Coating internal pore surfaces

Deposit quantity and shape can be varied in wide ranges. It is possible to recognize the following approaches doping, filling or coating of the internal pore surface, deposition of layer covering the external surface of PS with or without permeation into pores, or formation of composites (Fig. 2). 

By coating the internal pore surfaces of MCM-41 with functionalized self-assembled monolayers it is possible to impart excellent chemical selectivity for specific toxic metallic species (see Figure 2). For example, installation of a thiol-terminated monolayer provides unprecedented mercury sequestering capability [7,8]. Thiol-SAMMS also is effective for removing other soft heavy metals, such as Cd, Ag ami Au. In fact, the metallated thiols themselves (e.g. Hg and Ag) are also excellent sorbents for sequestering soft anions (e.g. radioiodide) as weU [9]. 

The sol-gel derived ceramic granular particles described above may be directly used as the adsorbents for certain applications. More often, however, these granular particles are used as the support body on which an active species is coated. The active species coated on the internal pore surface of the support body provides active sites for preferential adsorption of selected gas and liquid species. Because of the unique coating process and microstructure of the sol-gel derived support body, the sol-gel derived supported adsorbents are expected to exhibit better properties than similar adsorbents prepared by the conventional methods. This section describes synthesis and adsorption properties of the sol-gel derived granular adsorbents. 

Coating a metal or oxide species on the surface of adsorbent supports have been traditionally accomplished by the solid dispersion method [68] and wet-impregnation method [65, 69]. The first method is done by mixing physically the support body and solid active specie followed by heat-treatment to disperse the active species on the internal pore surface of the support. In the second method, the support body is brought in contact with a liquid solution containing the precursor of the active species. The precursor is transported into the pores of the support by capillary force. Solvent is removed by drying. The adsorbent is then calcined under specific atmosphere to convert the precursor to the desired active species. 

Researchers found that zeolite particles can be treated with high-molecular-weight surfactants [such as hexadecyltrimethylammonium (HDTMA)]. HDTMA does not penetrate into the internal pore structure of the zeolite, but coats the outer surface. The outer surface then develops hydrophobic anionic exchange properties, while the inner surface retains the capacity to adsorb cations. Researchers claim that SMZ can be used for the three major classes of water contaminants inorganic cations, inorganic anions, and nonpolar organics. 

Thin-layer MIP composite membranes (cf. Section III.E, see Table 2) had been prepared by surface functionalization of various commercial porous membranes, which had already been optimized towards high performance in microfiltration. These membranes have also a moderate specific surface area (e.g., 0.22 pm PVDF 4m /g, 0.2 pm PP 20m /g). By photo-initiated graft copolymerization [81] or cross-linking polymerization [76], the entire internal pore structure could be coated evenly with thin MIP layers without formation of agglomerates. Most important, at suited degrees of functionalization, no pore blocking occurred as indicated by the preserved high membrane permeability and specific surface area [81]. 

Internal pores in SiC/RBSN composites are unavoidable and reduce their oxidation resistance and thermal conductivity. Functionally graded oxidation resistant surface coatings appear to avoid internal oxidation problems for unstressed conditions. 

It is believed that in the presence of dampproofing admixtures, the surfaces of the concrete, and the internal surfaces of the pores become coated with either a layer of molecules in the case of stearic acid and other fatty acids (Fig. 4.5b) or a layer of coalesced or separate particles of material in the case of waxes and bitumens, etc. (Fig. 4.5c). The end result in both cases is the production of hydrophobic surfaces exhibiting high contact angles to water, as shown in Fig. 4.6. 

Organically modified MCM-41 can be prepared directly by using alkoxysilanes or organosiloxanes in the synthesis mixture thus coating the internal wall of the pores with functional groups. An example of a condensation reaction of an alcohol with the surface silanol groups to modify the pore wall is shown in Figure 7.22. 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

In order to facilitate the coating of a homogeneous thin layer on the support, the pore size must be adapted to the grain size of the layer which is to be deposited. The presence of large pores on the internal surface of channels could lead to penetration of the grains into the support and thus to defects in the membrane. The density of the support must be sufficient to ensure an excellent mechanical resistance. However, a low density shows a resistance to the flux through the support, so a compromise must be found for the choice of grain size. 

Hybrid models are usuaDy used to study defective surfaces. In these models the internal surfaces of a slit pore, defined by a stack of mean-field layers, are coated by one or more graphene layers with explicit atomic structure. To generate defects one or more carbon atoms are removed from one or more explicit graphene layers [19, 20]. 

The solid phase is a porous silica particle with a pore diameter of 80 A. The internal surface of the pores are coated with reversed-phase moieties, while the exterior of the particle is coated with a polar group. Interferences such as proteins cannot pass through the small pores, do not interact with the polar exterior of the particles, and pass through the sorbent nonretained. Smaller analytes of interest enter the interior of the particle where they are sorbed by the reversed-phase sorbent (Fig. 12.6). 

The size and shape of the void spaces within the zeolite depends on the particular material selected. Figure 6.17 shows a representative structure of this type. This structure gives the material an enormous internal surface area (typically hundreds of square meters per gram), access to which is restricted by the size of the apertures between the pores (typically 0.3-0.8 nm in diameter). Zeolites have been coated onto surfaces in a number of ways. Probably the simplest approach... 

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