Colloidal systems characterization methods

For advanced students a book is needed to describe the connection between techniques to functionalize colloids, the characterization methods, the physical fundamentals of structure formation, diffusion dynamics, transport properties in equilibrium, the physical fundamentals of nonequilibrium systems, the measuring principles to exploit these properties in applications, the differences in designing lab experiments and devices, and a few selected application examples. 

The different growth modes discussed above have been exemplified also from structural studies. Froment and Lincot [247] used structural characterization methods, such as TEM and HRTEM, to determine the formation mechanisms and habits of chemically deposited CdS, ZnS, and CdSe thin film at the atomic level. These authors formulated reaction schemes for the different deposition mechanisms and considered that these should be distinguished to (a) atom-by-atom process, providing autoregulation in normal systems (b) aggregation of colloids (precipitation) ... 

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

The description of a colloid should include particle size, mobility, charge and their distributions, charge/mass ratio, electrical conductivity of the media, concentration and mobility of ionic species, the extent of a double layer, particle-particle and particle-substrate interaction forces and complete interfacial analysis. The application of classical characterization methods to nonaqueous colloids is limited and, for this reason, the techniques best suited to these systems will be reviewed. Characteristic results obtained with nonaqueous dispersions will be summarized. Physical aspects, such as space charge effects and electrohydrodynamics, will receive special attention while the relationships between chemical and physical properties will not be addressed. An application of nonaqueous colloids, the electrophoretic development of latent images, will also be discussed. 

The sedimentation method belongs to the classical methods of characterization of the colloidal properties of disperse systems. These methods can be used for the analysis of colloidal solutions with size of colloidal particles between 1 and 100 micrometer. The analysis of solutions with smaller particles leads to relatively high errors as a result of Brownian motion. 

One of the most important developments in the 1980s in the search for novel concepts to characterize colloidal systems such as silica sols and gels was the advent of the fractal approach. Another very important development in the past 10 years was the application of 29Si CP MAS NMR methods to the study of the silica surface. This technique made it possible, for example, to identify without ambiguity the presence of silanediol groups on the silica surface (55). 

Information about the bound water fraction in some colloid systems, silica gels, and biological systems is usually inferred on die basis of the frequency- and time-domain DS measurements from the analysis of the dielectric decrements or die relaxation times (64, 150-152). However, the nonionic microemulsions are characterized by a broad relaxation specfrum as can be seen from the Cole-Cole plot (Fig. 33). Thus, these dielectric methods fail because of the difficulties of deconvoluting die relaxation processes associated widi the relaxations of bound water and surfactant occurring in the same frequency window. 

The properties of polymer mixtures depend on the method by which they are obtained and are determined by many factors by sizes of particles of the dispersed phase, by their shape and number in bulk, and by the thermodynamic affinity of the components for one another [19]. Linear polymers blend either in the course of their mutual dissolution or, in two-phase systems, under conditions of thermodynamic incompatibility of the components, when the dispersion is forced. The mixtures formed can be compatible (forming true solutions of one polymer in another), incompatible (representing a typical colloid system), quasicompatible (characterized by microscopic homogeneity at a level above heterogeneity on the molecular level), or pseudocompatible (with a strong adhesion interaction on the boundary) [106]. 

Three branches of science deal with colloids and macromolecules colloid science, surface science, and macromolecular science. CoUoid science is the study of physical, mechanical, and chemical properties of colloidal systems. Surface science deals with phenomena involving macroscopic surfaces. Macromolecular science investigates the methods of syntheses in the case of synthetic polymers (or isolation and purification in the case of natural products such as proteins, nucleic acids, and carbohydrates) and the characterization of macromolecules. It includes, for example, polymer chemistry, polymer physics, biophysical chemistry, and molecular biology. These three branches of science overlap. What one learns from one branch can often be apphed to the others. 

Unlike most other characterization techniques introduced in this chapter that only allow conclusions to be drawn on the bulk level, self-diffusion NMR gives detailed information on colloidal systems by focussing on the molecular level. This complex technique will therefore be introduced in detail. NMR spectroscopy, based on physical properties of the molecular spin, is a very powerful method for the measurement of self-diffusion of small molecules in complex solution [67] with direct insight into general aspects of the solution structure [68]. 

Other methods that detect a sphere of influence include those based on the Coulter principle which will also be reviewed at this conference. Here the data is reported in terms of a sphere of equivalent volume, irrespective of the shape or, in some situations, the state of the particulate interface. The method depends, essentially, on measuring the increase in resistance experienced between two electrodes as a particle passes between them and an essential requirement, therefore, is the presence of electrolyte in the measurement system. The method is realistically limited to particles down to about 1/jm in diameter, and there is no practical upper limit to the principle. The presence of electrolyte in the environment is an advantage in some situations since the effect is to suppress charge effects at the particle interface and this simplifies the measurement of the size of colloidal dispersions. Submicrometre dispersions can be measured but it should be noted that interference effects become more pronounced and there is less certainty about the magnitude of coincidence effects, quite apart from the intrinsic experimental difficulties of keeping orifices with diameters of less than 50um clean and operationally effective. Nevertheless, the Coulter principle has proved to be an invaluable technique for the detailed characterization of biological systems such as blood cells and, in some instances, bacterial suspensions. 

At first, an overview of the experimental methods which are suitable to characterize the CNT aggregation state in general, and thus are suitable to monitor CNT debundling in aqueous medium and in presence of surfactant, will be reviewed. Three main streams will be presented (i) "direct imaging of the CNT dispersions by m icroscopic techniques (ii] spectroscopic techniques, such as Raman spectroscopy, which exploit the difference in electronic properties between bundled and individualized CNTs and (iii] depolarized dynamic light scattering which is commonly used to characterize colloidal systems. 

Since latex dispersion application properties are related to the surface properties of the latex particles, there is a need for surface characterization of the particles at large. Historically, these types of systems have been applied as model colloids (Hearn et al, 1981) and therefore required well-characterized surfaces but as the sophistication of new coatings increase, the latex particle surfaces become more important from an industrial perspective. In addition to these applications the utilization of latex particles in pharmaceutical and biomedical applications has also contributed to the development of new surface characterization methods. The surface engineering, that is, variations in size, surface charge and surface hydrophobicity, of latex particles as colloidal carriers has been demonstrated to provide opportunities for the site-specific delivery of drugs (Ilium Davis, 1982). Surface... 

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