Supersaturated state, creating

A solid phase is precipitated from solution if the chemical potential of the solid phase is less than that of the dissolved component. A solution in which the chemical potential of the solute is the same as that of the corresponding solid phase and is in equilibrium with the solid phase under the given conditions (temperature, pH, and concentration) is called a saturated solution. For crystallization to occur, however, this equilibrium concentration or solubility must be exceeded. This excess concentration or chemical potential is called supersaturation. Supersaturated states may be created by increasing the solute concentration or decreasing the solute solubility using a variety of methods including the following ... 

Beer is capable of holding carbon dioxide in a supersaturated state so that rapid release of pressure or increase in temperature does not lead to the immediate attainment of the appropriate equilibrium. This has the advantage that when bottled beer with 3 vol. of carbon dioxide is poured into a glass, the gas does not normally gush uncontrollably but releases its carbon dioxide slowly. Considerable energy is required to create bubbles but they arise normally because of the presence of suspended solids, imperfections of the container, or mechanical agitation. 

Solubilization capacity is defined as solubilization effect of polymers on active ingredients in an aqueous solution. Needless to say, if the polymer can retain the drug in supersaturation state in the GI tract, it will significantly enhance bioavailability. Of all polymers used in ASD, amphiphilic polymers such as Soluplus have better solubilization capacity due to their ability to create micellar structures. On the other hand, most of ionic polymers such as methacrylate copolymers can create complexes with the drug and thus increase its solubility. 

When creating supersaturation levels sufficient to induce particle formation, precipitation of sparingly soluble salts and sol-gel processes are viewed differently. Precipitation normally involves mixing a cation solution with a precipitant solution. For example, consider preparation of an oxalate precursor to a CoO- and MnO-doped ZnO powder. In this process, the Zn, Mn, and Co are coprecipitated with controlled stoichiometry and the precipitate is calcined to the oxide. To form the oxalate, a state of supersaturation is created by mixing an aqueous solution of the metal nitrates or chlorides with an oxalate precipitant solution. The system is supersaturated with respect to the different metal oxalate phases and a crystalline coprecipitate forms. Depending on precipitation conditions (pH, concentrations, temperature, etc.), different metal complexes are present in solution. The form and concentration of these complexes determine the phase, morphology, and particle size distribution of the resulting precipitate. 

Suppose now that the rate of development of supersaturation is very rapid. Start with a saturated system a = 1 and suddenly create a supersaturation, say a = 4. The distribution of sizes that was characteristic of the saturated state has to alter and readjust itself to a new distribution that is characteristic of the supersaturated state. 

When a gas bubble has tom away, usually the small nucleus of a new bubble is left behind in its place. Therefore, in gas evolution an appreciable supersaturation is needed only for creating an initial set of nuclei, and subsequent processes require less supersaturation. Hence, in a galvanostatic transient the electrode s polarization will initially be higher but will then fall to a lower, steady-state value (Fig. 14.10). Such a time dependence of polarization is typical for many processes involving formation of a new phase. 

One of the most important results of including the bubble mechanism in the description of gas exchange is that it explains why gases are supersaturated when there is no net gas flux at the air-water interface. This situation is illustrated by considering a body of water in contact with the atmosphere at constant temperature where there is a sustained, constant brealdng of waves that creates a transfer of gases by bubbles (Fig. 10.11). At steady state there will be no net flux of inert gases across the air-water interface. The flux to the water by bubbles will be exactly balanced by the flux out via diffusive transfer across the surface (Ft = 0, Fawi=EB) and from Eqs. (10.1) and (10.33)... 

Table 10.3. I The relative degree of supersaturation 0/N2, Ar, and Ne at 20 °C created by bubble processes at steady state when there is no net gas flux across the air-water interface...

The proportion of substances in a solution depends on their limits of solution. The solubility of one substance in another is the maximum amount that can be dissolved at a given temperature and pressure. A solution containing such a maximum amount is saturated. A state of supersaturation can be created, but such solutions are unstable and may precipitate spontaneously. 

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