Porous membranes characterization

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. 

Fig. 3. Microporous membranes are characterized by tortuosity, T, porosity, S, and their average pore diameter, d. (a) Cross-sections of porous membranes containing cylindrical pores, (b) Surface views of porous membranes of equal S, but differing pore size.

In the third part of the chapter the solid state properties of our block copolymer are examined. The surface energies of these materials are characterized by contact angle measurements. The organization of the polymer chains in the solid state phase is investigated by small-angle X-ray scattering (SAXS) and the gas selectivity of porous membranes coated with these block copolymers is characterized by some preliminary permeation measurements. 

There are several cell monolayer models that are frequently used for the evaluation of drug permeability and absorption potential (Table 18.1). For a more detailed discussion, please refer to Chap. 8. Caco-2 cells (adenocarcinoma cells derived from colon) are the most extensively characterized and frequently used of the available cell lines [5-9], A unique feature of Caco-2 cells is that they undergo spontaneous enterocyte differentiation in cell culture. Unlike intestinal enterocytes, Caco-2 cells are immortalized and replicate rapidly into confluent monolayers. When the cells reach confluency during culture on a semi-porous membrane, they start to polarize and form tight junctions, creating an ideal system for permeability and transport studies. During the past decade, use of... 

Contacting ozone gas with water can be achieved with every kind of gas diffuser, which is made of a material resistant to ozone. Ring pipes, porous diffusers and porous membranes, injector nozzles as well as static mixers can be employed. The different types of diffusers are mainly characterized by the diameter of the bubbles produced, e. g. micro (dB = 0.01 — 0.2 mm), small (dB 1.0 mm) or big (dB - 2.5 mm) bubbles (Calderbank, 1970 Hughmark, 1967). 

Decatungstate, in the form of a lipophilic tetrabutilamonium salt ((n-C4H9N)4 W10O32), has been homogeneously dispersed in porous membranes made of PVDF (PVDF-W10). Solid-state characterization techniques confirmed that catalyst structure and spectroscopic properties of decatungstate have been preserved once immobilized within the membranes [42-44]. 

Gaseous permeation can be used for the characterization of porous membranes using an apparatus working with the technique of fixed volume-variable pressure as shown in Fig. 3.70. The technique, which was initially developed for dense polymer membranes, is based on the recording of the pressure evolution with time of a downstream compartment, which is separated from an upstream compartment filled with a pure gas by a flat membrane. Before starting the experiments, both compartments are put under very low pressure and, at the initial time of the measurements, a relatively high pressured pure gas is introduced into the upstream compartment [3.59]. 

In the characterization of porous membranes by liquid or gaseous permeation methods, the interpretation of data by the hyperbolic model can be of interest even if the parabolic model is accepted to yield excellent results for the estimation of the diffusion coefficients in most experiments. This type of model is currently applied for the time-lag method, which is mostly used to estimate the diffusion coefficients of dense polymer membranes in this case, the porosity definition can be compared to an equivalent free volume of the polymer [4.88, 4.89]. 

With an experimental protocol in place that facilitated studies aimed at characterizing the porous permeation pathway, a systematic study of polar compound permeation through HEM was undertaken (Peck et al., 1994). As has already been described, there is a void in the literature with respect to the passive permeation of polar solutes through skin. The initial purpose of the studies outlined in this section was to add to the polar solute permeation database. An effort was again made to determine the degree to which the barrier characteristics of skin with respect to polar compounds approach, or deviate from, those of an ideal porous membrane. 

Based upon Eq. 4 a systematic study was performed with four polar permeants (urea, mannitol, sucrose, and raffinose) in an effort to characterize further the porous permeation pathway through HEM (Peck et al., 1994). Dual-labeled liquid scintillation counting and an experimental protocol that incorporated successive permeability experiments, as outlined in the previous sections, allowed the permeability coefficients for each permeant to be determined for each HEM sample studied. Again, Eq. 4 predicts that, for a porous membrane, the permeability coefficient ratio should be equal to the ratio of the diffusion coefficients for the solutes in the membrane. As a first approximation, if the relative radii of the solutes and the membrane pore radii Rp are such that hindrance considerations are negligible (Deen, 1987), then the ratio PJPy should approach the ratio of the free diffusion coefficients D of the solutes in bulk solution. 

Membrane morphology and, in the case of porous membranes, pore size and orientation and porosity are vital to the separation properties of inorganic membranes. As the general characterization techniques evolve, the understanding of these miciostnictures improves. 

