Colloidal systems stable

In colloid science, colloidal systems are commonly classified as being lyophilic or lyophobic, based on the interaction between the dispersed phase and the dispersion medium. In lyophilic dispersions, there is a considerable affinity between the two constituent phases (e.g., hydrophilic polymers in water, polystyrene in benzene). The more restrictive terms hydrophilic and oleophilic can be used when the external phase is water and a nonpolar liquid, respectively. In contrast, in lyophobic systems there is little attraction between the two phases (e.g., aqueous dispersions of sulfur). If the dispersion medium is water, the term hydrophobic can be used. Resulting from the high affinity between the dispersed phase and the dispersion medium, lyophilic systems often form spontaneously and are considered as being thermodynamically stable. On the other hand, lyophobic systems generally do not form spontaneously and are intrinsically unstable. 

Not all colloid systems are stable. The most stable involve solid dispersion media, since movement through a solid host will be slow. Emulsions also tend to be stable think, for example, about a glass of milk, which is more likely to decompose than undergo the destructive process of phase separation. Aerosols are not very stable although a water-based polish generates a liquid-in-air colloid, the particles of liquid soon descend through the air to form a pool of liquid on the table top. Smoke and other solid-in-gas aerosols are never permanent owing to differences in density between air and the dispersed phase. 

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). 

FIGURE 7.1 Stability criteria of any colloidal system metastable-unstable-stable states. 

A colloidal suspension may be unstable and exhibit separation of particles within a very short time, or it may be stable for a very long time, such as for over a year. In between, a metastable state can be found. This is an oversimplified example, but it shows that any colloidal system should be analyzed following these three criteria. 

In real systems, both stable colloidal systems (as in paints, creams) and unstable systems (as in wastewater treatment) are of interest. It is thus seen that, from DLVO considerations, the degree of colloidal stability will be dependent on the following factors 1 2 3 4 5... 

A soil is a condensed colloid system because the negatively charged, plateshaped crystals are assembled in parallel or near-parallel alignment, to form stable operational entities, described as clay domains. The crystals within a day domain can be represented by a three-plate crystal model in which one crystal separates the other two crystals to produce a slit-shaped pore, where the crystals overlap. This situation is illustrated in Figure 3.4. 

The colloidal structures described above are dictated by thermodynamics, and the resulting structures are thermodynamically stable. Similar thermodynamically stable structures can develop even in a copolymer melt (i.e., there is no other polymer or solvent). Such colloidal systems differ from kinetically stable lyophobic dispersions of the type discussed in Vignettes 1.4 and 1.5. 

Douroumis, D. and A. Fahr (2007). Stable carbamazepine colloidal systems using the cosolvent technique. Eur. J. Pharm. Sci., 30 367-374. 

Although most colloidal systems are metastable or unstable with respect to the separate bulk phases, they can have an appreciable kinetic stability. That is, the state of dispersion can exist for an appreciable length of time. Colloidal species can come together in very different ways therefore, kinetic stability can have different meanings. A colloidal dispersion can be kinetically stable with respect to coalescence but unstable with respect to... 

In colloid science, the terms thermodynamically stable and metastable mean that a system is in a state of equilibrium corresponding to a local minimum of free energy (Ref. [978]). If several states of energy are accessible, the lowest is referred to as the stable state and the others are referred to as metastable states unstable states are not at a local minimum. Most colloidal systems are metastable or unstable with respect to the separate bulk phases. See also Colloid Stability, Kinetic Stability. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

G. Tovar describes one of the novel chemical applications of modern colloidal systems by using such miniemulsions (in addition to classical suspension polymerization) for molecular imprinting. Here, the stable nanoreactor situation is used to synthesize particle surfaces with molecular sized cavities for biomedically relevant species or species to be separated from each other. Such receptor sites are nowadays preferentially made by the pathways of modern colloid chemistry. 

Aggregation of liposomes both in vitro and in vivo is one of their main stability problems. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, or theory of colloidal stability, a colloidal system is stable if the electrostatic repulsion forces between two particles are larger than the attraction van der Waals forces. Therefore charged liposomal formulations are highly desirable. Manipulation of... 

Kinetically (or sedimentationally) stable systems are those in which the particles do not settle out appreciably under the force of gravity. Kinetic stability depends on the particle size and on the difference in density between the particles and the environment. All colloidal systems have very high kinetic stability. 

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