Pore entrance

Capillarity. The outer surface of porous material has pore entrances of various sizes. As surface Hquid is evaporated during constant rate drying, a meniscus forms across each pore entrance and interfacial forces are set up between the Hquid and material. These forces may draw Hquid from the interior to the surface. The tendency of Hquid to rise in porous material is caused pardy by Hquid surface tension. Surface tension is defined as the work needed to increase a Hquid s surface area by one square meter and has the units J/m. The pressure increase caused by surface tension is related to pore size ... 

To reduce further the pore size and/or to introduce specific interactions between the solid surface and the liquid or gaseous medium in the pores, sol-gel layers need to be modified. In principle this is done by precipitation or by adsorption of components from a gaseous or liquid medium followed by heat treatment of the formed products inside the pores or the pore entrance. This will be further discussed in Section 2.7. 

This type of coke formation occurs in the reaction zone and leads to coverage of the zeolite crystallites and closure of the pore entrances. 

Our results show that there is a clear difference between the sorption of /ert-butyl alcohol by the microporous phenyl-modified silica compared with the sorption of w-butyl alcohol. Comparison of total pore volumes and monolayer capacities with those for other sorptives leads us to conclude that sorption of the straight-chained isomer is significantly more sterically hindered than its branched analogue. This may be explained by the difference in the shapes of the two molecules. The spherical ten-butyl alcohol molecule is able to easily penetrate into the micropores of the unswollen sample without blocking the pore-entrance, while the straight-chain w-butyl alcohol molecule may block the pores upon adsorption. 

The terms 5 and M are, respectively, the number of metal sulfide molecules per organometallic compound and the molecular weight of the deposit. The rate of deposit formation is highest at the pore entrance where C is largest. The intrinsic activity of the surface was assumed to be unaffected... 

A membrane is usually seen as a selective barrier that is able to be permeated by some species present into a feed while rejecting the others. This concept is the basis of all traditional membrane operations, such as microfiltration, ultrafiltration, nanofil-tration, reverse osmosis, pervaporation, gas separation. On the contrary, membrane contactors do not allow the achievement of a separation of species thanks to the selectivity of the membrane, and they use microporous membranes only as a mean for keeping in contact two phases. The interface is established at the pore mouths and the transport of species from/to a phase occurs by simple diffusion through the membrane pores. In order to work with a constant interfacial area, it is important to carefully control the operating pressures of the two phases. Usually, the phase that does not penetrate into the pores must be kept at higher pressure than the other phase (Figure 20.1a and b). When the membrane is hydrophobic, polar phases can not go into the pores, whereas, if it is hydrophilic, the nonpolar/gas phase remains blocked at the pores entrance [1, 2]. 

External fouling is caused by the formation of a cake layer of cells or other materials on the membrane surface, leading to a reduction in permeate flux (defined as the volume of permeate produced per time and membrane area). Internal fouling is caused mainly by proteins and particles smaller than membrane pores. Proteins and protein aggregates can adsorb or deposit at the pore entrance or inside the pores and cause pore blockage or narrowing, leading to increased hydraulic resistance (2). 

Mesoporous silicas are characterized by surface areas 10 to 20 times larger than that of zeolites, typically around 1000 cm2 g. The pores can be adjusted in a range between 2 and 30 nm, and the pore volumes range between 0.5 and 2.5 cm3 g . The mesopore sizes and volumes are ideal for chemical reactions to take place in their interior, therefore the ability to block the pore entrances with stimuli responsive gates enables a control in chemical reactivity suitable for a variety of applications, including selective sensing, catalysis, and delivery, among others. 

The equilibration time for the adsorption in some microporous materials, like CMS and carbonized chars, may be extremely long that may be a source of error for the evaluation of microporosity. For example, this occurs for N2 at 77 K in samples with narrow microporosity (size below 0.7 nm), where the size of the adsorbate molecule is similar to the size of the pore entrance. In this case, contrary to the exothermic nature of the adsorption process, an increase in the temperature of adsorption leads to an increase in the amount adsorbed. In this so-called activated diffusion process, the molecules will have insufficient kinetic energy, and the number of molecules entering the pores during the adsorption equilibrium time will increase with temperature [9,23],... 

Diffusion of the products from the catalyst surface to the pore entrance... 

The complex isomerization A= B=C in a 21x21 network is simulated using the Monte Carlo method, and the effects of the frequency of the pore entrance deactivation on the product selectivity and effectiveness of catalysts for different values of Thile modulus are investigated. The parameters employed in the simulations are listed in Table 1. 

The values of the product selectivity and effectiveness, vary with the fraction of deactivated pore entrance (x), but also show some dependence on the particular configuration of the deactivated grid, i.e, the exact location of the deactivated pore... 

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