Thermodynamically stable colloidal system

Microemulsions. Unlike emulsions, microemulsions are transparent and thermodynamically stable colloidal systems, formed under certain concentrations of surfactant, water, and oil (Fig. 18.8). The transparency is because the droplet size of the microemulsions is small enough (<100 nm) that they do not reflect light. Because of its thermodynamic stability, microemulsions may have long shelf lives and spontaneously form with gentle agitation. However, microemulsions are not infinitely stable upon dilution because dilution... 

IV. Detergents are surface active substances that have features of all three surfactant groups described above, and in addition they are able to spontaneously form thermodynamically stable colloidal systems (for micellization in surfactant solutions please refer to Chapter VI). The particles that are washed away may become incorporated into the nuclei or micelles, i.e. solubilization (See Chapter VI) takes place. Various anionic, cationic, and nonionic surfactants that are encountered further in this section are typically members of this surfactant group. 

In this chapter, we will address the thermodynamic and kinetic aspects of colloid stability in free-disperse systems. We will discuss the concept of the factors for weak and strong stabilization, the possibility of spontaneous dispersion, and the conditions necessary to form thermodynamically stable colloidal systems. Furthermore, we will discuss the necessary conditions for the coagulation-peptization (dispersion) transition and the equilibrium between a coagulate comprising the connected-disperse system and the free-dispersed system formed in the course of dispersion. The fundamentals of colloid stability have been partially discussed in Chapters 1 and 2 and are covered to a great detail in textbooks on colloid and surface science [1-29]. We will address here the subject of colloid stability to the extent appropriate to the general scope of this book. 

Let us now address an interesting problem in colloid science and physical-chanical mechanics related to the contact interactions in disperse systems, namely, the possibility of spontaneous dispersion and the formation of thermodynamically stable colloid system. Originally, this problem was formulated by Max Volmer in 1927 [60,61] and later addressed by Rehbinder and Shchukin. Shchukin has made two principal contributions to the analysis of this problem [33,62-69]. The first is the detailed analysis of the conditions that make the process of spontaneous dispersion (at constant volume of the disperse phase, constant particle size, or constant number of particles) possible. Second, he proposed incorporating the entropy of mixing into the description of the conditions of spontaneous dispersion. The latter allows one to quantitatively estimate the concentration of the disperse phase in the disperse system formed. The analysis of the thermodynamics of spontaneous dispersion has important implications in the analysis of colloidal stability and in the control of various technological processes. 

Shchukin, E. D. 2004. Conditions of spontaneous dispersion and formation of thermodynamically stable colloid systems. J. Dispers. Sci. Technol. 25 875-893. 

The formation of thermodynamically stable colloidal systems by spontaneous dispersion is very common in nature, for example, in biological processes. However, in most cases, one encounters a... 

Microemulsions are a convenient medium for preparing microgels in high yields and rather uniform size distribution. The name for these special emulsions was introduced by Schulman et al. [48] for transparent systems containing oil, water and surfactants, although no precise and commonly accepted definitions exist. In general a microemulsion may be considered as a thermodynamically stable colloidal solution in which the disperse phase has diameters between about 5 to lOOnm. 

In contrast to the above-described kinetic stability, colloids may also be thermodynamically stable. A stable macromolecular solution is an example we have already discussed. Formation of micelles beyond the critical micelle concentration is another example of the formation of a thermodynamically stable colloidal phase. However, when the concentration of the (say, initially spherical) micelles increases with addition of surfactants to the system, the spherical micelles may become thermodynamically unstable and may form other forms of (thermodynamically stable) surfactant assemblies of more complex shapes (such as cylindrical micelles, liquid-crystalline phases, bilayers, etc.). 

Ti icroemulsions are transparent thermodynamically stable colloidal dispersions containing high amounts of both water and hydrocarbons. The colloidal state is stabilized by a proper balance between a hydrophobic and a hydrophilic surfactant. Initially microemulsions were considered to be different from colloidal solutions (I, 2, 3, 4, 5) an opinion that is still held by some (6) although it is accepted generally that microemulsions belong to micellar systems (7, 8, 9, 10). 

