Polymers stabilizing colloids

A similar polymer-stabilized colloidal system is described by James and coworkers [66]. Rhodium colloids are obtained by reducing RhCls, 3H2O with ethanol in the presence of PVP. The monophasic hydrogenation of various substrates such as benzyl acetone and 4-propylphenol and benzene derivatives was performed under mild conditions (25 °C and 1 bar H2). The nanoparticles are poorly characterized and benzyl acetone is reduced with 50 TTO in 43 h. 

As a final example of catalytic hydrogenation activity with polymer-stabilized colloids, the studies of Cohen et al. should be mentioned [53]. Palladium nanoclusters were synthesized within microphase-separated diblock copolymer films. The organometallic repeat-units contained in the polymer were reduced by exposing the films to hydrogen at 100 °C, leading to the formation of nearly monodisperse Pd nanoclusters that were active in the gas phase hydrogenation of butadiene. 

P. Omarjee Direct Force Measurements Between Polymer Stabilized Colloids. Ph.D thesis, Bordeaux I University (1999). 

Sample preparation for TEM analysis of colloidal metals is usually a simple procedure, involving the evaporation of a drop of suitably diluted colloid suspension onto a microscope grid. For polymer stabilized colloids, the polymer forms a thin transparent film by this procedure. Thin films can be alternatively prepared by ultramicrotomography of thick polymer/metal samples produced by evaporation of the liquid suspension. 

FIG. 4 Sterically stabilized colloidal particles are coated with short polymer brushes. A hard sphere-like interaction arises. 

Other applications. Displacement must also have its impact on colloidal stability. The relation between the adsorption of polymers on colloidal particles and the resultant steric stability... 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

Akashi and coworkers prepared small platinum nanoparticles by ethanol reduction of PtCl in the presence of various vinyl polymers with amide side chains [49]. These authors studied the effects of molecular weight and molar ratio [monomeric unit]/[Pt] on the particle sizes and size distributions by electron microscopy, and in some cases by the dispersion stability of the Pt colloids. The hydrogenation in aqueous phase of allyl alcohol was used as a model reaction to examine the change in catalytic activity of polymer-stabilized Pt colloids upon addition of Na2S04 to the reaction solution. The catalytic tests were performed in water or in Na2S04 aqueous solution at 25 °C under atmospheric pressure of... 

A. Mayer et al. examined poly(dimethylsiloxane)-bIock-poly(ethylene oxide) (PDMS-b-PEO), poly(styrene)-f)Iock-poly(ethylene oxide) (PS-b-PEO), polystyrene-block-poly(methacryhc acid) (PS-b-PMAA) as amphiphihc block copolymers with regard to their properties in stabilizing colloidal metal nanoparticles [37, 49]. All three polymers are successfully used to stabihze various transition metal coUoids... 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

The main advantages in using polymers for colloidal stability are... 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

Synthetic polymers stabilize metal colloids as important catalysts for multi-electron reactions. Polynuclear metal complexes are also efficient catalysts for multielectron processes allowing water photolysis. 

Dickinson, E. (1993). Protein-polysaccharide interactions. In Dickinson, E., Walstra, P. (Eds). Food Colloids and Polymers Stability and Mechanical Properties, Cambridge, UK Royal Society of Chemistry, pp. 77-93. 

The role of polymers on colloid stability is considerably more complicated than electrostatic stability due to low molecular weight electrolytes considered in Chapter 11. First, if the added polymer moieties are polyelectrolytes, then we clearly have a combination of electrostatic effects as well as effects that arise solely from the polymeric nature of the additive this combined effect is referred to as electrosteric stabilization. Even in the case of nonionic... 

Napper, D. H., Polymeric Stabilization of Colloidal Dispersions, Academic Press, London, 1983. (Graduate-level monograph. An advanced and in-depth treatment of the role of polymers in colloid stability.)... 

Bouldin M, Kulicke W-M, Kehler H (1988) A prediction of the non-Newtonian viscosity and shear stability of polymer solutions Colloid Polym Sci 266 793... 

The number of papers dealing with catalysis by Au was more or less than 5 a year in the 1980s but reached 700 in 2005 and 600 in 2006. There are three major streams in current research activities on Au catalysts expansion of applications, especially to liquid-phase organic reactions [4], discussion on the active states of Au [5], and exploration of new forms of Au catalysts. The last stream has emerged recently and is represented by Au submicron tube [6], nanoporous Au [7, 8], polymer stabilized Au colloids [9] and Au on solid polymers [10, 11], which in turn provide valuable information for determining what states of Au are surprisingly active and selective. 

Both groups claimed that the mechanism of nanoparticle stabilization of blue phases is similar to that reported for polymer stabilized blue phases [412], with the nanoparticles accumulating or being trapped in the lattice disclinations, which finds support from numerical modeling of colloidal particles in blue phases recently described by Zumer and co-workers [429]. 

Proteins are both colloids and polymers. Therefore, attempts have been made to understand the phenomenon of protein aggregation with the help of models from the polymer and colloid fields such as DLVO theory, describing the stability of colloidal particles, or phase behavior and attraction-repulsion models from polymers (De Young, 1993). For faster progress, more phase diagrams for equilibrium protein precipitation, in both the crystalline and the non-crystalline state, as well as more data on observations of defined protein oligomers or polymers, are required. 

GREGORY, J., Effects of polymers on colloid stability , in Scientific Basis of Flocculation (see reference 100), pp. 101-130... 

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