Lyophilic systems

In colloid science, colloidal systems are commonly classified as being lyophilic or lyophobic, based on the interaction between the dispersed phase and the dispersion medium. In lyophilic dispersions, there is a considerable affinity between the two constituent phases (e.g., hydrophilic polymers in water, polystyrene in benzene). The more restrictive terms hydrophilic and oleophilic can be used when the external phase is water and a nonpolar liquid, respectively. In contrast, in lyophobic systems there is little attraction between the two phases (e.g., aqueous dispersions of sulfur). If the dispersion medium is water, the term hydrophobic can be used. Resulting from the high affinity between the dispersed phase and the dispersion medium, lyophilic systems often form spontaneously and are considered as being thermodynamically stable. On the other hand, lyophobic systems generally do not form spontaneously and are intrinsically unstable. 

A colloid is defined as a system consisting of discrete particles in the size range of 1 nm to 1 pm, distributed within a continuous phase [153], On the basis of the interaction of particles, molecules, or ions of the disperse phase with molecules of the dispersion medium-, colloidal systems can be classified as being lyophilic or lyophobic. In lyophilic systems, the disperse phase molecules are dissolved within the continuous phase and in the colloidal size range or spontaneously form aggregates in the colloidal size range (association systems). In lyophobic systems, the disperse phase is very poorly soluble or insoluble in the continuous phase. During the last several decades, the use of colloids in... 

Above we used the words continuous phase and dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range. It should be understood that these are solvent and solute in lyophilic systems. In micellar systems, the micelles are dispersed in an aqueous continuous phase. Furthermore, the system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. This terminology is by no means universal. Lyophilic dispersions are true solutions and may be called such, although this term ignores the colloidal size of the solute molecules. 

Until now, we have been primarily concerned with the definition and measurement of viscosity without regard to the nature of the system under consideration. Next we turn our attention to systems containing dispersed particles with dimensions in the colloidal size range. Viscosity measurements can be used to characterize both lyophobic and lyophilic systems we discuss both in the order cited. 

It is important whether the various structural elements can be said to constitute phases or not. If not, we generally have a so-called lyophilic system, which is in principle in equilibrium. In most foods, structural elements do constitute phases, which implies that they have phase boundaries in which free energy is accumulated. This means an excess of free energy, hence a lyophobic system it costs energy to make it, and it is inherently unstable. The properties of such foods thus depend on the manufacturing and storage history and, for natural foods, on growth conditions. 

The possibility of existence of lyophilic systems in equilibrium with macroscopic phases is determined by the nature of dispersed phase and its interaction with dispersion medium. In systems consisting of simple molecules without strong diphilic features the formation of equilibrium colloidal systems occurs only within narrow temperature range in a direct vicinity of the critical point. These systems are referred to as critical emulsions. 

At the same time, within the same approximation, at low values of interfacial tension, a, such as tenths and hundredths of mJ m 2, i.e., when A 10 21 - 10 22 J, for particles of r 1 pm, one obtains p] slO10 N and uc 10 19 - 10 2° J. These numbers indicate that in such lyophilic system adhered particles can be separated by the energy of Brownian motion, which would oppose structure formation. 

At the same time, liquid medium of a similar nature as well as the adsorption of surfactants may lower the interfacial energy, a, and complex Hamaker constant, A, by 2 - 3 or more orders of magnitude. In such lyophilized system the adhesion forces and energy are lowered by several orders of magnitude [17]. Under these conditions a system with low concentration of dispersed phase remains stable towards aggregation (see... 

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

In the first place one should distinguish reversible and irreversible systems, that is to say, colloid systems which can undergo phase change or phase separation reversibly or otherwise. A thermodynamically definable stability difference is thus the basis of this classification. It is for this reason more logical than the old classification into lyophobic and lyophilic systems (and to a still greater degree than that into suspensoids and emulsoids). 

The optical properties of lyophilic systems are also different. Tyndall effect for these systems is minimum. Besides the size of the particles, the difference in refiractive indices between the dispersed phase and the dispersion medium ailso affect TyndalUzatlon. In an emulsoid this difference is not appreciable because of the high solvation character of this system. Therefore the Tyndall effect is not so prominent. 

We have a little earlier discussed the nature of lyophobic systems which are inherently unstable and the lyophilic systems which are thermodynamically stable md are made up of macromolecules having great solvation power. There exists another class of colloidal systems which, though thermodynamically stable, are made up of micelles. A micelle is a spontaneously formed aggregate of micromolecules. This aggregate achieves colloidal dimensions and is a thermodynamically stable structure. The dispersion finally formed is called the association colloidal system. 

Here, we have a typical example of a lyophilic system, in which the particles (and their surfaces) of the solid dispersion phase and the liquid dispersion medium are quite similar in their physical-chemical properties. The term lyophilic (from Greek, likes to dissolve ) was introduced by Freundlich. 

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). 

FIGURE 4.28 Molecular dynamics simulation of the spontaneous rupture of a contact between a large (convex) particle and a substrate in a lyophilic system initial condition (a), the rupture of a contact (25, 30, 35 units of time, respectively) (b-d), the turning of the particle (10, 20, 30, and 40 units of time, respectively) (e-h). (Redrawn from Yushchenko, V.S. et al.. Colloids Surf., 110, 63, 1996.)... 

FIGURE 4.29 The separation of a small (flat) particle from a substrate in lyophilic system in the course... 

Petroleum systems contain complex matter which is the subject of new field of condensed matter physics. Petroleum systems are typical oleo-dispersed, lyophilic systems, or systems of low polarity dispersive media. The medium is in dynamic balance with elements of a dispersed phase. Traditional tools for studying petroleum systems are limited only by the... 

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