Lateral phase separation of confined membranes

We consider membranes interacting via short, intermediate and long stickers. The effects of the intermediate stickers on the lateral phase separation of the membranes are studied via mean-field approximation. The critical potential depth of the stickers increases in the presence of the intermediate sticker. The lateral phase separation of the membrane thus suppressed by the intermediate stickers. Considering membranes interacting with short and long stickers, the effect of confinement on the phase behavior of the membranes is also investigated analytically

Permeation and separation studies on microporous sol-gel modified ceramic membranes

Permeation and separation experiments with H2, CO2, O2, N2, CH4 and isobutane with microporous sol-gel modified supported ceramic membranes were performed to determine the gas transport characteristics and the hydrogen separation performance of these membranes. It is found that the permeation is activated, and for defectfree membranes the apparent activation energies are in the ranges 13¿15 and 5¿6 kJ mol¿1 for H2 and CO2, respectively. Correction for the pressure drop over the support results in apparent activation energies for the silica top-layer on the order of 17¿22 and 10¿15 kJ mol¿1 for H2 and CO2 respectively. Due to the very thin top-layer, the permeation is relatively high, with representative values of 6·10¿7 and 20·10¿7 mol m¿2s¿1 Pa¿1 for H2 at 25 and 200°C, respectively. The H2 permeation is almost pressure-independent up to pressures of at least 5 bar. Typical separation factors for H2---CH4 and H2---isobutane are approximately ¿40 and ¿200, respectively, at 200°C for high-quality membranes. For moderate-quality membranes the H2---CH4 separation factor is around 10, while the H2---isobutane separation factor remains at a high value of around 100 at 200°C and 120 at 300°C

Polyelectrolyte multilayer films as backflushable nanofiltration membranes with tunable hydrophilicity and surface charge

A diverse set of supported polyelectrolyte multilayer (PEM) membranes with controllable surface charge, hydrophilicity, and permeability to water and salt was designed by choosing constituent polyelectrolytes and by adjusting conditions of their deposition. The membranes were characterized in terms of their water and MgSO4 permeabilities and resistance to colloidal fouling. The commercial nanofiltration membrane (NF270) was used as a comparative basis. Highly hydrophilic and charged PEMs could be designed. For all membranes, MgSO4 permeability coefficients of NF270 and all PEM membranes exhibited a power law dependence on concentration: Ps [is proportional to] C-[tau], 0.19 < [tau] < 0.83. PEM membranes were highly selective and capable of nearly complete intrinsic rejection of MgSO4 at sufficiently high fluxes. With the deposition of colloids onto the PEM surface, the separation properties of one type of polyelectrolyte membrane showed similar rejection and superior flux properties compared to NF270 membranes. We hypothesize that a PEM-colloid nanocomposite was formed as a result of colloidal fouling of these PEM films. The feasibility of regenerating the PEM membranes fouled by colloids was also demonstrated. In summary, the PEM-based approach to membrane preparation was shown to enable the design of membranes with the unique combination of desirable ion separation characteristics and regenerability of the separation layer

Core-sheath structured electrospun nanofibrous membranes for oil-water separation

In recent years, both the increasing frequency of oil spill accidents and the urgency to deal seriously with industrial oil-polluted water, encouraged material scientists to design highly efficient, cost effective oil-water separation technologies. We report on electrospun nanofibrous membranes which are composed of core-sheath structured cellulose-acetate (CA)-polyimide (PI) nanofibers. On the surface of the CA-PI fibers a fluorinated polybenzoxazine (F-PBZ) functional layer, in which silica nanoparticles (SNPs) were incorporated, has been applied. Compared with F-PBZ/SNP modified CA fibers reported before for the separation of oil from water, the PI-core of the core-shell F-PBZ/SNP/CA-PI fibers makes the membranes much stronger, being a significant asset in their use. Nanofibrous membranes with a tensile strength higher than 200 MPa, a high water contact angle of 160 degrees and an extremely low oil contact angle of 0 degrees were obtained. F-PBZ/SNP/CA-PI membranes seemed very suitable for gravity-driven oil-water separation as fast and efficient separation (>99%) of oil from water was achieved for various oil-water mixtures. The designed core-sheath structured electrospun nanofibrous membranes may become interesting materials for the treatment of industrial oil-polluted water

Phase separation processes in polymer solutions in relation to membrane formation

