Free-disperse systems thermodynamic factor

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. 

It has been repeatedly emphasized that lyophobic disperse systems are thermodynamically unstable as compared to macroheterogeneous systems. The cause for this instability is a high excess of surface free energy at the interfaces. At the same time, many lyophobic colloids are stable towards aggregation and may maintain such stability for infinite periods of time. Let us now discuss the basic thermodynamic and kinetic factors that favor stabilization in disperse systems. In this sections we will restrict ourselves to just naming some of these factors, and will return to their detailed discussion later on. 

We stress that the foregoing thermodynamic notions for aqueous dispersions are quite speculative. A better understanding awaits the formulation of theories of polymer solution thermodynamics that comprehend aqueous systems. These may well include factors that are absent from the free volume approach. 

Within a restriction to the case of dilute monodisperse systems, the analysis of the ratio between the elementary work of dispersion, w, conceived as the work of particle isolation from a compact phase or disperse strnctnre, and the entropy factor, kT ln [l/C]-i-l, represents a general, universal approach to the evaluation of the possibility of the spontaneous dispersion of a macroscopic phase into colloid size particles (Figure 4,39), In the analysis of the behavior of the AF function, while setting the various variables, r, a, C, T, v, and n constant, the function is determined by the work of the dispersion factor, which is equal to the entropy factor, The transition in the dispersion process from positive values for the system free energy to negative ones determines the possibility of the formation of a thermodynamically stable system. 

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