Direct-chemical bonding

Bioglasses are surface-active ceramics that can induce a direct chemical bond between an implant and the surrounding tissue. One example is 45S5 bioglass, which consists of 45% Si02, 6% 4.5% CaO, and 24.5% Na20. The various calcium phosphates have exceUent compatibUity with bone and... 

Chemical covalent bonding. The formation of covalent chemical bonds between elements at an interface may be an important factor. Such direct chemical bonding would greatly enhance interfacial adhesion, but specific chemical functional groups are required for the reactions to occur. 

Cement formation requires a continuous structure to be formed in situ from a large number of nuclei. Moreover, this structure must be maintained despite changes in the character of the bonds. These criteria are, obviously, more easily satisfied by a flexible random structure than by one which is highly-ordered and rigid. Crystallinity implies well-satisfied and rigidly-directed chemical bonds, exact stoichiometry and a highly ordered structure. So unless crystal growth is very slow a continuous molecular structure cannot be formed. 

Isomer shift and quadmpole splitting of salts, [Ru(C5H5)X] Y (X = Cl, Br Y = PFg and X = I, Y = I3) are larger compared to those of ruthenocene. This indicates direct chemical bonding between Ru and Cl, Br and I and that the Ru ion in each salt is in an oxidation state higher than Ru(II) in ruthenocene... 

Hench, L.L. and Paschall H.A. (1973) Direct chemical bond of bioactive glass-ceramic materials to bone and muscle. Journal of Biomedical Materials Research Symposium, 4, 25. 

The adsorption of saturated hydrocarbons on metallic substrates is typically considered as an example of a weak physical interaction, which is dominated by van der Waals forces. The classification of this type of interaction, denoted physisorption where no direct chemical bonds are formed between the adsorbate and substrate, has been based on the heat of adsorption. A physisorbed state is considered to be one in which the heat of adsorption is comparable to the heat of vaporization or... 

The pHPZC of ferric hydroxide surfaces is about 8 [127], so aqueous Pb2+ should be electrostatically repelled from these surfaces at pH values less than 8. However, as seen in Figure 7.6(a), the Pb2+ present in this aqueous solution is sorbed essentially completely to ferric hydroxide surfaces at pH 6. This behavior suggests that Pb2+ forms direct chemical bonds to these surfaces in order to overcome the repulsive electrostatic forces below the pHpzc of ferric hydroxide. This conclusion based on macroscopic uptake data has been confirmed by direct spectroscopic observation using X-ray absorption fine structure (XAFS) spectroscopy under in situ conditions (i.e., with aqueous solution in contact with a-FeOOH surfaces at ambient temperature and pressure) [133,134]. These studies showed that the aquated Pb(II) ion forms dominantly inner-sphere, bidentate complexes on a-FeOOH surfaces. 

The surface groups S with the highest free energy should be most efficient in the processes of chemical modification. In this case, one can expect the formation of a great variety of modification products (SR groups). For the compounds with the directional chemical bonds between the neighboring atoms in the main crystalline and amorphous silica modifications, these are the products of bond rupture and/or its rearrangement. 

A completely different, nevertheless intriguing concept, for the Jacketing of a macromolecular chain has been developed based on rotaxane complexes. In a rotaxane a macrocycle is thread on a linear molecule [368]. If the ends of the linear molecule get blocked by sterically demanding substituents, both molecules are permanently linked without formation of a direct chemical bond [368-370]. 

A better approach for that purpose is a ball-and-stick model, where all atoms or ions, and their relative positions, are represented [19]. (The one misleading feature of such models for ionic crystals is that the sticks which hold the balls together may be mistaken for directional chemical bonds.) Another good way to schematically show crystal structures is with juxtaposed spheres having the proper relative ionic radii [1,19], such as those in Figs. 3-5 below this method is used extensively in other chapters in this volume. 

The characteristics of QCM sensors containing mono- or multilayered DNA probe constructed by direct chemical bonding, avidin-biotin interaction or electrostatic adsorption on polyelectrolyte films were compared by Zhou et al. [60]. The use of the polyethyleneimine adhesion, glutaraldehyde cross-linking (PEI-Glu) method to immobilize hepatitis B virus DNA onto gold QCM quartz crystals, enabling the sensor to be regenerated five times, was reported by Hu and co-workers [61],... 

Solids are distinguished from liquids and gases by their rigidity. Solids retain their shape and exhibit structural strength when external forces are applied. These properties originate in the strong, directional chemical bonds between the atoms in solids. 

