Membranes characterization

Membrane characterization by CSLM has been rather limited when compared with other microscopic techniques such as SEM and atomic force microscopy (AFM). The earliest work found in the literature [13] records how van den Berg et al. used a combination of AFM and CSLM to study qualitative differences in the pore geometry of different brands of polypropylene membranes. The first reported applications that used only CSLM for membrane characterization [14,15] were by Charcosset et al. who used CSLM to characterize microporous membrane morphologies and to obtain values of surface porosity and pore size. The conclusions of those studies were that CSLM gave some characteristics on membrane morphology that SEM, which views only surfaces, cannot provide. However, as also mentioned previously in this chapter, they pointed out low resolution for membrane characterization as the main drawback of CSLM. This restricts the use of CSLM to the characterization of microfiltration membranes if measurements on pore size and surface porosity have to be performed. 

Thomas et al. [16] used CSLM to observe in situ the formation of polyamide membranes and the measurements were used to study polymer precipitation kinetics. Turner and Cheng [17] applied CSLM and hydrophilic fluorescent probes of varying molecular weights to image the size distribution of poly(methacrylic acid) (PMAA) hydrogel domains in polydimethylsiloxane (PDMS)-PMAA interpenetrating polymer networks. The combination of CSLM with AFM, SEM and X-ray spectroscopy allowed characterization of the structure of stimuli-responsive polymeric composite membranes [18]. 

Most of the studies reviewed in this section apply CSLM to morphological membrane characterization and relate the results with the membrane formation parameters and transport properties. Considering that the type, nature, preparation method and final use of the membrane can differ significantly, the CSLM analysis protocol has to be particularized for each application. 

The ionic resistance of a polymer electrolyte membrane is an important parameter in determining the mobility of protons through the membrane and the corresponding voltage loss across the membrane. Currently, the most commonly used membranes in PEM fuel cells are Nafion membranes produced by DuPont. However, these membranes are limited to low-temperature uses (usually below 80°C) because membrane dehydration at high temperatures can lead to reduced water content and then a lower proton transfer rate, resulting in a significant decrease in conductivity. The relationship between conductivity and the diffusion coefficient of protons can be expressed by the Nemst-Einstein equation  

The conductivity of the membrane can be calculated based on measured resistance by the following equation  

L Distance of two current injectors/ voltage-sensing probes in 2-probe 

In principle, the four-probe method is more accurate than the two-probe, because in the former the interface impedance can be eliminated. However, it was reported that in different frequency ranges the membrane resistance and contact resistance could be easily separated. For example, in the two-probe method, the favourite frequency range is 100 to 500 kHz, while in the four-probe method the range becomes 1 to 100 kHz [11], Thus, both methods can yield reliable results, and the probe distance can affect measurement accuracy. Our experiments using both methods indicated that a large probe distance is required to obtain accurate results. 

In fuel cells, through-plane conductivity is more meaningful than in-plane because the proton transfer occurs in the through-plane direction. Several methods have been developed to measure the through-plane conductivity, and these will now be described. 

Forrrier Transform Infrared Spectroscopy (FUR) is a useful tool to detect functional groups in a carbon membrane. Samples of the carbon membrane are grormd into fine powder and then mixed with a small amount of KBr. A hydraulic die is loaded with the powder mixtirre to form a transparent disc. The IR spectrum of the sample disc is directly recorded using Perkin Elmer 2,000 FUR spectrometer. 

3 Attenuated Total Reflection-Fouiier Transform Infrared Spectroscopy (ATR-FTIR) 

Molecular orientation in the active layer of the membrane is directly detected using plane polarized reflectance infrared spectroscopy. This technique can reveal 

Elemental analysis is used to determine the composition of the carbon membrane prepared at different pyrolysis temperatures. The percentage composition of carbon, hydrogen, nitrogen and oxygen in the PAN carbon membrane is measured by El-ementar Vario EL 111 elemental analytical equipment. 

Before the gas permeation test is carried out, the PAN hollow fiber membranes are potted at both ends to form abimdle consisting of 5-10 fibers. One end of the fiber bundle is sealed into a stainless steel tube of 5/8 in. outer diameter, while the other end is potted in an aluminium cap. Loctite E-30CL epoxy adhesive is used as a potting resin. This bimdle is then inserted in a suitable module as shown in Fig. 5.4. All 

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Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

Membrane Characterization The two important characteristics of a UF membrane are its permeability and its retention characteristics. Ultrafiltration membranes contain pores too small to be tested by bubble point. Direc t microscopic observation of the surface is difficult and unreliable. The pores, especially the smaller ones, usually close when samples are dried for the electron microscope. Critical-point drying of a membrane (replacing the water with a flmd which can be removed at its critical point) is utihzed even though this procedure has complications of its own it has been used to produce a Few good pictures. 

