Metal colloid particles, electrostatic stabilization

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

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

Controlled hydrolysis is one of the most popular methods for processing silica spheres in the range of 10-1,000 nm. The method was developed by Stober, Fink, and Bohn (SFB) [226-229] and is based on the hydrolysis of TEOS in a basic solution of water and alcohol. Particle size depends on the reactant concentration, i.e., the TEOS/alcohol ratio, water concentration, and pH (>7). This method has been extended to other metal oxide systems with similar success, particularly for Ti02 synthesis [85,230]. The hydrous oxide particles precipitated by the hydrolysis of an alkoxide compound have the same tendency to agglomerate as that described for metal colloid systems. Different stabilizers can be used to stabilize these particles and prevent coagulation (step 2). These stabilizers control coagulation by electrostatic repulsion or by steric effects [44], similarly to the metal colloid systems. 

Electrostatic stabilization of metal colloid particles. Attractive van der Waals forces are outweighed by repulsive electrostatic forces between adsorbed ions and associated counterions at moderate interparticle separation (Schmid, 2006). 

The technique of alternating polyelectrolyte film construction has also been adapted to incorporate semiconductors into layered films. For example, multilayer films have been constructed by alternately dipping a quartz substrate into a solution of poly(diallylmethylammonium chloride) and then a solution of a stabilized CdS or PbS colloid (41). The layer-by-layer self-assembly of alternating polymer and metal sulfide is at least partially driven by the electrostatic attraction of the cationic polymer and the negative charge of the stabilized MC colloid particles. 

Coagulation-Precipitation The nature of an industrial wastewater is often such that conventional physical treatment methods will not provide an adequate level of treatment. Particularly, ordinary settling or flotation processes will not remove ultrafine colloidal particles and metal ions. In these instances, natural stabilizing forces (such as electrostatic repulsion and physical separation) predominate over the natural aggregating forces and mechanisms, namely, van der Waals forces and Brownian motion, which tend to cause particle contact. Therefore, to adequately treat such particles in industrial wastewaters, coagulation-precipitation may be warranted. 

Only a few studies concerning the synthesis of ruthenium particles have been published, most of them reporting the use of RuCh as metal precursor. For example, stable colloidal solutions of monodisperse Ru particles can be obtained by reduction of RuCh in polyols. The stabilization is then achieved by addition of poly (vinyl) pyrrolidone (PVP) (steric stabilization) or of sodium acetate (electrostatic stabilization) [165]. 

Figure 4 Steric stabilization of colloidal metal particles by a polymer layer (left) and electrostatic stabilization of colloidal particles (right).

In this chapter we report some results for several nonionic polymers and cationic polyelectrolytes and their ability to stabilize platinum colloids. Both steric and electrostatic stabilization of the metal colloids can be combined by the use of polyelectrolytes (5). The materials have been examined by transmission electron microscopy (TEM) in order to determine the average particle size, size distribution and particle pe. The catalytic activity of these polymer-protected platinum nanoparticles has been tested by the hydrogenation of cyclohexene, d cyclooctene, and 1-hexene. 

The main role of stabilizers (surfactants or polymers) is to provide a steric or an electrostatic barrier between particles, thereby preventing inhibition of aggregation. Furthermore, stabilizers play an essential role in the control of both size and shape of nanoparticles. Generally, polymers are recommended as stabilizers for metal colloids due to their transparent, permeable, and nonconductive properties and also because they do not influence the optical, electrical, and catalytic properties of the nanoparticles. In addition, investigation of polymer-stabilized MNPs appears as a suitable way for solving the stability of MNPs. For this reason, great attention has been focused on the incorporation of MNPs into a polymer matrix, a procedure based on the synthesis of nanometer-sized metallic filler particles (Giannazzo et al. 2011). 

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