Free-disperse systems

Preparation of Free Disperse System Using Liquid Hydrocarbons... 

The work by group of Kozyuk [84—87] has illustrated the use of hydrodynamic cavitation for obtaining free disperse system in liquids, particularly in liquid hydrocarbons. It has been found that, there is substantial improvement in the quality of the obtained free dispersion, even in the absence of any catalyst. Also the geometry of a flow-constricting baffle body [84] effectively increases the degree of cavitation to substantially improve the quality of obtained free disperse system. 

Kozyuk OV (1996) Method and device for obtaining free disperse system in liquid. US Patent application No. 602069... 

Kozyuk OV (1998) Method of obtaining a free disperse system in liquid and device for effecting the same. US Patent US 5810 052... 

Kozyuk, O. V., Method of obtaining free disperse system and device for effecting same, U.S. Patent 5,492,654, Exclusively Licensed to Five Star Technologies (Feb. 10,1996). 

Kozyuk, O.V. Method of Obtaining a Free Disperse System in Liquid and Device for Effecting the Same. U.S. Patent 5,810,052, September 22, 1998. 

For transfer processes involving the motion of dispersed particles taking place in free-disperse systems, one can write the general relationship between the flux, /, and the average velocity of independent particles, u, i.e. ... 

Establishing the values of coefficient, k, and force, F, constitutes the major task in the theoretical treatment of transfer processes in free-disperse systems. 

The involvement of dispersed particles in thermal motion, which causes the entropy of the system to increase upon the dispersion of the substance, accounts for similarities in the properties of free disperse systems and molecular solutions. The term colloidal solutions was therefore... 

Let us continue the discussion of transfer processes taking place in the free disperse systems. We will primarily focus on the role that electrokinetic phenomena and electrical double layer play in these processes. 

The action of external electric field on the free disperse system results in particle motion (electrophoresis). The electrophoretic velocity, vE, is not a function of ( -potential only, but also depends on the particle radius, r, and the type of electrolyte present in the system. However, it turns out (see fine print further down) that all of these factors can be simultaneously accounted for by the numerical coefficient, kt, introduced into the Helmholtz-Smoluchowski equation (V.26). If the particles are spherical, k, changes from 2/3 for particles smaller compared to the ionic atmosphere thickness (kt 1) to 1 for large particles ( kt 1). Consequently, the particle flux due to the applied electric... 

Along with electrophoresis, the electric field applied to free disperse system causes the flow of electric current, related to both the motion of ions in dispersion medium and charge transfer by particles moving with velocity o . The specific electric conductance of the free disperse system, v, is equal to phenomenological coefficient a22 and includes the specific electric conductance of the dispersion medium,, and the additional conductance caused by moving charged particles. A more detailed consideration shows that for free disperse system... 

Among various electrokinetic phenomena that take place in free disperse systems, electrophoresis is the most significant one for the scientific studies and practical applications. 

Let us now discuss in some detail the peculiarities of particle motion during electrophoresis and some other electrical properties of free disperse systems. Electrophoresis usually takes place in a stationary liquid. In a moving fluid the motion of particles occurs only in thin flat gaps and capillaries (microelectrophoresis), where the fluid motion is caused by electroosmosis. If fairly large non-conducting particles are dispersed in a rather dilute electrolyte solution, the ratio of particle radius to the double layer thickness may be substantially greater than 1, i.e., r/8 = kt 1. The streamlines of outer electric field surround the particle and are parallel to most of its surface, as shown in Fig. V-9. In this case the particle velocity, v0, can be with good precision described by Helmholtz-Smoluchowski equation. 

Structural losses are characteristic of structured disperse systems. In free disperse systems these losses play a role at high concentration of dispersed phase. Structural losses are related to the oscillations of the network of particles, which may be viewed as a number of interconnected oscillators. As pointed out by Dukhin [26,27], structural losses provide a link that bridges acoustics with rheology. At this time there is no adequate theoretical description of this energy loss mechanism. 

Xv specific conductance of the free disperse system (section V.4)... 

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... 

At the same time, the transfer of particles from structured disperse system (the aggregate) into free disperse system (the sol) is followed by an involvement of freed particles into Brownian motion, leading to entropy increase. The gain in entropy, A J] may be described by expression, similar to eq. (IV.6), i.e. ... 

The equilibrium between aggregation and peptization of dispersed particles is given by u] (3 k77 Vi Z condition, which corresponds to a particular particle concentration in free disperse system, equilibrium with respect the to sediment (aggregate) ... 

In free disperse systems Brownian motion, along with stabilizing action, may also reveal a destabilizing one. Such destabilizing action is typical for truly lyophobic systems, i.e. the systems that are unstable with respect to aggregation and do not belong to the class of pseudolyophilic ones. We will further show that in these systems Brownian motion is indeed the mechanism responsible for particle coagulation. 

It has been already pointed out that the energy of interaction between dispersed particles depends on the particle size. As a result, for large particles, and especially for anisometric ones, oriented in a certain way with respect to each other, the presence of a secondary minimum may be of importance. For such particles this secondary minimum may be sufficiently deep in comparison with kT. In some cases these systems may experience a peculiar colloid phase transition from a free disperse system (at low concentrations of dispersed phase) to crystal-like periodic structures consisting of colloidal particles in equilibrium with the dilute sol consisting of single particles. Such periodic structures are observed in some biological systems, e.g. in tobacco mosaic virus, in V205 sols and in latexes. 

This description corresponds to the case of disperse structures of globular type in which the strength originates from a continuous skeleton that forms due to adhesion of individual particles upon the conversion of free disperse system into structured disperse system. There are, however, other types of structures, such as, e.g., cellular structures (in solidified foams and emulsions), in which the skeleton consists of continuous films of solid-like dispersion medium. Such structures, typical for some polymeric systems, may... 

It was pointed out in Chapter VIII that the studies of long range forces in dilute (free disperse) systems require the measurements of force as a... 

The viscosity of dilute free disperse systems is mainly determined by the viscosity of dispersion medium which may vary within several orders of magnitude, depending on the nature of the medium. For instance, viscosity of gases is on the order of 10 5 Pa s, for a large number of liquid-like objects it is between 10 2 and 1010 Pa s, and for glasses and solids the viscosity is 1015 - 1020 Pa s or higher. 

Fig. IX-23. Rheological curve of a free disperse system containing anisometric particles cot...

A high free energy excess, particularly in systems with a fine degree of dispersion, is the cause of thermodynamic instability, which is the most important feature of a majority of disperse systems. Thermodynamic instability in turn entails various processes aimed at decreasing the surface energy, which results in the saturation of surface forces. Such processes may occur in a number of ways. For example, in a free disperse system partial saturation of the surface forces may take place in the contact zone between 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). 

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... 

A Contact Interactions and the Stability of Free-Disperse Systems... 

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). 

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