Structural-mechanical barrier

Izmailova, V.N., Yampolskaya, G.P., Tulovskaya, Z.D. (1999). Development of Reh-binder s concept on structure-mechanical barrier in stability of dispersions stabilized with proteins. Colloids and Surfaces A Physicochemical and Engineering Aspects, 160, 89-106. 

First, it is the experimental and theoretical (including computer modeling) investigation of adsorption layers formed on solid surfaces by natural and synthetic polymers, especially by poly electrolytes. Such studies, and in particular those involving the use of Atomic Force Microscopy (AFM, see Chapter VII), provide important information regarding the optimal conditions for the use of polymers for flocculation or stabilization of disperse systems (Chapter VII), and establish the theoretical basis for understanding the mechanism behind the action of structural-mechanical barrier. 

Structural-mechanical barrier (after Rehbinder) is a factor of strong stabilization, capable of promoting essentially unlimited stability towards aggregation and coalescence of disperse systems, including the concentrated ones (see Chapter VII,5). 

The term structural-mechanical barrier was for the first time introduced by P. A. Rehbinder [2,46-48]. This is a strong factor of stabilization of colloidal systems related to the formation of interfacial adsorption layers of low and high molecular weight surfactants which lyophilize interfaces. The structure and mechanical properties of such adsorption layers are able to ensure very high stability of dispersion medium interlayers between dispersed particles. 

According to Rehbinder, the structural-mechanical barrier appears due to the adsorption of surfactant molecules that are capable of forming g e 1 -1 i ke structured layer at the interface, but are not necessarily highly surface... 

Thus, the important features of the structural-mechanical barrier are the rheological properties (See Chapter IX,1,3) of interfacial layers responsible for thermodynamic (elastic) and hydrodynamic (increased viscosity) effects during stabilization. The elasticity of interfacial layers is determined by forces of different nature. For dense adsorption layers this may indeed be the true elasticity typical for the solid phase and stipulated by high resistance of surfactant molecules towards deformation due to changes in interatomic distances and angles in hydrocarbon chains. In unsaturated (diffuse) layers such forces may be of an entropic nature, i.e., they may originate from the decrease in the number of possible conformations of macromolecules in the zone of contact or may be caused by an increase in osmotic pressure in this zone due to the overlap between adsorption layers (i.e., caused by a decrease in the concentration of dispersion medium in the zone of contact). 

Often one relates this type of stabilization to the so-called steric factor [48-51], the notion of which was introduced much later than Rehbinder s concept of lyophilic structural-mechanical barrier. Steric factor primarily reflects configurational elasticity of tails and loops of adsorbed macromolecules as well as osmotic effects. Steric factor represents only an entropic ( generally speaking, small) contribution to the elastic resistance of film, and by itself can not account for the strong stabilization. 

Fig. VII-12. The stabilization of emulsion droplets by structural-mechanical barrier...

As one can see, the structural-mechanical barrier is a complex factor of colloid stability, which includes the contribution from a number of different thermodynamic, kinetic and structural-rheological (i.e., related to peculiarities in structure of adsorption layers) factors. 

Stabilization of emulsions by powders can be viewed as a simple example of structural- mechanical barrier, which is a strong factor of stabilization of colloid dispersions (see Chapter VIII, 5). The stabilization of relatively large droplets by microemulsions, which can be formed upon the transfer of surfactant molecules through the interface with low a (Fig. VII-10), is a phenomenon of similar nature. The surfactant adsorption layers, especially those of surface active polymers, are also capable of generating strong structural mechanical barrier at interfaces in emulsions. Many natural polymers, such as gelatin, proteins, saccharides and their derivatives, are all effective emulsifiers for direct emulsions. It was shown by Izmailova et al [49-52]. that the gel-alike structured layer that is formed by these substances at the surface of droplets may completely prevent coalescence of emulsion drops. 

The use of substances that due to their ability to form structural-mechanical barrier are capable of very strong stabilization of emulsions (and especially of concentrated ones), allows one to prepare many commercial emulsions that are used e.g. in emulsion polymerization [55], lubricantcooling liquids, etc. Such surfactants, and especially natural ones, are widely used in food and pharmaceutical applications [56-58]. These surfactants are, for instance, formed as a result of chemical reaction between dextrins and their derivatives (generated by thermal decomposition and partial oxidation of starch) and oils. 

When present at high concentration, polymeric surfactant, due to its high adsorption, may form a dense lyophilizing adsorption layer at the particle surface. Under these conditions the same polymer acts as a stabilizer of colloid dispersion, stabilizing the latter by means of structural-mechanical barrier (Chapter VII). 

In the opposite case of adsorption from non-polar medium, e.g. adsorption of octadecylamine on glass in heptane, the adsorption layer reveals a certain finite strength, i.e. has properties of structural-mechanical barrier (Chapter VII, 5). 

