Colloidal systems, destabilization

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

These results demonstrate that nonadsorbing polymer can induce phase separations in colloidal systems with the nature of the phases depending primarily on the ratio of the particle and polymer sizes. Since the strength of the attraction is not necessarily a monotonic function of the polymer concentration, e.g., because of penetration of the free polymer into a grafted layer, both destabilization and restabilization are possible. 

In true solutions at a concentration c = 102 mol m 3 (n = 6xl025 particles per m 3) the osmotic pressure reaches 6xl025 m 3 xl.38xl0 23 JxK 1 x 298 K = 2.4 x 105 Pa (2.4 atm). For lyophobic colloidal systems the number of particles per m3 does not as a rule exceed 1021, and hence the osmotic pressure is not greater than a fraction a millimeter of water column. Besides the experimental difficulties in measuring values that are so small, the presence of electrolytes introduces a considerable source of errors. The complete removal of electrolytes may lead to the destabilization of a colloidal system (see Chapter VIII). The presence of electrolytes leads to high osmotic pressures and in the presence of membranes results in the establishment of membrane (Donnan) equilibrium. 

Emulsions and suspensions of solid particles are common examples of colloidal systems that are not in the equilibrium state. As the systems destabilize and approach the equilibrium state, several processes will be involved. Typically, flocculation, sedimentation, and coalescence, etc., take place simultaneously at more or less well-defined rates, continuously changing the properties of the system. 

For many reasons, it is important to understand the causes of stability in colloidal systems. Manufacturers of cosmetics and paints, for example, want longterm stability so that their products do not separate into two or more bulk phases during the time period between manufacture and use. The operator of an oil field, on the other hand, needs to know how to rapidly destabilize the emulsion of oil and water produced by his wells. 

Recent studies revealed that cadmium-based semiconductor nanocrystals did not affect the biological functions if they were completely coated with organic ligands [58, 59]. After the ligands were detached, the nanocrystals became extremely toxic [59]. The detachment of the ligands will not only destabilize the colloidal system but also cause possible cytotoxic problems [56]. 

In hydrophobic colloidal systems, the water molecules have a higher affinity for one another than for the colloidal particles. Consequently, the particles would stick together on each encounter, nnless they repel each other. It was discovered by H. Schulze in 1883 that addition of electrolyte destabilizes hydrophobic colloidal dispersions, and W.D. Hardy in 1900 showed that the destabilization is accompanied by a reduction in the electrophoretic mobility of the particles. From this, it was inferred that colloidal stability is maintained by electrostatic repulsion between charged particles. 

In summary High surface potentials stabilize colloidal systems. The addition of inert salt leads to stronger screening and destabilizes the system. The point at which rapid coagulation (case b) sets in is defined as the critical coagulation concentration (ccc). One key result of DLVO theory is the explanation of the Schultze-Hardy rule, which states that the ccc depends on the counterion valency z like 1/z . 

Figure 10.1 Colloidal dispersions are Inherently unstable systems and in the long run the attractive forces will dominate and the colloidal system will destabilize. However, colloid stability depends on the attractive van der Waals and the repulsive electrical or steric (polymeric) forces. The repulsive forces stabilize a dispersion if they are larger than the van der Waals (vdW) ones (and the total potential is larger than the "natural" kinetic energy of the particles). Surfaces are Inherently unstable and the van der Waals forces "take the system" back to its stable (minimum surface area) condition and contribute to instability (aggregation)...

Figures 10.20 illustrates the various destabilization mechanisms in colloidal systems. The terminology can be sometimes confusing and very often we use the term aggregation (or coagulation or...

When the colloid destabilization occurs, the kinetics of collapse is often fast. Let us consider that we have a colloid system containing originally n particles per volume. Once destabilized, the particles aggregate by colliding with each other and thus the concentration of particles drops with time. Typieally this phenomenon is represented by the seeond-order equation, often called Smoluchowski model ... 

Adding polymers can result in both stabilization and destabilization of colloidal systems (Figure 12.12). 

Not affected by presence of electrolytes Equally good in aqueous and non-aqueous dispersions Equally good in dilute and dense colloid systems Often results in reversible flocculation Good stability with temperature changes Electrolytes result in destabilization Effective especially for aqueous colloidal dispersions Best for dilute colloidal dispersions Often irreversible coagulation is obtained Coagulation often upon freezing... 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

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