Phase Stability in Closed Systems

Stability criteria are economically posed in terms of conceptual, such as S, G, or A, but before we can test for stability, we must connect the stability criteria to measm-ables. The cormections can be achieved either via models for volumetric equations of state, say P(T, v, x ), or (in cases of mixtures) via models for (T, P, x)). Both approaches are viable when all phases are fluid however, a model should be used for any solid phase. In general, then, we continue to face the ever-present thermodynamic problem of establishing useful relations between conceptual and measurables. 

The thermodynamics in this book is restricted to a description of well-defined states and to analyses of processes that change the system from one state to another. Thermodynamics deals mainly with equilibrium states, which were discussed in a qualitative way in 1.2.2 and in a quantitative way in 7.1. In both 1.2.2 and 7.1 we tacitly assumed that the situations xmder discussion were stable equilibrium states. But in general a stable state is only one of several possible kinds of states that are available to systems. In 8.1.1 we describe the kinds of states that can be legitimately proposed for thermodynamic systems, and we identify those that are observed in practice. 

Once we know the states that are available, then we want quantitative criteria that enable us to identify the state actually assumed by the system. Formally, the criteria are contained in 7.1 for example, if the system is maintained at a constant T and P, then the observed equilibrium state will be the one that satisfies (7.1.40)—the state that minimizes the Gibbs energy. So if at fixed T and P, a system can possibly exist as one phase or as two phases, the observed equilibrium situation will be the one with the lower Gibbs energy. For example, when 

By a well-defined state we mean a state to which property values can be assigned. The class of well-defined states contains the observable equilibrium states discussed in 1.2.2 but in addition, the class includes hypothetical states that are not observable but that nevertheless can be identified as points on phase diagrams. Often we need to determine whether a hypothetical state is in fact observable thermod)mamics pro- 

In the general case (considering not just thermodynamic systems), well-defined states can be divided into two types static and dynamic. Static well-defined states are always equilibrium states in which all forces acting on a system are balanced at every instant however, static states are not accessible to thermodynamic systems, so we do not discuss them further here. 

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In systems with liquid dispersion medium, i.e. in foams, emulsions, sols and suspensions, there is a broad variety of means to control colloid stability. In these systems the nature of colloid stability depends to a great extent on the aggregate state of dispersed phase. Similar to aerosols, foams are lyophobic, but in contrast to them can be effectively stabilized by surfactants. Properties of emulsions, and, to some extent, those of sols may be quite close to the properties of thermodynamically stable lyophilic colloidal systems. In such systems a high degree of stability may be achieved with the help of surfactants. 

The above use of "stable coexisting minerals" is of course based upon the fundamental consideration that the chemical system is "closed" that is, the chemical components K, Si and OH are "inert", their relative proportions, mass, in the system determines the phases formed. This can be assumed valid for many argillaceous sediments and rocks. However, in some geological environments, aqueous solutions containing alkalis and hydrogen ions in various concentrations (whose activities, therefore, are variables but constant throughout a given system) react with kaolinite or other minerals to influence its stability under otherwise constant physical and chemical parameters. 

Any surfactant adsorption will lower the oil-water interfacial tension, but these calculations show that effective oil recovery depends on virtually eliminating y. That microemulsion formulations are pertinent to this may be seen by reexamining Figure 8.11. Whether we look at microemulsions from the emulsion or the micellar perspective, we conclude that the oil-water interfacial free energy must be very low in these systems. From the emulsion perspective, we are led to this conclusion from the spontaneous formation and stability of microemulsions. From a micellar point of view, a pseudophase is close to an embryo phase and, as such, has no meaningful y value. 

The investigation of viscosities, electrical conductivities, refractive indexes and densities of binary liquid systems of sulphuric acid with nitromethane, nitrobenzene and 0-, m and p-nitrotoluene was made in order to obtain a clearer picture of the behaviour of these binary mixtures, regarding the stability of the addition compounds formed between the components. The application of these methods of physicochemical analysis to a number of binary systems with sulphuric acid [1, 2, 3] has enabled us to get some idea of the way in which the formation and stability of addition compounds affects the liquid phase properties of these systems. The binary systems of sulphuric acid with mononitrocompounds are particularly suitable for comparison with each other, because of the close similarity of the liquid media in these systems, due to comparable values of dielectric constants and liquid phase properties of the mononitrocompounds. The stability of the addition compounds in these systems in the crystalline phase [4] has... 

It should also be noted that the stability of the distinct mesophases can be quite different. It seems that there is a significant effect of molecular shape and topology, stabilizing SmA phases in the system 41/43 and Colhex phases in the system 35/37. In addition, the mesophase stability is often reduced close to the transition to another mesophase (see Fig. 15). Hence, the order-disorder temperatures can only be roughly estimated based on segmental solubility parameters [24, 25]. 

Foam films and a foam from the aqueous and organic phases of an extraction system containing a 30% solution of tri-buthyl phosphate (TBP) in kerosene and nitric acid (1 mol dm 3) have been studied in a parallel mode [137]. The reasons for foaming and the effect of emulsion formation on foam stability were elucidated. Thus, a foam with a measurable lifetime was obtained when TBP was in concentration of 0.8 mol dm 3 which corresponded to the concentration of black spot formation. When the volume ratio of the organic to the aqueous phase was 1 5, the foam formed in the system was stabilised additionally by a highly disperse O/W emulsion. This was due to the reduced rate of drainage. These results are confirmed by the experimental data acquired with a specially constructed centrifugal extractor [136]. It makes it possible to perform an extraction process under conditions close to those in industry. 

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