Colloidal dispersion additive

Chalcogenides of Cd are similar to those of Zn and display the same duality in their structures. The sulfide and selenide are more stable in the hexagonal form whereas the telluride is more stable in the cubic form. CdS is the most important compound of cadmium and, by addition of CdSe, ZnS, HgS, etc., it yields thermally stable pigments of brilliant colours from pale yellow to deep red, while colloidal dispersions are used to colour transparent glasses. 

Herein we briefly mention historical aspects on preparation of monometallic or bimetallic nanoparticles as science. In 1857, Faraday prepared dispersion solution of Au colloids by chemical reduction of aqueous solution of Au(III) ions with phosphorous [6]. One hundred and thirty-one years later, in 1988, Thomas confirmed that the colloids were composed of Au nanoparticles with 3-30 nm in particle size by means of electron microscope [7]. In 1941, Rampino and Nord prepared colloidal dispersion of Pd by reduction with hydrogen, protected the colloids by addition of synthetic pol5mer like polyvinylalcohol, applied to the catalysts for the first time [8-10]. In 1951, Turkevich et al. [11] reported an important paper on preparation method of Au nanoparticles. They prepared aqueous dispersions of Au nanoparticles by reducing Au(III) with phosphorous or carbon monoxide (CO), and characterized the nanoparticles by electron microscopy. They also prepared Au nanoparticles with quite narrow... 

Principally purification and characterization methods of monometallic nanoparticles are directly applied to those of bimetallic nanoparticles. Purification of metal nanoparticles dispersed in solution is not so easy. So, in classical colloid chemistry, contamination is carefully avoided. For example, people used pure water, distilled three times, and glass vessels, cleaned by steam, for preparation of colloidal dispersions. In addition, the reagents which could not byproduce contaminates were used for the preparation. Recently, however, various kinds of reagents were used for the reaction and protection. Thus, the special purification is often required especially when the nanoparticles are prepared by chemical methods. 

Monitoring the pH value during the preparation of gold sol, which leads to the below reported results, it has been observed that pH moves from ca. 3.2, before NaBH4 addition, to ca. 6.9, after NaBH4 addition. In this section a discussion of the influence of the initial pH value on the properties of the colloidal dispersion stabilized by a large amount (PVA/Au = 0.67) or a low amount (PVA/ Au = 0.05) of stabilizer is presented. Proper amounts of HCl or NaOH were used to produce the reported pH values. 

A further improvement in flame-retardant efficiency is observed when a colloidal dispersion of tin(IV) oxide is incorporated into the polyester. At a 1.5% addition level, colloidal SnO gives an 01 value (49.0) which is markedly higher than that obtained with 5% loadings of either anhydrous SnO, (47.7) or 8-stannic acid (47.9), as powdered additives (Figure 2). In addition to its increased flame-retardant ability, colloidal Sn02 offers the further advantages of translucency in the cured plastic, ease of incorporation and nonsettling in the resin prior to cure. 

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... 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

Biodegradable polyester-based nanoparticles have also been studied, especially in the biomedical domain. Like microelectronics, biomedical research follows the rule smaller is better . A typical example of nanoparticles based on the aliphatic polyester engineering by living ROP is provided by the poly(CL-h-GA) copolymers which form stable colloidal dispersions in organic solvents such as toluene and THF without the need of any additional surfactant [27]. The poly(CL-h-GA) particles form a new class of stable non-aqueous dispersions in... 

One-Step Activation Process. In a one-step activation process, the sensitizing and nucleating solutions are combined into one solution. It is assumed that when this solution is made up, it contains various Sn-Pd chloride complexes (24). These complexes may subsequently transform into colloidal particles of metallic Pd or a metallic alloy (Sn/Pd) to form a colloidal dispersion (19,28). This dispersion is unstable. It may be stabilized by addition of an excess of Sn ions. In this case, Pd particles adsorbed on the nonconductor surface are surrounded by Sn ions. The latter must be removed by solubilizing before electroless plating so that the catalytic Pd on the surface will become exposed, freely available, to subsequent plating. An example of such a solubilizing solution is a mixture of fluoroboric and oxalic acids in a dilute solution, or just plain NaOH or HCl. 

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

Next, we consider another difference between lyophobic and lyophilic colloids in addition to the presence or absence of surfaces between the continuous and dispersed species. This difference deals with the stability of the dispersion, and we examine the meaning(s) of this term in more detail below. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Emulsions are colloidal dispersions of liquid droplets in another liquid phase, sometimes stabilized by surface active agents. Emulsions thus consist of a discontinuous phase, dispersed in a continuous phase. The most common types of emulsions are water-in-oil (W/O) in which oil is the continuous phase, and oil-in-water (OAV) in which water forms the continuous phase. However, this traditional definition of an emulsion is too narrow to include most food emulsions. For example, in foods the dispersed phase may be partially solidified, as in dairy products or the continuous phase may contain crystalline material, as in ice cream. It may also be a gel, as in several desserts. In addition to this, air bubbles may have been incorporated to produce the desired texture. 

Suspensions and colloidal dispersions differ from true solutions in that they are systems with more than one phase. This means that the substances present do not mix very well. The system is said to be heterogeneous and is characterized by interfaces between the phases, for instance between the water and a clay particle in muddy water. However, true solutions are one-phase systems and as a result homogeneous. In addition, they differ because in suspensions and dispersions the solid phase can be separated by means of filtration. 

In certain cases, colloidal dispersions are made more sensitive to aggregation by the addition of small quantities of materials which, if used in larger amounts, would act as stabilising agents. Several factors may contribute to such observations ... 

In the early work of Schulze ( 0, Linder and Picton (2) and Hardy (3) the sensitivity of colloidal dispersions to the addition of electrolytes was clearly demonstrated. Then in 1900 Hardy (4) showed that the stability of sols was connected with the electrophoretic mobility of the particles and he demonstrated, i) that the valency of the ion opposite in charge to that of the sol particles determined the ability of an electrolyte to coagulate a sol and that, ii) the effectiveness of the electrolyte increased rapidly with increase in valency of the counter-ion. These observations formed the basis of the so-called Schulze-Hardy rule. 

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