Membrane Characterization MF membranes are rated by flux and pore size. Microfiltration membranes are uniquely testable by... 

Boulanger, Y. Schreier, S. Leitch, L. C. Smith, I. C. R, Multiple binding sites for local anesthetics in membranes Characterization of the sites and their equilibria by deuterium NMR of specifically deuterated procaine and tetracaine, Can. J. Biochem. 58, 986-995 (1980). 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Gorin, M. B., Yancey, S. B., Cline, J., Revel, J. P. and Horwitz, J. (1984). The major intrinsic protein (MIP) of the bovine lens fiber membrane characterization and structure based on cDNA cloning, Cell, 39, 49-59. 

If the ion-exchanger ion forms an ion pair in the membrane, characterized by the ion-pair formation constant... 

Materials Science of Reverse Osmosis Membranes - Characterization of Polymeric Membrane Materials... 

Dense Membrane Characterization. Small pieces of membranes (about 2 cm ) were kept in desiccators at relative humidities of 0, 18.8, 47.2, 80.5 and 100.0%. The 0% relative humidity was achieved with Drierite and the various relative humidities were obtained by varying the composition of aqueous H2S0ij solutions. 

Considerable progress has recently been made in developing the theoretical background necessary for the application of the above method of transient kinetic analysis. An important step in this direction was the use of WKB asymptotics to derive approximate analytical expressions for short- and long-time transient sorption and permeation in membranes characterized by concentration-independent continuous S(X) and Dt(X) functions 150-154). The earlier papers dealing with this subject152 154) are referred to in a recent review 9). The more recent articles 1S0 1S1) provide the correct asymptotic expressions applicable to all kinetic regimes listed above the usefulness... 

In some cases, the heat dissipated in an exothermic reaction can be used in an endothermic reaction taking place at the opposite side of the membrane. Typical examples are hydrogenation/dehydrogenation reactions carried out by palladium or Pd-alloy membranes characterized by a 100% theoretical selectivity towards the hydrogen. 

The performance of a specific already produced membrane characterized by a defined (known or unknown) thickness depends on permeance, which can be evaluated by means of a permeation measurement. 

The transport of some solutes across membranes does not resemble diffusion and suggests a temporary, specific interaction of the solute with some component (protein) of the membrane characterized as carrier, e.g., the small-peptide carrier of the intestinal epithelium. The rate of transport increases in proportion to concentration only when this is small, and it attains a maximal rate that cannot be exceeded even with a large further increase in concentration. The kinetics of carrier-mediated transport is theoretically treated by considering carrier-solute... 

Mayer, C., G. Grobner, K. Muller, K. Weisz, and G. Kothe (1990) Orientation-dependent deuteron spin-lattice relaxation times in bilayer membranes characterization of the overall hpid motion. Chem. Rhys. Lett. 165, 155-161. 

Goedert M, Pittaway K, Williams BJ, Emson PC (1984) Specific binding of tritiated neurotensin to rat brain membranes characterization and regional distribution. Brain Res 304 71-81. 

Membrane Characterization MF membranes are rated by flux and pore size. Microfiltration membranes are uniquely testable by direct examination, but since the number of pores that may be observed directly by microscope is so small, microscopic pore size determination is mainly useful for membrane research and verification of other pore-size-determining methods. Furthermore, the most critical dimension may not be observable from the surface. Few MF membranes have neat, cylindrical pores. Indirect means of measurement are generally superior. Accurate characterization of MF membranes is a continuing research topic for which interested parties should consult the current literature. 

Courel, M. The problem of membrane characterization for the process of osmotic distillation. Desalination, 140, 15, 2001. 

The first applications of CMRs have concerned high temperature reactions. The employed inorganic membranes, characterized by higher chemical and thermal stability with respect to polymeric membranes, still today suffer from some important drawbacks high cost, limited lifetime, difficulties in reactor manufacturing (delamination of the membrane top-layer from the support due to the different thermal expansion coefficients). 

The relationship between separation properties and casting parameters depends on the membrane structure. According to the Bokhorst - Altena — Smolders theory of phase separation [ 8 ], when PS concentration in the casting solution is increased and the time of solvent evaporation is extended, pore diameter decreases, thus improving the selectivity of the membranes. However, the increase of temperature to a critical value accounts for the increase of pore diameter, which brings about a decrease of the separation factor value. Further increase in temperature brings about a rapid evaporation of the solvent to yield small pore diameter membranes characterized by better separation properties. 

The membrane characterization data reported in this section have been obtained by means of a home-made apparatus which is made of stainless steel and can operate from high vacuum up to 70 bars [17], It is characterized by the unique capability of performing a broad range of porous membrane characterization and evaluation measurements, namely equilibrium isotherms, absolute (integral and differential) and relative gas and condensed vapor permeabilities and selectivities. 

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