Colloidal systems electron microscopy

A new development is biphasic hydrogenation using solvent-stabilized colloid (SSCs) catalysts [39-41]. Palladium colloid systems, especially, were proven to give high reactivity and selectivity. Best solvents are dimethylformamide and particularly the two cyclic carbonic acid esters, ethylene carbonate and 1,2-propene carbonate. In these solvents sodium tetrachloropalladate - stabilized by a sodium carbonate buffer - is reduced with hydrogen to yield the solvent-stabilized palladium colloid. Transmission electron microscopy of the palladium colloid demonstrates that the colloid particles are spherical with an average diameter of 4 nm. 

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. 

Two techniques for overcoming the limitations of optical microscopy are of particular value in the study of colloidal systems. They are electron microscopy36-37, in which the limit of resolution is greatly extended, and dark-field microscopy, in which the minimum observable contrast is greatly reduced. 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

Two main techniques have been used to determine the particle size distribution of colloidal systems PCS and electron microscopy including both SEM and TEM. The QELS technique for Brownian moment measurement, offers an accurate procedure for measuring the size distribution of nanoparticles. The PCS technique does not require any particular preparation for analysis and is excellent due to its efficiency and accuracy. However, its dependency on the Brownian movement of particles in a suspended medium may affect the particle size determination. 

We further demonstrated the importance of this repulsive electrostatic interaction by using colloidal particles whose surface were terminated in the amine group (-NH2). For this system, we could control the sign of interaction between the colloidal particles and template surface by changing the pH value of dispersion medium. Figure 11.3 shows the scanning electron microscopy (SEM) images of two samples that were prepared from colloidal dispersions with their pH values adjusted to 6.5 and 8.5, respectively. At pH = 6.5, there existed an attractive interaction between the positively... 

The typical results will be illustrated by one LC and one IC colloid. An example of an LC system (29) with background conductivities below 10 n "1 m-l will be carbon black dispersed in aliphatic hydrocarbon with polymeric materials and charged with a metal salt of stearic acid. Electron microscopy revealed that the particles are agglomerates in the... 

We have chosen hematite oxalate as a model system, since the photochemical properties of colloidal hematite (Stramel and Thomas, 1986) and the photochemistry of iron(III) oxalato complexes in solution (Parker and Hatchard, 1959) have been studied extensively. The experiments presented in this section were carried out as batch experiments with monodispersed suspensions of hematite (diameter of the particles 50 and 100 nm), synthesized according to Penners and Koopal (1986) and checked by electron microscopy and X-ray diffraction. An experimental technique developed for the study of photoredox reactions with colloidal systems (Sulzberger, 1983) has been used. A pH of 3 was chosen to maximize the adsorption of oxalate at the hematite surface. This case study is described in detail by Siffert (1989) and Siffert et al. (manuscript in preparation). 

The methods of scanning and transmission electron microscopy are broadly used for the investigation of various objects of colloidal nature [40], It is worth mentioning here the technique of the preparation of replica of rapidly frozen sols, which allows one to freeze the system at a given moment of time. The surface structure can be effectively analyzed by such methods as Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), Secondary Ion Mass Spectroscopy (SIMS), and others. 

Donald A M 1998 Environmental scanning electron microscopy for the study of wet systems Curr. Op. Colloid Interface Scl. 3 143-7... 

While electron microscopy has been widely used for the characterisation of colloidal systems in conjunction with natural organics (Leppard et al (1990), Wilkinson et al (1995)), its quantitative use for... 

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