Spectrum colloidal systems

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

Concerning the slow dynamics below the crossover temperature Tc, the predictive power of the theory seems to be rather limited. In particular, the emergence of intrinsic slow secondary processes, which seems to be associated with the dynamic crossover in the experimental spectra, is not contained even in the extended versions of the theory consequently, the slow dynamics spectrum is not reproduced correctly. In this respect, the extended theory introducing the hopping mechanism for describing the susceptibility minimum below Tc is misleading. On the other hand, the most prominent prediction of MCT below Tc is the anomaly of the nonergodicity parameter, which, as discussed, is found by different model-independent approaches. However, within the framework of MCT, this anomaly is closely connected with the appearance of a so-called knee feature in the spectral shape of the fast dynamics spectrum below Tc. This feature, however, has not been identified experimentally in molecular liquids, and only indications for its existence are observed in colloidal systems [19]. In molecular systems, merely a more or less smooth crossover to a white noise spectrum has been reported in some cases [183,231,401]. Thus, it may be possible that the knee phenomenon is also smeared out. 

The catalytic activity toward hydrogenation reactions has been indeed observed in such bimetallic colloidal systems. The SERS spectrum of p-nitrobenzoate (PNBA) adsorbed on colloidal silver and the SERS spectra observed on Ag/Pd nanoparticles, immediately after the addition of PNBA and after 1 week, are shown in Fig. 20.7 A, B, and C, respectively. In the bimetallic colloid, instead of the SERS spectrum of PNBA (spectrum A), a different spectrum is obtained (spectrum B) that slowly evolves toward a different spectral feature, which becomes predominant after a week (spectrum C). This modification may be related to the initial formation of p-aminobenzoate as a result of the catalytic reduction of the nitro group, followed by slow oxidation to azodibenzoate by atmospheric oxygen (see Fig. 20.8). 

FIGURE 16,5. If one constructs a spectrum of colloidal systems, it can be seen that the microemulsions and swollen micelles he in the range between the simple aggregate structures such as micelles and the larger emulsion and dispersion systems. 

Khlebtsov et al. (1991) have theoretically and experimentally studied the dispersion effect of the refractive indices of particles and dispersion medium in the turbidity spectrum method. A new approach is put forward to estimate the optical dispersion of the components of a colloidal system and to consider it when the system s parameters are determined from the wavelength exponent. The method has been verified on PS lattices with the particle diameter from 80 to 800 nm. The elaborated version of the turbidity spectrum method can be used as a metrological test for particle sizes, not inferior to electron microscopy in accuracy. 

In generally encountered disperse systems, the particle sizes and characteristics of the particle-particle interaction can cover a very broad spectrum of values, which explains the large variation in the rheological properties of different colloidal systems utilized in different areas of technology. At the same time, disperse systems are the main carriers of mechanical properties in both inanimate and live nature [10-17]. 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

In the remainder of this section we see how the theoretical calculations of Mie account for the observed spectrum of colloidal gold. In the next section we consider the inverse problem for a simpler system how to interpret the experimental spectrum of sulfur sols in terms of the size and concentration of the particles. Both of these example systems consist of relatively monodisperse particles. Polydispersity complicates the spectrum of a colloid since the same x value will occur at different X values for spheres of different radii according to Equations (99M101). 

The first part of the chapter reviews progress in the synthesis of monodisperse semiconductor NCs and gives a basic introduction to their specific physical properties. In conformity with the literature, the term monodisperse is used here to describe colloidal samples, in which the standard deviation of the particle diameter does not exceed 5%. Throughout the text we will restrict ourselves to the description of binary II-VI (CdSe, CdS, CdTe, ZnSe, etc.), III-V (InP, InAs), and IV-VI (PbS, PbSe, PbTe) semiconductor NCs. These systems exhibit optical properties that can be varied in the visible part of the spectrum, the near UV or near IR by changing the NC size and/or composition. 

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