Field of Supersaturation

The first consideration of the equipment designer is the control of the formation of a saturated solution. In order to do this, it is necessary to understand the field of supersaturation. 

The solubility chart divides the field of the solution into two regions the subsaturated region where the solution will dissolve more of the solute at fhe existing conditions, and file supersaturated region. 

Before Miers identified the metastable field, it was thought that a solution with a concentration of solute greater than the equilibrium amount would immediately form nuclei. Miers research and the findings of subsequent researchers determined that the field of supersaturation actually consists of at least three loosely identified regions (Fig. 1)  

Metastable region— where solute in excess of the equilibrium concentration will deposit on existing crystals, but no new nuclei are formed. 

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Fig. 30.2. Saturation states (Q/K) of supersaturated sulfate minerals over the courses of simulations in which seawater mixes at reservoir temperature with formation fluids from three North Sea oil fields. Reaction paths are the same as shown in Figure 30.1, except that minerals are not allowed to precipitate.

The chiton tooth, dentin and the sea urchin larval spicule reflect the enormous diversity of the field of biomineralization. They differ with respect to the nature of their mineral and macromolecular components, as well as their structures. Few underlying common strategies can be recognized the delineation of a dedicated space in which the mineralized tissue forms, the formation of mineral in a preformed framework within this space, and the precipitation of mineral from a supersaturated phase. In this section we will reexamine some of these underlying issues, focussing in particular on the microenvironment in which mineralization occurs. 

Work has been done to find whether the very expensive treatment of wines with artificial cold could be advantageously replaced by the addition of inhibitors of the crystallization process of tartrate salts such as metatartaric acid, yeast mannopro-teins or carboxymethylcellulose (CMC). Such inhibitors indeed increase the width of the supersaturation field of both KHT and TCa in the wine, thus delaying tartrate salt precipitation in the bottle. Metatartaric acid is currently the product most widely used for this purpose, though its efficacy is low as this compound does not remain stable over time. Yeast mannoproteins possess stabilizing properties, which result in the spontaneous improvement of protein and tartaric salt stability, as can be... 

The formation of ultrathin Me films on foreign substrates S (metals, superconductors, and semiconductors), S/Me, plays an important role in modern fields of technology such as micro- and nano-electronics, sensorics, electrocatalysis, etc. The process is often carried out by physical or chemical vapor deposition (PVD or CVD) of metals [6.152]. However, the difficult adjustment and control of the supersaturation via the gas flux is a great disadvantage of vapor deposition techniques. The situation becomes even more complicated, if more than one metal is deposited to form metallic sandwich layers and/or surface alloys. Therefore, electrochemical processes for the formation of ultrathin metal films and heterostructures became of great interest in modern thin layer technology. 

The reader will note many uses of qualitative terms to predict the behavior of these complex systems. As in the entire field of crystallization, these wide brackets around possibilities (e.g., will it crystallize, will it form an oil first, will it stay amorphous, will it grow, will it nucleate, what is good mixing, what is low supersaturation, etc. ) are necessary because of the extreme species and conditions dependency of the crystallization of organic molecules. The guidelines offered are intended as such and, in addition, to provide a framework for experimentation to determine where a particular system may fit in the wide scope of crystallization behavior possibilities. 

The mechanism for the formation of metal hydroxide surface precipitates is not clearly understood. It is clear that the type of metal ion determines whether metal hydroxide surface precipitates form, and the type of surface precipitate formed (i.e., metal hydroxide or mixed metal hydroxide) is dependent on the sorbent type. The precipitation could be explained by the combination of several processes (Yamaguchi et al., 2001). First, the electric field of the mineral surface attracts metal ions (e.g., Ni) through adsorption, leading to a local supersaturation... 

The technique of thermoparticulate analysis (TPA) consists of the detection of evolved particulate material in the evolved gases as a function of temperature. In the presence of supersaturated water vapor, these particles provide condensation sites for water, and hence can be detected by light-scattering techniques. Water droplets grow very rapidly on the particulate matter (condensation nuclei) until they are of a sufficient size to scatter light. The scattered light, as detected by a phototube in a dark-field optical system, is proportional to the number of condensation nuclei initially present. It is an extremely sensitive measurement, with the capability of detecting one part of material in 1015 parts of air. The technique was first employed by Doyle (90) and has been reviewed by Murphy (91. 92). 

Indeed, projecting from the intersections between the straight lines indicating conductivity and the two exponentials (A) and (B) to the temperature axis, produces temperatures Tsat and Tcsn respectively. The difference, Tsat — Tcs , defines the width of the supersatnration field of the wine in which i added KTH has been dissolved, expressed in degrees Celsius. The width of the supersaturation field is independent of the addition value i, as exponents (A) and (B) are roughly parallel. Thus, in the example described (Figure 1.11), the width of the supersaturation field is close to 21°C, whether 1.1 g/1 (Tsatu — Tcsi, =... 

The spontaneous crystallization temperature of each sample of treated wine (Table 1.16) was also determined using the same procedure. Examination of the results shows that a wine filtered on a 10 Da Millipore membrane, i.e. a wine from which all the colloids have been removed, has the lowest value for the supersaturation field (Tsat - Tcso), closest to that of the model dilute alcohol solution. Therefore, the difference between the results for this sample and the higher values of the supersaturation fields of fined samples define the effect of the protective colloids. It is interesting to note that the sample treated with metatartaric acid had the widest supersaturation field, and cold stabilization was completely ineffective in this case. This clearly demonstrates the inhibiting effect this polymer has on crystallization and, therefore, its stabilizing effect on wine (Section 1.7.6). Stabilization by this method, however, is not permanent. 

A method based on exposing a solution of a substrate to a very high, static electric field of -- 500 000 V/m to induce crystallization of a specific polymorph has been described [18] and shown to effect nucleation of the y-polymorph of glycine (Figure 1, 4). The same polymorph has been shown to crystallize selectively when a supersaturated solution of glycine is irradiated with laser light [19], Though currently limited in their application, these methods may likewise find more widespread use in the future. 

Closed dislocation loops as well as helicoids are observed very often in structures of high-temperature tested metals. Dark-field analysis makes it possible for one to determine the sign and the type of loops. Undoubtedly, the loops have been found to be of a vacancy type. Helicoids are known to be formed with screw dislocations usually, imder conditions of supersaturation with point crystalline defects. 

A qualitative interpretation of these results is that the hot-wire is heated by collision of supersaturated vapor molecules and clusters which will condense and discharge condensation heat energy to the wire surface. Although we have not directly identify a formation of clusters or liquid droplets in the vapor flow yet, we have another clue to imply their formation in the flow field measurement by laser beam attenuation method. A typical signal output from photodiode detector of R-11 vapor is shown in Fig. 4 (as symbolized by "LB"). The intensity of transmitted laser beam starts decreasing at the incidence of shock wave... 

The phenomenon of spontaneous condensation in a supersaturated vapour is of interest in different fields of natural science and in various technical applications, e.g. wind tunnels and steam turbines. Expecially with respect to Organic Rankine Cycles for heat recovery there exists some interest in the conditions for the onset of spontaneous condensation in the so-called Wilson point and in the influence of the released heat of condensation on the flow field for different working fluids. From this point of view, we started systematic investigations on spontaneous condensation of pure vapours in stationary supersonic nozzle flow with the intention to cover a wide range of thermodynamic state for substances of different molecular structure. In this paper we present experimental results for carbon dioxide and compare them in a first step with results... 

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