It is importcuit, from a practical point of view, that extremely thin black films are ruptured under the condition of reduced humidity, since evaporation of the dispersion medium leads to their ultimate thinning and appearance of high de- wedging pressures. Here are two possibilities to produce stable (under the conditions of dry air) foams. On the one hand, this is reduction of permeability of adsorption layers for water vapours and thus inhibition of the evaporation rate. On the other hand, it is the use of surfactants for foam stabilisation leading to the formation of a structure-mechanical barrier, i.e. gel-like interlayer. 

The coalescence rate, i.e. formation of larger droplets after collision of two droplets, depends on the number of collisions and on the properties of the adsorption layers. For dilute emulsions, as well as emulsions having V2 = 0.3—0.74, coalescence is the main process leading to the disturbance of their aggregative stability. Hence, prerequisite for the production of appreciable volumes of any type of emulsions suitable for practical application is provision of their coalescence stability. In case of o/w emulsions, maximum stability against coalescence is achieved through the formation, on the surface of the disperse phase particles, of structured adsorption layers, a structure-mechanical barrier defined by Rehbinder [8]. Such layers are... 

The plasticising activity of ethoxylated alkylphenols and EO/PO block-copolymers depends on the number (m) of the EO units in a surfactant molecule. Ethoxylated alkylphenols with m>40 have a substantial effect on the viscosity reduction of HWCS. It may be connected with the fact that they can form rather thick hydrated layers when adsorbing on the particles, which play the role of a structure-mechanical barrier. The addition of ethoxylated alkylphenols with m<40 produce a thickening effect on the suspensions which may be connected with the features of coal particle wetting by aqueous surfactant solutions. 

Recently, PPy nanopartides with the diameter of 60-90 nm were polymerized with FeCla in aqueous solutions containing PVA as a stabilizer [212]. At room temperature (RT), the polymerization of pyrrole occurred at a high rate. When the concentration of pyrrole increased, the resultant PPy nanoparticles became coarser with broadening the particle size distribution. Furthermore, the increase in concentration of PVA resulted in faster polymerization and finer PPy nanopartides. Such a phenomenon was due to the reinforcement of the structural-mechanical barrier formed by the stabilizer at the surface of the nanopartide, preventing the growth of PPy nanopartides during the polymerization process. 

According to Rhebinder (1), stable concentrated emulsions may be obtained only when there is a structural-mechanical barrier at the interface. The structural-mechanical barrier may result from adsorption of surfactants forming a structure of sufficiently high mechanical strength at the interface. In previous studies (2-4), it was shown that SFA aluminum and iron soaps (C] 7-C2q) can be used to obtain stable water-in-oil concentrated emulsions as models of hydrocarbon-based drilling muds. 

REHBINDER S LYOPHILIC STRUCTURAL-MECHANICAL BARRIER AS A FACTOR OF STRONG COLLOID STABILITY... 

FIGURE 4.10 Schematic illustration of Rehbinder s structural-mechanical barrier as a strong factor of colloid stability. 

There are a number of experimental methods that can be used to evaluate these two factors of a lyophilic structural-mechanical barrier separately. We will briefly review these methods in the succeeding text and then focus on specific experimental data that have been collected by employing these methods. 

In the section that follows, we will address in some detail the stability of anulsions against coalescence and discuss the experimental results within the framework of the structural-mechanical barrier. We will devote special attention to systems with lluorinated interfaces. The most noteworthy result was that the most effective stabilizers for fluorinated hydrocarbons in aqueous solutions were nonfluorinated surfactants, while fluorinated surfactants were very effective for stabilizing emulsions of nonfluorinated hydrocarbons. 

SlABItlTY OF FtUORINATED SYSTEMS STRONG STABILIZATION BY THE Structural-Mechanical Barrier... 

It is worth emphasizing here that while the structural-rheological properties (i.e., mechanical strength) of the interfacial adsorption layer play a determining role in the stability of the system toward coalescence, they alone may not be sufficient for complete stabilization. The prevention of coagulation also requires that the structural-mechanical barrier formed is lyophilic (hydrophilic) with respect to the surrounding polar liquid. The latter can be achieved by the introduction of common surfactants, for example, sodium dodecyl sulfate (SDS). 

In Rehbinder s concept of the stability of emulsions and other disperse systems, the focus is on the lyophilic structural-mechanical barrier as a factor responsible for the strong stabilization of disperse systems. This barrier is manifested with the interfacial surfactant adsorption layer formed predominantly with high molecular weight substances (the so-called protective colloids). This barrier on the one hand promotes the formation of a system with substantial mechanical strength that is capable of resisting coalescence and the rupture of the droplets and, on the other hand, is lyophilic with reference to the dispersion medium. The lyophilic nature of the barrier is characterized by a low value of the interfacial energy, o, on the side facing the dispersion medium. One can thus... 

Shchukin, E. D., Amelina, E. A., and V. N. Izmaylova. 2003. The lyophilic structure-mechanical barrier as a factor of dispersion strong stabilization. In Role of Interfaces in Environmental Protection, S. Barany (Ed.), pp. 81-90. Amsterdam, the Netherlands Kluwer Academic Publishers. 

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