Pore size distributions of membranes

Although most particles, larger than a given pore size, are normally retained, many smaller particles (sometimes 10 - 1000 times smaller than the pore size) may also be retained. If above and within a pore depth filtration occurs (see Chapter 7.6 and Fig. 7.12) colloidal particles smaller than the pore size become attached to the larger particles. Furthermore the pore size distribution of membrane filters is often non-narrow. 

Separation processes as a whole have grown in importance because of increasingly stringent requirements for product purity [1]. Among the different membrane techniques, pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) were the first to undergo rapid commercialization [2-A], These processes basically differ in pore size distribution of membranes used and the types of compounds recovered. A typical schematic of the exclusion of various compounds through different membrane processes is illustrated in Figure 42.1. 

Pore Size Distribution of Membrane Prepared Through Micro-phase... 

The pore size and the pore size distribution of membranes can be determined using AFM in both the contact and the tapping mode in air, and by the contact mode in liquid [18-20]. The abihty to measure the size of pores by AFM can obviously be enhanced when a good image is produced. In this field, the school of Bowen, Swansea, UK, has made a remarkable achievement. 

The following paragraphs will be devoted to the methods for determination of the pore size distributions of membranes. As previously mentioned, these materials ean be divided into two groups (presenee ofmieropores or mesopores). Flowever, as a first step we will eonsider the determination of the total adsorption surfaee of these materials by gas adsorption, not only beeause this parameter is an important straetural eharaeteristie, but also due to the relevanee of the models used in its determination as the basis of some of the usual methods to ealeulate pore size distributions. 

W.B. Krantz, A.R. Greenberg, E. Kujundzic, A. Yeo, S.S. Hosseini, Evapoporometry A novel technique for determining the pore-size distribution of membranes, Journal of Membrane Science, 438 (2013) 153-166. 

C. S. Kong, D.-Y. Kim, H.-K. Lee, Y.-G. Shul, and T.-H. Lee. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells. Journal of Power Sources 108 (2002) 185-191. 

Figure 3A Pore size distribution of a four-layered alumina membrane (Hsieh, Bhave and Fleming 1988).

Hgure 3.9. Dynamic pore size distributions of (a) two sol-gel alumina membranes and (b) two anodized alumina membranes (Fain 1990). 

Particles smaller than the largest pores, but larger than the smallest pores are partially rejected, according to the pore size distribution of the membrane. Particles much smaller than the smallest pores will pass through the membrane. Thus, separation of solutes by microporous membranes is mainly a function of molecular size and pore size distribution. In general, only molecules that differ considerably in size can be separated effectively by microporous membranes, for example, in ultrafiltration and microfiltration. 

Table 1 summarises the most important results from the investigation of metal doping. In this table the results of MAP treatment are combined with effects of firing temperature and doping. As can be seen in Table 1, y-alumina membranes with pore radii as low as 2.0 nm (Kelvin radius) may be obtained after firing at 600°C. Note that an instrumental standard error of 0.5 nm (90% reliability) is common in permporometry. This technique should therefore only be used for comparison purposes and to obtain a qualitative impression of the pore-size and pore-size distribution of the material under investigation. 

Pore size distribution of a SASRA-treated y-alumina membrane. The support was treated with 5 mol-% MAP (MAP 10). The y-alumina was doped with 6 mol-% La and sintered at 1000°C for three hours. 

The properties of the membranes prepared by sol-gel processes are crucially dependent on the control of the characteristics of the particles contained in the sol. The homogeneity of the particle size in the sol determines the pore size distribution of the membrane. 

Figure 6.15. Influence of GDL pore-former content on cell performance of a H2/02 single cell (0) 0 mg/cm2, (o) 3 mg/cm2, ( ) 5 mg/cm2, (A) 7 mg/cm2, and (V) 10 mg/cm2 pore-former loading 5 mg/cm2 carbon loading in the GDL and 0.4 mg Pt/cm2 in the catalyst layer [15]. (Reprinted from Journal of Power Sources, 108(1-2), Kong CS, Kim DY, Lee HK, Shul YG, Lee TH. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells, 185-91, 2002, with permission from Elsevier and the authors.)...

The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 

Figure 4.10 Pore size distribution of an alumina membrane by mercury porosimetry...

Figure 4.11 Differential pore size distribution of a Y-AI2O3 membrane by the nitrogen adsorption/desorption method [Anderson etal., 1988]...

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