Formation of a Supersaturated Solution

If a solution is to have only a slight degree of supersaturation, then a cyclic system in which large quantities of liquor are supersaturated uniformly is required. The solution must then be brought back to saturation before feed liquor is allowed to enter the system and the mixture is again supersaturated in the next cycle. 

The removal of the metastable supersaturation is a slow process. A large amount of crystal surface is required to allow for the large number of random collisions necessary to remove the supersaturation generated during the cycle. The proper orientation of both the molecules in solution and the molecule on the crystal surface is required for deposition, and the increased complexity of the molecule increases the number of collisions required for proper orientation. 

If the supersaturation generated during the cycle is not completely removed, the level of supersaturation attained during the following cycle is increased. This increase from cycle to cycle will continue until the supersaturation level of the solution exceeds the metastable region and enters the labile region, where spontaneous nucleation occurs. The occurrence of spontaneous nucleation means loss of control of crystal size. 

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A kinetic study36 carried out in the range 858 to 1000 K indicates the evolution of NOz, with the initial formation of a supersaturated solution of CaO in Ca(N03)2. 

Ic) The compound remains n solution after cooling the solvent fs unsuitable or a supersaturated solution has been formed. To check if the formation of a supersaturated solution has occurred, gently scratch the inside of the test tube at the surface of the solution with a Pyrex rod. The scfatchesprovide points of nucleation for crystal growth and crystals should form rapidly if the solution is supersaturated. Take care when scratching the test tube with the glass rod (see p. 100). If a supersaturated solution is formed, then the solvent is su/fabte forrecrystallization. 

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

Cooling solutions to below their freezing point results in the formation of ice. If solutions of sugars are cooled rapidly, non-equilibrium ice formation occurs. This is the most common form of ice in frozen dairy products (e.g. ice-cream). Rapid freezing of ice-cream mixes results in the freeze concentration of lactose and other sugars, resulting in supersaturated solutions if the temperature is too low to permit crystallization. The rapid cooling of lactose results in the formation of a supersaturated, freeze-concentrated amorphous matrix. 

The microscopic image shows a juxtaposition of differently orientated areas whose sizes, varying between a few microns and several tens of microns, are associated particularly with the elementary composition of the initial carbonaceous material (4, 18, 19). The formation of a texture of this type, often called a mosaic structure, can be compared (20) to the crystallization of a supersaturated solution areas, each characterized by a definite orientation, develop from nuclei up to the total consumption of the isotropic material surrounding them. 

Also, formation of a solid solution is often considered to be a prerequisite for the occurrence of a chemical compound layer, with the latter being a result of supersaturation of the former. In fact, however, these are two concurrent, competing processes, if both solid solutions and chemical compounds are present on the phase diagram of a binary system. In any... 

As mentioned earlier, the composition of natural groundwaters depends on the composition of the geological formations where they originate from they contain dissolved rock and soil components that were soluble under the conditions (such as temperature and pressure) of their formation. Their dissolution is governed by the law of thermodynamics that is, dissolution occurs when the solution is undersaturated with respect to components such as rocks and soils. Provided that the solid components are present in sufficient quantity and there is no kinetic barrier, this process may lead to a thermodynamic equilibrium. The reversed process of dissolution is precipitation, that is, the formation of a solid phase from the dissolved components of a supersaturated solution. The composition of the... 

In principle, the crystallization of a protein, nucleic acid, or virus (as exemplified in Figure 2.2) is little different than the crystallization of conventional small molecules. Crystallization requires the gradual creation of a supersaturated solution of the macromolecule followed by spontaneous formation of crystal growth centers or nuclei. Once growth has commenced, emphasis shifts to maintenance of virtually invariant conditions so as to sustain continued ordered addition of single molecules, or perhaps ordered aggregates, to surfaces of the developing crystal. 

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 relative growths of the faces of a crystal can be altered, and often controlled, by a number of factors. Rapid crystallization, such as that produced by the sudden cooling or seeding of a supersaturated solution, may result in the formation of needle crystals impurities in the crystallizing solution can stunt... 

Models used to describe the growth of crystals by layers call for a two-step process (/) formation of a two-dimensional nucleus on the surface and (2) spreading of the solute from the two-dimensional nucleus across the surface. The relative rates at which these two steps occur give rise to the mononuclear two-dimensional nucleation theory and the polynuclear two-dimensional nucleation theory. In the mononuclear two-dimensional nucleation theory, the surface nucleation step occurs at a finite rate, whereas the spreading across the surface is assumed to occur at an infinite rate. The reverse is tme for the polynuclear two-dimensional nucleation theory. Erom the mononuclear two-dimensional nucleation theory, growth is related to supersaturation by the equation. 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

The process of homogeneous nucleation is determined by the formation of stable nuclei in a supersaturated solution. 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

Detailed calculations show that the work of formation of a single nucleus in a supersaturated solution is determined by the expression... 

Thus, we see that the digestive ripening process leads to highly monodispersed nanoparticles that can come together to form ordered superstructures similar to atoms or molecules that form crystals from a supersaturated solution. Then if the superstructure formation can indeed be related to atomic/molecular crystallization, it should also be possible to make these supercrystals more soluble in the solvent with a change of temperature. Indeed, the optical spectra of the three colloids prepared by the different thiols discussed above exhibit only the gold plasmon band at 80 °C suggesting the solubilization of these superlattices at the elevated temperatures [49]. 

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