Particles, colloidal kinetic properties

The significant characteristics of colloids are particle size and shape, scattering of radiation, and kinetic properties. 

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

The participation of colloidal particles in thermal motion (the entropic factor) was taken into consideration, mostly indirectly, in earlier studies dealing with the molecular-kinetic properties of disperse systems. Volmer was the first to realize the importance of the role that the thermal motion of colloidal particles played in controlling the formation and stabilization of disperse systems. However, the attempt to compare the work of surface formation and the entropic factor directly, undertaken by March, was not successful, since it was applied only to systems with high interfacial energy. 

It should be noted, however, that some typically colloidal phenomena, such as light scattering, are exhibited (though very weakly) by systems in which the microheterogeneity arises from random kinetic fluctuations in density in an otherwise uniform system of small molecules such as a gas or a liquid, while in some cases (e.g. suspensions of relatively coarse solid particles) certain colloid-like properties may persist to particle sizes much larger than the above maximum. 

This chapter will present the very important kinetic properties of coUoids, and their motion under the influence of gravitational and centrifugal fields. Kinetic properties are important for many reasons, for example they provide methods for determining the molecular weight (molar mass) of colloidal particles, their size and shape - i.e. for the characterization of colloidal particles. Many of the methods we will see in this chapter have found widespread use in the study of biological and polymeric molecules. 

We now proceed with a thorough investigation of the kinetic properties of colloidal particles. Our discussion starts with the Brownian motion (Figure 8.2). 

A discussion on kinetic properties and characterization of colloid particles would not be complete without a few words about the rheology of colloids. 

The macroscopic properties of a material are related intimately to the interactions between its constituent particles, be they atoms, ions, molecules, or colloids suspended in a solvent. Such relationships are fairly well understood for cases where the particles are present in low concentration and interparticle interactions occur primarily in isolated clusters (pairs, triplets, etc.). For example, the pressure of a low-density vapor can be accurately described by the virial expansion,1 whereas its transport coefficients can be estimated from kinetic theory.2,3 On the other hand, using microscopic information to predict the properties, and in particular the dynamics, of condensed phases such as liquids and solids remains a far more challenging task. In these states... 

Colloidal solutions are characterized by the degree of stability or instability. This is related to the fact that both kinds of properties in everyday phenomena need to be understood. The kinetics of coagulation is studied using different methods. The number of particles, Np, at a given time is dependent on the diffusion-controlled process. The rate is given by... 

Cerpa, A. Garcia-Gonzalez, M.T. Tartaj, P. Re-quena, J. Garcell, L. Serna, C.J. (1999) Mineral-content and particle-size effects on the colloidal properties of concentrated lateri-tic suspensions. Clays Clay Min. 47 515-521 Cervini-Silva, J. Sposito, G. (2002) Steady-state dissolution kinetics of aluminum-goethite in the presence of desferrioxamine-B and oxalate. Environ. Sci. Technol. 36 337-342 Cesco, S. Rdmheld.V. Varanini, Z. Pinton,... 

In discussing the mechanisms of the formation of monodispersed colloids by precipitation in homogeneous solutions, it is necessary to consider both the chemical and physical aspects of the processes involved. The former require information on the composition of all species in solution, and especially of those that directly lead to the solid phase formation, while the latter deal with the nucleation, particle growth, and/or aggregation stages of the systems under investigation. In both instances, the kinetics of these processes play an essential role in defining the properties of the final products. 

Studies on luminescence of CdS colloids provide useful knowledge on the energy and nature of recombination sites of charge carriers in the colloidal particles. The regularities of the colloid photoluminescence quenching provide the information on the dynamics of electrons and holes in semiconductor particles as well as on the kinetics of interfacial electron transfer. Of a particular interest are studies on the luminescence of colloidal solutions of the so-called Q-semiconductors, their properties depending on the size of semiconductor particles due to the quantum size effects. 

Since the surface properties of the colloid have a strong influence on the photoreduction kinetics, it seemed interesting to elucidate the effect of the added surface-active substances on the kinetic regularities of the reactions photosensitized by semiconductor colloid. The surfactant molecules are known to concentrate near the surface of the colloidal particle, so they may affect strongly the kinetics of photocatalytic reactions proceeding at the particles surface [52,53]. Fig. 2.30 presents the temperature dependencies of the initial rate of the MV photoreduction over colloidal CdS prepared at the excess of the sulfide ions. These dependencies were obtained at the addition of different amounts of PAA. One may see that both the initial rate and the observed activation energy of the methylviologen photoreduction do not depend, within the experimental error, on the concentration of... 

Summing up the results presented here it may be concluded that in the polymerization rate equation the reaction order with respect to emulsifier is a variable and a function of the area occupied by the emulsifier molecule in the saturated adsorption layer on particles of the resultant latex x=/(i4s,im)- The basic ififierence of this equation from the equations proposed earlier [Eqs. (I) and (2)] is the presence of a parameter that characterizes the properties of the interface (adsorption capacity), which is necessary for a kinetic equation of polymeriza tion occurring in a colloidal... 

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