Methods for Preparing Colloidal Dispersions

Globally speaking, techniques for preparing hydrophobic colloids fall into two main categories, depending on the size of the initial objects  

---

The class of methods used for preparing colloidal dispersions in which precipitation from either solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. An empirical or qualitative term referring to the relative ease with which a material can be deformed or made to flow. It is a reflection of the cohesive and adhesive forces in a mixture or dispersion. See also Atterberg Limits. 

The class of mechanical methods used for preparing colloidal dispersions in which particles or droplets are progressively subdivided. See also Condensation Methods. 

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... 

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). 

Preparation of Emulsions. An emulsion is a system in which one liquid is colloidally dispersed in another (see Emulsions). The general method for preparing an oil-in-water emulsion is to combine the oil with a compatible fatty acid, such as an oleic, stearic, or rosin acid, and separately mix a proportionate quantity of an alkali, such as potassium hydroxide, with the water. The alkali solution should then be rapidly stirred to develop as much shear as possible while the oil phase is added. Use of a homogenizer to force the resulting emulsion through a fine orifice under pressure further reduces its oil particle size. Liquid oleic acid is a convenient fatty acid to use in emulsions, as it is readily miscible with most oils. 

Tachibana T, Nakamura A. A method for preparing an aqueous colloidal dispersion of organic materials by using water-soluble polymers dispersion of beta-carotene by polyvinylpyrrolidone. Kolloid-Z Polym 1965 203 130-133. 

The methods for preparation of niosomes are similar and as complicated as those used for liposomes. One of the most frequently utilized techniques consists of the hydration of a mixture of the surfactant-lipid at elevated temperature followed by optional size reduction (by sonication, extrusion, homogenization, etc.) to obtain a homogeneous colloidal dispersion and separation of the unentrapped drug [36,40,41,52,55],... 

In this chapter we describe various methods for preparing molecularly imprinted polymers (MIP) in a colloidal state. The resulting materials consist of submicron scale particles dispersed in a liquid acting as fully functional synthetic affinity receptors. The synthesis, characterization, and performance of colloidal molecularly imprinted polymers are described and applications with this new class of affinity receptors is discussed. 

Other methods for preparation of thiol-stabilized colloids have followed. Straightforward reduction of tetrachloroaurate can be performed in ethanol in the presence of an ethanol-soluble thiol329,483. Gold colloids can be prepared first by any conventional method, and then the colloid dispersion in water is mixed with the ethanol solution... 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Scheme 13.3 was then adopted by us and other different research groups also for gas-phase applications. Coprecipitation, deposition-precipitation and colloidal particles immobilisation, discussed in Section 13.2, were the preferred methods for preparing catalysts. In our case, supporting finely dispersed gold on carbon by means of metal sol immobilisation led to the discovery of an active and selective catalyst for liquid phase oxidation. 

An alcohol reduction method in which alcohol like ethanol can work both as a reductant and a solvent has been applied to the synthesis of polymer-capped bimetallic nanoclusters. They have been prepared by the simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoclusters can be prepared by refluxing... 

Dispersion, or Peptisation, of Insoluble Particles.—By mechanical disintegration in a colloid mill it is possible to render insoluble substances, such as graphite or chalk, sufficiently fine to form colloidal solutions in water, oil and other bqnids. While it is also possible by means of an electric arc formed between electrodes of a metal, immersed in water or other liquids, to disperse the metal in the form of a sol. These are industrial methods used for preparing colloidal lubricants and therapeutic agents. 

Conducting polymer nanocomposites consist of a conducting polymer or copolymer having nanofiUers in the form of particulate (OD) or fiber (ID) or flake (2D) dispersed in the conducting polymer matrix. These may be of different shape, but at least one dimension must be in the nanoscale. Nanocomposites of conducting polymers have been prepared by various methods such as colloidal dispersions, electrochemical encapsulation, coating of inorganic polymers, in situ polymerization with nanoparticles and have opened new avenues for material synthesis. 

We prepared ceria on Ni substrate by sol-gel coating method. Ceria sol solution was prepared with ceria sol solution (Alfa, 20% in H2O, colloidal dispersion) mixed with ethanol (99.9%, Hayman) with weight ratio (1 2) and stirred. Ceria was deposited on Ni substrate by dip coating method. The variation number of dipping was carried out to obtain different coating ratio. The anode was completely dipped into the ceria sol solution for several seconds and dried at a temperature of 50 C for 24 hours in air atmosphere followed by calcination at 700 C for 30 minutes in 5%H2-N2 atmosphere. 

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. 

---