Crystallization of a Nucleus
Crystallization from solution involves a stepwise process where molecules of dissolved compounds become supersaturated upon cooling and subsequently condense into a more ordered form. An essential component in the crystallization process is the formation of nuclei, which are cores created around which molecules condense and become organized into the crystalline structure. The nucleation process begins when small clusters of molecules form a crystal lattice, which creates a kinetic barrier which must be overcome in order to form a true nucleus. Knowing the rate of nucleation allows researchers to control the rate at which a crystalline structure forms.
Nucleation rate, or the speed at which a nucleus forms, is determined by several factors that significantly influence the homogeneity of the crystalline structure. The first factor is the concentration of atom clusters, or alternatively, the degree of supersaturation. Higher concentrations will yield a faster rate of nucleation, while lower concentrations will slow down the rate. This can be further modulated by increasing or decreasing the cooling rate, with rapidly cooled solutions providing higher concentrations of atom clusters and therefore a higher nucleation rate.
The second factor that affects nucleation is the nature of the solution itself. Factors such as the presence of impurities or certain types of ions, can act as heterogeneous nucleation sites. These sites provide an increased rate of nucleation, as molecules are more likely to be “attracted” to them rather than forming an ordered arrangement among themselves.
The last main factor that affects nuclear growth is the presence of external energy. Energy can come from a multitude of sources, including thermal, electrical and light energy, and can be tuned to aid the formation of the crystal nucleus. This energy can be used to induce vibrational motion of the atoms, increasing the likelihood of interatomic interactions, and in turn, yielding an increased nucleation rate.
Thus, when investigating crystallization from solution, it is important to take into consideration the main variables that influence the rate of nucleation. By doing so, researchers can gain insight into the precise conditions that yield the most desirable features of the resulting crystalline structure. With the advancement of modern technology, crystallization processes can be very precisely controlled, allowing scientists to produce crystalline structures of greater homogeneity and quality than was ever possible before.
