filtration of oil-in-water emulsions.
However, surfactant adsorption to polyelectrolyte brushes may overcome
this fouling resistance. This work examines adsorption of cationic
and anionic surfactants in polyanionic brushes and the effect of these
surfactants on emulsion filtration. <i>In situ</i> ellipsometry
with films on flat surfaces shows that brushes composed of poly(3-sulfopropyl
methacrylate salts) (pSPMK) swell 280% in water and do not adsorb
sodium dodecyl sulfate (SDS). pSPMK-modified microfiltration membranes
reject >99.9% of the oil from SDS-stabilized submicron emulsions,
and the specific flux through these modified membranes is comparable
to that through NF270 nanofiltration membranes. Moreover, the brush-modified
membranes show no decline in flux over a 12 h filtration, whereas
the flux through NF270 membranes decreases by 98.7%. In contrast,
pSPMK brushes adsorb large quantities of cetyltrimethylammonium
bromide (CTAB), and at low chain densities the brushes collapse in
the presence of this cationic surfactant. Filtration of CTAB-stabilized
emulsions through pSPMK-modified membranes gives minimal oil rejection,
presumably due to the brush collapse. Thus, the fouling resistance
of polyelectrolyte brush-modified membranes clearly depends on the
surfactant composition in a particular emulsion