Solution crystallisation

Driving force for melt crystallisation is melting point (mp) difference of impurities and desired product feed must meet mp and crystal characteristics for feasibUity (see also solution crystallisation). 

A liquid solution may be separated into its constituents by crystallising out either pure solvent or pure solute, the latter process occurring only with saturated solutions. (At one special temperature, called the cryohydric temperature, both solvent and solute crystallise out side by side in unchanging proportions.) We now consider what happens when a small quantity of solute is separated from or taken up by the saturated solution by reversible processes. Let the saturated solution, with excess of solute, be placed in a cylinder closed below by a semipermeable septum, and the w7hole immersed in pure solvent. The system is in equilibrium if a pressure P, equal to the osmotic pressure of the saturated solution when the free surface of the pure solvent is under atmospheric pressure, is applied to the solution. Dissolution or precipitation of solute can now be brought about by an infinitesimal decrease or increase of the external pressure, and the processes are therefore reversible. If the infinitesimal pressure difference is maintained, and the process conducted so slowly that all changes are isothermal, the heat absorbed when a mol of solute passes into a solution kept always infinitely... 

Solution crystallisers are usually classified according to the method by which supersaturation is achieved, that is by cooling, evaporation, vacuum, reaction and salting out. The term controlled denotes supersaturation control whilst classifying refers to classification of product size. 

Capobianco, C.H., Hervig, R.L., Drake, M. 1994. Experiments on crystal/liquid partitioning of Ru, Rh and Pd for magnetite and hematite solid solutions crystallised from silicate melt. Chemical Geology, 113, 23-43. 

Cocrystals are often prepared by a traditional solution crystalhsation approach such as solvent evaporation, coohng, or anti-solvent addition. There are a number of reasons for the popularity of the solution-based approach. Solution crystallisation can yield large, well-formed single crystals, from which one may easily evaluate crystal habit and surface features. Analysis of the diffraction pattern of a single crystal is typically the best means of obtaining an absolute crystal structure determination. Further, solution crystalhsation is an established and effective purification step. 

Interestingly, in the same work [23] Etter and Adsmond reported two polymorphs of a 1 1 cocrystal containing 25 and malonic acid 27. Both crystallised from solution, but only one was obtainable from solid-state grinding. Contrary to the examples mentioned above, this instance suggests that solution crystallisation may at times offer more product diversity than grinding. 

The structure solution of the cocrystal of 37 and 29 from powder XRD illustrates the current utility of this approach in the field of crystal engineering. While the material above was prepared from solution crystallisation, it is clear that this approach directly impacts cocrystals prepared by sohd-state grinding. 

Solution crystallisation (adiabatic evaporation (vacuum cooling))... 

The normal sulphates show a marked tendency to the formation of double salts,8 the best known case being that of the alums, which are isomorphous compounds of the general formula M2(S04)3.X2S04. 24H20, where M and X represent a tervalent and univalent metal, respectively in aqueous solution these double salts are almost entirely resolved into the ions of their constituent salts,9 recombination taking place as the solution crystallises. Double salts are also formed by the crystallisation of fused mixtures of anhydrous sulphates, the freezing-... 

Supersaturated Solutions.—When a solution of a solid in water, already saturated at a given temperature, is heated up with more of the solid until the whole of the latter has passed into solution, crystallisation or precipitation of the excess of solute does not always take place upon cooling the system to the original temperature. It is clear that the solution must now hold a greater quantity of substance than corresponds to the ordinary solubility and is said to be supersaturated. Such solutions can readily be prepared by heating up sodium thiosulphate, sodium acetate, or sodium sulphate with water, and allowing to cool without agitation. 

The nascent powder sample has quite a different morphology compared to the solution-crystallised or melt-crystallised samples. The nascent powder morphology mainly consists of particles connected by fibrils, which is called the "cobweb" structure.26 27 The nascent powder does not have any typical lamellar morphology but has a domain structure where crystalline domains distribute within the whole powder globule (Figure 3C). The domain size has a wide range of several tens of nanometres radius. 

Figure l.28 The beats, showing a combination of intensity drop and subsequent recovery, are clearly observed in the 20-30 /rs region on these FIDs for the higher crystalline solution-crystallised materials and the nascent powder. These two samples undergo a rapid decay, indicating that the solution-crystallised and nascent powder samples are more rigid than the melt-crystallised ones. 

The integral width except for the longest component of the melt-crystallised sample is plotted in Figure 5 for the solution-crystallised, melt-crystallised and... 

The melt-crystallised sample skipped Process 1 due to the limited relaxation of its entangled molecules on the lamellar surfaces, rather than the adjacent re-entry folding for the solution-crystallised sample. 

Annealing of the nascent powder sample passed through Process 1, and above 90 °C, dynamic molecular motion started, as defined by Process 2. This critical temperature is slightly higher than that of the solution-crystallised sample. This difference indicates the restricted crystalline chain motion for the domain network structure crystallised during polymerisation. In Process 2, the crystallinity remained at a constant level for the nascent powder sample. This shows that the lamellar thickening is limited for the nascent powder morphology. 

Such a difference of the amorphous characteristic was accountable by NMR relaxation analysis. Figure 10 compares the FIDs for a series of solution-crystallised materials prepared from different prior polymer concentrations.40... 

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