E Pore size

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

The performance of the separation system is mainly determined by kinetic aspects, e.g. in LC by physical properties of the chromatographic support material and the packed bed (particle si/e. pore size), by the flow rate, the diffusion coefficients, whereas in CE other criteria, including the applied voltage and diffusion coefficient, play a significant role. 

Characterisation of porous texture of solids is of relevance because their properties are determined, or at least, influenced by this characteristic [1-3]. A number of techniques exist to characterise the porous texture of solids. Among them, physical adsorption of gases is the most widely used due to its simplicity [1-11], N2 adsorption at 77K [3] is, undoubtedly, the most used. One of its main advantages is that it covers reduced pressures from 10 to 1, being sensitive to the whole range of porosity. However, N2 adsorption at 77K has some limitations when used to characterise solids containing ultramicroporosity (i.e., pore sizes lower than 0.7 nm). It can be influenced by diffusional limitations in this range of porosity [4]. 

Support Embedded species/loading Surface area e Pore size nm Metal size nm... 

The nitrogen adsorption isotherms of compounds synthesized with (Fig. 4B) and without (Fig. 4A) surfactant are not well defined in the way that they exhibit a linear region from p/po = 0.1 to 0.4 before reaching a plateau until a relative pressure p/po equal to 0.85. This kind of isotherm is situated between the type I, related to microporpus materials, and the type IV, characteristic of mesoporous materials. The analysis of the pore size distribution shows that these materials possess supermicropores, i.e. pore sizes centered at about 1.7 nm. According to Dubinin [13], these materials belong to super-microporous family. Super microporous materials form an... 

The mesoporous silicates MCM-41 and MCM-48 are members of the family M41S, which are of interest for adsorption and catalysis because their textural properties, i.e., pore size between 35 and 50 A and surface areas of about 1000 m /g [1-2]. These materials are being investigated as supports and catalysts for the processing of complex hydrocarbons, but they exhibit some drawbacks, such as a low hydrothermal stability and structural degradation in the presence of water. Thus, this work explores the structural reinforcement of the silicate walls of mesoporous MCM-41 materials by means of the coating with polymeric carbon, which can also modify their physicochemical properties... 

Porosity of nanoporous carbonaceous materials is usually analyzed on the basis of nitrogen adsorption isotherms, which reflect the gradual formation of a multilayer film on the pore walls followed by capillary condensation in the unfilled pore interior. The pressure-dependence of the film thickness is affected by the adsorbent surface. Hence, an accurate estimation of the pore-size distribution (i.e., pore-size analysis) requires a correction for the thickness of the film formed on the pore walls. The latter (so-called t-curve) is determined on the basis of adsorption isotherms on non-porous or macroporous adsorbents of the surface properties analogous to those for the adsorbent studied. 

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

Fig 2 1 e Pore size distnbutions of Molecular sieve zeolite 5A (A) Davidson Z-lOO (Binderless), (B) Davidson Regular. 

Figure 13.5 Droplets of aqueous dyestuff solution as employed for the TEGEWA drop penetration test on a hydrophobized PET fabric. The photographs clearly show a significant difference in the contact angles of the two droplets, which might be due to the clearly visible fabric distortion as well as local variations in microscopic features of the fabric geometry, i.e. pore sizes and capillary system.

Figure 13.7 shows the effect of the porosity (e), pore size (rfp) and thickness (S) ofthe different membranes on the distillate flux the trans-membrane flux inereased with the sdpS ratio, due to the lower membrane resistance. The highest fluxes were obtained with the membrane M4 (around 3 kg m h at 20°C and 12.5 kg m hat 40°C), while the lowest values occurred with the Ml (2.3 and 9 kg m h at 20°C and 40°C, respectively). 

Figure 1. SEM micrographs for (a) Al, (b) A2, (c) A3 and (d) A4 foams, (e) Pores size distribution obtained by SEM image analysis...

Gelb L. D. and Gubbins K. E., Pore size distributions in porous glasses a computer simulation study, Langmuir 15 (1999) 305-308. 

Figure 10.11 which shows different successive slices of the white square shown in Figure 10.10. Two successive slices are separated by 0.7 pm. In Figure 10.11, the external skin appears as a white layer. From the Z = 0 image, one can see a black line across the skin linking the exterior to the macrovoid. This black line is not present on the Z=—l and Z= + 2 images and is visible on the Z=—l and Z= +1 images. This line is the image of a pore which is considered a defect its size is within the micrometer range whereas the MWCO of the membrane (21 kDa, PEG) is low (i.e. pore sizes in the nanometer range).

Pore texture of a catalyst (i.e. pore size distribution P.S.D., total pore volume and surface of the pores) governs catalytic performance such as activity and selectivity, through diffusion of reactants and products in the pore system of the solid, density, mechanical strength and thermal stability (5,11, 12). 

---