Solids, metastable

An example is illustrated in Figure 8 for W/2.303 RT = 1.4. Two ranges of x values occur near the borders at x = 0 and x = 1, where the solid solutions are stable. The range of stability end at the x values which apply to the minima in the enthalphy curve, which are known as the limits of solid solubility Xgg. The two ranges of x values between the minima and the points of inflection in the enthalphy curve pertain to metastable solid solutions. The points of inflection occur at the so-called spinodal compositions X p (61). Between the two spinodal compositions any solid solution is unstable and will disproportionate into two solid solutions of the compositions Xgg. 

Vol. 9 The Local Chemical Analysis of Materials by J. W. Martin Vol. 10 Metastable Solids from Undercooled Melts by D. M. Herlach, P. Galenko and D. Holland-Moritz... 

The basic question is how to perform extrapolations so as to obtain a consistent set of values, taking into account various complications such as the potential presence of mechanical instability. Additional complications arise for elements which have a magnetic component in their Gibbs energy, as this gives rise to a markedly non-linear contribution with temperature. This chapter will concern itself with various aspects of these problems and also how to estimate the thermodynamic properties of metastable solid solutions and compound phases, where similar problems arise when it is impossible to obtain data by experimental methods. 

RH (e.g., Oatis et al., 1998). In addition, there is evidence that new metastable solid states can form in these droplets (Tang et al., 1995). Similar studies of ammonium bisulfate, NH4HS04, have been carried out over a range of temperatures and relative humidities relevant to the atmosphere, and its phase diagram, including metastable states and a crystalline hydrate phase at low temperatures has been reported (Imre et al, 1997). 

Despite the complexities introduced by metastable solid and liquid phases, topological features of the phase diagram can be thermodynamically interpreted in a standard manner from Clapeyron-type equations. Thus, from the forward slopes of a-fi, /3-liquid (stable), and a-liquid (metastable) phase boundaries, we can infer from (7.32) that... 

Reactive sputter deposition is a useful technique not only for coating but also for obtaining metastable phases, especially nitrides. Nitrogen gas is activated in a plasma to enhance nitride formation and heating at high temperature is not required. It is possible to obtain metastable nitrides by sputter deposition in which species in the plasma are quenched on substrates. Compounds with different crystal structures normally do not form solid solutions, but by co-sputter deposition metastable solid solutions may form. 

Compared with the solid and gaseous phases, the aqueous solution has numerous complications. Therefore, we will first mention methods by which one can predict the composition of the solution in the presence of a given solid and gaseous phase. Later we shall remember that the formulae already attributed to the solid phases are inadequate. There is sometimes a multiplicity of solids, each having the same formula but differing from each other in chemical behavior. Within this multiplicity of solid phases, all are metastable but one. Some kinds of metastable solids and metastable equilibria will be discussed later. 

Although the difference in solubility (and Gibbs energy) may be rather large, it is often observed that some metastable solids show a high inertia in that their rate of conversion to stable solids may be extremely small. We shall return to this later. 

Now, let us return to a consideration of the relative inertia of some metastable solids. As stated above, the change of the solubility product of ZnO with particle size is given by the empirical equation... 

Examining metastable solids has been restricted so far to pure phases. Still to be considered are systems consisting of an aqueous solution and a solid solution, the latter being represented by the simplified formula (A,B)L. The equilibrium is given by Equation 6. 

The solutions in the region inside the spinodal domain are unstable, whereas the solid solutions in the region between the binodal and spinodal lines are metastable. The presence of a miscibility gap limits the potential usefulness of these materials in device applications. Solutions with compositions lying inside the spinodal domain cannot be grown by LPE, whereas metastable solid solutions have a tendency toward phase separation and, eventually, device degradation. 

Strategy for V Mer-lnsoluble Drug Formulation Using Metastable Solids. 560... 

Advantages of Using Metastable Solids Dissolution Rate Improvement... 

In contrast to the solubility enhancing effect of metastable solids, there are some potential disadvantages that must be taken into account. In most cases, suspensions of metastable solids are n feasible owing to rapid transformation of the excess solid to the stable (low solubility) form. Shefter and Higuchi (1963) showed that many nonsolvated forms transform to the hydrated form in aqueous... 

Clements and Popli (1973) summarized transformation times during dissolution of pharmaceutical compounds from the literature. Theirstudy with meprobamate found that it took 168 h to convert to form I. Other than the example of chloramphenicol palmitate, this was much longer than most systems, which converted to the stable modiLcation in 300 s (theophylline) to 24 h (ampicillin). This is generally slow enough to allow therapeutic advantages to dosing the metastable solid orally, but too rapid to market suspensions of these forms with adequate physical stability. 

STRATEGY FOR WATER-INSOLUBLE DRUG FORMULATION USING METASTABLE SOLIDS... 

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