Free-disperse systems colloid stability

In free disperse systems, in particular those with low concentration of dispersed phase, the nature of colloid stability and conditions under which the collapse occurs, are to a great extent dependent on thermal motion of dispersed particles, which may contribute to both stability and destabilization. For example, the necessary condition for sedimentation stability is sufficiently small particle size, so that the tendency of particles to distribute within the entire volume of disperse system due to the Brownian motion (an increase in entropy) would not be affected by gravity. As a quantitative criterion for the presence of noticeable amount of dispersed particles in equilibrium with a sediment one, for instance, may use the... 

It is worth recalling here that a dispersion medium akin to the particles, as well as surfactant adsorption, can lower both the interfacial energy, o, and the complex Hamaker constant. A by two to three orders of magnitude. In such a lyophilized system, the adhesive energy and force are also lowered by several orders of magnitude. In a concentrated disperse system in which the dispersed particles are mechanically forced to come into contact with each other, the lyophilization manifests itself as a decrease in the resistance to strain t. This means that in concentrated colloidal systems, plasticizing takes place, while in systems with a low concentration of dispersed particles, the lyophilization results in enhanced colloid stability of the free-disperse system (see Chapter 4). 

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. 

FIG U RE 4.2 Loss of colloid stability in free-disperse system. 

The presence of an essentially similar liquid medium and of surfactant adsorption influences the magnitude and nature of the surface forces and may result in weakened cohesion in the contacts by two to three orders of magnitude. In a lyophilized, highly concentrated system in which the particles are brought into mechanical contact, this is revealed through lower resistance to deformation, x, and results in a plasticizing of the system (see Chapters 2 and 3). When the disperse phase concentration is low, lyophilization leads to the preservation of the colloidal stability of a free-disperse system, that is, the resistance of the system to coagulation (see Chapter 4). 

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. 

The introduction of free polymer into the dispersion medium appears to make no significant difference to the number of phases accessible to the system. This can be discerned from the following simplistic considerations using the Phase. Rule, which will be assumed to be applicable. We consider a dispersion of colloidal particles sterically stabilized by terminally grafted polym chains. Free polymer is dissolved into the dispersion medium. 

Recent experimental studies (1-3), on systems of sterically stabilized colloidal particles that are dispersed in polymer solutions, have highlighted the role played by the free polymer molecules. These experiments are particularly relevant because the systems chosen are model dispersions in which the particles can be well approximated as monodisperse hard spheres. This simplifies the interpretation of the data and leads to a better understanding of the intcrparticle forces. DeHek and Vrij (1, 2) have added polystyrene molecules to sterically stabilized silica particles dispersed in cyclohexane and observed the separation of the mixtures into two phases—a silica-rich phase and a polystyrene-rich phase—when the concentration of the free polymer exceeds a certain limiting value. These experimental results indicate that the limiting polymer concentration decreases with increasing molecular weight of... 

---