Recrystallization is a broad class of chemical purification techniques characterized by the dissolution of an impure sample in a solvent or solvent mixture, followed by some change in conditions that encourages the formation of pure isolate as solid crystals. Recrystallization as a purification technique is driven by spontaneous processes of self-assembly that leverage the highly ordered (i.e. low-entropy) and periodic characteristics of a crystal's molecular structure to produce purification.
The driving force of this purification emerges from the difference in molecular interactions between the isolate and the impurities: if a molecule of the desired isolate interacts with any isolate crystal present, it is likely the molecule deposits on the crystal's ordered surface and contributes to the crystal's growth; if a molecule of the impurity interacts with any isolate crystal present, it is unlikely to deposit on the crystal's ordered surface, and thus stays dissolved in the solvent. Initial crystals of isolate form by processes of stochastic nucleation and grow to macroscopic sizes when isolate molecules in solution deposit on them.
The simplest example of recrystallization is by temperature manipulation of a solution where the isolate compound has an endothermic dissolution (ÃÂH > 0) and a solubility product K<sub>sp</sub> that increases with temperature. A saturated solution of the impure sample (usually in a disordered state of matter, such as a solid powder or a viscous liquid) is prepared near or at the boiling point of the solvent, and then the solution is slowly cooled to form a supersaturated solution where crystal nucleation (and thus formation) is imminent.
The importance of crystallized compounds is so great that considerable effort and many reports describe methods for crystallization. Among the more popular methods are:
In a simple case, solution of a solid is cooled to below the stage of saturation. In some cases, the solution is prepared with a hot solvent. In some cases, a mixed solvent is employed, for example aqueous ethanol. Some of the solute will crystallize upon cooling. Ideally the precipitate will be absent some or most of impurities, which are more soluble in the solvent.
Two solvent recrystallization relies on the product being far more soluble in one solvent than a second solvent, which is called the antisolvent. The solvent and antisolvent must be miscible. The volume ratio between the solvent and antisolvent is important as well as the concentration of the sample. The antisolvent is added to the solution of the solute until incipient precipitation of the solid. The solution is then cooled or simply allowed stand to further induce further crystallization. In one variation of this method, the solution is layered with the antisolvent.
Recrystallized products are often subject to X-ray crystallography for purity assessment. The technique requires crystallized products to be singular, and absent of clumps. Several approaches to this phenomenon are listed below.