Crystallisation from supersaturated solution

Increase in temperature causes a slight decrease in refractive index. The heat of dissolution 4 of the octahedral form is - 7530 calories at 18° C. The velocity of crystallisation from supersaturated solutions corresponds with 5 - dcjdt = fre4, where c is the concentration the temperature coefficient for the interval 0 to 25° C. is zero. 

With regard to the limits of supersaturation and the spontaneous crystallisation of the solute from supersaturated solutions, see Jaffe, Z. physikaL Chem., 1903, 43, 565 j Miers and Isaac,y. Chem, Soc, 1906, 89, 413 1908, 93, 384 Hartley, Jones, and Hutchinson, Jones, 1739 Fouquet, Compt rena.j 1910,... 

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. 

Occasionally substances form supersaturated solutions from which the first crystals separate with difficulty this is sometimes caused by the presence of a little tar or viscous substance acting as a protective colloid. The following methods should be tried in order to induce crystallisation —... 

General Considerations.—Solid crystallisable substances are usually obtained at the end of a reaction in the form of a crude product which separates in more or less pure form from the solvent on cooling, either directly or after concentration. The rate at which organic substances crystallise varies within very wide limits, and their tendency to form supersaturated solutions is extraordinarily great. But even when supersaturation is counteracted by dropping a crystal into the solution—by seeding —the attainment of equilibrium in the cold saturated solution is often exceedingly slow. The cause is indeed the slow rate of crystallisation. Hence the full yield of crude product is often obtained only after the solution has been left for many hours. 

In the rate of crystallisation of a substance from a supersaturated solution two independent factors have to he considered, firstly the rate of nucleus formation from which crystallisation may proceed and secondly the rate of growth of a nucleus once it is formed. 

Co crystallisation is mainly done from supersaturated sugar solutions [15]. Aggregated particles (of 3-30 pm) of sugar crystals are formed which entrap guest molecules. The sugars form an oxygen barrier, thereby extending the shelf life of aroma chemicals. The procedure is simple and inexpensive, because relatively cheap encapsulation matrices can be used, such as sucrose. 

Because of the difficulties associated with the characterisation of heteronuclei in solution, few studies have attempted to explain experimental results in a quantitative way. If it is assumed that, once nucleation occurs, the particles grow without recrystallisation, then it is possible to get information about the particle density from a consideration of the geometry of the particles and the growth kinetics. One approach is to add heteronuclei to supersaturated solutions and measure the crystallisation kinetics and, from the data obtained, estimate the surface area of the growing crystals. In this way, it is feasible to obtain information about the nucleation capability of different heteronuclei and the effects of pretreatments on the nucleation capability. An example of such an application will be discussed in Sect. 5.4. 

Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles. Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. 

If a liquid solution having a composition represented by a point lying to the left of the eutectic point C be cooled down, the solid component A will crystallise out (supersaturation supposed excluded) when the temperature reaches the point on the curve AC corresponding with the initial composition of the solution. If the temperature be allowed to fall still further, more and more of the component A will crystallise out, and the composition of the liquid solution will alter in the direction of C. When the composition of the point C is reached, solid B, also, can begin to crystallise out. If one continues to withdraw heat from the system, solid A and solid B will separate out... 

Rapid crystallisation from super-cooled melts and supersaturated solutions often results in a "fluffy" tree-like crystal formation called dendrites. The main crystal... 

The process of crystallisation from waxy hydrocarbons may be regarded as similar in some respects to precipitation from a normal solubility salt. The requirement for supersaturation to exist before crystallisation can begin, is less certain than for inorganic solutions, but it is likely to be related to the concentration of the material in solution. 

An important feature of transient supersaturation is its duration or, in other words, the rates of solute nucleation and crystal growth. Some of the factors that influence these rates have already been discussed for the case of pure water. For crystallisation from solutions, two additional factors are the viscosity of the residual freeze-concentrated liquid phase and probably also the conflgurational complexity of the crystal structures of the solutes involved. Thus, for NaCl, a degree of supersaturation of up to 6 M, reached at ca. —25°C, seems reasonable, at which temperature the salt is likely to crystallise spontaneously. For a typical cooling rate of 10°C min the duration of supersaturation would then be ca. 5 min, with the NaCl concentration transient reaching 6 M ... 

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