Supersaturation stability states

Amorphous form provide the most rapid dissolution and the most often increased solubility by supersaturation however, practical usefulness is limited by stability issues, including transformation of the solid state form. 

The dipping technique is used (13), and because of the great stability of the solutions in the supersaturated state, large numbers of substrates, up to 30 at a time (19), can be dipped simultaneously under nearly isothermal conditions. 

In the aluminum oxide system the precipitation pH is one of the variables which controls the nature of the phase eventually obtained. However, aging conditions of the initially formed amorphous precipitate are at least equally important. In general, it can be stated that precipitation above pH = 8 leads to the formation of bayerite, while precipitation under more acidic conditions favors the subsequent formation of boehmite. Hydrargillite is formed as the product of the Bayer process by seeding a supersaturated alkali containing aluminum solution. The formation of bayerite is strongly facilitated by the presence of alkali cations which stabilize the structure. 

The concentration threshold above which crystallization is observed at times shorter than the processing time or desired product shelf-life or GI transit time, is determined by the kinetic stability of supersaturated states and is regulated by the nucleation mechanisms and kinetics. Nucleation phenomena are equally important in the control of micrometric properties and in the selective crystallization of a particular polymorph. 

Upon release of supersaUiration, the initially dissolved compound will be separated from the solution and form a secondary phase, which could be either oil, amorphous solid, or crystalline solid. Crystalline materials are solids in which molecules are arranged in a periodical three-dimensional pattern. Amorphous materials are solids in which molecules do not have a periodical three-dimensional pattern. Under some circumstances with very high supersaturation, the initial secondary phase could be a liquid phase, i.e., oil, in which molecules could be randomly arranged in three-dimensional patterns and have much higher mobility than solids. Generally, the oil phase is unstable and will convert to amorphous material and/or a crystalline solid over time. At a lower degree of supersaturation, an amorphous solid can be generated. Like the oil, the amorphous solid is unstable and can transform into a crystalline solid over time. Even as a crystalline solid, there could be different solid states with different crystal structures and stability. The formation of different crystalline solid states is the key subject of polymorphism, which will be mentioned below and... 

As this test is based on measuring the wine s electrical conductivity, it has the tremendous advantage that there is no need to collect the precipitate by filtration and determine the increase in weight. This new mini-contact test, measuring conductivity, is much faster (5-10 min instead of 2 h). Furthermore, by comparison with the first variant of the mini-contact test, as the contact surface (A) and, consequently, the state of supersaturation of the wine are multiplied by 2.5 (adding 10 g/1 of KTH instead of 4 g/1), it gives a more accurate assessment of a wine s stability. 

The throughput, i.e. the average time in the crystallizer, is defined according to the wine s initial state of supersaturation, as well as the type of preparatory treatment (fining, bentonite, etc.) the wine received prior to artificial cold stabilization. The importance of preparation has already been mentioned (Section 1.6.4). 

In practice, however, the dynamic behaviour of a continuous crystallizer is not so simple. The above stabilizing effect is subject to a considerable time lag, because newly formed nuclei have no appreciable surface area for a long time. Before stabilization can occur, therefore, large numbers of nuclei might be formed which will later reduce the supersaturation below its steady-state value. The resulting slow nucleation will lead to a decrease of the total crystal surface area, below its steady-state value, and this in turn will cause an increase of the supersaturation above its steady-state value, and so on. 

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