Th. van den Boomgaard

Microstructures in phase inversion membranes. Part II. The role of a polymeric additive

Membranes were prepared from a casting solution of a water-soluble polymer, poly (vinyl pyrrolidone) (PVP), and a membrane forming polymer, poly (ether sulfone), in 1-methyl-2-pyrrolidone (NMP) as solvent by immersing them in mixtures of water and NMP. It was found that the addition of PVP to the ternary system suppresses the formation of macrovoids in the sub-layer, while the ultrafiltration-type top-layer consists of a closely packed layer of nodules. Using a model for pass transfer in this quaternary system, it is possible to explain the effects of the additive on macrovoid formation. Strong indications are found that the appearance of a nodular structure in the top-layer follows a mechanism of spinodal decomposition during the very early stages of the immersion step.

Stability of Supported Liquid Membranes: State of the Art

Abstract This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed. ∗ Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The Netherlands.

Fouling of ultrafiltration membranes. The role of protein adsorption and salt precipitation

Abstract During ultrafiltration (UF) of whey, membrane/solute interactions appear to have a considerable effect on the separation characteristics of the membrane. This is primarily caused by precipitation of poorly soluble salts and/or adsorption of whey proteins, taking place inside the membranes and affecting the pore size distribution. Evidence for location of precipitation and adsorption inside the membrane is obtained by measuring the amounts of salts precipitated (using radiolabelling procedures) and protein adsorbed, and by comparing the protein adsorption with the adsorbed amounts on model surfaces having membrane-like characteristics. To quantify the influence of fouling in terms of pore narrowing a characteristic pore size for UF membranes is defined, based on the retention of low-molecular-mass saccharides. Membrane morphology and surface properties appear to have a considerable influence on the amount and type of protein adsorbed, suggesting a potential for controlling membrane fouling by modification of membrane properties.

Mass transfer and thermodynamics during immersion precipitation for a two-polymer system: Evaluation with the system PES—PVP—NMP—water

An extended version of the mass transfer model by Reuvers et al. for a four-component system is evaluated, which is shown to be generally valid for short times. The thermodynamics under these circumstances are evaluated, together with the kinetics. Initial composition paths (concentration profiles) are calculated. It appears that delay of demixing is not possible when a polymeric additive is used, which is soluble in the nonsolvent, while the velocity of demixing decreases. The calculations are evaluated for the system poly(ether sulfone)-poly(vinylpyrrolidone)-N-methylpyrrolidone-water by means of light transmission measurements during immersion precipitation, for a wide range of compositions of the polymer solution and coagulation bath.

Stabilization of supported liquid membranes by interfacial polymerization top layers

In this paper, a new method of stabilizing supported liquid membranes is presented. The stabilization is based on the application of polymeric top layers to the surface of microfiltration membranes, preventing loss of the liquid membrane phase out of the support pores. The modified microfiltration membranes were used as supports for supported liquid membranes and tested on selective nitrate transport and stability. Screening experiments revealed that most applied top layers did not hinder the transport of nitrate ions. However, a few were able to improve the stability of the liquid membranes. Best results were obtained when piperazine (PIPA) and trimesoyl chloride (TMCl) were used as monomers. For Accurel polypropylene supports with PIPA/TMCl top layer, nitrate flux was constant at the initial 18 × 10−10 mol cm−2 s−1 for 350 h of simulated operation. For uncoated supported liquid membranes (SLMs), the flux decreased within one day from 18 × 10−10 to almost 0 mol cm−2 s−1. Scanning electron microscopy investigations revealed a particular, rippled surface texture of layers prepared with these monomers.

Linearized cloudpoint curve correlation for ternary systems consisting of one polymer, one solvent and one non-solvent

A linear correlation function is found for cloudpoint composition curves of ternary systems consisting of one polymer, one solvent and one non-solvent. The conditions for validity of this correlation function appear to be that the polymer is strongly incompatible with the non-solvent, and that only liquid-liquid demixing occurs. The linearized cloudpoint (LCP) curve is interpreted in terms of the various parameters occurring in the Flory-Huggins theory. The slope of the LCP line appears to be only dependent on the molar volumes of the components. Information about the binary Flory-Huggins interaction parameters and their concentration dependence can be obtained from the intercept of the linearized curve. Cloudpoints induced by crystallization do not follow the correlation. This gives an opportunity to distinguish between crystallization and liquid-liquid demixing without any additional experiments.

Current-voltage curve of a bipolar membrane at high current density

The potential drop across a bipolar membrane was measured as a function of the applied current density. As a result, an inflection point was observed in the obtained current-voltage curve at high current density. This inflection point indicates that at high current densities water supply from outside of the bipolar membrane to the interface between a cation and an anion exchange layer is the rate limiting step for the water dissociation process. The dependence of the current-voltage curve on external conditions such as temperature and feed solution concentration was also investigated. The position of the inflection point changed with the temperature and the concentration of the external solution.

Characterization of Clean and Fouled Ultrafiltration Membranes

Much research into the fundamentals of membrane formation and separation has been performed in order to improve the efficiency of the manufacture of ultrafiltration membranes. Determination of the membrane characteristics is a key problem in these investigations. In this paper, we report on a study of membrane morphology by fractional rejection measurements, using low molecular weight saccharides as the test solute, and by electron microscopy. Using a simple model for solute/solvent transport through cylindrical pores, a “characteristic pore size” was derived from saccharide rejection data. This pore size of a hypothetical isoporous membrane, interpreting the measured separation characteristics, provides a promising means of describing differences between membranes with respect to pore size and pore size changes caused by solute adsorption. From high resolution electron micrographs, information was obtained on the skin layer morphologies and, for some membranes the sizes of the larger pores could be estimated.

Physicochemical aspects of polymer selection for ultrafiltration and microfiltration membranes

The concept of additivity of surface tension components has been used to predict the adsorptive fouling tendency of membranes. The calculated value for the free energy of adhesion ΔGLWS is taken as a measure for this fouling tendency. ΔGLWS values can be determined from the surface tension components of the solid and liquid. The determination of the surface tension components of the solid surfaces has been carried out using sessile drop contact angle measurements with water, glycerol and α-bromonaphthalene. ΔGLWS values have been determined for human serum albumin (HSA) and polyethyleneglycol (PEG) on ten different non-porous polymer surfaces and it has been calculated that cellulose acetate (CA) and polyacrylonitrile (PAN) showed the lowest fouling tendency, whereas polyethersulfone (PES) and polyvinylidenedifluoride (PVDF) showed the highest fouling tendencies.

Spinning of hollow fiber ultrafiltration membranes from a polymer blend

In this study the dry-wet spinning technique is used for the preparation of hollow fiber membranes. In the polymer solution a blend of two polymers, poly(ether sulfone) and poly(vinyl pyrrolidone), is used. The morphology of the membranes obtained is related to rheological characteristics and phase behavior of the polymer solution during spinning. The outer surface pore structure is mainly dependent on the conditions in the airgap. The typical performance of the membranes lies in the ultrafiltration region.