This review covers new experimental and theoretical physical research related to the formation of polymeric membranes by phase separation of a polymer solution, and to the morphology of these membranes. Two main phase separation processes for polymeric membrane formation are discussed: thermally induced phase separation and immersion precipitation. Special attention is paid to phase transitions like liquid-liquid demixing, crystallization, gelation, and vitrification, and their relation to membrane morphology. In addition, the mass transfer processes involved in immersion precipitation, and their influence on membrane morphology are discussed

Adhesion-induced phase separation of multiple species of membrane junctions

A theory is presented for the membrane junction separation induced by the adhesion between two biomimetic membranes that contain two different types of anchored junctions (receptor/ligand complexes). The analysis shows that several mechanisms contribute to the membrane junction separation. These mechanisms include (i) the height difference between type-1 and type-2 junctions is the main factor which drives the junction separation, (ii) when type-1 and type-2 junctions have different rigidities against stretch and compression, the ``softer'' junctions are the ``favored'' species, and the aggregation of the softer junction can occur, (iii) the elasticity of the membranes mediates a non-local interaction between the junctions, (iv) the thermally activated shape fluctuations of the membranes also contribute to the junction separation by inducing another non-local interaction between the junctions and renormalizing the binding energy of the junctions. The combined effect of these mechanisms is that when junction separation occurs, the system separates into two domains with different relative and total junction densities.Comment: 23 pages, 6 figure

Adhesion-Induced Lateral Phase Separation in Membranes

Adhesion between membranes is studied using a phenomenological model, where the inter-membrane distance is coupled to the concentration of sticker molecules on the membranes. The model applies to both for adhesion of two flexible membranes and to adhesion of one flexible membrane onto a second membrane supported on a solid substrate. We mainly consider the case where the sticker molecules form bridges and adhere directly to both membranes. The calculated mean-field phase diagrams show an upward shift of the transition temperature indicating that the lateral phase separation in the membrane is enhanced due to the coupling effect. Hence the possibility of adhesion-induced lateral phase separation is predicted. For a particular choice of the parameters, the model exhibits a tricritical behavior. We also discuss the non-monotonous shape of the inter-membrane distance occurring when the lateral phase separation takes place. The inter-membrane distance relaxes to the bulk values with two symmetric overshoots. Adhesion mediated by other types of stickers is also considered.Comment: 13 pages, 9 PostScript figures included. To be published in Euro. Phys. J - E. Minor revision

Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes

Sol-gel modification of mesoporous alumina membranes is a very successful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and molecular sieve-like separation factors. This paper concentrates on the mechanism of activated transport (also often referred to as micropore diffusion or molecular sieving). Based on a theoretical analysis, results from permeation and separation experiments with H2, CO2, O2, N2, CH4 and iso-C4H10 on microporous sol-gel modified supported ceramic membranes are integrated with sorption data.\ud \ud Gas permeation through these membranes is activated, and for defect-free membranes the activation energies are in the order of 13¿15 kJ.mol¿1 and 5¿6 kJ.mol¿1 for H2 and CO2 respectively. Representative permeation values are in the order of 6×10¿7 mol.m¿2.s¿1.Pa¿1 and 20×10¿7 mol.m¿2.s¿1.Pa¿1 for H2 at 25°C and 200°C, respectively. Separation factors for H2/CH4 and H2/iso-butane are in the order of 30 and 200 at 200°C, respectively, for high quality membranes.\ud \ud Processes which strongly determine gas transport through microporous materials are sorption and micropore diffusion. Consequently, the activation energy for permeation is an apparent one, consisting of a contribution from the isosteric heat of adsorption and the activation energy for micropore diffusion. An extensive model is given to analyse these contributions.\ud \ud For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rate determining. The calculated activation energies for micropore diffusion are 21 kJ.mol¿1 and 32 kJ.mol¿1 for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpore=4Å), indicating that the average pore size of the sol-gel derived membranes is probably smaller

Synthesis, characterisation and gas permeation studies on microporous silica and alumina-silica membranes for separation of propane and propylene

Microporous silica membranes are known to exhibit molecular sieving effects. However, separation of nearly equal sized molecules is difficult to carry out by size exclusion. Introducing sorption selectivity and keeping the kinetics favourable to facilitate a good contribution of permeation from sorption is a possible solution to enhance selectivity of adsorbing molecules. Results are presented in this paper on the synthesis of a microporous silica membrane with commendable permselectivity between helium and propylene. Modifications are performed on the membrane to improve its almost non-selective nature to propylene/propane mixtures to give practical separation values. Gas separation results on the modified membranes are presented. Surface selectivity on the newly added alumina surface layer is identified as the helping mechanism in realising this separation