Microemulsions, like micelles, are considered to be lyophilic, optically isotropic and thermodynamically stable colloidal dispersions (the first microemulsions were termed oleopathic hydromicelles [128]). In some systems, the addition of a fourth component, a cosurfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to zero or near-zero values, easily on the order of 10 -10 mN m or lower, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about lO-lOOnm [129]. > The droplets can be so small that they scatter little light, so the emulsions appear transparent. Unlike coarse emulsions, micro emulsions are thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence and possibly transient negative interfacial tensions, but this remains an area of continued research [130-134]. It is sometimes helpful to think of microemulsions as systems of very highly swollen micelles. 

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. 

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. 

Micelles are formed by association of molecules in a selective solvent above a critical micelle concentration (one). Since micelles are a thermodynamically stable system at equilibrium, it has been suggested (Chu and Zhou 1996) that association is a more appropriate term than aggregation, which usually refers to the non-equilibrium growth of colloidal particles into clusters. There are two possible models for the association of molecules into micelles (Elias 1972,1973 Tuzar and Kratochvil 1976). In the first, termed open association, there is a continuous distribution of micelles containing 1,2,3,..., n molecules, with an associated continuous series of equilibrium constants. However, the model of open association does not lead to a cmc. Since a cmc is observed for block copolymer micelles, the model of closed association is applicable. However, as pointed out by Elias (1973), the cmc does not correspond to a thermodynamic property of the system, it can simply be defined phenomenologically as the concentration at which a sufficient number of micelles is formed to be detected by a given method. Thermodynamically, closed association corresponds to an equilibrium between molecules (unimers), A, and micelles, Ap, containingp molecules ... 

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. 

Microemulsions Microemulsions (MEs) are colloidal dispersions composed of an oil phase, an aqueous phase, and one or more surfactants. They are optically isotropic and thermodynamically stable and appear as transparent liquids as the droplet size of the dispersed phase is less than 150 nm. One of their main advantages is their ability to increase the solubilization of lipophilic and hydrophilic drugs accompanied by a decrease in systemic absorption [217]. Moreover, MEs are transparent systems thus enable monitoring of phase separation and/or precipitation. In addition, MEs possess low surface tension and therefore exhibit good wetting and spreading properties. 

A definite prediction of DLVO theory is that charge-stabilized colloids can only be kinetically, as opposed to thermodynamically, stable. The theory does not mean anything at all if we cannot identify the crystalline clay state (d 20 A) with the primary minimum and the clay gel state (d 100 to 1000 A) with the secondary minimum in a well-defined model experimental system. We were therefore amazed to discover a reversible phase transition of clear thermodynamic character in the n-butylammonium vermiculite system, both with respect to temperature T and pressure P. These results rock the foundations of colloid science to their roots and... 

Microstmctures are frequently present in a kinetically trapped nonequilibrium state, and their structures depend on the components and colloidal interactions based on their different chemical and physical properties, as well as on the procedure by which these components have been assembled. These structures are thermodynamically unstable and tend to reduce their free energy (surface area) with time. On the contrary, self-assembly nanostructures are thermodynamically stable, unless the molecules react with the environment or degrade. Most food systems are based on an interplay of kinetically stabilized and thermodynamic equilibrium structures. Some typical examples of structures at different length scales formd in food systems are shown in Figure 11.1. 

Polymerization in microemulsion systems has recently gained some attention as a consequence of the numerous studies on microemulsions developed after the 1974 energy crisis (1,2). This new type of polymerization can be considered an extension of the well-known emulsion polymerization process (3). Hicroemulsions are thermodynamically stable and transparent colloidal dispersions, which have the capacity to solubilize large amounts of oil and water. Depending on the different components concentration, microemulsions can adopt various labile structural organizations -globular (w/o or o/w tyne), bicontinuous or even lamellar -Polymerization of monomers has been achieved in these different media (4-18),... 

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

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