Surface models, silica surfaces

An unprecedented functional Yb silsesquioxane was obtained as outlined in Scheme 29.12. Trisilanol 1 was lithiated in situ using an excess of LiN(SiMe3)2, followed by treatment with YbCfi. In bimetallic 44, a reactive ytterbium bis(trimethylsilyl)amide unit resides on a model silica surface formed by two lithium-linked silsesquioxane cages. 

Figure 6.1 Retention of pyridine and phenol on a model silica surface, (a) The model silanol phase, (b) the siloxane phase, (c) the ionized silicon oxide phase, (d) adsorption of pyridine on a silanol phase, and (e) adsorption of phenol on a silanol phase. White, light-gray, gray, and black balls represent hydrogen, nitrogen, silicon, and ojiygen, respectively. Reproduced by permission of Springer, ref. 22.

FIG. 26 Surface-charge density as a function of pH as calculated with a purely difiuse-layer model (silica surface) for different particle geometries and fixed specific surface areas of (a) 100 m /g and (b)... 

Advances have been made in directly measuring the forces between two surfaces using freshly cleaved mica surfaces mounted on supports (15), and silica spheres in place of the sharp tip of an atomic force microscopy probe (16). These measurements can be directly related to theoretical models of surface forces. 

Figure 8 (a) Schematic diagram showing distribution of fillers in different parts of anionic elastomer [27]. (b) Proposed structural model showing interaction of silanol groups on silica surface with carboxylale groups [27]. 

The use of FOSS polyhedra as models for silica surfaces or as secondary building units in inorganic materials such as zeolites or other porous solids is likely to increase rapidly as more is understood about the mechanisms by which the polyhedra may be constructed. It will be of particular interest to see if the larger structures such as TeoHeo or T240H240 or their derivatives (Section VII.C) and analogous to carbon structures such as Cgo or nanotubes, can be prepared. 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

Figure 4.16 A model for the surface of silica gel in equilibrium with a mobile phase of (55 40 5 methanol-water-0.2 M potassium phosphate buffer (pH 7.5) with the addition ot 2.5 aM of cetyltrimethylammonium bromide. (Reproduced with permission from ref. 279. Copyright pergamon Journals Ltd).

The use of small polyhedral silsesquioxanes as molecular models of silica surfaces has been studied increasingly in recent years, because catalysts, etc., attached to such species are relatively easy to handle and characterize when compared to bulk silica. The preparation and structures of silsesquioxanes used in this type of work are described later in Sections II,C, IV,E, and IV,F. Further recent examples may be found in Ref. 5. 

Nucleic acids, DNA and RNA, are attractive biopolymers that can be used for biomedical applications [175,176], nanostructure fabrication [177,178], computing [179,180], and materials for electron-conduction [181,182]. Immobilization of DNA and RNA in well-defined nanostructures would be one of the most unique subjects in current nanotechnology. Unfortunately, a silica surface cannot usually adsorb duplex DNA in aqueous solution due to the electrostatic repulsion between the silica surface and polyanionic DNA. However, Fujiwara et al. recently found that duplex DNA in protonated phosphoric acid form can adsorb on mesoporous silicates, even in low-salt aqueous solution [183]. The DNA adsorption behavior depended much on the pore size of the mesoporous silica. Plausible models of DNA accommodation in mesopore silica channels are depicted in Figure 4.20. Inclusion of duplex DNA in mesoporous silicates with larger pores, around 3.8 nm diameter, would be accompanied by the formation of four water monolayers on the silica surface of the mesoporous inner channel (Figure 4.20A), where sufficient quantities of Si—OH groups remained after solvent extraction of the template (not by calcination). 

Figure 9.26 Cr 2p spectra and negative SIMS spectra of two model catalysts and a blank reference. The blank (bottom) shows only Si,0, fragments on a chromium-loaded catalyst CriCf fragments appear after thermal activation (Ar/02). If desorption of chromium is made impossible (in oxygen-free argon), Cr2Or clusters can also be detected. In combination, this is strong evidence that chromate anchors to the silica surface as a monomer (courtesy of P.C. Thiine and R. Linke, Eindhoven).

Figure 9.28 RBS proves that chromate can indeed desorb from the model catalyst during thermal activation. A wafer with 10 Cr/nm2 loading features 7.0 Cr/nm2 after calcination at 550 °C. The desorbing chromate readily readsorbs on an empty silica surface placed opposite to the loaded wafer (courtesy of...

The incompletely condensed silsesquioxanes can be considered as intermediates in the synthesis of the whole family of silsesquioxanes, but at the same time they are the building blocks (e.g., initiators) for the new polymeric architectures.66 67 The importance of this class of materials in model studies of silica surfaces and silicate defects,68 and in creating various oligosilanols with novel frameworks,69 70 makes them the subject of extensive research.71 The use of the incompletely condensed silsesquioxanes in forming dimeric silsesquioxanes and metallasilsesquioxanes in attempts to design new routes to the well-defined Si-O assemblies has recently been reviewed in a well-illustrated research report.72... 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

We also examined oxygen quenching of Ru(II) complexes bound to a silica surface—a potentially useful sensor support. 33 Figure 4.9 shows typical quenching curves. We explore the modeling of these systems later. 

Controlled hydrolysis of RSiX3 compounds gives so-called silsesquioxanes or POSS compounds (Polyhedral Oligomeric SilSesquioxane), which can be used as models for silica surfaces or supports for catalysts [4] (Figure 18.2, schematic structure on the right). 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

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