Connected-disperse systems

The viscosity in connected-disperse systems with coagulation structures changes more abruptly than the viscosity in free-disperse systems. In this case, one can encounter an entire spectrum of states between two limiting cases that of a completely intact structure and one corresponding to the... 

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. 

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

Free-disperse systems are typically represented by dilnte sols having low concentrations of particles, while the connected-disperse systems are those in which one typically encounters high particle concentrations, such as concentrated sediments or pastes (Figure 1.2). 

FIGURE 1.2 Free-disperse systems and connected disperse systems. 

This description holds true for disperse systems of the globular type, in which a continuous backbone is formed due to the cohesion of the individual particles in the course of transformation of a free disperse system into a connected disperse system. In such systems, the backbone formed is the main carrier of the strength. At the same time, there are also other types of systems, for example, those with a cellular structure (solidified foams or emulsions). Such structures are typical in polymeric systems and may form in the course of new phase formations by condensation in mixtures of polymers. An individual approach also needs to be employed in the description of the mechanical properties of structures with anisometric particles, due to the specifics of the cohesive forces in such systems. In addition to porous structures, we also consider various compact microheterogeneous structures, such as mineral rocks, modern composite materials, and natural materials such as bone and wood. 

Different kinds of dispersions can be formed. Most of them have important applications and have special names (Table 1.1). While there are only five types of interface, we can distinguish ten types of disperse system because we have to discriminate between the continuous, dispersing (external) phase and the dispersed (inner) phase. In some cases this distinction is obvious. Nobody will, for instance, mix up fog with a foam although in both cases a liquid and a gas are involved. In other cases the distinction between continuous and inner phase cannot be made because both phases might form connected networks. Some emulsions for instance tend to form a bicontinuous phase, in which both phases form an interwoven network. 

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. 

Low-dispersion HPLC systems are necessitated by the increasing trend of using shorter and narrower HPLC columns, which are more susceptible to the deleterious effects of extra-column band-broadening. HPLC manufacturers are designing newer analytical HPLC systems with improved instrumental bandwidths compatible with 2-mm i.d. columns by using micro injectors, smaller i.d. connection tubing, and detector flow cells. A new generation of ultra-low dispersion systems dedicated for micro and nano LC is also available. 

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... 

The description of mechanics of continuous medium of multi-phase reaction mixtures accordingly with Eiler s approach is connected with introduction of conception of multispeed continuum and determination of interpenetrative motion composing disperse system. Multispeed continuum is totality of N continuums, each of them relates to its composing mixture (phase or component) and fills up one and the same volume. Density p, continuum rate and then other parameters were determined for each composing continuum in every point by usual method. Thus, in each volume point filling up by mixture N densities, N rates etc. are determined. Furthermore, parameters characterizing components mixture as a whole such as density and bulk mixture flow rate can be determined on the base of these magnitudes. 

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. 

Instruments that are designed to reduce unwanted radiation to an absolute rninimum will place two monochromators in tandem with an intermediate sUt connecting the dispersing systems. In the case illustrated in Figure 8.43 the first monochromator uses a prism, while the second uses a grating. The two monochromators, however, must be in perfect synchronization or no fight at aU will be transmitted. 

This unusual freezing behavior was explained in terms of the size of the domains formed by the water when it froze. At T = Tp the domain is linked with the size of the cluster connecting the system as well as with the sizes of smaU, unconnected clusters of droplets. AtT> Tp, the system is multiconnected. For temperatures well below Tp only one exothermic peak is associated with the dispersed phase, as shown in Figs. 21-24. 

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 described approach introduces a principally new way of looking at the problem of the transition between lyophilicity and lyophobicity in fine disperse colloidal systems, both free disperse and connected disperse. 

When the contacts between the particles in a free-disperse system are established, the transition of the system into a connected-disperse state takes place. This transition is associated with the development of a spatial network of particles in which the cohesive forces between the particles forming a network are sufficiently strong to resist thermal motion and the action of external forces. As a result of the transition, the system acquires a set of new structnral-mechanical (rheological) properties that characterize the ability of the syston to resist deformation and separation into individual parts. That is, the system acquires mechanical strength, which is the principal and universal characteristic of all solid and solid-like materials. For many materials, their mechanical strength defines the conditions of their use. 

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