Physical adsorption (physisorption) occurs when an adsorptive comes into contact with a solid surface (the sorbent) [1]. These interactions are unspecific and similar to the forces that lead to the non-ideal behavior of a gas (condensation, van der Waals interactions). They include all interactive and repulsive forces (e.g., London dispersion forces and short range intermolecular repulsion) that cannot be ascribed to localized bonding. In analogy to the attractive forces in real gases, physical adsorption may be understood as an increase of concentration at the gas-solid or gas-liquid interface imder the influence of integrated van der Waals forces. Various specific interactions (e.g., dipole-induced interactions) exist when either the sorbate or the sorbent are polar, but these interactions are usually also summarized under physisorption unless a directed chemical bond is formed. 

The forces responsible for adhesion/release are intermolecular, and are usually referred to as secondary valence forces. In many cases, however, they can also be primary, that is, those of direct chemical bond. In any event, the force of separation Fs is shown to be a function of two factors work of adhesion Wa, and the distance of bond separation d. Taylor and... 

Direct measurements on the work of adhesion Wa have also been summarized by Krauss (11). Values of Wa are also calculatable with reasonable accuracy for systems free of direct chemical bond as is shown later in "Equation 3 . The separation distance d for high energy systems where Fmax exists has been estimated by McKelvey... 

When Avogadro first mooted the notion of molecule there was no evidence for molecular structure or directed chemical bonding. Molecules, not atoms, were identified as the particles that feature in the various gas laws, without reference to internal structure, and assumed to be spherical. 

In systems with the upper critical temperature, rcu, the surface tension decreases with increasing temperature, and consequently the excess of entropy within the surface layer is positive (Fig. III-l, a). In systems with the lower critical temperature, TCL, (Fig. III-l, b) the increase in interfacial tension is observed above the point at which system separates into two phases the value of tj is hence negative. The latter may serve as evidence for the existence of strong coorientation between molecules within the interfacial layer, which is due to the presence of directed chemical bonding, such as hydrogen bonding. 

Before discussing various high-nuclearity clusters and cluster compounds we want to address the general question of what can be defined as a metal cluster. To this end we consider two examples of low-nuclearity Au compounds. According to a widely used definition, a metal cluster is characterized by direct chemical bonds between metal atoms.Here, we want to correlate the metal-metal distance in the cluster with the strength of the metal-metal bond. 

It is important to realize that the valencies of the oxygens of the Si20a layer (the so-called planar oxygens) are completely satisfied, since each O atom is joined to two Si atoms, and so it cannot be joined by direct chemical bonds to any other atoms— in other words, the Si203 layer has no tendency to extend itself perpendicularly. Horizontally, however, it can extend indefinitely precisely the same applies to the planar OH groups in the gibbsite layer. 

The adsorption of anions on metal electrodes has been one of the major topics in surface electrochemistry. Specific adsorption of anions occurs when the anion loses aU or part of its solvation shell and forms a direct chemical bond with the substrate. In this situation the surface coverage by anions can be high and the adlayer tends to form a close-packed structure that depends critically on the surface atomic geometry of the underlying substrate and the balance between the anion-metal and anion-anion interaction energies. The structures of halide anions adsorbed onto Au(Jtkl), Ag(hkl), and Pt(hkl) low-index surfaces have been the most widely studied systems by SXS, and a comprehensive review of ordered anion adlayers on metal electrodes is given by Magnussen [57]. 

Besides the purely electronic effects that we have discussed up to now, the promoter can also form direct chemical bonds with the adsorbate. An example is the influence of alkali metal cations on the synthesis of methanol with copper catalysts. Sodium and potassium hydroxide can react with CO under relatively mild conditions to form alkali metal formates, which are hydrogenated to methanol by hydrogen dis-sociatively adsorbed on copper. 

Among the most important properties of hydroxyapatite as a biomaterial is its excellent biocompatibility. Hydroxyapatite appears to form a direct chemical bond with hard tissues [Piattelli andTrisi, 1994]. On implantation of hydroxyapatite particles or porous blocks in bone, new lamellar cancellous bone forms within 4 to 8 weeks [ Bajpai and Fuchs, 1985]. Scanning electron micrograph (500 x) of a set and hardened hydroxyapatite-cysteine composite is shown in Figure 39.7. The composite sets and hardens on addition of water. 

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