Colloidal Stability

One of the basic requirements for a protein-latex complex to be applicable to clinical diagnostics is colloidal stability under immunological conditions. However, as indicated in the introduction, the most serious problem in latex immunoagglutination assays is that the system can lose its colloidal stability after antibody adsorption step. This low colloidal stability of latex-antibody complexes in the reaction medium may provoke the nonspecific agglutination of particles. The isoelectric point of most polyclonal IgG molecules used in latex immunoassays tests is in the range 6.5-8.5; in addition, they present a low charge density. Therefore, when the particles are covered by IgG the nonspecific agglutination process takes places under physiological conditions (pH 7.4 and ionic strength 150 mM), since there is almost no electrostatic repulsion between them. Antibody-coated particles must be completely stable in the absence of the antigen. That is, agglutination must only be triggered by the presence of the specific antigen rather than by the experimental conditions of the test. But, what is colloidal stability? When a cube with 1 cm of edge inmersed in a fluid medium is divided into many small colloidal cubes with 10 nm of edge, the surface of the system increases from 6 cm2 to 600 m2 (Fig. 18). This increasing area process is accompanied by a change in the free energy given by the expression dG = jshdA, where ySL is the solid-liquid interfacial surface tension expressed in J/m2. If the interfacial surface tension is positive, the colloidal dispersion is thermodynamically unstable (AG > 0) and the particles tend to assemble to reduce the interfacial area (aggregation phenomena). These colloids are generally called lyophobic. On the other hand, if the interfacial surface tension is negative, the colloid is said to be lyophilic, the free energy of the system is negative, and the particles are thermodynamically stable. An example of lyophobic colloid is latex particles, whereas a typical lyophilic system is microgel. The term colloidal stability is refer to the ability of a suspension to resist aggregation. The colloidal stability may be either thermodynamic or kinetic. Lyophilic colloids are systems thermodynamically stable whereas lyophobic colloids are kinetically stabilized. The kinetic stability is a consequence of an energy barrier opposing collisions between the particles and possible aggregation subsequently. The stability control of suspensions warrants detailed attention because development of different applications of these systems to biophisics, pharmacy, agriculture, medicine, and modern technologies is dependent to a large extent on a better understanding and manipulation of the colloidal stability.

The tendency to aggregation of lyophobic colloids is attributable to the universal attractive van der Waals forces. In some cases, this attractive force between particle and medium is stronger than that between particles, with the result that the colloidal state is preferred, i.e., the system is lyophilic [143]. For lyophobic colloids only when the attractive van der Waals force is counteracted by a repulsive force can some degree of stability be obtained. When the particles have charges on the surface the colloid may be electrostatically stabilized. In some cases, a suitable polymer can be adsorb on the particle surfaces. As the two surfaces are brought closer together the concentration of polymer units increases in the overlap region with a resulting increase in the osmotic pressure.

FIG. 18 Fragmentation process of a cube.

This tends to bring in solvent from the surrounding medium, with a consequent repulsive force to separate the particles. This polymer-induced stability is referred to as steric stability.

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability occupies a central position in colloid science. According to this theory, the stability is predicated on the notion that two independent types of forces govern the interaction between similar colloidal particles immersed in polar (especially aqueous) solutions: attractive van der Waals forces and repulsive electrostatic forces due to the net charge of the particle [144,145]. Electrostatic repulsion decays approximately exponentially with the separation distance H between two particles, whereas the van der Waals forces are proportional to H l. As a consequence, the interaction-energy distance curve is characterized by the presence of a shallow, secondary minimum at longer separation distances, an interaction barrier closer to the surface, and a deep primary minimum at short separation. The maximal potential represents the energy barrier opposing aggregation. If particles approach each other with sufficient kinetic energies to overcome this energy potential, aggregation occurs and the suspension is destabilized. Adding salt to a dispersion initiates aggregation by suppressing the electrostatic repulsion between particles. The energy barrier decreases with increasing electrolyte concentration and disappears above certain salt concentration called critical coagulation concentration (CCC). The study of this concentration is a practical way to determine colloidal stability.

The DLVO theory has been extensively tested and reviewed [146-149], and it stands today as the only quantitative theory of the colloidal and biocolloidal sciences. However, experimental investigations of the aggregation properties of a wide range of colloidal dispersions suggest that not all systems can be explained using the DLVO theory. When water is the dielectric medium in which colloidal particles are suspended, the theory generally fails to predict the stabilities of very hydrophobic or very hydrophilic particle suspensions. For example, the colloidal stability of silica at its isoelectric point [150], prevention of bubble coalescence at high ionic strength [151], deposition of PS latex on glass surfaces [152], swelling of clays [153], and many hydrophilic colloidal particles, and most biological surfaces and macromolecules, remain separated in aqueous solution even in high salt or in the absence of any net surface charge [154,155]. Reported results with amphoteric charged latex [156] also points to a deviation in the behavior respect to the classical DLVO theory. Many studies based on atomic force microscopy have thrown light on the limitations of the classical DLVO theory. Direct investigations of the interaction potential between silica surfaces [157-159] and mica surfaces [160] in aqueous electrolyte solutions have revealed agreement with DLVO at separations above a few nanometers, but at smaller separations a short-range repulsive force appears, often termed a "hydration" interaction.

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