On the crystallization kinetics of highly soluble salts

Abstract

AbstractFor the spontaneous crystallization of highly soluble salts, a sufficiently high concentration of certain ionic species (complexes) or clusters has to be created in the solution, so that their grouping could yield a suitable crystal nucleus in a reasonably short time. The lowest critical supersaturation needed for nucleation and the highest rate of crystallization are displayed by those salts whose complexes in the solution have analogues in the crystal structure of the crystallizing salt, i.e., when the structure and the composition of the complexes enable their incorporation into the crystal lattice of the crystallizing salt with minimum changes. From this point of view a crystallochemical nucleation mechanism for explaining the Ostwald step rule is advanced. Concerning the rate of crystallization this concept was confirmed by studies on the system Na+, Mg2+/Cl–, SO42–//H2O and by parallel Raman spectroscopic studies of the microstructure of these solutions. It was established that upon increasing the concentration of Mg2+ ions and respective lowering of the water activity in the solution, the variety of the SO42– complexes increases. A direct correlation was found between the presence of various SO42– associations and the rate of crystallization of the corresponding salts in these systems.