Fluorinated polymer zwitterions on gold nanoparticles: patterned catalyst surfaces guide interfacial transport and electrochemical CO2 reduction.

We report the use of fluorinated polymer zwitterions to build hybrid systems for efficient CO2 electroreduction. The unique combination of hydrophilic phosphorylcholine and hydrophobic fluorinated moieties in these polymers creates a fractal structure with mixed branched cylinders on the surface of gold nanoparticles (AuNPs). In the presence of these polymers, the CO faradaic efficiency improves by 50-80% in the range of -0.7 V to -0.9 V. The fractal structures have a domain size of ∼3 nm, showing enhanced mass transfer kinetics of CO2 approaching the catalyst surfaces without limiting ion diffusion. The phase-separated hydrophilic and hydrophobic domains offer separated channeling to water and CO2, as confirmed by attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and molecule dynamic (MD) simulations. H2O molecules permeate extensively into the polymer layer that adsorbs on zwitterions, forming continuous chains, while CO2 molecules strongly associate with the fluorinated tails of fluorinated polyzwitterions, with oxygen facing the positively charged amine groups. Overall, this coupling of zwitterion and fluorocarbon in a polymer material creates new opportunities for defining microenvironments of metallic nanocatalysts in hybrid structures.

Mesoscale Polymer Surfactants: Photolithographic Production and Localization at Droplet Interfaces.

Stabilization of fluid droplets, classically as oil-in-water or water-in-oil emulsions, is typically conducted using molecular surfactants or small particulates that localize at oil-water interfaces. In this paper, we describe a method whereby thin polymer films are converted photolithographically to ribbon-like mesoscale objects, which, in turn, adsorb to fluid interfaces where they extend as appendages, or arms, from the droplet surface. These "mesoscale polymer surfactants", or MPSs, were prepared from thin polymer films containing reactive functional moieties, including coumarin for photo-cross-linking, triphenylsulfonium for photoacid generation, and tert-butyl ester for solubility switching. The resultant MPSs, prepared initially on Si substrates, were released into water to reveal an exquisite shape sensitivity (forming straight, bent, or helical structures) and affinity for droplet interfaces based on their preparation conditions and the properties of the surrounding liquid. Notably, the lithographic techniques employed were amenable to differentiating the wettability of MPS segments, affording access to diblock-like MPSs which adhered to dispersed droplets via their hydrophobic segments, allowing their hydrophilic segments to extend into the continuous phase.

Functional Polymer Zwitterions as Reactive Surfactants for Nanoparticle Capture.

We describe the synthesis of sulfothetin (ST)-containing polymer zwitterions and their use as polymer surfactants for stabilizing emulsion droplets and for capturing and transporting nanoparticles (NPs) through a flowing aqueous fluid. In contrast to conventional zwitterions, which are chemically inert, the multifunctional ST-containing copolymers we describe both participate in droplet stabilization and embed reactive functionality directly into the zwitterionic framework. Advantageously including these ST zwitterions in phosphorylcholine (PC)-containing copolymers proved particularly useful for producing surfactants that contributed characteristics of droplet stabilization and interfacial reactivity. This was demonstrated by NP pickup, or "capture", experiments that were performed by circulating ST-coated emulsion droplets across a substrate, in a flow cell, containing amine-functionalized silica NPs. The resultant NP adherence to the fluid-fluid interface of the droplets hinged on the available reactivity of both the electrophilic (from ST) and nucleophilic (from the NPs) components as well as the solution pH and extent of amine functionality on the NPs.

Oil droplet behavior on model nanofiltration membrane surfaces under conditions of hydrodynamic shear and salinity.

HYPOTHESES: Oil droplet stability and electrical charge, and membrane's affinity for oil govern droplet attachment to a membrane surface. Moderate droplet-surface affinity encourages surface coalescence and removal of droplets to help maintain the membrane relatively oil-free. EXPERIMENTS: Droplet attachment onto model nanofiltration membranes was studied, in situ and in real time, using the Direct Observation Through the Membrane method. Optically transparent nanofiltration membranes were designed by forming polyelectrolyte multilayer films, with either positively or negatively charged surfaces, on Anopore ultrafilters. Crossflow across the membrane surface employed hexadecane-in-water emulsions stabilized by an anionic surfactant (sodium dodecylsulfate) in model sea water or aqueous solutions containing NaCl or MgSO4. FINDINGS: Moderate affinity between oil and the polyelectrolyte-coated surface promotes crossflow controlled coalescence to remove droplets larger than a critical size, ddropcrit, in the crossflow shear. The torque balance on a sessile oil droplet in a linear shear field overpredicted ddropcrit pointing to a need for more accurate estimates of lift and drag forces on a droplet. In the presence of divalent cations, lower electrostatic repulsion between droplets facilitated droplet-droplet adhesion and led to rapid coalescence that resulted in membrane fouling. The most significant fouling appeared in tests with positively charged and less oleophobic coatings.

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. In situ 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.

Preparation and functions of hybrid membranes with rings of Ag NPs anchored at the edges of highly ordered honeycomb-patterned pores.

We report a new, simple strategy to apply honeycomb films for the patterning of colloidal particles. By combination of a "bottom-up" breath figure method and the electrochemical properties of the honeycomb films of ferrocenyl-based oligomers, highly ordered hybrid membranes coated with ring-like patterning of 0D- and 1D-Ag nanoparticles (NPs) have been fabricated. One interesting phenomenon is that the nucleation and adsorption of Ag dots occurred preferentially at the edges of the micropores. The hybrid membranes exhibited richly electrochemical activities towards reduction of iodate and enhanced effectively catalytic reduction of organic dyes. We believe that this method can be used to decorate and/or assemble functional metal NPs such as Au, Pd, and Cu on honeycomb-patterned materials for the further applications of photonics, sensors, and catalysis.