Pore channel-like

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

Concerning the nature and structure of such amyloid peptide or protein channels, oligomers with annular morphologies have in fact been observed by EM for a-synuclein (Lashuel et al., 2002) and equine lysozyme (Malisauskas et al., 2003) even in the absence of any lipids or membranes. Channel-like structures have also been reconstituted in liposomes and observed by SFM for A/ i 4o, A/ j 42, human amylin, a-synuclein, ABri, ADan, and serum amyloid A (Fig. 5A Lin et al., 2001 Quist et al., 2005). Doughnut-shaped structures with a diameter of 10-12 nm and a central hole size of 1-2 nm (Fig. 5B) were imaged on top of lipid membranes (Quist et al., 2005). However, the radius of curvature of the SFM tips meant that it is not possible to say whether the pores were really traversing the lipid bilayer. 

Pore diameter (dp) for channel-like PMSs according to the BJH pore size distribution, which, however, underestimates the effective pore diameter by circa 1.0 nm, as shown by theoretical and geometrical calculations [48, 49] pore diameter of cage-like PMSs according to Ravikovitch and Neimark [50],... 

Mordenite has a channel-like pore structure in which the basic building blocks consist of five-membered rings. A view of the mordenite structure perpendicular to the main channels is shown in Fig. 3. 

Another early theory, which also attracted a great deal of attention, was the ink-bottle theory this was originally put forward by Kraemer (1931) and subsequently developed by McBain (1935). Kraemer pointed out that the rate of evaporation of a liquid in a relatively large pore is likely to be retarded if the only exit is through a narrow channel. This argument led Brunauer (1945) to conclude that the liquid in the pore cannot be in true equilibrium with its vapour during the desorption process and therefore it is the adsorption branch of the loop which represents thermodynamic reversibility. 

Powder X-ray diffraction patterns for calcined pure MCM-41, p-MCM41 and p-tb-MCM-41 are shown in Fig. 1. They all exhibited three peaks characteristic of the MCM-41 structure that is known to consist of hexagonal arrays of uniform channel-like pores. An intensive peak observed at 26=2.1-2.4 for these samples conesponds to the (100) diffraction peak. The XRD pattern is in very good agreement with that reported by Kresge et al. However, after MCM-41 was modified with monomer PABI and Tb, the intensity of the (100) diffraction peaks of p-MCM-41, p-tb-MCM-41 are lower and broader than that of MCM-41, indicating that the mesoporous structures of MCM-41 become less uniform upon the introduction of monomer PABI and ion. 

Based on these characteristics, porous silicon may be described as a random array of channel-like pores or etch tunnels growing in <100) directions. For the case of n-type silicon these channels are isolated from each other and, for etching in the dark, the pore spacing is approximately equal to the depletion layer width at a planar surface [83-86]. For the case of p-type silicon the channels are interconnected. The... 

Q cm in 10 M HF at 100 mA cm [94]. The primary pores are highly oriented and channel like, propagating perpendicular to the surface in the [100] direction. The pores are polygonal in cross-section and do not exhibit any obvious anisotropy. The pores are packed closely together with interpore regions of silicon on the order of 100 A and a pore density of 2xl0 °cm. ... 

Heavily doped n-type silicon channel-like pores propagate in the [100] direction. The pores are typically 10 nm or smaller in diameter and do not exhibit the characteristic anisotropy of low donor concentrations. These structures are similar to those seen in highly doped p-type silicon. 

FIGURE 43.7 Examples of porous structures produced in thin polymeric films using various methods of irradiation and chemical treatment. (Reprinted from Apel, P., Radial. Meas., 34, 559, 2001. With permission from Elsevier.) (A) Cross section of a polycarbonate membrane with cylindrical nonparallel pore channels (B) polypropylene membrane with slightly conical parallel pores (C) polyethylene terephthalate membrane with cigar-like pores and (D) polyethylene terephthalate membrane with bow-tie pores. 

Monocrystalline, macro- and mesoporous silicon were used for the electrochemical deposition of Pt. A 10 pm thick macroporous silicon layer was formed by anodizing of p-type Si wafers of 12 Ohm-cm resistivity in an aqueous solution of HF acid and DMSO (10 46 by volume parts) at the current density of 8 mA-cm [1]. Pore channels distributed with the surface density of 6T0 cm look like long straight holes with inlet diameters of 1.5 pm. An uniform 1 pm thick mesoporous silicon layer was fabricated by anodizing of n" -type Si wafers of 0.01 Ohm-cm resistivity in a solution of HF acid, water and isopropanol (1 3 1 by volume parts) at the current density of 60 mA-cm . The mesoporous silicon sample formed looks like Si layer perpendicularly pierced through by pore channels with diameter of about 20 nm. The number of pores per square centimetre is up to 2-10 [2]. 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

Fig. 11. Classification of adsorption-desorption hysteresis loops. HI are typical for materials containing agglomerates and for materials with cylindrical pore geometry and a high degree of pore size uniformity. The type H2 is characteristic of materials with relatively uniform channel-like pores with the pore connectivity effects. H3 type is attributed to aggregates (loose assemblages) of platelike particles forming slitlike pores. The type H4 loop is proposed to be attributed to large mesopores embedded in a matrix with pores of much smaller size. Reprinted with permission from [76]. Copyright (2001) American Chemical Society...

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