Adsorption of Anionic or Cationic Surfactants in Polyanionic Brushes and Its Effect on Brush Swelling and Fouling Resistance during Emulsion Filtration

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

Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants

Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly­[2-(methacryloyloxy)­ethyl­dimethyl-(3-sulfopropyl)­ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethyl­ammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution

Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants

Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly­[2-(methacryloyloxy)­ethyl­dimethyl-(3-sulfopropyl)­ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethyl­ammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution