Lutz Krasemann

Self-assembled polyelectrolyte multilayer membranes with highly improved pervaporation separation of ethanol/water mixtures

Abstract Ethanol/water pervaporation through ultrathin polyelectrolyte multilayer membranes is described. The membranes were prepared by the layer-by-layer technique, i.e. by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous support. The separation capability was optimized by variation of the chemical structure of the polyelectrolytes, by variation of pH and ionic strength of the polyelectrolyte solutions used for membrane preparation and by annealing of the polyelectrolyte membranes. It was found that the separation is mainly affected by the charge density of the polyelectrolytes which is controlled by the chemical structure and the degree of ionisation of the polar groups. Selectivity for water was highest, if polyelectrolytes of high charge density such as polyethyleneimine (PEI), polyvinylamine (PVA) and polyvinylsulfate (PVS) were used and if the pH of the polyelectrolyte solutions was equal to the mean of the p K a values of the corresponding cationic and anionic polyelectrolyte. Best results were obtained for PVA/PVS and PEI/PVS membranes which are characterized in detail with regard to their separation behavior.

Ultrathin membranes for gas separation and pervaporation prepared upon electrostatic self-assembly of polyelectrolytes

Abstract Composite membranes with ultrathin separation layer were prepared by alternating electrostatic adsorption of cationic and anionic compounds (polyelectrolytes and bolaamphiphiles) at porous supporting membranes such as Celgard 2400, PAN/PET (a polyethyleneterephthalate fleece coated with a thin layer of polyacrylonitrile) and Isopore (an etched ion-track polycarbonate membrane). As the ionic compounds poly(allylamine hydrochloride) (PAH), poly(styrenesulfonate sodium salt) (PSS), 1.4-diketo-3.6-diphenylpyrrolo-[3,4- c ]-pyrrole-4,4′-disulfonic acid and 10,12-docosadiyne-1,22-disulfate disodium salt were used. The composite membranes were studied on their gas permeation and pervaporation properties. The adsorption of 20 layer pairs of PAH/PSS at PAN/PET membranes was sufficient to reduce the argon flow to 7% of the initial value, while 60 bilayers reduced the flow to 0.1%. Gas flow rates of oxygen, nitrogen and argon were nearly identical, while the flow rate of carbon dioxide was higher by a factor of up to 2.4. Toluene/heptane pervaporation through composite membranes of 60 bilayers of PAH/PSS on PAN/PET gave a toluene enrichment from 20% in the feed to 24% in the permeate, while for ethanol/water permeation a water enrichment from 3% in the feed up to 50% in the permeate was found.

Polyelectrolyte multilayer membranes for pervaporation separation of alcohol/water mixtures

Abstract Composite membranes with ultrathin polyelectrolyte separation layer were studied on their use for alcohol/water separation under pervaporation conditions. The separating layer was prepared by alternate electrostatic layer-by-layer adsorption of cationic and anionic polyelectrolytes onto a porous polymer support. Alcohol/water pervaporation led to water enrichment in the permeate, which was highest for polyelectrolytes of high charge density such as polyvinylamine (PVA), polyethyleneimine (PEI) and polyvinylsulfate (PVS), and if the preparation conditions of the separation layer (pH, ionic strength of polyelectrolyte solutions) were properly chosen. If a PVA/PVS separation layer was used for ethanol/water pervaporation, a separation factor α up to 700 was found, while the flux was about 0.5 kg m −2 h −1 . For 1-propanol/water and t -butanol/water pervaporation, the α -values were up to 7×10 3 and 3.2×10 4 , respectively.

Composite membranes with ultrathin separation layer prepared by self-assembly of polyelectrolytes

Abstract New composite membranes with ultrathin self-assembled polyelectrolyte separating layer and their use in gas, liquid and ion separation are described. Composite membranes were prepared by alternating electrostatic adsorption of polycations and polyanions on a porous PAN/PET supporting membrane (a polyethylenenterephthalate fleece coated with a thin layer of polyacrylonitrile). As the polycations poly(allylamine hydrochloride) (PAH), poly(ethylenimine) (PEI), poly(diallyldimethyl–ammonium chloride) (PDADMAC), poly(4-vinylpyridine) (P4VP) and chitosan (CHI) were used, the polyanion was always poly(styrene sulfonate sodium salt) (PSS). Studying CO2 and N2 permeation rates, selectivities for CO2 (α=1.5) were only found with P4VP/PSS separation layer. Studying ethanol–water pervaporation, separation factors α′ up to 80 were found, when the separation layer was made of PAH/PSS and annealed at T≥60°C. For non-annealed samples, the highest α′ value was 20. It was found for a separating membrane of PEI/PSS. Various effects of preparation and operation conditions on flux and separation factor are discussed. Studying permeation of NaCl and MgCl2 in aqueous solution, a selective transport of the monovalent Na+ was found, if the separation layer was PAH/PSS. The permeation rate for Na+ was 15.1 times higher than for Mg2+. Our study indicates that composite membranes with self-assembled polyelectrolyte separation layer are useful in materials separation, although individual polyions may exhibit a very different separation capability.

Self-assembled mono- and multilayers of monodisperse cationic and anionic latex particles

Abstract Monolayer formation upon self-assembly of monodisperse anionic latex particles and multilayer formation upon alternating self-assembly of cationic and anionic latex particles at positively charged glass supports were studied using scanning electron microscopy (SEM). Particles were prepared by soap-free emulsion copolymerization of styrene with acrylic acid (1a, 1b), and styrene with 2-acryloxyethyltrimethylammonium bromide (2) having diameters of 440 (1a) and 200 nm (1b, 2). Interfacial adsorption of 1a leads to adsorption of isolated particles and small two-dimensional (2D) aggregates. A normalized surface coverage Cs of 30% is reached, the average particle aggregate size being 5. Alternate dipping into dispersions of particles 1b and 2 with identical diameter of 200 nm leads to successive formation of a highly porous, amorphous 3D film. The thickness increases with the number n of dipping processes, but is only 60% of the value calculated for a densely packed array of n layers.

Tailor‐made membranes for alcohol/water pervaporation and ion separation prepared upon layer‐by‐layer adsorption of polyelectrolytes

The paper is concerned with separating membranes prepared upon layer-by-layer adsorption of cationic and anionic polyelectrolytes on a porous substructure. The use of the resulting composite membranes for alcohol/water separation under pervaporation conditions, and for the separation of mono- and divalent ions is described. It is demonstrated that a suitable choice of the polyelectrolytes and the optimisation of the preparation conditions allow to tailor membranes with excellent separation capability.