Supersaturated liquid

It is a well-known fact that substances like water and acetic acid can be cooled below the freezing point in this condition they are said to be supercooled (compare supersaturated solution). Such supercooled substances have vapour pressures which change in a normal manner with temperature the vapour pressure curve is represented by the dotted line ML —a continuation of ML. The curve ML lies above the vapour pressure curve of the solid and it is apparent that the vapour pressure of the supersaturated liquid is greater than that of the solid. The supercooled liquid is in a condition of metastabUity. As soon as crystallisation sets in, the temperature rises to the true freezing or melting point. It will be observed that no dotted continuation of the vapour pressure curve of the solid is shown this would mean a suspended transformation in the change from the solid to the liquid state. Such a change has not been observed nor is it theoretically possible. 

As soon as S > 0, a supersaturated liquid enters a metastable state and must degas to recover a supersaturating ratio equal to zero. In the case of Champagne wines, just after uncorking the bottle, Cl is the equilibrium concentration of CO2 in the liquid bulk corresponding to a partial... 

The factor a is difficult to evaluate and depends upon a number of conditions. It may be considered the percentage of supersaturated liquid which crystallizes. Eq (12-16) must be considered approximate, in view of the assumptions made, but it should be useful in determining the time required to crystallize a product of required size from a known size-distribution. 

Crystallization is the formation of solid particles from supersaturated liquid solution. Supersaturation is produced by the following ways. 

Several grams of the semi>fluorinated compound were placed in a windowed Jergeson Cell. The cell was placed in a horizontal position and the FnHm distributed on the window so that visual observations could easily be made. CO2 was then compressed into the windowed cell until the F Hm dissolved completely. In order to obtain a supersaturated liquid phase the vapor phase above the solution was vented. The progression from liquid to gel occurred in both of the compounds tested as shown in Figure 7. Unfortunately not enough of the FnHm was available to conduct PVT... 

In conventional crystallization processes, supersaturation (and hence nucleation) are caused by a thermal perturbation. Because of the inherently low thermal diffusivities of liquids, this approach is ahva3rs accompanied by the existence of temperature non-uniformities within the supersaturated liquid. This, in turn, gives rise to rather wide product size distributions. In the rapid expansion of a highly compressible supercritical mixture, on the other hand. 

Heterogeneous nudeation. Nncleation induced by foreign matter in a supersaturated liquid. Homogeneous nudeation. Spontaneous nuclemion caused by supersaturation only. 

Secondary nuelanlana. Nudeation of a supersaturated liquid caused directly or indirectly by the presence of crystals of the same species as the solute. 

VARIATIONS IN CRYSTALLIZERS. Commercial crystallizers may also be differentiated in several other ways. One important difference is in how the crystals are brought into contact with the supersaturated liquid. In the first technique, called the circidating-liquid method, a stream of supersaturated solution is passed through... 

As in any crystallization, the production of protein crystals requires bringing the protein into a supersaturated liquid state. The degree of supersaturation determines the rate of nucleation as well as crystal growth rate. Each of these phenomena are... 

What role does preordering (molecular aggregation) in the supersaturated liquid phase determine the subsequent nucleation and growth processes in polymorphic systems ... 

Heterogeneous nucleation. Nucleation induced by fin n matter in a supersaturated liquid. Homogeneous nucleation. Spontaneous nucleation caused by siqiersatination only. 

SiC nanofibers were synthesized by Honda et cd. on Si substrates covered by Ni thin films using high-power microwave plasma CVD under hydrogen gas [117]. The resultant fibrous material was identified as (3-SiC with a high crystallinity. SiC nanofibers were also fabricated by these authors using the VLS mechanism, in which SiC is precipitated from supersaturated liquid Ni nanoballs. 

Circulating-liquid evaporator-crystallizer. In a combination evaporator-crystallizer shown in Fig. 12.1 l-3a, supersaturation is generated by evaporation. The circulating liquid is drawn by the screw pump down inside the tube side of the condensing steam heater. The heated liquid then flows into the vapor space, where flash evaporation occurs giving some supersaturation. The vapor leaving is condensed. The supersaturated liquid... 

Answer by author No evidence of non-equilibrium caused by rapid liquid withdrawal was observed. If the liquid was subcooled initially, the pressure would decrease until it was in equilibrium with the liquid temperature. Circulation caused by the changing liquid level probably prevented our achieving the unstable condition of a supersaturated liquid. 

Simulating the crystallization process is a computational challenge, precisely because crystal nucleation is an activated process. This implies that the formation of small crystal nuclei in a supersaturated liquid is infrequent but, when it happens, the process is quite fast, i.e. it proceeds on a time scale that can be followed in a molecular simulation. For instance, experimentally measured nucleation rates are typically on the order of (9(10 ) to (9(10 ) nuclei per cm per sec. We can estimate the number of time steps needed in a molecular dynamics (MD) simulation to observe one nucleation event. In a large-scale computer simulation, it is feasible to study the dynamics of (9(10 ) particles, but the number of particles in a typical simulation is some two to three order of magnitude less. For an atomic liquid, the volume of a simulation box containing one million particles is of order (9(10 ) cm. If a million nuclei form per second in one cubic centimeter, then it will take, on average, 10 seconds for a nucleus to form in a system of a million particles. As the typical time step in a molecular simulation (MD) is on the order of femto seconds, this implies that it would take some 10 " MD time-steps to observe a single nucleation event under experimental conditions. 

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