Nucleation and the Crystal Growth Process

Solution crystallisation can be viewed as a two-step process involving first the dissolution of a material, and then changing some attribute of the crystallising system, such as temperature, solubility or solvent content to induce aystallisation. At a given temperature and pressure there is a fixed amount of solute that can dissolve in a given amount of solvent to achieve chemical equilibrium. When this amount of solute is added, the solution is said to be saturated. The amount of solute required to make a saturated solution at a specified temperature and pressure is equivalent to the solubility [43]. 

In the nucleation stage, small clusters of solute molecules are formed some of these clusters may grow sufficiently to form stable nuclei and subsequently form crystals. Others fail to reach adequate dimensions before they dissolve again. Within the metastable zone width (MSZW), the induction time to the onset of crystallisation has an inverse relationship with the supersaturation [44-47]. 

Following their nucleation, crystals grown from solution typically exhibit regular, planar facets characterised by their Miller indices. Although appearing flat to the naked eye, these crystalline surfaces are rarely so at the molecular level. The various features which make up the nanoscale surface topography of crystal faces are intimately involved in the mechanisms by which crystals grow [48]. 

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Clontz and McCabe (1971) showed that at moderate levels of supersaturation, crystal contacts readily caused secondary nucleation of MgS04 7H2O, but crystal-crystal contacts gave up to five times as many nuclei as did crystal-metal rod contacts. Furthermore, the faster growing faces produced fewer nuclei than did the slower growing faces (Johnson, Rousseau and McCabe, 1972) indicating a connection between secondary nucleation and the crystal growth process. 

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