Metal electrostatic stabilization

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

Divalent Co substitution in copper amine oxidase revealed 19% of the native specific activity (for MeNH2) and 75% of the native reactivity toward phenylhydrazine. The major cause of this was a 68-fold increase in Km for 02. These investigations support the idea that electrons flow directly to bound 02 without the need for a prior metal reduction and that the Cu does not redox cycle but simply provides electrostatic stabilization during reduction of 02 to 02-. 1211... 

Small metal particles are unstable with respect to agglomeration to the bulk. At short interparticle distances, two particles would be attracted to each other by van der Waals forces and, in the absence of repulsive forces to counteract this attraction, an unprotected sol would coagulate. To counteract this, stabilization can be achieved in two ways electrostatic stabilization and steric stabilization. 

Scheme 9.1 Schematic representation of electrostatic stabilization a coulombic repulsion between metal colloid particles.

Parameter B (electrostatic term) reflects the difference between the abilities of L and L to electrostatically stabilize the metal d orbitals (difference between the net electron-acceptor characters of these ligands), whereas parameter C (bonding term) is determined by the difference between the abilities of L... 

The composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective maimer. Very stable covalent metal-phosphorus or metal sulfur bonds lead to such strong ligand shells that in some cases the protected particles can even be isolated in solid state, which can never be done with electrostatically stabilized particles. The chemical nature of the protecting ligand molecules is responsible for the solubility of the particles. Thus, the use of organic solvents has become very useful for several reasons. Figme 3 shows a sketch of the three types of steric stabihzations of metal nanoparticles. 

Figure 2 Electrostatic stabilization of metal nanoparticles. Repulsive electrostatic forces outweigh attractive van der Waals forces...

Figure 9 (a) Electrostatic stabilization of nanostructured metal colloids, (b) Steric... 

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