Stability of Colloid Systems

The stability of colloidal systems is subject to the state of their configuration, very roughly comparable to a bucket that is stable when standing up but if tilted beyond a certain angle, topples and comes to rest on its side (Figure 7.1). 

Here, the quantities jn ° and ji are, respectively, the chemical potentials of pure solvent and of the solvent at a certain biopolymer concentration V is the molar volume of the solvent and n is the biopolymer number density, defined as n C/M, where C is the biopolymer concentration (% wt/wt) and M is the number-averaged molar weight of the biopolymer. The second virial coefficient has (weight-scale) units of cm mol g. Hence, the more positive the second virial coefficient, the larger is the osmotic pressure in the bulk of the biopolymer solution. This has consequences for the fluctuations in the biopolymer concentration in solution, which affects the solubility of the biopolymer in the solvent, and also the stability of colloidal systems, as will be discussed later on in this chapter. 

A. Impact of Physical Interactions between Biopolymers on Structure and Stability of Colloidal Systems... 

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

Another point of view concerning foam stability appeared in relation to the development of the general theory of stability of colloid systems (DLVO-theory). It has already been noted that this theory was verified for the first time with foam films [35]. This gave rise to the concept of foam stabilisation on the account of the electrostatic component of disjoining pressure [e.g. 24, 32, 36],... 

The stability of colloidal systems consisting of charged particles can be essentially explained by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1-7]. According to this theory, the stability of a suspension of colloidal particles is determined by the balance between the electrostatic interaction and the van der Waals interaction between particles. A number of studies on colloid stability are based on the DLVO theory. In this chapter, as an example, we consider the interaction between lipid bilayers, which serves as a model for cell-cell interactions [8, 9]. Then, we consider some aspects of the interaction between two soft spheres, by taking into account both the electrostatic and van der Waals interactions acting between them. 

According to kinetics, the instability or stability of colloidal systems is determined by the balance of the forces of attraction and repulsion between the individual particles. The forces of attraction, causing the particles to stick together, are of the same nature as intermolecular forces and increase very rapidly as the particles approach each other. The forces of repulsion may be electrical, arising as a result of selective adsorption by the phase interface of one of the ions of an electrolyte present in the system. One of the factors keeping colloidal particles apart may be the formation on the interface of a solvate shell of molecules of the environment. 

Stability of colloidal systems. Two types of stabihty are distinguished, kinetic and aggregative (Pisarenko et al., 1964). 

The physical stability of a colloidal system is determined by the balance between the repulsive and attractive forces which is described quantitatively by the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. The electrostatic repulsive force is dependent on the degree of double-layer overlap and the attractive force is provided by the van der Waals interaction the magnitude of both are a function of the separation between the particles. It has long been realized that the zeta potential is a good indicator of the magnitude of the repulsive interaction between colloidal particles. Measurement of zeta potential has therefore been commonly used to assess the stability of colloidal systems. 

Derjaguin (25) distinguished three types of stability of colloidal systems. 

In the case when the depth of potential minimum is smaller than several kT, the coagulation (i.e., the combination of two particles) becomes thermodynamically unfavorable even at low height of the potential barrier (Chapter VII, 1), and the stability of colloidal system towards coagulation is of thermodynamic nature. This is confirmed by observed peptization of coagulated precipitates upon washing out the excess of coagulating electrolyte and by stabilization of sols by specifically adsorbed ions. 

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. 

A general article on the stability of colloidal systems is given by H. van Olphen in Enriching Topics, Ch. 2. 

DLVO theory has often been applied to gain insight into the stability of colloidal systems. For colloidal montmorillonite with a particle radius of R = 200 x 10 m in an aqueous NaCl solution, the electrical double layer repulsion can be approximated as... 

As is known in the colloid chemistry, the electrolyte concentration Q necessary for fast coagulation of colloids, strongly depends on charge of the "anti-ions , that is, the ions with the charge opposite to that of the particles in question. On the other hand, stability of colloid system practically does not depend on the ions charge and on concentration of colloid particles. This is consistent with the Schulze-Hardy law, according to which, it is the valence of the anti-ions that exerts the basic influence on stability of colloidal systems. 

Particle charge plays a major role on the stabilization of colloidal systems. Especially when nanoparticles are stabilized by an adsorption layer of polyelectrolytes, zeta potential measurements are very useful. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. The zeta potential can be detected by means of electro-osmosis, electrophoresis, streaming potential, and sedimentation potential measmements. The potential drop across the mobile part of electric donble layer can be determined experimentally, whenever one phase is made... 

Macromolecular species have played an indispensable role in the stabilization of colloidal systems since the first prelife protein complexes came into existence. We (humans) have consciously (although usually without knowing why) been making use of their properties in that context for several thousand years. Today macromolecules play a vital role in many important industrial processes and products, including as dispersants, stabilizers, and flocculants as surface coatings for protection, lubrication, and adhesion for the modification of rheological properties and, of course, for their obvious importance to biological processes. 

The main forces that rule colloidal stability, that is, the tendency of particles to aggregate, are the topic of Chapter 16. Colloid stability appears to be the net outcome of a subtle interplay between dispersion, osmotic, and steric forces. Evaluation of these forces provides the clue for manipulating the stability of colloidal systems. 

It has long been recognized that the zeta potential is a very good index of the magnitude of the interaction between colloidal particles, and this parameter is often measured to assess the stability of colloidal systems [32-34]. 

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