Free-disperse systems lyophilicity

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 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). 

Because the lower limit of the colloidal range is just larger than the size of some molecules and solvated species it is difficult to determine exactly where the distinction between surface and bulk ends and a molecularly dispersed system begins. For macromolecular systems, of course, the molecular size is such that even a molecular dispersion or solution easily falls into the size range of colloids. For that reason, primarily, such systems are referred to as lyophilic colloids, even though the properties of such systems are governed for the most part by phenomena distinct from the classic surface interactions considered in lyophobic colloids. It is no trivial matter, therefore, to decide just where surface effects end and the characteristics of the individual free, solvated units begin. 

At some even smaller particle sizes and under certain other conditions (e.g., an increase in a on approaching molecular dimensions), a negative minimum in the free energy can be observed. This minimum corresponds to the formation of a thermodynamically stable, lyophilic disperse system. These concepts were further developed in several directions in theoretical and experimental studies by Shchukin, Pertsov, and Kochanova [33,66-69], and in works by Rusanov et al. [63-65]. 

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. 

The conditions needed for a spontaneous dispersion of the condensed phase and the formation of a lyophilic colloidal system were analyzed by Rehbinder and Shchukin back in 1958, and a quantitative description of this problem was proposed. That original analysis was based on the estimation of the changes in the free energy, AF, upon dispersing a condensed phase in a given dispersion medium [62]. Let s assume that, as a result of dispersion, n particles have separated from the condensed phase. Due to the participation of these separated particles in Brownian motion, the work of dispersion, wa5 o, is balanced by the gain in entropy, AS... 

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