In liquid filtration using micro-, ultra-, and nanofiltration porous membranes, the driving force for transport is a pressure gradient. Solvent permeability and separation selectivity are the two main factors characterizing membrane performance. Convective flux is predominant with macroporous and mesoporous membrane strucmres, the selectivity being controlled by a... 

Ryi S-K, Park J-S, Choi S-H, Cho S-H, and Kim S-H. Fabrication and characterization of metal porous membrane made of Ni powder for hydrogen separation. Sep. Purif. Technol. 2006 47 148-155. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

Membrane characterization means the determination of structural and morphological properties of a given membrane. Because membranes range from porous to nonporous depending on the type of separation problem involved, different characterization techniques are required in each case. For example, in MF or UF membranes, fixed pores are present. MF membranes have macropores (pore diameter > 50 mn), while UF membranes have mesopores (2 mn < pore diameter < 50 nm). The pore size (and size distribution) mainly determines which particles or molecules are retained or pass through. On the other hand, for dense or nonporous membranes, no fixed pores are present and the material chemistry itself mainly determines the performance. 

Two different types of characterization method for porous membranes can be distinguished (13) ... 

There are a number of characterization techniques available for porous membranes, the following methods are usually used ... 

Structural models emerge from the notion of membrane as a heterogenous porous medium characterized by a radius distribution of water-filled pores. This structural concept of a water-filled network embedded in the polymer host has already formed the basis for the discussion of proton conductivity mechanisms in previous sections. Its foundations have been discussed in Sect. 8.2.2.1. Clearly, this concept promotes hydraulic permeation (D Arcy flow [80]) as a vital mechanism of water transport, in addition to diffusion. Since larger water contents result in an increased number of pores used for water transport and in larger mean radii of these pores, corresponding D Arcy coefficients are expected to exhibit strong dependencies on w. 

P. Sneider and P. Uchytil, Liquid expulsion permporometry for characterization of porous membranes. /. Membr. Sci., 95 (1994) 29. 

Other reactor configurations and concepts have also been discussed in the technical literature. Most commonly dted are hybrid concepts, where the membrane reactor is used as an add-on stage to an already existing conventional reactor. This particular configuration has a number of attractive features, especially for applications involving conventional type porous membranes, which are characterized by moderate (Knudsen-type) permselective properties. Staged membrane reactors have received mention and so have reactors with multiple feed-ports and recycle. To facilitate the transport across the membrane in laboratory studies one often applies a sweep gas or a vacuum in the permeate side or a pressure gradient across the membrane. It is unlikely that the first two approaches, effective as they may be in laboratory applications, will find widespread commercial application. 

R364 W. Heink, J. Karger and S. Vasenkov, Application of Pulsed Field Gradient NMR to Characterize the Transport Properties of Micro-porous Membranes , Membr. Sci. Technol Ser., 2000,6, 97 R365 P. J. F. Henderson, C. K. Hoyle and A. Ward, Expression, Purification and Properties of Multidrug Efflux Proteins , Biochem. Soc. Trans., 2000,28, 513... 

Methods of quantitative characterization of porous membrane structures have been explored. It is believed that knowledge obtained through these and similar methods will lead to a better understanding of both membrane formation and membrane function processes. 

Gates, B., Yin, Y, and Xia, Y, Eabrication and characterization of porous membranes with highly ordered three-dimensional periodic structures, Chem. Mater, 11, 2827, 1999. 

Baltus, R.E. 1997. Characterization of the pore area distribution in porous membranes using transport measurements. J. Membrane Sci. 123 165-184. 

Many characterization techniques developed for the characterization of meso-porous and microporous materials have been adapted to membrane characterization (e.g., mercury porosimetry, adsorption and desorption isotherms, and thermoporometry). These techniques are related to morphological parameters... 

Small Intestinal Cell Lines. The most notable, and certainly best characterized, in vitro tool for studying absorption and, to a less extent, intestinal metabolism utilizes Caco-2 cells grown in a confluent monolayer on porous membrane filters and, for the experiments, mounted in diffusion chambers. Under these growing conditions they differentiate spontaneously into polarized enterocyte-like cells possessing an apical brush border and tight Junctions between adjacent cells, thereby retaining many characteristics of the intestinal brush border. The permeability of a compound is determined by the rate of its appearance in the basolateral compartment. 

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