Shape thermodynamic/kinetic stability

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.). 

A template approach has been invoked by Stack et al. to immobilize a metastable [Fe"(OTf)2(phen )2] (phen , substituted 9,10-phenanthroline) complex 12 in a preformed binding pocket shaped by a thermodynamically stable [Cu(phen )2] template. After replacing Cu+ by Fe +, the iron(II)-containing polymer 12 was obtained. A [Fe (OTf)2(phen )2] intermediate is unstable in a homogeneous solution and rapidly leads to the tris(chelate) [Fe(phen )3] + complex and Fe + ions [21]. Thus, the matrix prevents ligand scrambling and again kinetically stabilizes a metastable species by site isolation. 

In contrast to natural structures the morphological features of structures in fabricated foods are in principle within our control. The source of the many structures of foods, even those made from a single raw material (e.g., wheat flour), lies in the ingredient mix and the fact that thermodynamic equilibrium is practically never required or achieved during processing. These metastable structures can be attained because they are favored kinetically, that is, the approach to equilibrium is slow. At any point during the development of a particular structure a process of shape stabilization sets in, usually by vitrification, partial crystallization, phase separation and/or formation of a network (Figure 12.5). 

The main difference between emulsions and microemulsions lies in the size and shape of the droplets of dispersed phase, which causes the differences in the thermodynamic stability of the two systems. Emulsions allow the drug to be administered as a dispersed oil solution and thus are kinetically stable but thermodynamically unstable. After storage or aging, droplets will coalesce and the two phases separate. Unlike emulsions, microemulsions are thermodynamically stable and phases do not separate on storage. Another important difference between the two systems is their appearance emulsions have a cloudy appearance, while microemulsions are transparent because of the lower dispersed phase size than macroemulsions. 

While analyzing the above-presented models, one realizes that the problem of mode choice cannot be unambiguously solved within the solution of mass transfer equation. This makes it necessary to consider thermodynamic or kinetic approaches to the analysis of transformation front stability and to choose a certain contact zone morphology. From the point of view of kinetics, the interphase boundary instability may be caused either by instabihty with respect to fluctuations of the boundary shape [15-17] or by the failure of balance equations for fluxes at the moving boundaries [16]. From a thermodynamic viewpoint, the problem of choice of one kinetically allowed mode can be solved using the variation principles of nonequilibrium thermodynamics [18-29],... 

---