Connected-disperse systems stability

Connected-disperse system Stability=resistance to the applied stress, P... 

An electrical double layer (edl) existing on the solid-solution interface is essentially connected with the surface properties of the system. The amount of accumulated charge influences the adsorption of ions and molecules. In the latter case it also influences the configuration of the adsorbed species. On the other hand, the adsorption of the ions and molecules varies surface properties of the interface (functional groups) and thus, the distribution of the charge in the interfacial region. The existence of the electric charge at the interface influences the dispersed system stability. 

In this chapter, we will address the thermodynamic and kinetic aspects of colloid stability in free-disperse systems. We will discuss the concept of the factors for weak and strong stabilization, the possibility of spontaneous dispersion, and the conditions necessary to form thermodynamically stable colloidal systems. Furthermore, we will discuss the necessary conditions for the coagulation-peptization (dispersion) transition and the equilibrium between a coagulate comprising the connected-disperse system and the free-dispersed system formed in the course of dispersion. The fundamentals of colloid stability have been partially discussed in Chapters 1 and 2 and are covered to a great detail in textbooks on colloid and surface science [1-29]. We will address here the subject of colloid stability to the extent appropriate to the general scope of this book. 

This Chapter describes preparation, structure, and properties of different colloidal systems. A lot of attention will be devoted to the connection between particular properties of disperse systems (and possible ways that can be used to monitor colloid stability) and the aggregate states of both the dispersed matter and dispersion medium. 

Macromolecular colloid solutions also play an important role in ensuring the stability of disperse systems (e.g. suspensions, emulsions). In the case of emulsions the polymer decreases the rate of separation by increasing viscosity on the one hand, and it has an enthalpy stabilizing effect by adsorption on the surface of the droplets on the other hand [3, 4, 7]. Depending on the concentration of the polymer, a protecting and flocculating effect can be observed during the interaction between suspensions and polymers. If the polymer concentration is low, the polymer adsorbed on the surface of the particles connects the particles into loose floccules. Thereby, the rate of... 

It should be emphasized that stabihzation of metal nanoparticles by high-molecular compounds presents a major branch of polymer colloidal modem science. Modem polymer colloidal science studies generation regularities of dispersed systems with highly developed interfaces, their kinetic and aggregation stabilities, different surface phenomena arising at the interface, and adsorption of macromolecules from liquids on solid surfaces. The theory of improving stabihty of colloidal particles by polymers has been treated in detail elsewhere. This chapter focuses on basic questions that are connected with nanoparticles and nanocomposites. 

Free-disperse systems comprise dilute emulsions, sols, and suspensions in which the participation of particles in thermal Brownian motion plays a dominant role over the cohesive forces between them. In these systems, we are particularly interested in the stability resisting the transition from the free-disperse state to the connected-disperse state via aggregation, flocculation, or sedimentation (Figure 4.2). 

The basic mechanism of plasticizing effect on fresh cement mixes is explained by forming a temporarily stable double layer on cement particles. Since the formation of the double layer is also connected with the surface of particles, the increased demand for AEA, WRA and SP, in silica fume concrete and the decreased demand for AEA in the presence of a WRA or SP can be directly correlated to the specific surface increase of silica fume-cement blends and the dispersing action of WRAs and SPs in achieving a mortar consistency that enables the air-bubble-generating and stabilizing process [147, 149]. Concrete producers are now cognizant of the effect of these factors. Field silica fume concrete with a satisfactory, stable air-void system can therefore be produced consistently. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

A direct link between theoretical and experimental work on depletion-induced phase separation of a colloidal dispersion due to non-adsorbing polymers was made by De Hek and Vrij [56, 109]. They mixed sterically stabilized silica dispersions with polystyrene in cyclohexane and measured the limiting polymer concentration (phase separation threshold). Commonly, one uses the binodal or spinodal as experimental phase boundary. A binodal denotes the condition (compositions, temperature) at which two or more distinct phases coexist, see Chap. 3. A tie-line connects two binodal points. A spinodal corresponds to the boundary of absolute instability of a system to decomposition. At or beyond the spinodal boundary infinitesimally small fluctuations in composition will lead to phase separation. De Hek and Vrij [56] used the pair potential (1.21) to estimate the stability of colloidal spheres in a polymer solution by calculating the second osmotic virial coefhcient B2 ... 

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