No abstract

This study shows that alternating polyelectrolyte deposition on porous supports can yield nanofiltration membranes that allow high water flux along with selective ion transport. Membranes composed of 4.5-5 layer pairs of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina allow water fluxes of 1-2 m 3 m -2 day -1 at 4.8 bar while exhibiting MgSO4 rejections of 96%. Rejections of CaCl2 and Na2SO4 depend on polyelectrolyte deposition conditions and the composition of the outer layer of the membrane. In general, divalent-ion rejection increases when the charge of the outer layer of the membrane has the same sign as the divalent ion being rejected. Increasing the concentration of the supporting electrolyte present during deposition of the terminating PSS layer of PSS/PAH membranes results in a higher surface charge, and hence higher Na 2SO4 rejections (up to 95%). Nanofiltration with mixed solutions of NaCl and Na2SO4 yields Cl -/SO4 2selectivities of about 30 when the top layer of PSS/PAH membranes is deposited from a solution of high ionic strength. Capping PSS/PAH films with a layer of PAA increases Cl -/SO4 2selectivities to values as high as 80. Interestingly, Cl -/SO4 2selectivities in mixed solutions are higher than those determined from single-salt measurements, presumably because diffusion potentials are different in the two cases. The high selectivities, water fluxes, and ion rejections of PSS/PAH membranes make them potentially attractive for applications in water and salt purification.

Jeremy J. Harris, Jacqueline L. Stair, and Merlin L. Bruening, 'Layered Polyelectrolyte Films as Selective, Ultrathin Barriers for Anion Transport', Chemistry of Materials, Vol. 12 (7): 1941-1946, June 2000, doi: http://pubs.acs.org/doi/pdf/10.1021/cm0001004.Synthesis of high-flux composite membranes requires methods for deposition of ultrathin, defect-free films on highly permeable supports. Layer-by-layer deposition of polyelectrolytes on porous alumina (0.02 ??m pore diameter) produces such membranes. Electron microscopy shows that five bilayers (<25 nm) of poly(allylamine hydrochloride) (PAH)/poly(styrenesulfonate) (PSS) are sufficient to cover porous alumina and that underlying pores are not clogged during the deposition process. The selectivity of anion transport through these membranes increases with the number of bilayers until the substrate is fully covered. Fivebilayer PAH/PSS membranes have Cl-/SO4 2- and Cl-/Fe(CN)6 3- selectivity values of 7 and 310, respectively. PAH/poly(acrylic acid) membranes show selectivity values similar to those of PAH/PSS membranes but with a 3-fold decrease in anion flux. Selectivity in both of these systems likely results from Donnan exclusion

We report on the spontaneous organization of up to eight covalently attached layers formed on gold from solution phase (α,ω)-dithiols 1,6-hexanedithiol (C6), 1,8-octanedithiol (C8), and 1,9-nonanedithiol (C9). The linking chemistry between layers is the oxidative formation of a sulfur−sulfur bond that competes successfully with intralayer S−S bond formation. We have used optical null ellipsometry, FTIR, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) to characterize the multilayers. Once formed, the multilayers are stable when washed with 1 M KCl, water, ethanol, CHCl3 and n-hexane solutions, before and after prolonged exposure to ambient laboratory conditions. In addition to the formation of multilayers, our data point to the efficient oxidation of the interlayer disulfide bond to an oxidized sulfur moiety where the S−S bond remains intact. Extensive oxidation produces a sulfonate-terminated surface that reacts with Zr4+ and alkanebisphosphonates to form a hybrid multilayer assembly.

Deposition of hyperbranched poly(acrylic acid) (PAA) films on porous supports is an attractive method for synthesizing derivatizable, ultrathin composite membrane skins. We previously synthesized these films by sputtering a thin gold layer onto a porous alumina support and then grafting several layers of PAA to a self-assembled monolayer of mercaptoundecanoic acid on the gold. This paper demonstrates grafting of PAA onto PAA/poly(allylamine hydrochloride) (PAH) films that were prepared by alternating polyelectrolyte deposition. This procedure overcomes the inconvenience associated with sputtering of gold and allows synthesis of defect-free membranes using only one grafted PAA layer. Membrane skins consisting of 2.5 bilayers of PAA/PAH plus 1 layer of grafted PAA (total thickness of ∼14 nm) effectively cover the underlying pores of porous alumina supports without filling them, as shown by field-emission scanning electron microscopy (FESEM) images. When derivatized with H2NCH2(CF2)6CF3, these composite membranes show an ideal selectivity of 2.4 for O2 over N2, demonstrating the absence of defects in this system. The chemistry developed for the grafting process also allows modest derivatization of PAA/PAH films with H2NCH2(CF2)6CF3 to increase hydrophobicity.